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Research ArticleMean-Field Backward Stochastic Evolution Equations inHilbert Spaces and Optimal Control for BSPDEs
Ruimin Xu12 and Tingting Wu3
1 School of Mathematics Shandong University Jinan 250100 China2 School of Mathematics Shandong Polytechnic University Jinan 250353 China3 School of Mathematical Sciences Shandong Normal University Jinan 250014 China
Correspondence should be addressed to Ruimin Xu ruiminx126com
Received 31 March 2014 Revised 23 May 2014 Accepted 18 June 2014 Published 13 July 2014
Academic Editor Guangchen Wang
Copyright copy 2014 R Xu and T Wu This is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited
We obtain the existence and uniqueness result of the mild solutions to mean-field backward stochastic evolution equations (BSEEs)in Hilbert spaces under a weaker condition than the Lipschitz one As an intermediate step the existence and uniqueness result forthe mild solutions of mean-field BSEEs under Lipschitz condition is also established And then a maximum principle for optimalcontrol problems governed by backward stochastic partial differential equations (BSPDEs) of mean-field type is presented In thiscontrol system the control domain need not to be convex and the coefficients both in the state equation and in the cost functionaldepend on the law of the BSPDE as well as the state and the control Finally a linear-quadratic optimal control problem is given toexplain our theoretical results
1 Introduction
Backward stochastic evolution equations (BSEEs) in theirgeneral nonlinear form were introduced by Hu and Peng [1]in 1991 By the stochastic Fubini theorem and an extendedmartingale representation theoremHu andPeng [1] obtainedthe existence and uniqueness result of a so-called ldquomildsolutionrdquo under Lipschitz coefficients for semilinear BSEEsSince then BSEEs have been studied by a lot of authors andhave found various applications namely in the theory ofinfinite dimensional optimal control and the controllabilityfor stochastic partial differential equations (see eg [1ndash4] andthe papers cited therein) To relax the Lipschitz condition ofthe coefficients Mahmudov andMckibben [2] studied BSEEsunder a weaker condition than the Lipschitz one in Hilbertspaces Their approach extended the method proposed byMao [5] in which the author investigated BSDEs under aweaker condition which contains Lipschitz condition as aspecial case Our present work also investigates backwardstochastic evolution equations but with one main differ-ence to the setting chosen by the papers mentioned above
the coefficients of the BSEEs are allowed to depend on thelaw of the BSEEs
Recently mean-field approaches which can be usedto describe particle systems at the mesoscopic level haveattracted more and more researchersrsquo attention because oftheir great importance in applications For example mean-field approach can be used in statistical mechanics andphysics quantum mechanics and quantum chemistry eco-nomics finance game theory and optimal control theory(refer to [6ndash8] and the references therein) Mean-field BSDEswere deduced by Buckdahn et al [9] when they investi-gated a special mean-field problem in a purely stochasticapproach Buckdahn et al [7] studied the well posedness ofmean-field BSDEs and gave a probabilistic interpretation tosemilinear McKean-Vlasov partial differential equations Togive a probabilistic representation of the solutions for a classofMckean-Vlasov stochastic partial differential equations Xu[10] investigated the well-posedness of mean-field backwarddoubly stochastic differential equations with locally mono-tone coefficients
Hindawi Publishing CorporationMathematical Problems in EngineeringVolume 2014 Article ID 718948 18 pageshttpdxdoiorg1011552014718948
2 Mathematical Problems in Engineering
In this paper we investigate a new type of backwardstochastic evolution equations inHilbert spaces whichwe callmean-field BSEEs Mean-field implies that the coefficient ofthe BSEE depends on the law of the BSEE Specifically theBSEE we consider is defined as
119889119884 (119904) = minus 119860119884 (119904) 119889119904
minus E1015840[119891 (119904 119884
1015840
(119904) 1198851015840
(119904) 119884 (119904) 119885 (119904))] 119889119904
+ 119885 (119904) 119889119882 (119904)
119884 (119879) = 120585 119904 isin [0 119879]
(1)
in a Hilbert space 119867 where 119891 denotes a given measurablemapping 119879 is a fixed positive real number 119882(119904) is acylindrical Wiener process and 119860 represents the generatorof a strongly continuous semigroup 119890
119905119860 in 119867 with 119905 ge 0Precise interpretation of E1015840[119891(119904 1198841015840
(119904) 1198851015840(119904) 119884(119904) 119885(119904))] is
given in the following sections Based on the contractionmapping we firstly prove that (1) admits a unique mildsolution if the function 119891 is Lipschitz continuous Secondlyunder non-Lipschitz assumptions we obtain the existenceand uniqueness of the mild solution for mean-field BSEEby constructing a special Cauchy sequence The Lipschitzcondition is a special case of this non-Lipschitz condition (seeMao [5]) In addition we investigate the well-posedness ofmean-field stochastic evolution equations
We also study optimal control problems of stochasticsystems governed by mean-field BSPDEs in Hilbert spacesOur objective is to formulate a stochastic maximumprinciple(SMP) for the optimal control problem with an initial stateconstraint There is a vast literature on the theory of SMPAmong these papers Andersson and Djehiche [8] studiedthe optimal control problem for mean-field stochastic systemwhen the control domain is convex They obtained themaximum principle by a convex variational method By aspike variational technique Buckdahn et al [11] obtained ageneral maximum principle for a special mean-field stochas-tic differential equation (SDE) where the action space is notconvex Later Li [12] investigated the maximum principlefor more general SDEs of mean-field type with a convexcontrol domain Wang et al [13] were concerned with apartially observed optimal control problem of mean-fieldtype By using Girsanovrsquos theorem and convex variationthey derived the correspondingmaximum principle and gavean illustrative example to demonstrate the application ofthe obtained SMP Hafayed studied the mean-field SMP forsingular stochastic control in [14] and mean-field SMP forFBSDEs with Poisson jump processes in [15]
For the case of stochastic control systems in infinitedimensions on the assumption that the control domain isnot necessarily convex while the diffusion coefficient doesnot contain the control variable Hu and Peng [16] usedspike variation approach and Ekelandrsquos variational principleto establish the maximum principle for semilinear stochas-tic evolution control systems with a final state constraintMahmudov and Mckibben [2] obtained an SMP for stochas-tic control systems governed by BSEEs in Hilbert spacesRecently Fuhrman et al [17] deduced themaximumprinciple
for optimal control of stochastic PDEs when the controldomain is not necessarily convex
We establish necessary optimality conditions for thecontrol problem in the form of a maximum principle on theassumption that the control domain is not necessarily convexDue to the initial state constraint we first need to applyEkelandrsquos variational principle to convert the given controlproblem into a free initial state optimal control problemThenspike variation approach is used to deduce the SMP in themean-field framework In our control system not only thestate processes which are the unique mild solution of thegiven BSPDE but also the cost functional are of mean-fieldtype In other words they depend on the law of the BSPDEas well as the state and the control For this new controlledsystem the adjoint equation will turn out to be a mean-fieldstochastic evolution equation
Theplan of this paper is organized as follows In Section 2we introduce some notations which are needed in whatfollows In Section 3 the well-posedness of mean-field BSEE(1) is studied we first prove the existence and uniqueness of amild solution under the Lipschitz condition and investigatethe regular dependence of the solution (119884 119885) on (120585 119891)And then under the assumption that the coefficient is non-Lipschitz continuous a new result on the existence anduniqueness of the mild solution to (1) in Hilbert space isestablished which generalizes the result for the Lipschitzcase Section 4 is devoted to the regularity of mean-fieldstochastic evolution equations In Section 5 we derive thestochastic maximum principle for the BSPDE systems ofmean-field type with an initial state constraint and at thelast part of Section 5 an LQ example is given to show theapplication of our maximum principle An explicit optimalcontrol is obtained in this example
2 Preliminaries
The norm of an element 119909 in a Banach space 119865 is denotedby |119909|
119865or simply |119909| if no confusion is possible Γ 119867 and
119870 are three real and separable Hilbert spaces Scalar productis denoted by ⟨sdot sdot⟩ with a subscript to specify the space ifnecessaryL(Γ 119870) is the space of Hilbert-Schmidt operatorsfrom Γ to 119870 endowed with the Hilbert-Schmidt norm
Let (ΩFP) be a complete probability space A cylin-drical Wiener process 119882(119905) 119905 ge 0 in a Hilbert space Γ is afamily of linear mappings Γ rarr 119871
2(ΩFP) such that
(i) for every 119906 isin Γ 119882(119905)119906 119905 ge 0 is a real (continuous)Wiener process
(ii) for every 119906 V isin Γ and 119905 119904 ge 0 E(119882(119905)119906 sdot 119882(119904)V) =(119905 and 119904)⟨119906 V⟩
Γ
By F119905 119905 isin [0 119879] we denote the natural filtration of 119882
augmented with the familyN of P-null sets ofF119879
F119905= 120590 (119882 (119904) 119904 isin [0 119905]) orN (2)
The filtration (F119905)119905ge0
satisfies the usual conditions Allthe concepts of measurability for stochastic processes (egadapted etc) refer to this filtration
Mathematical Problems in Engineering 3
Next we define several classes of stochastic processes withvalues in a Hilbert space119867
(I) H2
F ([0 119879]119867) denotes the set of (classes of 119889P times
119889119905 ae equal) measurable random processes 120595119905 119905 isin
[0 119879] which satisfy
(i) Eint1198790|120595
119905|2119889119905 lt +infin
(ii) 120595119905isF
119905measurable for ae 0 le 119905 le 119879
Evidently H2
F (0 119879119867) is a Banach space en-dowed with the canonical norm
10038171003817100381710038171205951003817100381710038171003817 = Eint
119879
0
100381610038161003816100381612059511990410038161003816100381610038162
119889119904
12
(3)
(II) S2
F ([0 119879]119867) denotes the set of continuous randomprocesses 120595
119905 119905 isin [0 119879] which satisfy
(i) E(sup0le119905le119879
|120595119905|2) lt +infin
(ii) 120595119905isF
119905measurable for ae 0 le 119905 le 119879
(III) 1198710(ΩFP 119867) denotes the space of all 119867 valued F-measurable random variables
(IV) For 1 le 119901 lt infin 119871119901(ΩFP 119867) is the space of allF-measurable random variables such that E[|120585|119901] lt infin
(V) For any 120573 isin R introduce the norm
1003817100381710038171003817(119910 119911)10038171003817100381710038172
120573119905= Eint
119879
119905
1198902120573119904
(1003816100381610038161003816119910 (119904)
10038161003816100381610038162
+ |119911 (119904)|2) 119889119904 (4)
on the Banach space
K120573[119905 119879] = S
2
F ([119905 119879] 119867) timesH2
F ([119905 119879] L (Γ119867)) (5)
For 0 lt 119879 lt infin all the norms sdot 120573119905
with different 120573 isin R areequivalentK[0 119879] = K
0[0 119879] is the Banach space endowed
with the norm
1003817100381710038171003817(119910 119911)10038171003817100381710038172
= Eint119879
0
(1003816100381610038161003816119910 (119904)
10038161003816100381610038162
+ |119911 (119904)|2) 119889119904 (6)
The following result on BSEEs (see Lemma 2 in Mahmu-dov and McKibben [2]) will play a key role in proving thewell-posedness of mean-field BSEEs
Lemma 1 Let 119867 be a Hilbert space and let 119860 119863(119860) sub
119867 rarr 119867 be a linear operator which generates a 1198620-semigroup
119878(119905) 0 le 119905 le 119879 on 119867 For any (120585 119891) isin 1198712(ΩF
119879P 119867) times
H2
F ([0 119879]119867) the following equation
119884 (119905) = 119878 (119879 minus 119905) 120585 + int
119879
119905
119878 (119904 minus 119905) 119891 (119904) 119889119904
+ int
119879
119905
119878 (119904 minus 119905) 119885 (119904) 119889119882 (119904) 119875-119886119904(7)
has a unique solution inK120573[0 119879] moreover
E sup119905le119904le119879
1198902120573119904
|119884 (119904)|2+ Eint
119879
119905
1198902120573119904
|119885 (119904)|2119889119904
le 241198722
119878(119890
2120573119879E100381610038161003816100381612058510038161003816100381610038162
+1
2120573int
119879
119905
1198902120573119903
E1003816100381610038161003816119891 (119903)
10038161003816100381610038162
119889119903)
(8)
where119872119878= sup119878(119905)B(119867) 0 le 119905 le 119879 andB(119867) is the space
of bounded linear operators on119867
3 Mean-Field Backward StochasticEvolution Equations
In this section we study the existence and uniqueness resultof mild solutions to mean-field BSEEs in a Hilbert space 119867To this end we firstly recall some notations introduced byBuckdahn et al [7]
Let (ΩFP) = (Ω times ΩF otimes FP otimes P) be the(noncompleted) product of (ΩFP) with itself and wedefine F = F
119905= F otimes F
119905 0 le 119905 le 119879 on this product
space A random variable 120585 isin 1198710(ΩFP 119867) originally
defined on Ω is extended canonically to Ω 1205851015840(120596
1015840 120596) =
120585(1205961015840) (120596
1015840 120596) isin Ω = Ω times Ω For any 120578 isin 119871
1(ΩFP) the
variable 120578(sdot 120596) Ω rarr 119870 belongs to 1198711(ΩFP)P(119889120596) aswhose expectation is denoted by
E1015840[120578 (sdot 120596)] = int
Ω
120578 (1205961015840 120596)P (119889120596
1015840) (9)
Note that E1015840[120578] = E1015840[120578(sdot 120596)] isin 1198711(ΩFP) and
E [120578] (= intΩ
120578119889P = intΩ
E1015840[120578 (sdot 120596)]P (119889120596)) = E [E
1015840[120578]]
(10)
Themean-field BSEE we consider has the following formfor any given measurable mapping 119891 [0 119879] times119867timesL(Γ119867)times
119867 timesL(Γ119867) rarr 119867 and 120585 isin 1198712(ΩF119879P 119867)
119889119884 (119904) = minus 119860119884 (119904) 119889119904
minus E1015840[119891 (119904 119884
1015840
(119904) 1198851015840
(119904) 119884 (119904) 119885 (119904))] 119889119904
+ 119885 (119904) 119889119882 (119904)
119884 (119879) = 120585 119904 isin [0 119879]
(11)
where 119860 119863(119860) sub 119867 rarr 119867 is the generator of a stronglycontinuous semigroup 119890119905119860 119905 ge 0 in the Hilbert space119867 withthe notation119872
119860≜ sup
119905isin[0119879]|119890119905119860|
4 Mathematical Problems in Engineering
Definition 2 We say that a pair of adapted processes (119884 119885)is a mild solution of mean-field BSEE (11) if (119884 119885) isin
S2
F ([0 119879]119867) timesH2
F ([0 119879]L(Γ119867)) and for all 119905 isin [0 119879]
119884 (119905) = 119890119860(119879minus119905)
120585
+ int
119879
119905
119890119860(119904minus119905)
E1015840[119891 (119904 119884
1015840
(119904) 1198851015840
(119904) 119884 (119904) 119885 (119904))] 119889119904
minus int
119879
119905
119890119860(119904minus119905)
119885 (119904) 119889119882 (119904)
(12)
Remark 3 We emphasize that the coefficient of (11) can beinterpreted as
E1015840[119891 (119904 119884
1015840
(119904) 1198851015840
(119904) 119884 (119904) 119885 (119904))] (120596)
= E1015840[119891 (119904 119884
1015840
(119904) 1198851015840
(119904) 119884 (120596 119904) 119885 (120596 119904))]
= intΩ
119891 (1205961015840 120596 119904 119884 (120596
1015840 119904) 119885 (120596
1015840 119904) 119884 (120596 119904) 119885 (120596 119904))
times P (1198891205961015840)
(13)
31 Lipschitz Case Now we study the existence and unique-ness ofmild solutions tomean-field BSEE (11) under Lipschitzconditions For119891 [0 119879] times 119867 times L(Γ119867) times 119867 times L(Γ119867) rarr
119867 assume the following(A1) There exists an 119871 gt 0 such that
10038161003816100381610038161003816119891 (119905 119910
1015840
1 119911
1015840
1 119910
1 119911
1) minus 119891 (119905 119910
1015840
2 119911
1015840
2 119910
2 119911
2)10038161003816100381610038161003816
2
le 119871 (100381610038161003816100381610038161199101015840
1minus 119910
1015840
2
10038161003816100381610038161003816
2
+100381610038161003816100381610038161199111015840
1minus 119911
1015840
2
10038161003816100381610038161003816
2
+10038161003816100381610038161199101 minus 1199102
10038161003816100381610038162
+10038161003816100381610038161199111 minus 1199112
10038161003816100381610038162
)
(14)
for all 119905 isin [0 119879] 1199101015840119894 119910
119894isin 119867 119911
1015840
119894 119911
119894isin L(Γ119867) (119894 =
1 2)(A2) 119891(sdot 0 0 0 0) isin H2
F ([0 119879]119867)We have the following theorem
Theorem 4 For any random variable 120585 isin 1198712(ΩF
119879P 119867)
under (A1) and (A2) mean-field BSEE (11) admits a uniquemild solution (119884 119885) isin S2
F ([0 119879]119867) timesH2
F ([0 119879]L(Γ119867))
Proof Consider the following
Step 1 For any (119910 119911) isin S2
F ([0 119879]119867) timesH2
F ([0 119879]L(Γ119867))BSEE119884 (119905) = 119890
119860(119879minus119905)120585
+ int
119879
119905
E1015840[119890
119860(119904minus119905)119891 (119904 119910
1015840
(119904) 1199111015840
(119904) 119884 (119904) 119885 (119904))] 119889119904
minus int
119879
119905
119890119860(119904minus119905)
119885 (119904) 119889119882 (119904) 0 le 119905 le 119879
(15)
has a unique solution In order to get this conclusion wedefine
119891(119910119911)
(119904 120583 ]) = E1015840[119891 (119904 119910
1015840
(119904) 1199111015840
(119904) 120583 ])] (16)
Then (15) can be rewritten as
119884 (119905) = 119890119860(119879minus119905)
120585
+ int
119879
119905
119890119860(119904minus119905)
119891(119910119911)
(119884 (119904) 119885 (119904)) 119889119904
minus int
119879
119905
119890119860(119904minus119905)
119885 (119904) 119889119882 (119904)
(17)
Due to (A1) for all (1205831 ]
1) (120583
2 ]
2) isin 119867timesL(Γ119867)119891 satisfies
10038161003816100381610038161003816119891(119910119911)
(1205831 ]
1) minus 119891
(119910119911)(120583
2 ]
2)10038161003816100381610038161003816
2
le 119871 (10038161003816100381610038161205831 minus 1205832
10038161003816100381610038162
+1003816100381610038161003816]1 minus ]
2
10038161003816100381610038162
)
(18)
According to Theorem 31 in [1] BSEE (15) has a uniquesolution
Step 2 FromStep 1 we can define amappingΦ (119884(sdot) 119885(sdot)) =
Φ[(1199101015840(sdot) 119911
1015840(sdot))] K[0 119879] rarr K[0 119879] through
119884 (119905) = 119890119860(119879minus119905)
120585
+ int
119879
119905
E1015840[119890
119860(119904minus119905)119891 (119904 119910
1015840
(119904) 1199111015840
(119904) 119884 (119904) 119885 (119904))] 119889119904
minus int
119879
119905
119890119860(119904minus119905)
119885 (119904) 119889119882 (119904) 0 le 119905 le 119879
(19)
For any (119910119894 119911119894) isin K[0 119879] we set (119884119894 119885
119894) = Φ[(119910
119894 119911
119894)] 119894 =
1 2 (119910 119911) = (1199101minus119910
2 119911
1minus119911
2) and (119884 119885) = (119884
1minus119884
2 119885
1minus119885
2)
Then from Lemma 1 we have
E sup0le119904le119879
119890212057311990410038161003816100381610038161003816
119884 (119904)10038161003816100381610038161003816
2
+ Eint119879
0
119890212057311990410038161003816100381610038161003816
119885 (119904)10038161003816100381610038161003816
2
119889119904
le12119872
2
119860
120573E
Mathematical Problems in Engineering 5
times [int
119879
0
1198902120573119904
100381610038161003816100381610038161003816E1015840[119891 (119904 (119910
1
(119904))1015840
(1199111
(119904))1015840
1198841
(119904) 1198851
(119904))
minus 119891 (119904 (1199102
(119904))1015840
(1199112
(119904))1015840
1198842
(119904) 1198852
(119904))]100381610038161003816100381610038161003816
2
119889119904]
le12119872
2
1198601198712
120573E
times [int
119879
0
1198902120573119904
(E [1003816100381610038161003816119910 (119904)
10038161003816100381610038162
+ |119911 (119904)|2]
+10038161003816100381610038161003816119884 (119904)
10038161003816100381610038161003816
2
+10038161003816100381610038161003816119885 (119904)
10038161003816100381610038161003816
2
) 119889119904]
=12119872
2
1198601198712
120573E
times [int
119879
0
1198902120573119904
(1003816100381610038161003816119910 (119904)
10038161003816100381610038162
+ |119911 (119904)|2
+10038161003816100381610038161003816119884 (119904)
10038161003816100381610038161003816
2
+10038161003816100381610038161003816119885 (119904)
10038161003816100381610038161003816
2
) 119889119904]
(20)
If we set 120573 = 361198722
1198601198712max119879 1 then
Eint119879
0
1198902120573119904
(10038161003816100381610038161003816119884 (119904)
10038161003816100381610038161003816
2
+10038161003816100381610038161003816119885 (119904)
10038161003816100381610038161003816
2
) 119889119904
le 119879 sdot E sup0le119904le119879
119890212057311990410038161003816100381610038161003816
119884 (119904)10038161003816100381610038161003816
2
+ Eint119879
0
119890212057311990410038161003816100381610038161003816
119885 (119904)10038161003816100381610038161003816
2
119889119904
le12119872
2
1198601198712max 119879 1120573
E
times [int
119879
0
1198902120573119904
(1003816100381610038161003816119910 (119904)
10038161003816100381610038162
+ |119911 (119904)|2
+10038161003816100381610038161003816119884 (119904)
10038161003816100381610038161003816
2
+10038161003816100381610038161003816119885 (119904)
10038161003816100381610038161003816
2
) 119889119904]
=1
3E [int
119879
0
1198902120573119904
(1003816100381610038161003816119910 (119904)
10038161003816100381610038162
+ |119911 (119904)|2
+10038161003816100381610038161003816119884 (119904)
10038161003816100381610038161003816
2
+10038161003816100381610038161003816119885 (119904)
10038161003816100381610038161003816
2
) 119889119904]
(21)
That is
Eint119879
0
1198902120573119904
(10038161003816100381610038161003816119884 (119904)
10038161003816100381610038161003816
2
+10038161003816100381610038161003816119885 (119904)
10038161003816100381610038161003816
2
) 119889119904
le1
2Eint
119879
0
1198902120573119904
(1003816100381610038161003816119910 (119904)
10038161003816100381610038162
+ |119911 (119904)|2) 119889119904
(22)
The estimate (22) shows that Φ is a contraction on thespaceK
120573[0 119879] with the norm
(119884 119885)2
120573= Eint
119879
0
1198902120573119904
(|119884 (119904)|2+ |119885 (119904)|
2) 119889119904 (23)
With the contraction mapping theorem there admits aunique fixed point (119884 119885) isin K
120573[0 119879] such that Φ(119884 119885) =
(119884 119885) On the other hand from Step 1 we know thatif Φ(119884 119885) = (119884 119885) then (119884 119885) isin S2
F ([0 119879]119867) times
H2
F ([0 119879]L(Γ119867)) which is the unique mild solution of(11)
Arguing as the previous proof we arrive at the followingassertion in a straightforward way
Corollary 5 Suppose that for all 120572 in a metric space 119865 119891120572is
a given function satisfying (A1) and (A2) with 119871 independenton 120572 Also suppose that
E1015840[119891
120572(119904 119884
1015840
(119904) 1198851015840
(119904) 119884 (119904) 119885 (119904))]
997888rarr E1015840[119891
1205720(119904 119884
1015840
(119904) 1198851015840
(119904) 119884 (119904) 119885 (119904))]
(24)
in 1198712([0 119879]119867) as 120572 rarr 1205720for all (119884 119885) isin S2
F ([0 119879]119867) times
H2
F ([0 119879]L(Γ119867))If we denote by (119884(120585 120572) 119885(120585 120572)) the mild solution of (11)
corresponding to the functions 119891120572and to the final data 120585 isin
1198712(ΩF
119879P 119867) then the map (120572 120585) rarr (119884(120585 120572) 119885(120585 120572))
is continuous from 119865 times 1198712(ΩF
119879P 119867) to S2
F ([0 119879]119867) times
H2
F ([0 119879]L(Γ119867))
32 Non-Lipschitz Case This subsection is devoted to findingsome weaker conditions than the Lipschitz one under whichthe mean-field BSEE has a unique solution To state our mainresult in this section we suppose the following
(A3) For all 119905 isin [0 119879] 1199101015840119894 119910
119894isin 119867 1199111015840
119894 119911
119894isin L(Γ119867) (119894 =
1 2) there exists an 119897 gt 0 such that
10038161003816100381610038161003816119891 (119905 119910
1015840
1 119911
1015840
1 119910
1 119911
1) minus 119891 (119905 119910
1015840
2 119911
1015840
2 119910
2 119911
2)10038161003816100381610038161003816
2
le 120579 (100381610038161003816100381610038161199101015840
1minus 119910
1015840
2
10038161003816100381610038161003816
2
) + 120579 (10038161003816100381610038161199101 minus 1199102
10038161003816100381610038162
)
+ 119897 (100381610038161003816100381610038161199111015840
1minus 119911
1015840
2
10038161003816100381610038161003816
2
+10038161003816100381610038161199111 minus 1199112
10038161003816100381610038162
)
(25)
where 120579 R+rarr R+ is a concave increasing function such
that 120579(0) = 0 120579(119906) gt 0 for 119906 gt 0 and int0+(119889119906120579(119906)) = infin
InMao [5] the author gave three examples of the function120579(sdot) to show the generality of condition (A3) From theseexamples we can see that Lipschitz condition (A1) is a specialcase of the given condition (A3)
Since 120579 is concave and 120579(0) = 0 there exists a pair ofpositive constants 119886 and 119887 such that
120579 (119906) le 119886 + 119887119906 (26)
for all 119906 ge 0 Therefore under assumptions (A2) and(A3) 119891(sdot 1199101015840(sdot) 1199111015840(sdot) 119910(sdot) 119911(sdot)) isin H2
F ([0 119879]119867) whenever
6 Mathematical Problems in Engineering
1199101015840(sdot) 119910(sdot) isin S2
F ([0 119879]119867) and 1199111015840(sdot) 119911(sdot) isin H2
F ([0 119879]L(Γ119867))
By Picard-type iteration we now construct an approxi-mate sequence using which we obtain the desired result Let1198840(119905) equiv 0 and for 119899 isin N let 119884
119899 119885
119899 be a sequence in
S2
F ([0 119879]119867) timesH2
F ([0 119879]L(Γ119867)) defined recursively by
119884119899(119905) = 119890
119860(119879minus119905)120585
+ int
119879
119905
E1015840[119890
119860(119904minus119905)
times119891 (119904 1198841015840
119899minus1(119904) 119885
1015840
119899(119904) 119884
119899minus1(119904) 119885
119899(119904))] 119889119904
minus int
119879
119905
119890119860(119904minus119905)
119885119899(119904) 119889119882 (119904)
(27)
on 0 le 119905 le 119879 From Theorem 4 (27) has a unique mildsolution (119884
119899(119905) 119885
119899(119905))
In order to give the main result we need to prepare thefollowing lemmas about the properties of (119884
119899(119905) 119885
119899(119905)) 119905 isin
[0 119879]
Lemma 6 Under hypotheses (A2) and (A3) there existpositive constants 119862
1and 119862
2such that
(i)E( sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899 (119904)
10038161003816100381610038162
) le 21198621exp (119879 minus 119905)
(ii)Eint119879
119905
11989021205731199041003816100381610038161003816119885119899
(119904)10038161003816100381610038162
119889119904 le 21198621exp (119879 minus 119905)
(iii)E( sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899+1 (119904) minus 119884119899 (119904)
10038161003816100381610038162
)
le 1198622int
119879
119905
120579(E sup119904le119903le119879
11989021205731199031003816100381610038161003816119884119899 (119903) minus 119884119899minus1 (119903)
10038161003816100381610038162
)119889119904
(28)
for all 119905 isin [0 119879] and 119899 ge 1
Proof Using the hypotheses (A2) and (A3)with 120579(119906) le 119886 + 119887119906
yields
10038161003816100381610038161003816119891 (119904 119884
1015840
119899minus1(119904) 119885
1015840
119899(119904) 119884
119899minus1(119904) 119885
119899(119904))
10038161003816100381610038161003816
2
le 2120579 (100381610038161003816100381610038161198841015840
119899minus1(119904)10038161003816100381610038161003816
2
) + 2120579 (1003816100381610038161003816119884119899minus1 (119904)
10038161003816100381610038162
)
+ 2119897 (100381610038161003816100381610038161198851015840
119899(119904)10038161003816100381610038161003816
2
+1003816100381610038161003816119885119899
(119904)10038161003816100381610038162
) + 21003816100381610038161003816119891 (119904 0 0 0 0)
10038161003816100381610038162
le 4119886 + 2119887100381610038161003816100381610038161198841015840
119899minus1(119904)10038161003816100381610038161003816
2
+ 21198871003816100381610038161003816119884119899minus1 (119904)
10038161003816100381610038162
+ 2119897 (100381610038161003816100381610038161198851015840
119899(119904)10038161003816100381610038161003816
2
+1003816100381610038161003816119885119899
(119904)10038161003816100381610038162
) + 21003816100381610038161003816119891 (119904 0 0 0 0)
10038161003816100381610038162
(29)
Then it follows from Lemma 1 that
E( sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899 (119904)
10038161003816100381610038162
) + Eint119879
119905
11989021205731199041003816100381610038161003816119885119899
(119904)10038161003816100381610038162
119889119904
le 241198722
1198601198902120573119879
E100381610038161003816100381612058510038161003816100381610038162
+12119872
2
119860
120573E
times int
119879
119905
1198902120573119904 10038161003816100381610038161003816
E1015840[119891 (119904 119884
1015840
119899minus1(119904)
1198851015840
119899(119904) 119884
119899minus1(119904) 119885
119899(119904))]
10038161003816100381610038161003816
2
119889119904
le 241198722
1198601198902120573119879
E100381610038161003816100381612058510038161003816100381610038162
+12119872
2
119860
120573E
times int
119879
119905
E1015840[119890
2120573119904 10038161003816100381610038161003816119891 (119904 119884
1015840
119899minus1(119904)
1198851015840
119899(119904) 119884
119899minus1(119904) 119885
119899(119904))
10038161003816100381610038161003816
2
] 119889119904
le 1198621+24119872
2
119860
120573E
times int
119879
119905
E1015840[119890
2120573119904[119887100381610038161003816100381610038161198841015840
119899minus1(119904)10038161003816100381610038161003816
2
+ 1198871003816100381610038161003816119884119899minus1 (119904)
10038161003816100381610038162
+ 119897100381610038161003816100381610038161198851015840
119899(119904)10038161003816100381610038161003816
2
+ 1198971003816100381610038161003816119885119899
(119904)10038161003816100381610038162
]] 119889119904
= 1198621+48119872
2
119860
120573E
times int
119879
119905
1198902120573119904
[1198871003816100381610038161003816119884119899minus1 (119904)
10038161003816100381610038162
+ 1198971003816100381610038161003816119885119899
(119904)10038161003816100381610038162
] 119889119904
(30)
where
1198621= 24119872
2
1198601198902120573119879
E100381610038161003816100381612058510038161003816100381610038162
+24119872
2
119860
120573Eint
119879
119905
1198902120573119904
[2119886 +1003816100381610038161003816119891 (119904 0 0 0 0)
10038161003816100381610038162
] 119889119904 + 1
(31)
If we set 120573 = 961198722
119860max119887 119897 we can obtain
sup119899isinN
E( sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899 (119904)
10038161003816100381610038162
) +1
2int
119879
119905
sup119899isinN
E [11989021205731199041003816100381610038161003816119885119899
(119904)10038161003816100381610038162
] 119889119904
le 1198621+1
2int
119879
119905
sup119899isinN
E [11989021205731199041003816100381610038161003816119884119899minus1 (119904)
10038161003816100381610038162
] 119889119904
le 1198621+1
2int
119879
119905
sup119899isinN
E[ sup119904le119903le119879
11989021205731199031003816100381610038161003816119884119899minus1 (119903)
10038161003816100381610038162
]119889119904
(32)
An application of the Gronwall inequality now implies
sup119899isinN
E( sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899 (119904)
10038161003816100381610038162
) le 21198621exp (119879 minus 119905
2)
le 21198621exp (119879 minus 119905)
(33)
Point (i) of Lemma 6 is now proved
Mathematical Problems in Engineering 7
From formula (32) we know that
int
119879
119905
sup119899isinN
E [11989021205731199041003816100381610038161003816119885119899
(119904)10038161003816100381610038162
] 119889119904
le 21198621+ int
119879
119905
sup119899isinN
E[ sup119904le119903le119879
11989021205731199031003816100381610038161003816119884119899minus1 (119903)
10038161003816100381610038162
]119889119904
le 21198621+ 2119862
1int
119879
119905
exp (119879 minus 119904) 119889119904
= 21198621exp (119879 minus 119905)
(34)
This proves point (ii) of the LemmaTo prove point (iii) we note that
E1015840[10038161003816100381610038161003816119891 (119904 119884
1015840
119899(119904) 119885
1015840
119899+1(119904) 119884
119899(119904) 119885
119899+1(119904))
minus 119891 (119904 1198841015840
119899minus1(119904) 119885
1015840
119899(119904) 119884
119899minus1(119904) 119885
119899(119904))
10038161003816100381610038161003816
2
]
le E1015840[120579 (
100381610038161003816100381610038161198841015840
119899(119904) minus 119884
1015840
119899minus1(119904)10038161003816100381610038161003816
2
) + 119897100381610038161003816100381610038161198851015840
119899+1(119904) minus 119885
1015840
119899(119904)10038161003816100381610038161003816
2
]
+ 120579 (1003816100381610038161003816119884119899 (119904) minus 119884119899minus1 (119904)
10038161003816100381610038162
) + 1198971003816100381610038161003816119885119899+1
(119904) minus 119885119899(119904)10038161003816100381610038162
= E [120579 (1003816100381610038161003816119884119899 (119904) minus 119884119899minus1 (119904)
10038161003816100381610038162
) + 1198971003816100381610038161003816119885119899+1
(119904) minus 119885119899(119904)10038161003816100381610038162
]
+ 120579 (1003816100381610038161003816119884119899 (119904) minus 119884119899minus1 (119904)
10038161003816100381610038162
) + 1198971003816100381610038161003816119885119899+1
(119904) minus 119885119899(119904)10038161003816100381610038162
(35)
By Lemma 1 we have
E( sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899+1 (119904) minus 119884119899 (119904)
10038161003816100381610038162
)
+ Eint119879
119905
11989021205731199041003816100381610038161003816119885119899+1
(119904) minus 119885119899(119904)10038161003816100381610038162
119889119904
le12119872
2
119860
120573E
times int
119879
119905
1198902120573119904 10038161003816100381610038161003816
E1015840[119891 (119904 119884
1015840
119899(119904) 119885
1015840
119899+1(119904) 119884
119899(119904) 119885
119899+1(119904))
minus 119891 (119904 1198841015840
119899minus1(119904) 119885
1015840
119899(119904)
119884119899minus1
(119904) 119885119899(119904))]
10038161003816100381610038161003816
2
119889119904
le12119872
2
119860
120573E
times int
119879
119905
1198902120573119904
E1015840[10038161003816100381610038161003816119891 (119904 119884
1015840
119899(119904) 119885
1015840
119899+1(119904) 119884
119899(119904) 119885
119899+1(119904))
minus 119891 (119904 1198841015840
119899minus1(119904) 119885
1015840
119899(119904)
119884119899minus1
(119904) 119885119899(119904))
10038161003816100381610038161003816
2
] 119889119904
le24119872
2
119860
120573Eint
119879
119905
1198902120573119904
[120579 (1003816100381610038161003816119884119899 (119904) minus 119884119899minus1 (119904)
10038161003816100381610038162
)
+ 1198971003816100381610038161003816119885119899+1
(119904) minus 119885119899(119904)10038161003816100381610038162
] 119889119904
(36)
We can choose 120573 gt 0 sufficiently large such that
(1 minus24119872
2
119860119897
120573)Eint
119879
119905
11989021205731199041003817100381710038171003817119885119899+1
(119904) minus 119885119899(119904)10038171003817100381710038172
119889119904 ge 0 (37)
Then
E( sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899+1 (119904) minus 119884119899 (119904)
10038161003816100381610038162
)
le24119872
2
119860
120573Eint
119879
119905
1198902120573119904120579 (1003816100381610038161003816119884119899 (119904) minus 119884119899minus1 (119904)
10038161003816100381610038162
) 119889119904
le 1198622Eint
119879
119905
120579(E sup119904le119903le119879
11989021205731199031003816100381610038161003816119884119899 (119903) minus 119884119899minus1 (119903)
10038161003816100381610038162
)119889119904
(38)
where we set 1198622= (24119872
2
119860120573)119890
2120573119879
We divide the interval [0 119879] into subintervals 0 = 1205910lt
1205911lt sdot sdot sdot lt 120591
119898= 119879 by setting 120591
119896= 119896120575 119896 = 1 2 3 119898 with
120575 = 119879119898
Lemma 7 For all 119905 isin [120591119896minus1
120591119896] define
1198623= 119862
2120579 (2119862
1exp (119879))
1205931198961(119905) = 119862
3(120591119896minus 119905)
120593119896119899+1
(119905) = 1198622int
120591119896
119905
120579 (120593119896119899(119904)) 119889119904 119899 ge 1
(39)
Then for all 119899 ge 1 the following inequality holds for a suitable120575 gt 0
0 le 120593119896119899(119905) le 120593
119896119899minus1(119905) le sdot sdot sdot le 120593
1198961(119905) (40)
Proof Firstly it needs to be verified that for all 119905 isin [120591119896minus1
120591119896]
the following inequality
1205931198962(119905) = 119862
2int
120591119896
119905
120579 (1205931198961(119904)) 119889119904 = 119862
2int
120591119896
119905
120579 (1198623(120591119896minus 119904)) 119889119904
le 1198623(120591119896minus 119905) = 120593
1198961(119905)
(41)
holds provided 120575 gt 0 is chosen sufficiently smallActually this inequality holds provided that
1198622120579 (119862
3(120591119896minus 119905)) le 119862
3= 119862
2120579 (2119862
1exp (119879)) (42)
8 Mathematical Problems in Engineering
or
1198623(120591119896minus 119905) = 119862
2120579 (2119862
1exp (119879)) (120591
119896minus 119905) le 2119862
1exp (119879)
(43)
Since 1198621gt 1 from 120579(119906) le 119886 + 119887119906 the above inequality holds
if
1198622(119886 + 119887) (120591
119896minus 119905) le 1 (44)
Thus (41) holds for any 119905 isin [120591119896minus1
120591119896] 119896 = 1 2 119898 if 120591
119896minus
120591119896minus1
le 11198622(119886 + 119887) Therefore we can choose a sufficiently
large119898 isin N such that 120575 = 119879119898 le 11198622(119886 + 119887) Clearly such a
120575 only depends on 119886 119887 119897 119879 and119872119860
Now assume that (40) holds for some 119899 ge 2 Then wehave
120593119896119899+1
(119905) = 1198622int
120591119896
119905
120579 (120593119896119899(119904)) 119889119904 le 119862
2int
120591119896
119905
120579 (120593119896119899minus1
(119904)) 119889119904
= 120593119896119899(119905) forall119905 isin [120591
119896minus1 120591
119896]
(45)
This completes the proof
Now we can give the main result of this section
Theorem 8 Assume that (A2) and (A3) hold Then thereexists a unique mild solution (119884 119885) to (11)
Proof Consider the following
Uniqueness To show the uniqueness let both (119884 119885) and( 119885) be solutions of (11) For any 120573 gt 0 similar to the proofof (36) one can obtain
E( sup119905le119904le119879
119890212057311990410038161003816100381610038161003816
119884 (119904) minus (119904)10038161003816100381610038161003816
2
) + Eint119879
119905
119890212057311990410038161003816100381610038161003816
119885 (119904) minus 119885 (119904)10038161003816100381610038161003816
2
119889119904
le24119872
2
119860
120573E
times int
119879
119905
1198902120573119904
[120579 (10038161003816100381610038161003816119884 (119904) minus (119904)
10038161003816100381610038161003816
2
) + 11989710038161003816100381610038161003816119885 (119904) minus 119885 (119904)
10038161003816100381610038161003816
2
] 119889119904
(46)
That is if 120573 is sufficiently large
E( sup119905le119904le119879
119890212057311990410038161003816100381610038161003816
119884 (119904) minus (119904)10038161003816100381610038161003816
2
)
le24119872
2
119860
120573Eint
119879
119905
1198902120573119904
[120579 (10038161003816100381610038161003816119884 (119904) minus (119904)
10038161003816100381610038161003816
2
)] 119889119904
le 1198622Eint
119879
119905
120579(E sup119904le119903le119879
119890212057311990310038161003816100381610038161003816
119884 (119904) minus (119904)10038161003816100381610038161003816
2
)119889119904
(47)
An application of Bihari inequality yields
E( sup119905le119904le119879
119890212057311990410038161003816100381610038161003816
119884 (119904) minus (119904)10038161003816100381610038161003816
2
) = 0 (48)
So 119884(119905) = (119905) for all 119905 isin [0 119879] as It then follows from (46)that 119885(119905) = 119885(119905) for all 119905 isin [0 119879] as as well This establishesthe uniqueness
ExistenceWe claim that the sequence (119884119899 119885
119899) defined by (27)
satisfies
E sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899+1 (119904) minus 119884119899 (119904)
10038161003816100381610038162
997888rarr 0 forall0 le 119905 le 119879 (49)
as 119899 rarr infinIndeed for all 119905 isin [120591
119896minus1 120591
119896] we set 120593
119896119899(119905) =
E sup119904isin[119905120591119896]
1198902120573119904|119884119899+1
(119904) minus 119884119899(119904)|
2 By Lemmas 6 and 7
1205931198961(119905) = E sup
119904isin[119905120591119896]
119890212057311990410038161003816100381610038161198842 (119904) minus 1198841 (119904)
10038161003816100381610038162
le 1198622int
120591119896
119905
120579(E sup119904le119903le120591119896
119890212057311990310038161003816100381610038161198841 (119903) minus 1198840 (119903)
10038161003816100381610038162
)119889119904
le 1198622int
120591119896
119905
120579 (21198621exp (120591
119896minus 119905)) 119889119904
le 1198622120579 (2119862
1exp (119879)) (120591
119896minus 119905) = 119862
3(120591119896minus 119905) = 120593
1198961(119905)
(50)
Suppose that 120593119896119899(119905) le 120593
119896119899(119905) holds for some 119899 ge 1
According to Lemma 6(iii) and Lemma 7 for all 119905 isin [120591119896minus1
120591119896]
we obtain
120593119896119899+1
(119905) = E sup119904isin[119905120591119896]
11989021205731199041003816100381610038161003816119884119899+2 (119904) minus 119884119899+1 (119904)
10038161003816100381610038162
le 1198622int
120591119896
119905
120579(E sup119903isin[119904120591119896]
11989021205731199031003816100381610038161003816119884119899+1 (119903) minus 119884119899 (119903)
10038161003816100381610038162
)119889119904
= 1198622int
120591119896
119905
120579 (120593119896119899(119904)) 119889119904
le 1198622int
120591119896
119905
120579 (120593119896119899(119904)) 119889119904 = 120593
119896119899+1(119905)
(51)
This implies that for all 119899 isin N
120593119896119899(119905) le 120593
119896119899(119905) (52)
By definition 120593119896119899(sdot) is continuous on [120591
119896minus1 120591
119896] Note that
for each 119899 ge 1 120593119896119899(sdot) is decreasing on [120591
119896minus1 120591
119896] and for each
119905 119896 120593119896119899(119905) is a nonincreasing sequence Therefore we define
the function 120593119896(119905) by 120593
119896119899(119905) darr 120593
119896(119905) It is easy to verify that
120593119896(119905) is continuous and nonincreasing on [120591
119896minus1 120591
119896] By the
definitions of 120593119896119899(119905) and 120593
119896(119905) we get
120593119896(119905) = lim
119899rarrinfin
1198622int
120591119896
119905
120579 (120593119896119899(119904)) 119889119904 = 119862
2int
120591119896
119905
120579 (120593119896(119904)) 119889119904
(53)
for all 120591119896minus1
le 119905 le 120591119896 Since int
0+(119889119906120579(119906)) = infin the Bihari
inequality implies
120593119896(119905) = 0 for each 119905 isin [120591
119896minus1 120591
119896] (54)
For each 119896 isin 1 2 119898 (52) and (54) yield
lim119899rarrinfin
120593119896119899(119905) = 0 (55)
Mathematical Problems in Engineering 9
Then
E sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899+1 (119904) minus 119884119899 (119904)
10038161003816100381610038162
le max1le119896le119898
E sup119904isin[120591119896minus1120591119896]
11989021205731199041003816100381610038161003816119884119899+1 (119904) minus 119884119899 (119904)
10038161003816100381610038162
= max1le119896le119898
120593119896119899(119905) 997888rarr 0
(56)
as 119899 rarr infin and this proves the assertion (49)By (36) we obtain
E( sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899+1 (119904) minus 119884119899 (119904)
10038161003816100381610038162
) + (1 minus24119872
2
119860119897
120573)E
times int
119879
119905
11989021205731199041003816100381610038161003816119885119899+1
(119904) minus 119885119899(119904)10038161003816100381610038162
119889119904
le24119872
2
119860
120573Eint
119879
119905
1198902120573119904
[120579 (1003816100381610038161003816119884119899 (119904) minus 119884119899minus1 (119904)
10038161003816100381610038162
)] 119889119904
(57)
Applying (49) to the above formula we see that (119884119899 119885
119899) is
a Cauchy (hence convergent) sequence in S2
F ([0 119879]119867) times
H2
F ([0 119879]L(Γ119867)) denote the limit by (119884 119885) Now letting119899 rarr infin in (27) we obtain that
119884 (119905) = 119890119860(119879minus119905)
120585
+ int
119879
119905
E1015840[119890
119860(119904minus119905)119891 (119904 119884
1015840
(119904) 1198851015840
(119904) 119884 (119904) 119885 (119904))] 119889119904
minus int
119879
119905
119890119860(119904minus119905)
119885 (119904) 119889119882 (119904)
(58)
holds on the entire interval [0 119879]The theorem is nowproved
To illustrate the application of the obtained existenceand uniqueness result we consider the example of backwardstochastic partial differential equations (BSPDEs) of mean-field type
Example 9 Let O be an open bounded domain in R119899
with uniformly 1198622 boundary 120597O let 119861(119905) be a standard 119899-dimensional Brownian motion (equipped with the normalfiltration) and let 120585 O rarr R be an F
119879-measurable
random variable We also let 119871 denote the semielliptic partialdifferential operator on 1198622
(R) of the form
119871 =
119899
sum
119894119895=1
119886119894119895(119909)
1205972
120597119909119894120597119909
119895
+
119899
sum
119894=1
119887119894(119909)
120597
120597119909119894
(59)
The aim is to study the solvability of the following initialboundary value problem
119889119884 (119905 119909)
= (119871119884 (119905 119909) + E1015840
times [119892 (119905 119909 1198841015840
(119905 119909) 1198851015840
(119905 119909) 119884 (119905 119909) 119885 (119905 119909))]) 119889119905
+ 119885 (119905 119909) 119889119861 (119905) ae on (0 119879) times O
119884 (119905 119909) = 0 ae on (0 119879) times 120597O
119884 (119879 119909) = 120585 (119879 119909) ae onO(60)
where
119884 [0 119879] times O 997888rarr R
119885 [0 119879] times O 997888rarr L (R119899 119871
2
(O))
119892 [0 119879] times O timesR timesL (R119899 119871
2
(O))
timesR timesL (R119899 119871
2
(O)) 997888rarr R
(61)
The following assumptions will have to be in force
(H1) 119886119894119895 119887
119894 R119899
rarr R are uniformly continuous andbounded and satisfy the usual uniform ellipticitycondition sum119899
119894119895=1119886119894119895(119909)119908
119894119908119895ge 120582|119908|
2 for some 120582 gt 0
and all 119909 isin O 119908 isin R119899(H2) 119892 is measurable in (119905 119909 119910 119910 119911) and continuous in
( 119911) and there exists 119862 gt 0 such that1003816100381610038161003816119892 (119905 119909 1199101 1 1199101 1199111) minus 119892 (119905 119909 1199102 2 1199102 1199112)
1003816100381610038161003816
le 119862 [10038161003816100381610038161199101 minus 1199102
1003816100381610038161003816 +10038161003816100381610038161 minus 2
1003816100381610038161003816 +10038161003816100381610038161199101 minus 1199102
1003816100381610038161003816 +10038161003816100381610038161199111 minus 1199112
1003816100381610038161003816]
(62)
for all 0 le 119905 le 119879 119909 isin O 1199101 119910
2 119910
1 119910
2isin R
1
2 119911
1 119911
2isin
L(R119899 119871
2(O))
Then we are now in a position of showing existence anduniqueness of the solution of BSPDEs (60)
Theorem 10 If (H1) and (H2) are satisfied then the mean-field BSPDE (60) has a unique mild solution (119884 119885) isin
1198712(0 119879 119871
2(Ω 119871
2(O))) times 1198712F (0 119879 119871
2(R119899
1198712(Ω 119871
2(O))))
Proof Let119867 = 1198712(O) and119870 = R119899 Define the operator 119860 by
119860119884 (119905 sdot) = 119871119884 (119905 sdot) (63)
It is shown in [17] (see Example 21 in [17]) that 119860 generatesa strongly continuous semigroup on 119867 Define the maps 119891
[0 119879] times 119867 timesL(119870119867) times 119867 timesL(119870119867) rarr 119867 by
119891 (119905 1198841015840
(119905) 1198851015840
(119905) 119884 (119905) 119885 (119905)) (119909)
= 119892 (119905 119909 1198841015840
(119905 119909) 1198851015840
(119905 119909) 119884 (119905 119909) 119885 (119905 119909))
(64)
10 Mathematical Problems in Engineering
for all 0 le 119905 le 119879 119909 isin O With these identifications(60) can be written in the form of (11) By (H2) we know119891 satisfy condition (A1) Hence an application of Theorem 4concludes that (60) has a unique mild solution (119884 119885) isin
1198712(0 119879 119871
2(Ω 119871
2(O))) times 1198712F (0 119879 119871
2(R119899
1198712(Ω 119871
2(O))))
4 Mean-Field Stochastic Evolution Equations
Let 119882(119905) 119905 isin [0 119879] be a cylindrical Wiener process withvalues in a Hilbert space Γ defined on a probability space(ΩFP) We fix an interval [119905 119879] sub [0 119879] and considerthe stochastic evolution equations of mean-field type for anunknown process 119883(119904) 119904 isin [119905 119879] with values in a Hilbertspace 119870
119889119883 (119904) = 119861119883 (119904) 119889119904 + E1015840[119887 (119904 119883
1015840
(119904) 119883 (119904))] 119889119904
+ E1015840[120590 (119904 119883
1015840
(119904) 119883 (119904))] 119889119882 (119904)
119883 (119905) = 119909 isin 119870
(65)
where operator 119861 is the generator of a strongly continuoussemigroup 119890
119905119861 119905 ge 0 in the Hilbert space 119870 with 119872119861≜
sup119905isin[0119879]
|119890119905119861|
By a mild solution of (65) we mean an F119904-measurable
process 119883(119904) 119904 isin [119905 119879] with continuous paths in 119870 suchthat P-as
119883 (119904) = 119890119861(119904minus119905)
119909
+ int
119904
119905
119890119861(119904minus120591)
E1015840[119887 (120591 119883
1015840
(120591) 119883 (120591))] 119889120591
+ int
119904
119905
119890119861(119904minus120591)
E1015840[120590 (120591119883
1015840
(120591) 119883 (120591))] 119889119882 (120591)
119904 isin [119905 119879]
(66)
We suppose the following(A4) 119887 [0 119879] times 119870 times 119870 rarr 119870 is a measurable mapping
which satisfies10038161003816100381610038161003816119887 (119905 119909
1015840 119909) minus 119887 (119905 119910
1015840 119910)
10038161003816100381610038161003816
2
le 1198711(100381610038161003816100381610038161199091015840minus 119910
101584010038161003816100381610038161003816
2
+1003816100381610038161003816119909 minus 119910
10038161003816100381610038162
)
119905 isin [0 119879] 1199091015840 119909 119910
1015840 119910 isin 119870
(67)
for some constant 1198711gt 0
(A5) The mapping 120590 [0 119879] times 119870 times 119870 rarr L(Γ 119870) fulfillsthat for every V isin Γ the map 120590V [0 119879] times 119870 times 119870 rarr 119870
is measurable for every 119904 gt 0 119905 isin [0 119879] 1199091015840 1199101015840 119909 119910 isin 119870119890119904119861120590(119905 119909
1015840 119909) isin L(Γ 119870) and
10038161003816100381610038161003816119890119904119861120590 (119905 119909
1015840 119909)
10038161003816100381610038161003816L(Γ119870)le 119871
2119904minus120574(1 +
10038161003816100381610038161003816119909101584010038161003816100381610038161003816+ |119909|)
10038161003816100381610038161003816119890119904119861120590 (119905 119909
1015840 119909) minus 119890
119904119861120590 (119905 119910
1015840 119910)
10038161003816100381610038161003816L(Γ119870)
le 1198712119904minus120574(100381610038161003816100381610038161199091015840minus 119910
101584010038161003816100381610038161003816+1003816100381610038161003816119909 minus 119910
1003816100381610038161003816)
10038161003816100381610038161003816120590 (119905 119909
1015840 119909)
10038161003816100381610038161003816L(Γ119870)le 119871
2(1 +
10038161003816100381610038161003816119909101584010038161003816100381610038161003816+ |119909|)
(68)
for some constants 1198712gt 0 and 120574 isin [0 12)
Theorem 11 Under assumptions (A3) and (A4) (65) has aunique mild solution119883(sdot) isin S2
F ([119905 119879] 119870)
The proof is constructed in two steps like that ofTheorem 4 and it uses standard arguments for stochastic evo-lution equations introduced in the proof of Proposition 32 in[3] Since the proof is straightforward we prefer to omit it
Remark 12 In our paper Lipchitz condtion (A4) is givento get the well-posedness of mean-field stochastic evolutionequations In fact (A4) can be replaced by a weaker conditionsuch as (A3) We just give the condition (A4) for simplicity
From standard arguments we can also get the followingcontinuous dependence theorem
Corollary 13 Assume that for all 120572 in a metric space 119865(119887120572 120590
120572) satisfy (A4) and (A5)with119871
1and119871
2independent of 120572
Also assume that
Eint119879
0
10038161003816100381610038161003816E1015840[119887120572(119904 119883
1015840
(119904) 119883 (119904))]
minus E1015840[1198871205720(119904 119883
1015840
(119904) 119883 (119904))]10038161003816100381610038161003816
2
119889119904 997888rarr 0
Eint119879
0
10038161003816100381610038161003816E1015840[120590
120572(119904 119883
1015840
(119904) 119883 (119904))]
minusE1015840[120590
1205720(119904 119883
1015840
(119904) 119883 (119904))]10038161003816100381610038161003816
2
119889119904 997888rarr 0
(69)
as 120572 rarr 1205720for all119883 isin S2
F ([0 119879] 119870)If we denote by 119883120572
(sdot) the mild solution of mean-field SEE(65) corresponding to the functions (119887
120572 120590
120572) and to the initial
data 119909 then we have
sup119904isin[0119879]
E1003816100381610038161003816119883
120572
(119904) minus 1198831205720 (119904)
10038161003816100381610038162
997888rarr 0 119886119904 120572 997888rarr 1205720 (70)
5 Maximum Principle for BSPDEs ofMean-Field Type
51 Formulation of the Problem Let O isin R119899 be a boundedopen set with smooth boundary 120597O and let 119880 the space ofcontrols be a separable real Hilbert space We denote
U = V (sdot) isin 1198712F(0 119879 119880)
| V119905(120596
1015840 120596) [0 119879] times Ω times Ω
997888rarr 119880 is F otimesF119905-progressively measurable
(71)
Mathematical Problems in Engineering 11
An element ofU is called an admissible controlFor any V isin U we consider the following controlled
BSPDE system in the state space119867 = 1198712(O) (norm | sdot | scalar
product ⟨sdot sdot⟩)
119889119884119905(119909) = minus119860119884
119905(119909) 119889119905
minus E1015840[119891 (119905 119909 (119884
119905(119909))
1015840
(119885119905(119909))
1015840
119884119905(119909) 119885
119905(119909) V
119905)] 119889119905
+ 119885119905(119909) 119889119882 (119905) 119905 isin [0 119879]
119884119879(119909) = 120585 (119909) 119909 isin O
(72)
where 119860 is a partial differential operator 119891 [0 119879] timesO times119867 times
L(Γ119867)times119867timesL(Γ119867)times119880 rarr 119867 and 120585 isin 1198712(ΩF119879P 119867)
The cost functional is given by
119869 (V) = Eint119879
0
intO
E1015840[ℎ (119904 119909 (119884
119904(119909))
1015840
(119885119904(119909))
1015840
119884119904(119909) 119885
119904(119909) V
119904)] 119889119909 119889119904
+E1015840intO
119892 (119909 (1198840(119909))
1015840
1198840(119909)) 119889119909
(73)
where
ℎ [0 119879] times O times 119867 timesL (Γ119867) times 119867 timesL (Γ119867) times 119880 997888rarr R
119892 O times 119867 times119867 997888rarr R
(74)
Our purpose is to minimize the functional 119869(sdot) over Uadsubject to the following state constraint
EintO
Φ(119909 (1198840(119909))
1015840
1198840(119909)) 119889119909 = 0 (75)
where
Φ O times 119867 times119867 997888rarr R (76)
An admissible control 119906 isin Uad that satisfies
119869 (119906) = minVisinUad
119869 (V) (77)
is called optimalThrough what follows the following assumptions will be
in force
(L1) 119860 is a partial differential operator with appropri-ate boundary conditions We assume that 119860 is theinfinitesimal generator of a strongly continuous semi-group 119890119905119860 119905 ge 0 in119867 Moreover for every 119905 isin [0 119879]119890
119905119860119891
1198712(O) le 119872
119860119891
1198712(O) for some constant 119872
119860
independent of 119905 and 119891(L2) 119891 ℎ 119892 and Φ are continuously Gateaux differen-
tiable with respect to (1199101015840 1199111015840 119910 119911) 119891 is continuouslyGateaux differentiable with respect to V and ℎ iscontinuous with respect to V
(L3) The derivatives of 119891 ℎ 119892 and Φ are Lipschitzcontinuous and bounded by
100381610038161003816100381610038161198911199101015840
10038161003816100381610038161003816+10038161003816100381610038161198911199111015840
1003816100381610038161003816 +10038161003816100381610038161003816119891119910
10038161003816100381610038161003816+1003816100381610038161003816119891119911
1003816100381610038161003816 +1003816100381610038161003816119891V
1003816100381610038161003816 +10038161003816100381610038161003816Φ1199101015840
10038161003816100381610038161003816+10038161003816100381610038161003816Φ119910
10038161003816100381610038161003816le 119862
10038161003816100381610038161003816ℎ1199101015840
10038161003816100381610038161003816+1003816100381610038161003816ℎ1199111015840
1003816100381610038161003816 +10038161003816100381610038161003816ℎ119910
10038161003816100381610038161003816+1003816100381610038161003816ℎ119911
1003816100381610038161003816 +100381610038161003816100381610038161198921199101015840
10038161003816100381610038161003816+10038161003816100381610038161003816119892119910
10038161003816100381610038161003816
le 119862 (1 +10038161003816100381610038161003816119910101584010038161003816100381610038161003816+10038161003816100381610038161003816119911101584010038161003816100381610038161003816+10038161003816100381610038161199101003816100381610038161003816 + |119911| + |V|)
(78)
where 119862 is a positive constant
Obviously according to Theorem 4 state equation (72)has a unique mild solution under the above assumptions
Remark 14 We can define the second order differentialoperator
(119860119891) (119909) =
119899
sum
119894119895=1
119886119894119895(119909)
1205972119891
120597119909119894120597119909
119895
(119909) +
119899
sum
119894=1
119887119894(119909)
120597119891
120597119909119894
(119909) (79)
By Example 9 119860 fulfills assumption (L1) if 119886119894119895 119887
119894satisfy
condition (H1)
52 Variation of the Trajectory Let 119906 be an optimal controlwith (119884(sdot) 119885(sdot)) being the corresponding optimal state Let120576 gt 0 and [119903 119903 + 120576] sube [0 119879] For any given V isin Uad weintroduce the spike variation of the control 119906(sdot)
119906120576
119905=
V119905 119905 isin [119903 119903 + 120576]
119906119905 119905 isin [0 119879] [119904 119904 + 120576]
(80)
It is clear that 119906120576(sdot) isin UadLet (119884120576
(sdot) 119885120576(sdot)) be the trajectory corresponding to 119906120576(sdot)
We use the following short notation for brevity
119891 (119905) = 119891 (119905 119909 (119884119905(119909))
1015840
(119885119905(119909))
1015840
119884119905(119909) 119885
119905(119909) 119906
119905)
119891 (119906120576
119905) = 119891 (119905 119909 (119884
119905(119909))
1015840
(119885119905(119909))
1015840
119884119905(119909) 119885
119905(119909) 119906
120576
119905)
(81)
Consider the following equation
119889119870120576
119905(119909) = minus119860119870
120576
119905(119909) 119889119905
minus E1015840[119891
1199101015840 (119905) (119870
120576
119905(119909))
1015840
+ 119891119910(119905) 119870
120576
119905(119909)
+ 1198911199111015840 (119905) (119876
120576
119905(119909))
1015840
+ 119891119911(119905) 119876
120576
119905(119909)
+1
120576(119891 (119906
120576
119905) minus 119891 (119905))] 119889119905 + 119876
120576
119905(119909) 119889119882 (119905)
119870120576
119879(119909) = 0
(82)
Since the coefficients in (82) are bounded it is easy to checkthat there exists a unique mild solution such that
E[ sup119905isin[0119879]
1003816100381610038161003816119870120576
119905
10038161003816100381610038162
+ int
119879
0
1003816100381610038161003816119876120576
119905
10038161003816100381610038162
119889119905] lt infin (83)
We have the following estimate
12 Mathematical Problems in Engineering
Theorem 15 There holds
lim120576rarr0
E[ sup119904isin[119905119879]
1003816100381610038161003816100381610038161003816100381610038161003816
119884120576
119904minus 119884
119904
120576minus 119870
120576
119904
1003816100381610038161003816100381610038161003816100381610038161003816
2
] = 0 forall119905 isin [0 119879]
lim120576rarr0
Eint119879
0
1003816100381610038161003816100381610038161003816100381610038161003816
119885120576
119904minus 119885
119904
120576minus 119876
120576
119904
1003816100381610038161003816100381610038161003816100381610038161003816
2
119889119904 = 0
(84)
Proof We define
120578120576
119904=119884120576
119904minus 119884
119904
120576minus 119870
120576
119904 120577
120576
119904=119885120576
119904minus 119885
119904
120576minus 119876
120576
119904 119904 isin [0 119879]
(85)
For simplicity let us define
Λ120576
119904= ((119884
120576
119904)1015840
(119885120576
119904)1015840
119884120576
119904 119885
120576
119904)
119891 (119904 120582) = 119891 (119904 1198841015840
119904+ 120582(119884
120576
119904minus 119884
119904)1015840
1198851015840
119904+ 120582(119885
120576
119904minus 119885
119904)1015840
119884119904+ 120582 (119884
120576
119904minus 119884
119904)
119885119904+ 120582 (119885
120576
119904minus 119885
119904) 119906
120576
119904)
(86)
By the definition of (119884120576
119904 119885
120576
119904) (119884
119904 119885
119904) and (119870120576
119904 119876
120576
119904) (120578120576
119904 120577
120576
119904)
is the mild solution of
119889120578120576
119904= minus119860120578
120576
119904119889119904 minus E
1015840
[119871 (119904 120576)] 119889119904 + 120577120576
119904119889119882 (119904)
120578120576
119879= 0
(87)
with
119871 (119904 120576) =1
120576(119891 (119904 Λ
120576
119904 119906
120576
119904) minus 119891 (119906
120576
119904))
minus 1198911199101015840 (119904) (119870
120576
119904)1015840
minus 119891119910(119904) 119870
120576
119904minus 119891
1199111015840 (119904) (119876
120576
119904)1015840
minus 119891119911(119904) 119876
120576
119904
= ((120578120576
119904)1015840
+ (119870120576
119904)1015840
) int
1
0
1198911199101015840 (119904 120582) 119889120582 + (120578
120576
119904+ 119870
120576
119904)
times int
1
0
119891119910(119904 120582) 119889120582 minus 119891
1199101015840 (119904) (119870
120576
119904)1015840
minus 119891119910(119904) 119870
120576
119904
+ ((120577120576
119904)1015840
+ (119876120576
119904)1015840
)int
1
0
1198911199111015840 (119904 120582) 119889120582 + (120577
120576
119904+ 119876
120576
119904)
times int
1
0
119891119911(119904 120582) 119889120582 minus 119891
1199111015840 (119904) (119876
120576
119904)1015840
minus 119891119911(119904) 119876
120576
119904
= (120578120576
119904)1015840
int
1
0
1198911199101015840 (119904 120582) 119889120582 + 120578
120576
119904int
1
0
119891119910(119904 120582) 119889120582
+ (120577120576
119904)1015840
int
1
0
1198911199111015840 (119904 120582) 119889120582 + 120577
120576
119904int
1
0
119891119911(119904 120582) 119889120582 + 120574
120576
119904
(88)
where we denote
120574120576
119904= (119870
120576
119904)1015840
int
1
0
(1198911199101015840 (119904 120582) minus 119891
1199101015840 (119904)) 119889120582
+ 119870120576
119904int
1
0
(119891119910(119904 120582) minus 119891
119910(119904)) 119889120582
+ (119876120576
119904)1015840
int
1
0
(1198911199111015840 (119904 120582) minus 119891
1199111015840 (119904)) 119889120582
+ 119876120576
119904int
1
0
(119891119911(119904 120582) minus 119891
119911(119904)) 119889120582
(89)
For any 120573 gt 0 according to Lemma 1 we obtain
E sup119905le119904le119879
11989021205731199041003816100381610038161003816120578
120576
119904
10038161003816100381610038162
+ Eint119879
119905
11989021205731199041003816100381610038161003816120577
120576
119904
10038161003816100381610038162
119889119904
le12119872
2
119860
120573int
119879
119905
1198902120573119904
E [10038161003816100381610038161003816E1015840
[119871 (119904 120576)]10038161003816100381610038161003816
2
] 119889119904
le12119872
2
119860
120573Eint
119879
119905
1198902120573119904
E1015840[|119871 (119904 120576)|
2] 119889119904
(90)
By condition (L3) we have
E [E1015840[|119871 (119904 120576)|
2]]
= E [E1015840[1003816100381610038161003816119871 (119904 120576) minus 120574
120576
119904+ 120574
120576
119904
10038161003816100381610038162
]]
le 81198622E [E
1015840[10038161003816100381610038161003816(120578
120576
119904)101584010038161003816100381610038161003816
2
+1003816100381610038161003816120578120576
119904
10038161003816100381610038162
+10038161003816100381610038161003816(120577
120576
119904)101584010038161003816100381610038161003816
2
+1003816100381610038161003816120577120576
119904
10038161003816100381610038162
]]
+ 2E [E1015840[1003816100381610038161003816120574120576
119904
10038161003816100381610038162
]]
le 161198622E [
1003816100381610038161003816120578120576
119904
10038161003816100381610038162
+1003816100381610038161003816120577120576
119904
10038161003816100381610038162
] + 2E [E1015840[1003816100381610038161003816120574120576
119904
10038161003816100381610038162
]]
(91)
Combined with (91) (90) yields
E sup119905le119904le119879
11989021205731199041003816100381610038161003816120578
120576
119904
10038161003816100381610038162
+ (1 minus192119872
2
1198601198622
120573)Eint
119879
119905
11989021205731199041003816100381610038161003816120577
120576
119904
10038161003816100381610038162
119889119904
le192119872
2
1198601198622
120573Eint
119879
119905
11989021205731199041003816100381610038161003816120578
120576
119904
10038161003816100381610038162
119889119904
+24119872
2
119860
120573Eint
119879
119905
1198902120573119904
[E1015840[1003816100381610038161003816120574120576
119904
10038161003816100381610038162
]] 119889119904
(92)
We claim that
Eint119879
119905
E1015840[1003816100381610038161003816120574120576
119904
10038161003816100381610038162
] 119889119904 997888rarr 0 as 120576 997888rarr 0 (93)
From (89)
120574120576
119904= 119868
1
119904+ 119868
2
119904+ 119868
3
119904+ 119868
4
119904 (94)
Mathematical Problems in Engineering 13
where
1198681
119904= (119870
120576
119904)1015840
int
1
0
(1198911199101015840 (119904 120582) minus 119891
1199101015840 (119904)) 119889120582
1198682
119904= 119870
120576
119904int
1
0
(119891119910(119904 120582) minus 119891
119910(119904)) 119889120582
1198683
119904= (119876
120576
119904)1015840
int
1
0
(1198911199111015840 (119904 120582) minus 119891
1199111015840 (119904)) 119889120582
1198684
119904= 119876
120576
119904int
1
0
(119891119911(119904 120582) minus 119891
119911(119904)) 119889120582
(95)
Then
Eint119879
119905
E1015840[1003816100381610038161003816120574120576
119904
10038161003816100381610038162
] 119889119904
le 4Eint119879
119905
E1015840[100381610038161003816100381610038161198681
119904
10038161003816100381610038161003816
2
+100381610038161003816100381610038161198682
119904
10038161003816100381610038161003816
2
+100381610038161003816100381610038161198683
119904
10038161003816100381610038161003816
2
+100381610038161003816100381610038161198684
119904
10038161003816100381610038161003816
2
] 119889119904
(96)
Take Eint119879119905E1015840[|1198682
119904|2
]119889119904 for example
Eint119879
119905
E1015840[100381610038161003816100381610038161198682
119904
10038161003816100381610038161003816
2
] 119889119904
= Eint119879
119905
E1015840[(119870
120576
119904int
1
0
(119891119910(119904 120582) minus 119891
119910(119904)) 119889120582)
2
]119889119904
le Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119904 120582) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582] 119889119904
le 2Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119904 120582) minus 119891
119910(119906
120576
119905)10038161003816100381610038161003816
2
119889120582] 119889119904
+ 2Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119906
120576
119905) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582] 119889119904
(97)
Note that
Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119906
120576
119905) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582] 119889119904
= Eint119903+120576
119903
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119906
120576
119905) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582] 119889119904
le sup119904isin[119905119879]
E [E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119906
120576
119905) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582]] 120576
997888rarr 0 as 120576 997888rarr 0
(98)
The inequality above holds due to the boundedness of|119870
120576
119904|2
int1
0|119891119910(119906
120576
119905) minus 119891
119910(119904)|
2
119889120582 Indeed Assumption (L3) im-plies the boundedness of 119891
119910(119906
120576
119905) minus 119891
119910(119904) Meanwhile 119870120576
119904is
the solution of mean-field BSEE (82) It can be easy to check119870120576
119904is bounded since the coefficients in (82) are boundedOn the other hand
Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119904 120582) minus 119891
119910(119906
120576
119905)10038161003816100381610038161003816
2
119889120582] 119889119904
le 11986221205822Eint
119879
119905
1003816100381610038161003816119870120576
119904
10038161003816100381610038162
times int
1
0
1198641015840[100381610038161003816100381610038161003816(119884
120576
119904minus 119884
119904)1015840100381610038161003816100381610038161003816
2
+100381610038161003816100381610038161003816(119885
120576
119904minus 119885
119904)1015840100381610038161003816100381610038161003816
2
times10038161003816100381610038161003816119884120576
119904minus 119884
119904
10038161003816100381610038161003816
2
+10038161003816100381610038161003816119885120576
119904minus 119885
119904
10038161003816100381610038161003816
2
] 119889120582 119889119904
(99)
where (119884120576
119904 119885
120576
119904) is the mild solution of the following equation
119889119884120576
119905= minus 119860119884
120576
119905119889119905
minus E1015840[119891 (119905 (119884
120576
119905)1015840
(119885120576
119905)1015840
119884120576
119905 119885
120576
119905 119906
120576
119905)] 119889119905
+ 119885120576
119905119889119882 (119905) 119905 isin [0 119879]
119884120576
119879= 120585
(100)
and (119884119904 119885
119904) is the mild solution of
119889119884119905= minus 119860119884
119905119889119905
minus E1015840[119891 (119905 (119884
119905)1015840
(119885119905)1015840
119884119905 119885
119905 119906
119905)] 119889119905
+ 119885119905119889119882 (119905) 119905 isin [0 119879]
119884119879= 120585
(101)
By the definition of 119906120576119905 according to (L2) we have
E1015840[119891 (119904 119884
1015840
(119904) 1198851015840
(119904) 119884 (119904) 119885 (119904) 119906120576
119905)]
997888rarr E1015840[119891 (119904 119884
1015840
(119904) 1198851015840
(119904) 119884 (119904) 119885 (119904) 119906119905)]
(102)
in 1198712([0 119879]119867) as 120576 rarr 0 Using the continuous dependencetheorem Corollary 5 we obtain
(119884120576
119904 119885
120576
119904) 997888rarr (119884
119904 119885
119904) as 120576 997888rarr 0 (103)
Then
Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119904 120582) minus 119891
119910(119906
120576
119905)10038161003816100381610038161003816
2
119889120582] 119889119904
le 11986221205822Eint
119879
119905
1003816100381610038161003816119870120576
119904
10038161003816100381610038162
times int
1
0
E1015840[100381610038161003816100381610038161003816(119884
120576
119904minus 119884
119904)1015840100381610038161003816100381610038161003816
2
+100381610038161003816100381610038161003816(119885
120576
119904minus 119885
119904)1015840100381610038161003816100381610038161003816
2
times10038161003816100381610038161003816119884120576
119904minus 119884
119904
10038161003816100381610038161003816
2
+10038161003816100381610038161003816119885120576
119904minus 119885
119904
10038161003816100381610038161003816
2
] 119889120582 119889119904
997888rarr 0 as 120576 997888rarr 0
(104)
14 Mathematical Problems in Engineering
Combining (98) with (104) we finally haveEint
119879
119905E1015840[|1198682
119904|2
]119889119904 rarr 0 as 120576 rarr 0The required result (93) follows by using the similar
estimations for 1198681119904 1198683
119904 and 1198684
119904
Note that 1 minus 1921198722
1198601198622120573 gt 0 if 120573 is sufficiently large
Now to prove the desired result (84) it suffices to applyGronwallrsquos lemma and estimate (93) to inequality (92)
To deal with the state constraint (75) we need to recall theEkeland variational principle
Lemma 16 (Ekelandrsquos variational principle see [16 Lemma41]) Let (119878 119889) be a complete metric space and let 119865(sdot) 119878 rarr
R be lower semicontinuous and bounded from below If for 120588 gt0 there exists 119906 isin 119878 such that
119865 (119906) le infVisin119878
119865 (V) + 120588 (105)
then there exists 119906120588 isin 119878 satisfying
(i) 119865 (119906120588) le 119865 (119906)
(ii) 119889 (119906120588 119906) le 120588
(iii) 119865 (119906120588) le 119865 (V) + 120588 sdot 119889 (119906120588 119906) forallV = 119906120588
(106)
Now fix V isin Uad and set
119878 = V (sdot) isin Uad | sup0le119905le119879
E1003816100381610038161003816V11990510038161003816100381610038162
le E100381610038161003816100381611990611990510038161003816100381610038162
+ |V|2
119889 (V (sdot) V (sdot)) = 119898 119905 isin [0 119879] | E1003816100381610038161003816V119905 minus V
119905
10038161003816100381610038162
gt 0
forallV (sdot) V (sdot) isin 119878
(107)
where119898 denotes the Lebesgue measure on RThe following result is proved as Proposition 41 in [16]
Lemma 17 (119878 119889(sdot sdot)) is a complete metric space and 119869120588 is
continuous and bounded on 119878 where
119869120588
(V (sdot)) = (119869 (V (sdot)) minus 119869 (119906 (sdot)) + 120588)2
+
10038161003816100381610038161003816100381610038161003816Eint
O
Φ(119909 (1198840(119909))
1015840
1198840(119909)) 119889119909
10038161003816100381610038161003816100381610038161003816
2
12
forallV (sdot) isin 119878(108)
and (119884 119885) is the mild solution of (72) corresponding to thecontrol V
Now we consider the following free initial state optimalcontrol problem
infV(sdot)isin119878
119869120588
(V (sdot)) (109)
It is easy to check that
0 le infV(sdot)isin119878
119869120588
(V (sdot)) le 119869120588
(119906 (sdot)) = 120588 (110)
According to Ekelandrsquos variational principle there exists a119906120588(sdot) isin 119881 such that
(i) 119869120588 (119906120588 (sdot)) le 120588
(ii) 119889 (119906120588 (sdot) 119906 (sdot)) le 120588
(iii) 119869120588 (119906120588 (sdot)) le 119869120588
(V (sdot)) + 120588119889 (119906120588 (sdot) 119906 (sdot)) forallV (sdot) isin 119878(111)
Using the spike variationmethod we can construct 119906120576120588(sdot) isin 119878as follows
119906120576120588
119905=
V119905 119905 isin [119904 119904 + 120576]
119906120588
119905 119905 isin [0 119879] [119904 119904 + 120576]
(112)
It is clear that 119889(119906120576120588(sdot) 119906120588(sdot)) le 120576 Let (119884120576120588(sdot) 119885
120576120588(sdot)) (resp
(119884120588(sdot) 119885
120588(sdot))) be the solution of (72) with respect to the
control 119906120576120588(sdot) (resp 119906120588(sdot)) Following (82) (119870120576120588
119905 119876
120576120588
119905) is the
mild solution of
119870120576120588
119905= int
119879
119905
119890119860(119904minus119905)
E1015840
times [1198911199101015840 (119904 Λ
120588
119904 119906
120588
119904) (119870
120576120588
119904)1015840
+ 119891119910(119904 Λ
120588
119904 119906
120588
119904)119870
120576120588
119904
+ 1198911199111015840 (119904 Λ
120588
119904 119906
120588
119904) (119876
120576120588
119904)1015840
+ 119891119911(119904 Λ
120588
119904 119906
120588
119904) 119876
120576120588
119904
+1
120576(119891 (119904 Λ
120588
119904 119906
120576120588
119904) minus 119891 (119904 Λ
120588
119904 119906
120588
119904))] 119889119904
minus int
119879
119905
119890119860(119904minus119905)
119876120576120588
119904119889119882 (119904)
(113)
ByTheorem 15 we know that
lim120576rarr0
E[ sup119904isin[119905119879]
100381610038161003816100381610038161003816100381610038161003816
119884120576120588
119904minus 119884
120588
119904
120576minus 119870
120576120588
119904
100381610038161003816100381610038161003816100381610038161003816
2
] = 0 forall119905 isin [0 119879]
lim120576rarr0
Eint119879
0
100381610038161003816100381610038161003816100381610038161003816
119885120576120588
119904minus 119885
120588
119904
120576minus 119876
120576120588
119904
100381610038161003816100381610038161003816100381610038161003816
2
119889119904 = 0
(114)
The proof of the following proposition is technical butbased on the arguments above and we omit it
Proposition 18 One has
1
120576E [Φ ((119884
120576120588
0)1015840
119884120576120588
0) minus Φ((119884
120588
0)1015840
119884120588
0)]
= E [Φ1199101015840 ((119884
120588
0)1015840
119884120588
0) (119870
120576120588
0)1015840
+ Φ119910((119884
120588
0)1015840
119884120588
0)119870
120576120588
0]
+ 119900 (120576)
Mathematical Problems in Engineering 15
1
120576(119869 (119906
120576120588) minus 119869 (119906
120588))
= E [1198921199101015840 ((119884
120588
0)1015840
119884120588
0) (119870
120576120588
0)1015840
+ 119892119910((119884
120588
0)1015840
119884120588
0)119870
120576120588
0]
+ Δ120576+1
120576Eint
119879
0
E1015840[ℎ (119904 Λ
120588
119904 119906
120576120588
119904)
minus ℎ (119904 Λ120588
119904 119906
120588
119904)] 119889119904 + 119900 (120576)
(115)
where
Δ120576= Eint
119879
0
E1015840[ℎ
1199101015840 (119904 Λ
120588
119904 119906
120576120588
119904) (119870
120576120588
119904)1015840
+ ℎ1199111015840 (119904 Λ
120588
119904 119906
120576120588
119904) (119876
120576120588
119904)1015840
+ ℎ119910(119904 Λ
120588
119904 119906
120576120588
119904)119870
120576120588
119904
+ ℎ119911(119904 Λ
120588
119904 119906
120576120588
119904) 119876
120576120588
119904] 119889119904
(116)
53 Variational Inequality and Adjoint Equation In thissubsection the adjoint process is introduced to deduce thevariational inequality
If we set V(sdot) = 119906120576120588(sdot) in (111) and notice that
119889(119906120576120588(sdot) 119906
120588(sdot)) le 120576 we get
minus120588 le1
120576(119869
120588(119906
120576120588
(sdot)) minus 119869120588(119906
120588
(sdot))) (117)
By Lemma 17
1
120576(119869
120588(119906
120576120588
(sdot)) minus 119869120588(119906
120588
(sdot)))
=(119869
120588(119906
120576120588(sdot)))
2
minus (119869120588(119906
120588(sdot)))
2
120576 (119869120588 (119906120576120588 (sdot)) + 119869120588 (119906120588 (sdot)))
=119869 (119906
120576120588(sdot)) + 119869 (119906
120588(sdot)) minus 2119869 (119906 (sdot)) + 2120588
119869120588 (119906120576120588 (sdot)) + 119869120588 (119906120588 (sdot))
times119869 (119906
120576120588(sdot)) minus 119869 (119906
120588(sdot))
120576
+
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] + E [Φ ((119884
120588
0)1015840
119884120588
0)]
119869120588 (119906120576120588 (sdot)) + 119869120588 (119906120588 (sdot))
times
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] minus E [Φ ((119884
120588
0)1015840
119884120588
0)]
120576
997888rarr 119897120588
1
119869 (119906120576120588(sdot)) minus 119869 (119906
120588(sdot))
120576
+ 119897120588
2
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] minus E [Φ ((119884
120588
0)1015840
119884120588
0)]
120576
(118)
where we set
119897120588
1=119869 (119906
120588(sdot)) minus 119869 (119906 (sdot)) + 120588
119869120588 (119906120588 (sdot))
119897120588
2=
E [Φ ((119884120588
0)1015840
119884120588
0)]
119869120588 (119906120588 (sdot))
(119)
and use the limit
119869 (119906120576120588
(sdot)) 997888rarr 119869 (119906120588
(sdot))
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] 997888rarr E [Φ ((119884
120588
0)1015840
119884120588
0)]
(120)
as 120576 rarr 0 according to (115)As |119897120588
1|2
+ |119897120588
2|2
= 1 for all 120588 gt 0 we know that there existsa subsequence of 119897120588
1 119897120588
2 (still denoted by 119897120588
1 119897120588
2) such that
lim120588rarr0
119897120588
1 119897120588
2 = 119897
1 1198972
1003816100381610038161003816119897110038161003816100381610038162
+1003816100381610038161003816119897210038161003816100381610038162
= 1
(121)
Combining (115) (117) with (118) we get
minus120588 le 119897120588
1
119869 (119906120576120588(sdot)) minus 119869 (119906
120588(sdot))
120576
+ 119897120588
2
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] minus E [Φ ((119884
120588
0)1015840
119884120588
0)]
120576
= 119897120588
1Δ120576+ 119897
120588
1E [119892
1199101015840 ((119884
120588
0)1015840
119884120588
0) (119870
120576120588
0)1015840
+119892119910((119884
120588
0)1015840
119884120588
0)119870
120576120588
0]
+119897120588
1
120576Eint
119879
0
E1015840[ℎ (119904 Λ
120588
119904 119906
120576120588
119904) minus ℎ (119904 Λ
120588
119904 119906
120588
119904)] 119889119904
+ 119897120588
2E [Φ
1199101015840 ((119884
120588
0)1015840
119884120588
0) (119870
120576120588
0)1015840
+Φ119910((119884
120588
0)1015840
119884120588
0)119870
120576120588
0] + (119897
120588
1+ 119897
120588
2) 119900 (120576)
(122)
Next we introduce the adjoint equation corresponding tovariational equation (113) whose solution is denoted by119875120588(119905)
119889119875120588
(119905)
= 119860lowast119875120588
(119905) 119889119905
+ E1015840[119891
1199101015840 (119905 Λ
120588
119905 119906
120588
119905) (119875
120588
(119905))1015840
+ 119891119910(119905 Λ
120588
119905 119906
120588
119905) 119875
120588
(119905) + 119897120588
1ℎ1199101015840 (119905 Λ
120588
119905 119906
120588
119905)
+ 119897120588
1ℎ119910(119905 Λ
120588
119905 119906
120588
119905)] 119889119905
16 Mathematical Problems in Engineering
+ E1015840[119891
1199111015840 (119905 Λ
120588
119905 119906
120588
119905) (119875
120588
(119905))1015840
+ 119891119911(119905 Λ
120588
119905 119906
120588
119905) 119875
120588
(119905)
+ 119897120588
1ℎ1199111015840 (119905 Λ
120588
119905 119906
120588
119905) + 119897
120588
1ℎ119911(119905 Λ
120588
119905 119906
120588
119905)] 119889119882 (119905)
119875120588
(0) = 119897120588
1E [119892
1199101015840 ((119884
120588
0(119909))
1015840
119884120588
0(119909))
+ 119892119910((119884
120588
0(119909))
1015840
119884120588
0(119909))]
+ 119897120588
2E [Φ
1199101015840 ((119884
120588
0(119909))
1015840
119884120588
0(119909))
+ Φ119910((119884
120588
0(119909))
1015840
119884120588
0(119909))]
(123)
where 119860lowast is the 1198712(O)-adjoint operator of 119860 Under assump-tions (L1)ndash(L3) this is a linear mean-field SEE with boundedcoefficients An application ofTheorem 11 implies that it has aunique adaptedmild solution such that119875120588(119905) isin S2
F ([0 119879] 119870)When 120588 rarr 0 according to Corollaries 5 and 13 119875120588(119905)
converges to 119875(119905) where
119875 (119905) isin S2
F ([0 119879] 119870) (124)
is the solution of the following equation
119875 (119905) = 1198971E [119892
1199101015840 (119884
1015840
0 119884
0) + 119892
119910(119884
1015840
0 119884
0)]
+ 1198972E [Φ
1199101015840 (119884
1015840
0 119884
0) + Φ
119910(119884
1015840
0 119884
0)]
+ int
119905
0
119890119860lowast(119905minus119904)
E1015840[119891
1199101015840 (119904) 119875
1015840
(119904) + 119891119910(119904) 119875 (119904)
+ 1198971ℎ1199101015840 (119904) + 119897
1ℎ119910(119904)] 119889119904
+ int
119905
0
119890119860lowast(119905minus119904)
E1015840[119891
1199111015840 (119904) 119875
1015840
(119904) + 119891119911(119904) 119875 (119904)
+ 1198971ℎ1199111015840 (119904) + 119897
1ℎ119911(119904)] 119889119882 (119904)
(125)
The following proposition which formally follows fromProposition 18 gives the relation between 119875120588(119905) and119870120576120588
119905
Proposition 19 Consider the following
E [119875120588
(0)119870120576120588
0]
= Eint119879
0
1
120576119875120588
(119904)E1015840[119891 (119904 Λ
120588
119904 119906
120576120588
119904) minus 119891 (119904 Λ
120588
119904 119906
120588
119904)]
minus 119897120588
1119870120576120588
119904E1015840[ℎ
1199101015840 (119904 Λ
120588
119904 119906
120588
119904) + ℎ
119910(119904 Λ
120588
119904 119906
120588
119904)]
minus 119897120588
1119876120576120588
119904E1015840[ℎ
1199111015840 (119904 Λ
120588
119904 119906
120588
119904) + ℎ
119911(119904 Λ
120588
119904 119906
120588
119904)] 119889119904
(126)
The following theorem constitutes the main contributionof this section themaximumprinciple for the BSPDE controlsystem
Theorem 20 Let assumptions (L1)ndash(L3) hold Suppose 119906(sdot) isan optimal control and (119884(sdot) 119885(sdot)) is the corresponding optimalstate trajectory for the BSPDE control systems (72) and (73)with the initial state constraint (75) Then there exists 119875(119905) isinS2
F ([0 119879] 119870) which satisfies (125) such that
H (119905 1198841015840
119905 119885
1015840
119905 119884
119905 119885
119905 V
119905 119875 (119905))
ge H (119905 1198841015840
119905 119885
1015840
119905 119884
119905 119885
119905 119906
119905 119875 (119905))
119886119890 119886119904 forallV isin U119886119889
(127)
whereH [0 119879]times119867timesL(Γ119867)times119867timesL(Γ119867)times119880times119870 rarr R
is the Hamiltonian function defined by
H (119905 119910 119910 119911 V 119901) = 1198971ℎ (119905 119910 119910 119911 V)
+ 119901119891 (119905 119910 119910 119911 V) (128)
Proof By (122) and Proposition 19 we obtain
minus120588 le1
120576Eint
119879
0
119875120588
(119904)E1015840[119891 (119904 Λ
120588
119904 119906
120576120588
119904)
minus 119891 (119904 Λ120588
119904 119906
120588
119904)] 119889119904
+119897120588
1
120576Eint
119879
0
E1015840[ℎ (119904 Λ
120588
119904 119906
120576120588
119904)
minus ℎ (119904 Λ120588
119904 119906
120588
119904)] 119889119904 + (119897
120588
1+ 119897
120588
2) 119900 (120576)
(129)
Letting 120576 rarr 0+ in (129) we derive for ae 120591 isin [0 119879]
minus120588 le 119897120588
1E1015840[ℎ (120591 Λ
120588
120591 V
120591) minus ℎ (120591 Λ
120588
120591 119906
120588
120591)]
+ 119875120588
(119904)E1015840[119891 (120591 Λ
120588
120591 V
120591) minus 119891 (120591 Λ
120588
120591 119906
120588
120591)]
(130)
for all V isin UadFinally taking 120588 rarr 0 in (130) we derive the desired
result
Remark 21 We note that if the coefficients do not dependexplicitly on the marginal law of the underlying diffusionthe result reduces to the classical case that is the SMP forBSPDEs without mean-field term
Remark 22 When we remove the initial state constraint (75)we obtain the general maximum principle for the mean-fieldBSPDEs system (ie without the constraint) with 119897
1= 1
54 Application A Backward Linear Quadratic Control Prob-lem Now we apply our maximum principle to solve an LQproblem For notational simplicity we restrict ourselves tothe free case (ie without the initial state constraint (75)) thegeneral case being handled in a similar way
Mathematical Problems in Engineering 17
Consider the following problem
119869 (119906) =1
2E [(119884 (0))
2] +
1
2Eint
119879
0
119873V2 (119905) 119889119905 997888rarr min
(131)
subject to
119889119884 (119905) = minus119860119884 (119905) 119889119905
minus 119861119884 (119905) + 119861E [119884 (119905)]
+119862V (119905) + 119863119885 (119905) + 119863E [119885 (119905)] 119889119905
+ 119885 (119905) 119889119882 (119905)
119884 (119879) = 120585 119905 isin [0 119879]
(132)
where 119860 is a partial differential operator satisfying condition(L1) and 119861 119861 119862119863119863 and119873 are bounded and deterministicconstantsWe also assume that119873 gt 0 and V isin 1198712F (0 119879 119880) It iseasy to verify that BSPDE (132) admits a uniquemild solution(119884(119905) 119885(119905))
119875(119905) the adjoint process of state equation (132) is thesolution of
119889119875 (119905) = 119860lowast119875 (119905) 119889119905 + 119861119875 (119905) + 119861E [119875 (119905)] 119889119905
+ 119863119875 (119905) + 119863E [119875 (119905)] 119889119882 (119905)
119875 (0) = 119884 (0)
(133)
Let 119906(sdot) be an optimal control and let (119884(sdot) 119885(sdot)) bethe corresponding state process By maximum principle ofTheorem 20 (note that 119897
1= 1 in this problem)
1
2119873V2 (119905) + 119875 (119905) 119862V (119905) ge
1
2119873119906
2
(119905) + 119875 (119905) 119862119906 (119905) (134)
for all V isin 119880ad since the state equation has the form (132)Thisin turn implies
119906 (119905) = minus119862
119873119875 (119905) (135)
It is clear that (131) is a positive quadratic functional of controlbecause of 119873 gt 0 Hence an optimal control exists Thecandidate optimal control (135) is indeed an optimal controlof this LQ problem for it is the only control which satisfies themaximum principle
Next we want to obtain a more explicit representationof the optimal control (135) from the state equation (132)Substituting (135) into state equation (132) yields
119889119884 (119905) = minus119860119884 (119905) 119889119905
minus 119861119884 (119905) + 119861E [119884 (119905)] minus1198622
119873119875 (119905)
+119863119885 (119905) + 119863E [119885 (119905)] 119889119905 + 119885 (119905) 119889119882 (119905)
119884 (119879) = 120585 119905 isin [0 119879]
(136)
Combining the above equation with (133) we obtain thefollowing related feedback control system
119889119884 (119905) = minus119860119884 (119905) 119889119905
minus 119861119884 (119905) + 119861E [119884 (119905)] minus1198622
119873119875 (119905)
+ 119863119885 (119905) + 119863E [119885 (119905)] 119889119905 + 119885 (119905) 119889119882 (119905)
119884 (119879) = 120585
119889119875 (119905) = 119860lowast119875 (119905) 119889119905 + 119861119875 (119905) + 119861E [119875 (119905)] 119889119905
+ 119863119875 (119905) + 119863E [119875 (119905)] 119889119882 (119905)
119875 (0) = 119884 (0)
(137)
Looking at the terminal condition of 119875(119905) in (133) andconsidering the mean-field type of (132) it is reasonable toconjecture that 119875(119905) has the following form
119875 (119905) = 120593 (119905) 119884 (119905) + 120601 (119905)E [119884 (119905)] (138)
where120593(119905)120601(119905) are deterministic differential functionswhichwill be specified below Moreover 120593(0) = 1 120601(0) = 0
Inserting this form into adjoint equation (133) and notic-ing that 119884(119905) satisfies (136) we can compare the coefficientsof 119889119905 and 119889119882(119905) to obtain the following equation
2120593 (119905) 119860119884 (119905) + 2120601 (119905) 119860E [119884 (119905)] + 119861120593 (119905) 119884 (119905)
+ 2 (119861120601 (119905) + 119861120593 (119905) + 119861120601 (119905))E [119884 (119905)]
= minus120593 (119905) 119861119884 (119905) minus1198622
119873120593 (119905) 119884 (119905) + 119863119885 (119905)
+ 119884 (119905)119889120593 (119905)
119889119905+ E [119884 (119905)]
119889120601 (119905)
119889119905
+ (1206012
(119905)1198622
119873+ 2
1198622
119873120593 (119905) 120601 (119905))E [119884 (119905)]
minus (119863120601 (119905) + 119863120601 (119905) + 119863120593 (119905))E [119885 (119905)]
120593 (119905) 119885 (119905) = 119863120593 (119905) 119884 (119905)
+ (119863120601 (119905) + 119863120593 (119905) + 119863120601 (119905))E [119884 (119905)]
(139)
Then subtracting 119885(119905) we have
2120593 (119905) 119860119884 (119905) + 2120601 (119905) 119860E [119884 (119905)] + 119861120593 (119905) 119884 (119905)
+ 2 (119861120601 (119905) + 119861120593 (119905) + 119861120601 (119905))E [119884 (119905)]
18 Mathematical Problems in Engineering
= 120593 (119905) minus119861 +1198622
119873120593 (119905) minus 119863
2119884 (119905)
+ 119884 (119905)119889120593 (119905)
119889119905+ E [119884 (119905)]
119889120601 (119905)
119889119905
+ (1206012
(119905)1198622
119873+ 2
1198622
119873120593 (119905) 120601 (119905))E [119884 (119905)]
minus1
120593 (119905)(119863120601 (119905) + 119863120593 (119905) + 119863120601 (119905))
2
E [119884 (119905)]
minus 2119863 (119863120601 (119905) + 119863120593 (119905) + 119863120601 (119905))E [119884 (119905)]
(140)
Comparing the coefficients of 119884(119905) and E[119884(119905)] respec-tively we get
119889
119889119905120593 (119905) = 2119860
lowast120593 (119905) + 2119861120593 (119905) + 119863
2120593 (119905) minus
1198622
1198731205932
(119905)
120593 (0) = 1
(141)
119889
119889119905120601 (119905) = 2119860
lowast120601 (119905) + (
(119863 + 119863)2
120593 (119905)minus1198622
119873)120601
2
(119905)
+ 2119877 (119905) 120601 (119905) + (1198632+ 2119861 + 2119863119863)120593 (119905)
120601 (0) = 0
(142)
where 119877(119905) = 119861 + 119861 + (119863 + 119863)2
minus (1198622119873)120593(119905)
We solve (141) to get
120593 (119905) = (119890minus(2119860lowast+2119861+119863
2)119905(1 minus
1198622
119873(2119860lowast + 2119861 + 1198632))
+1198622
119873(2119860lowast + 2119861 + 1198632))
minus1
(143)
Then (142) exists a unique solution from the classical Riccatiequation theory
We now conclude the above discussions in the followingresult
Theorem 23 For onersquos linear quadratic stochastic partialdifferential control problem (131)-(132) the unique optimalcontrol 119906(sdot) isin 119880ad is given by
119906 (119905) = minus119862
119873(120593 (119905) 119884 (119905) + 120601 (119905)E [119884 (119905)]) (144)
with 120593(119905) satisfying (143) and 120601(119905) solving (142)
Conflict of Interests
The authors declare that they have no conflict of interestsregarding the publication of this paper
Acknowledgment
This research is partially supported by the Natural ScienceFoundation of China under Grant no 11301310
References
[1] Y Hu and S Peng ldquoAdapted solution of a backward semilinearstochastic evolution equationrdquo Stochastic Analysis and Applica-tions vol 9 no 4 pp 445ndash459 1991
[2] N IMahmudov andMAMcKibben ldquoOnbackward stochasticevolution equations in Hilbert spaces and optimal controlrdquoNonlinear Analysis vol 67 no 4 pp 1260ndash1274 2007
[3] M Fuhrman and G Tessitore ldquoNonlinear Kolmogorov equa-tions in infinite dimensional spaces the backward stochasticdifferential equations approach and applications to optimalcontrolrdquoThe Annals of Probability vol 30 no 3 pp 1397ndash14652002
[4] M Fuhrman and G Tessitore ldquoInfinite horizon backwardstochastic differential equations and elliptic equations inHilbert spacesrdquo Annals of Probability vol 32 no 1B pp 607ndash660 2004
[5] X Mao ldquoAdapted solutions of backward stochastic differentialequations with non-Lipschitz coefficientsrdquo Stochastic Processesand their Applications vol 58 no 2 pp 281ndash292 1995
[6] J Lasry and P Lions ldquoMean field gamesrdquo Japanese Journal ofMathematics vol 2 no 1 pp 229ndash260 2007
[7] R Buckdahn J Li and S Peng ldquoMean-field backward stochas-tic differential equations and related partial differential equa-tionsrdquo Stochastic Processes and their Applications vol 119 no 10pp 3133ndash3154 2009
[8] D Andersson and B Djehiche ldquoAmaximum principle for SDEsofmean-field typerdquoAppliedMathematics andOptimization vol63 no 3 pp 341ndash356 2011
[9] R Buckdahn B Djehiche J Li and S Peng ldquoMean-fieldbackward stochastic differential equations a limit approachrdquoThe Annals of Probability vol 37 no 4 pp 1524ndash1565 2009
[10] R Xu ldquoMean-field backward doubly stochastic differentialequations and related SPDEsrdquo Boundary Value Problems vol2012 article 114 2012
[11] R Buckdahn B Djehiche and J Li ldquoA general stochasticmaximum principle for SDEs of mean-field typerdquo AppliedMathematics and Optimization vol 64 no 2 pp 197ndash216 2011
[12] J Li ldquoStochasticmaximumprinciple in themean-field controlsrdquoAutomatica vol 48 no 2 pp 366ndash373 2012
[13] G Wang C Zhang and W Zhang ldquoStochastic maximumprinciple for mean-field type optimal control under partialinformationrdquo IEEE Transactions on Automatic Control vol 59no 2 pp 522ndash528 2014
[14] M Hafayed ldquoA mean-field necessary and sufficient conditionsfor optimal singular stochastic controlrdquo Communications inMathematics and Statistics vol 1 no 4 pp 417ndash435 2013
[15] M Hafayed ldquoA mean-field maximum principle for optimalcontrol of forward-backward stochastic differential equationswith Poisson jumpprocessesrdquo International Journal ofDynamicsand Control vol 1 no 4 pp 300ndash315 2013
[16] Y Hu and S Peng ldquoMaximum principle for semilinearstochastic evolution control systemsrdquo Stochastics and StochasticsReports vol 33 no 3-4 pp 159ndash180 1990
[17] M Fuhrman Y Hu and G Tessitore ldquoStochastic maximumprinciple for optimal control of SPDEsrdquo Comptes Rendus Math-ematique vol 350 no 13-14 pp 683ndash688 2012
Submit your manuscripts athttpwwwhindawicom
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Differential EquationsInternational Journal of
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Function Spaces
Abstract and Applied AnalysisHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of Mathematics and Mathematical Sciences
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Discrete Dynamics in Nature and Society
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Volume 2014 Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Stochastic AnalysisInternational Journal of
2 Mathematical Problems in Engineering
In this paper we investigate a new type of backwardstochastic evolution equations inHilbert spaces whichwe callmean-field BSEEs Mean-field implies that the coefficient ofthe BSEE depends on the law of the BSEE Specifically theBSEE we consider is defined as
119889119884 (119904) = minus 119860119884 (119904) 119889119904
minus E1015840[119891 (119904 119884
1015840
(119904) 1198851015840
(119904) 119884 (119904) 119885 (119904))] 119889119904
+ 119885 (119904) 119889119882 (119904)
119884 (119879) = 120585 119904 isin [0 119879]
(1)
in a Hilbert space 119867 where 119891 denotes a given measurablemapping 119879 is a fixed positive real number 119882(119904) is acylindrical Wiener process and 119860 represents the generatorof a strongly continuous semigroup 119890
119905119860 in 119867 with 119905 ge 0Precise interpretation of E1015840[119891(119904 1198841015840
(119904) 1198851015840(119904) 119884(119904) 119885(119904))] is
given in the following sections Based on the contractionmapping we firstly prove that (1) admits a unique mildsolution if the function 119891 is Lipschitz continuous Secondlyunder non-Lipschitz assumptions we obtain the existenceand uniqueness of the mild solution for mean-field BSEEby constructing a special Cauchy sequence The Lipschitzcondition is a special case of this non-Lipschitz condition (seeMao [5]) In addition we investigate the well-posedness ofmean-field stochastic evolution equations
We also study optimal control problems of stochasticsystems governed by mean-field BSPDEs in Hilbert spacesOur objective is to formulate a stochastic maximumprinciple(SMP) for the optimal control problem with an initial stateconstraint There is a vast literature on the theory of SMPAmong these papers Andersson and Djehiche [8] studiedthe optimal control problem for mean-field stochastic systemwhen the control domain is convex They obtained themaximum principle by a convex variational method By aspike variational technique Buckdahn et al [11] obtained ageneral maximum principle for a special mean-field stochas-tic differential equation (SDE) where the action space is notconvex Later Li [12] investigated the maximum principlefor more general SDEs of mean-field type with a convexcontrol domain Wang et al [13] were concerned with apartially observed optimal control problem of mean-fieldtype By using Girsanovrsquos theorem and convex variationthey derived the correspondingmaximum principle and gavean illustrative example to demonstrate the application ofthe obtained SMP Hafayed studied the mean-field SMP forsingular stochastic control in [14] and mean-field SMP forFBSDEs with Poisson jump processes in [15]
For the case of stochastic control systems in infinitedimensions on the assumption that the control domain isnot necessarily convex while the diffusion coefficient doesnot contain the control variable Hu and Peng [16] usedspike variation approach and Ekelandrsquos variational principleto establish the maximum principle for semilinear stochas-tic evolution control systems with a final state constraintMahmudov and Mckibben [2] obtained an SMP for stochas-tic control systems governed by BSEEs in Hilbert spacesRecently Fuhrman et al [17] deduced themaximumprinciple
for optimal control of stochastic PDEs when the controldomain is not necessarily convex
We establish necessary optimality conditions for thecontrol problem in the form of a maximum principle on theassumption that the control domain is not necessarily convexDue to the initial state constraint we first need to applyEkelandrsquos variational principle to convert the given controlproblem into a free initial state optimal control problemThenspike variation approach is used to deduce the SMP in themean-field framework In our control system not only thestate processes which are the unique mild solution of thegiven BSPDE but also the cost functional are of mean-fieldtype In other words they depend on the law of the BSPDEas well as the state and the control For this new controlledsystem the adjoint equation will turn out to be a mean-fieldstochastic evolution equation
Theplan of this paper is organized as follows In Section 2we introduce some notations which are needed in whatfollows In Section 3 the well-posedness of mean-field BSEE(1) is studied we first prove the existence and uniqueness of amild solution under the Lipschitz condition and investigatethe regular dependence of the solution (119884 119885) on (120585 119891)And then under the assumption that the coefficient is non-Lipschitz continuous a new result on the existence anduniqueness of the mild solution to (1) in Hilbert space isestablished which generalizes the result for the Lipschitzcase Section 4 is devoted to the regularity of mean-fieldstochastic evolution equations In Section 5 we derive thestochastic maximum principle for the BSPDE systems ofmean-field type with an initial state constraint and at thelast part of Section 5 an LQ example is given to show theapplication of our maximum principle An explicit optimalcontrol is obtained in this example
2 Preliminaries
The norm of an element 119909 in a Banach space 119865 is denotedby |119909|
119865or simply |119909| if no confusion is possible Γ 119867 and
119870 are three real and separable Hilbert spaces Scalar productis denoted by ⟨sdot sdot⟩ with a subscript to specify the space ifnecessaryL(Γ 119870) is the space of Hilbert-Schmidt operatorsfrom Γ to 119870 endowed with the Hilbert-Schmidt norm
Let (ΩFP) be a complete probability space A cylin-drical Wiener process 119882(119905) 119905 ge 0 in a Hilbert space Γ is afamily of linear mappings Γ rarr 119871
2(ΩFP) such that
(i) for every 119906 isin Γ 119882(119905)119906 119905 ge 0 is a real (continuous)Wiener process
(ii) for every 119906 V isin Γ and 119905 119904 ge 0 E(119882(119905)119906 sdot 119882(119904)V) =(119905 and 119904)⟨119906 V⟩
Γ
By F119905 119905 isin [0 119879] we denote the natural filtration of 119882
augmented with the familyN of P-null sets ofF119879
F119905= 120590 (119882 (119904) 119904 isin [0 119905]) orN (2)
The filtration (F119905)119905ge0
satisfies the usual conditions Allthe concepts of measurability for stochastic processes (egadapted etc) refer to this filtration
Mathematical Problems in Engineering 3
Next we define several classes of stochastic processes withvalues in a Hilbert space119867
(I) H2
F ([0 119879]119867) denotes the set of (classes of 119889P times
119889119905 ae equal) measurable random processes 120595119905 119905 isin
[0 119879] which satisfy
(i) Eint1198790|120595
119905|2119889119905 lt +infin
(ii) 120595119905isF
119905measurable for ae 0 le 119905 le 119879
Evidently H2
F (0 119879119867) is a Banach space en-dowed with the canonical norm
10038171003817100381710038171205951003817100381710038171003817 = Eint
119879
0
100381610038161003816100381612059511990410038161003816100381610038162
119889119904
12
(3)
(II) S2
F ([0 119879]119867) denotes the set of continuous randomprocesses 120595
119905 119905 isin [0 119879] which satisfy
(i) E(sup0le119905le119879
|120595119905|2) lt +infin
(ii) 120595119905isF
119905measurable for ae 0 le 119905 le 119879
(III) 1198710(ΩFP 119867) denotes the space of all 119867 valued F-measurable random variables
(IV) For 1 le 119901 lt infin 119871119901(ΩFP 119867) is the space of allF-measurable random variables such that E[|120585|119901] lt infin
(V) For any 120573 isin R introduce the norm
1003817100381710038171003817(119910 119911)10038171003817100381710038172
120573119905= Eint
119879
119905
1198902120573119904
(1003816100381610038161003816119910 (119904)
10038161003816100381610038162
+ |119911 (119904)|2) 119889119904 (4)
on the Banach space
K120573[119905 119879] = S
2
F ([119905 119879] 119867) timesH2
F ([119905 119879] L (Γ119867)) (5)
For 0 lt 119879 lt infin all the norms sdot 120573119905
with different 120573 isin R areequivalentK[0 119879] = K
0[0 119879] is the Banach space endowed
with the norm
1003817100381710038171003817(119910 119911)10038171003817100381710038172
= Eint119879
0
(1003816100381610038161003816119910 (119904)
10038161003816100381610038162
+ |119911 (119904)|2) 119889119904 (6)
The following result on BSEEs (see Lemma 2 in Mahmu-dov and McKibben [2]) will play a key role in proving thewell-posedness of mean-field BSEEs
Lemma 1 Let 119867 be a Hilbert space and let 119860 119863(119860) sub
119867 rarr 119867 be a linear operator which generates a 1198620-semigroup
119878(119905) 0 le 119905 le 119879 on 119867 For any (120585 119891) isin 1198712(ΩF
119879P 119867) times
H2
F ([0 119879]119867) the following equation
119884 (119905) = 119878 (119879 minus 119905) 120585 + int
119879
119905
119878 (119904 minus 119905) 119891 (119904) 119889119904
+ int
119879
119905
119878 (119904 minus 119905) 119885 (119904) 119889119882 (119904) 119875-119886119904(7)
has a unique solution inK120573[0 119879] moreover
E sup119905le119904le119879
1198902120573119904
|119884 (119904)|2+ Eint
119879
119905
1198902120573119904
|119885 (119904)|2119889119904
le 241198722
119878(119890
2120573119879E100381610038161003816100381612058510038161003816100381610038162
+1
2120573int
119879
119905
1198902120573119903
E1003816100381610038161003816119891 (119903)
10038161003816100381610038162
119889119903)
(8)
where119872119878= sup119878(119905)B(119867) 0 le 119905 le 119879 andB(119867) is the space
of bounded linear operators on119867
3 Mean-Field Backward StochasticEvolution Equations
In this section we study the existence and uniqueness resultof mild solutions to mean-field BSEEs in a Hilbert space 119867To this end we firstly recall some notations introduced byBuckdahn et al [7]
Let (ΩFP) = (Ω times ΩF otimes FP otimes P) be the(noncompleted) product of (ΩFP) with itself and wedefine F = F
119905= F otimes F
119905 0 le 119905 le 119879 on this product
space A random variable 120585 isin 1198710(ΩFP 119867) originally
defined on Ω is extended canonically to Ω 1205851015840(120596
1015840 120596) =
120585(1205961015840) (120596
1015840 120596) isin Ω = Ω times Ω For any 120578 isin 119871
1(ΩFP) the
variable 120578(sdot 120596) Ω rarr 119870 belongs to 1198711(ΩFP)P(119889120596) aswhose expectation is denoted by
E1015840[120578 (sdot 120596)] = int
Ω
120578 (1205961015840 120596)P (119889120596
1015840) (9)
Note that E1015840[120578] = E1015840[120578(sdot 120596)] isin 1198711(ΩFP) and
E [120578] (= intΩ
120578119889P = intΩ
E1015840[120578 (sdot 120596)]P (119889120596)) = E [E
1015840[120578]]
(10)
Themean-field BSEE we consider has the following formfor any given measurable mapping 119891 [0 119879] times119867timesL(Γ119867)times
119867 timesL(Γ119867) rarr 119867 and 120585 isin 1198712(ΩF119879P 119867)
119889119884 (119904) = minus 119860119884 (119904) 119889119904
minus E1015840[119891 (119904 119884
1015840
(119904) 1198851015840
(119904) 119884 (119904) 119885 (119904))] 119889119904
+ 119885 (119904) 119889119882 (119904)
119884 (119879) = 120585 119904 isin [0 119879]
(11)
where 119860 119863(119860) sub 119867 rarr 119867 is the generator of a stronglycontinuous semigroup 119890119905119860 119905 ge 0 in the Hilbert space119867 withthe notation119872
119860≜ sup
119905isin[0119879]|119890119905119860|
4 Mathematical Problems in Engineering
Definition 2 We say that a pair of adapted processes (119884 119885)is a mild solution of mean-field BSEE (11) if (119884 119885) isin
S2
F ([0 119879]119867) timesH2
F ([0 119879]L(Γ119867)) and for all 119905 isin [0 119879]
119884 (119905) = 119890119860(119879minus119905)
120585
+ int
119879
119905
119890119860(119904minus119905)
E1015840[119891 (119904 119884
1015840
(119904) 1198851015840
(119904) 119884 (119904) 119885 (119904))] 119889119904
minus int
119879
119905
119890119860(119904minus119905)
119885 (119904) 119889119882 (119904)
(12)
Remark 3 We emphasize that the coefficient of (11) can beinterpreted as
E1015840[119891 (119904 119884
1015840
(119904) 1198851015840
(119904) 119884 (119904) 119885 (119904))] (120596)
= E1015840[119891 (119904 119884
1015840
(119904) 1198851015840
(119904) 119884 (120596 119904) 119885 (120596 119904))]
= intΩ
119891 (1205961015840 120596 119904 119884 (120596
1015840 119904) 119885 (120596
1015840 119904) 119884 (120596 119904) 119885 (120596 119904))
times P (1198891205961015840)
(13)
31 Lipschitz Case Now we study the existence and unique-ness ofmild solutions tomean-field BSEE (11) under Lipschitzconditions For119891 [0 119879] times 119867 times L(Γ119867) times 119867 times L(Γ119867) rarr
119867 assume the following(A1) There exists an 119871 gt 0 such that
10038161003816100381610038161003816119891 (119905 119910
1015840
1 119911
1015840
1 119910
1 119911
1) minus 119891 (119905 119910
1015840
2 119911
1015840
2 119910
2 119911
2)10038161003816100381610038161003816
2
le 119871 (100381610038161003816100381610038161199101015840
1minus 119910
1015840
2
10038161003816100381610038161003816
2
+100381610038161003816100381610038161199111015840
1minus 119911
1015840
2
10038161003816100381610038161003816
2
+10038161003816100381610038161199101 minus 1199102
10038161003816100381610038162
+10038161003816100381610038161199111 minus 1199112
10038161003816100381610038162
)
(14)
for all 119905 isin [0 119879] 1199101015840119894 119910
119894isin 119867 119911
1015840
119894 119911
119894isin L(Γ119867) (119894 =
1 2)(A2) 119891(sdot 0 0 0 0) isin H2
F ([0 119879]119867)We have the following theorem
Theorem 4 For any random variable 120585 isin 1198712(ΩF
119879P 119867)
under (A1) and (A2) mean-field BSEE (11) admits a uniquemild solution (119884 119885) isin S2
F ([0 119879]119867) timesH2
F ([0 119879]L(Γ119867))
Proof Consider the following
Step 1 For any (119910 119911) isin S2
F ([0 119879]119867) timesH2
F ([0 119879]L(Γ119867))BSEE119884 (119905) = 119890
119860(119879minus119905)120585
+ int
119879
119905
E1015840[119890
119860(119904minus119905)119891 (119904 119910
1015840
(119904) 1199111015840
(119904) 119884 (119904) 119885 (119904))] 119889119904
minus int
119879
119905
119890119860(119904minus119905)
119885 (119904) 119889119882 (119904) 0 le 119905 le 119879
(15)
has a unique solution In order to get this conclusion wedefine
119891(119910119911)
(119904 120583 ]) = E1015840[119891 (119904 119910
1015840
(119904) 1199111015840
(119904) 120583 ])] (16)
Then (15) can be rewritten as
119884 (119905) = 119890119860(119879minus119905)
120585
+ int
119879
119905
119890119860(119904minus119905)
119891(119910119911)
(119884 (119904) 119885 (119904)) 119889119904
minus int
119879
119905
119890119860(119904minus119905)
119885 (119904) 119889119882 (119904)
(17)
Due to (A1) for all (1205831 ]
1) (120583
2 ]
2) isin 119867timesL(Γ119867)119891 satisfies
10038161003816100381610038161003816119891(119910119911)
(1205831 ]
1) minus 119891
(119910119911)(120583
2 ]
2)10038161003816100381610038161003816
2
le 119871 (10038161003816100381610038161205831 minus 1205832
10038161003816100381610038162
+1003816100381610038161003816]1 minus ]
2
10038161003816100381610038162
)
(18)
According to Theorem 31 in [1] BSEE (15) has a uniquesolution
Step 2 FromStep 1 we can define amappingΦ (119884(sdot) 119885(sdot)) =
Φ[(1199101015840(sdot) 119911
1015840(sdot))] K[0 119879] rarr K[0 119879] through
119884 (119905) = 119890119860(119879minus119905)
120585
+ int
119879
119905
E1015840[119890
119860(119904minus119905)119891 (119904 119910
1015840
(119904) 1199111015840
(119904) 119884 (119904) 119885 (119904))] 119889119904
minus int
119879
119905
119890119860(119904minus119905)
119885 (119904) 119889119882 (119904) 0 le 119905 le 119879
(19)
For any (119910119894 119911119894) isin K[0 119879] we set (119884119894 119885
119894) = Φ[(119910
119894 119911
119894)] 119894 =
1 2 (119910 119911) = (1199101minus119910
2 119911
1minus119911
2) and (119884 119885) = (119884
1minus119884
2 119885
1minus119885
2)
Then from Lemma 1 we have
E sup0le119904le119879
119890212057311990410038161003816100381610038161003816
119884 (119904)10038161003816100381610038161003816
2
+ Eint119879
0
119890212057311990410038161003816100381610038161003816
119885 (119904)10038161003816100381610038161003816
2
119889119904
le12119872
2
119860
120573E
Mathematical Problems in Engineering 5
times [int
119879
0
1198902120573119904
100381610038161003816100381610038161003816E1015840[119891 (119904 (119910
1
(119904))1015840
(1199111
(119904))1015840
1198841
(119904) 1198851
(119904))
minus 119891 (119904 (1199102
(119904))1015840
(1199112
(119904))1015840
1198842
(119904) 1198852
(119904))]100381610038161003816100381610038161003816
2
119889119904]
le12119872
2
1198601198712
120573E
times [int
119879
0
1198902120573119904
(E [1003816100381610038161003816119910 (119904)
10038161003816100381610038162
+ |119911 (119904)|2]
+10038161003816100381610038161003816119884 (119904)
10038161003816100381610038161003816
2
+10038161003816100381610038161003816119885 (119904)
10038161003816100381610038161003816
2
) 119889119904]
=12119872
2
1198601198712
120573E
times [int
119879
0
1198902120573119904
(1003816100381610038161003816119910 (119904)
10038161003816100381610038162
+ |119911 (119904)|2
+10038161003816100381610038161003816119884 (119904)
10038161003816100381610038161003816
2
+10038161003816100381610038161003816119885 (119904)
10038161003816100381610038161003816
2
) 119889119904]
(20)
If we set 120573 = 361198722
1198601198712max119879 1 then
Eint119879
0
1198902120573119904
(10038161003816100381610038161003816119884 (119904)
10038161003816100381610038161003816
2
+10038161003816100381610038161003816119885 (119904)
10038161003816100381610038161003816
2
) 119889119904
le 119879 sdot E sup0le119904le119879
119890212057311990410038161003816100381610038161003816
119884 (119904)10038161003816100381610038161003816
2
+ Eint119879
0
119890212057311990410038161003816100381610038161003816
119885 (119904)10038161003816100381610038161003816
2
119889119904
le12119872
2
1198601198712max 119879 1120573
E
times [int
119879
0
1198902120573119904
(1003816100381610038161003816119910 (119904)
10038161003816100381610038162
+ |119911 (119904)|2
+10038161003816100381610038161003816119884 (119904)
10038161003816100381610038161003816
2
+10038161003816100381610038161003816119885 (119904)
10038161003816100381610038161003816
2
) 119889119904]
=1
3E [int
119879
0
1198902120573119904
(1003816100381610038161003816119910 (119904)
10038161003816100381610038162
+ |119911 (119904)|2
+10038161003816100381610038161003816119884 (119904)
10038161003816100381610038161003816
2
+10038161003816100381610038161003816119885 (119904)
10038161003816100381610038161003816
2
) 119889119904]
(21)
That is
Eint119879
0
1198902120573119904
(10038161003816100381610038161003816119884 (119904)
10038161003816100381610038161003816
2
+10038161003816100381610038161003816119885 (119904)
10038161003816100381610038161003816
2
) 119889119904
le1
2Eint
119879
0
1198902120573119904
(1003816100381610038161003816119910 (119904)
10038161003816100381610038162
+ |119911 (119904)|2) 119889119904
(22)
The estimate (22) shows that Φ is a contraction on thespaceK
120573[0 119879] with the norm
(119884 119885)2
120573= Eint
119879
0
1198902120573119904
(|119884 (119904)|2+ |119885 (119904)|
2) 119889119904 (23)
With the contraction mapping theorem there admits aunique fixed point (119884 119885) isin K
120573[0 119879] such that Φ(119884 119885) =
(119884 119885) On the other hand from Step 1 we know thatif Φ(119884 119885) = (119884 119885) then (119884 119885) isin S2
F ([0 119879]119867) times
H2
F ([0 119879]L(Γ119867)) which is the unique mild solution of(11)
Arguing as the previous proof we arrive at the followingassertion in a straightforward way
Corollary 5 Suppose that for all 120572 in a metric space 119865 119891120572is
a given function satisfying (A1) and (A2) with 119871 independenton 120572 Also suppose that
E1015840[119891
120572(119904 119884
1015840
(119904) 1198851015840
(119904) 119884 (119904) 119885 (119904))]
997888rarr E1015840[119891
1205720(119904 119884
1015840
(119904) 1198851015840
(119904) 119884 (119904) 119885 (119904))]
(24)
in 1198712([0 119879]119867) as 120572 rarr 1205720for all (119884 119885) isin S2
F ([0 119879]119867) times
H2
F ([0 119879]L(Γ119867))If we denote by (119884(120585 120572) 119885(120585 120572)) the mild solution of (11)
corresponding to the functions 119891120572and to the final data 120585 isin
1198712(ΩF
119879P 119867) then the map (120572 120585) rarr (119884(120585 120572) 119885(120585 120572))
is continuous from 119865 times 1198712(ΩF
119879P 119867) to S2
F ([0 119879]119867) times
H2
F ([0 119879]L(Γ119867))
32 Non-Lipschitz Case This subsection is devoted to findingsome weaker conditions than the Lipschitz one under whichthe mean-field BSEE has a unique solution To state our mainresult in this section we suppose the following
(A3) For all 119905 isin [0 119879] 1199101015840119894 119910
119894isin 119867 1199111015840
119894 119911
119894isin L(Γ119867) (119894 =
1 2) there exists an 119897 gt 0 such that
10038161003816100381610038161003816119891 (119905 119910
1015840
1 119911
1015840
1 119910
1 119911
1) minus 119891 (119905 119910
1015840
2 119911
1015840
2 119910
2 119911
2)10038161003816100381610038161003816
2
le 120579 (100381610038161003816100381610038161199101015840
1minus 119910
1015840
2
10038161003816100381610038161003816
2
) + 120579 (10038161003816100381610038161199101 minus 1199102
10038161003816100381610038162
)
+ 119897 (100381610038161003816100381610038161199111015840
1minus 119911
1015840
2
10038161003816100381610038161003816
2
+10038161003816100381610038161199111 minus 1199112
10038161003816100381610038162
)
(25)
where 120579 R+rarr R+ is a concave increasing function such
that 120579(0) = 0 120579(119906) gt 0 for 119906 gt 0 and int0+(119889119906120579(119906)) = infin
InMao [5] the author gave three examples of the function120579(sdot) to show the generality of condition (A3) From theseexamples we can see that Lipschitz condition (A1) is a specialcase of the given condition (A3)
Since 120579 is concave and 120579(0) = 0 there exists a pair ofpositive constants 119886 and 119887 such that
120579 (119906) le 119886 + 119887119906 (26)
for all 119906 ge 0 Therefore under assumptions (A2) and(A3) 119891(sdot 1199101015840(sdot) 1199111015840(sdot) 119910(sdot) 119911(sdot)) isin H2
F ([0 119879]119867) whenever
6 Mathematical Problems in Engineering
1199101015840(sdot) 119910(sdot) isin S2
F ([0 119879]119867) and 1199111015840(sdot) 119911(sdot) isin H2
F ([0 119879]L(Γ119867))
By Picard-type iteration we now construct an approxi-mate sequence using which we obtain the desired result Let1198840(119905) equiv 0 and for 119899 isin N let 119884
119899 119885
119899 be a sequence in
S2
F ([0 119879]119867) timesH2
F ([0 119879]L(Γ119867)) defined recursively by
119884119899(119905) = 119890
119860(119879minus119905)120585
+ int
119879
119905
E1015840[119890
119860(119904minus119905)
times119891 (119904 1198841015840
119899minus1(119904) 119885
1015840
119899(119904) 119884
119899minus1(119904) 119885
119899(119904))] 119889119904
minus int
119879
119905
119890119860(119904minus119905)
119885119899(119904) 119889119882 (119904)
(27)
on 0 le 119905 le 119879 From Theorem 4 (27) has a unique mildsolution (119884
119899(119905) 119885
119899(119905))
In order to give the main result we need to prepare thefollowing lemmas about the properties of (119884
119899(119905) 119885
119899(119905)) 119905 isin
[0 119879]
Lemma 6 Under hypotheses (A2) and (A3) there existpositive constants 119862
1and 119862
2such that
(i)E( sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899 (119904)
10038161003816100381610038162
) le 21198621exp (119879 minus 119905)
(ii)Eint119879
119905
11989021205731199041003816100381610038161003816119885119899
(119904)10038161003816100381610038162
119889119904 le 21198621exp (119879 minus 119905)
(iii)E( sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899+1 (119904) minus 119884119899 (119904)
10038161003816100381610038162
)
le 1198622int
119879
119905
120579(E sup119904le119903le119879
11989021205731199031003816100381610038161003816119884119899 (119903) minus 119884119899minus1 (119903)
10038161003816100381610038162
)119889119904
(28)
for all 119905 isin [0 119879] and 119899 ge 1
Proof Using the hypotheses (A2) and (A3)with 120579(119906) le 119886 + 119887119906
yields
10038161003816100381610038161003816119891 (119904 119884
1015840
119899minus1(119904) 119885
1015840
119899(119904) 119884
119899minus1(119904) 119885
119899(119904))
10038161003816100381610038161003816
2
le 2120579 (100381610038161003816100381610038161198841015840
119899minus1(119904)10038161003816100381610038161003816
2
) + 2120579 (1003816100381610038161003816119884119899minus1 (119904)
10038161003816100381610038162
)
+ 2119897 (100381610038161003816100381610038161198851015840
119899(119904)10038161003816100381610038161003816
2
+1003816100381610038161003816119885119899
(119904)10038161003816100381610038162
) + 21003816100381610038161003816119891 (119904 0 0 0 0)
10038161003816100381610038162
le 4119886 + 2119887100381610038161003816100381610038161198841015840
119899minus1(119904)10038161003816100381610038161003816
2
+ 21198871003816100381610038161003816119884119899minus1 (119904)
10038161003816100381610038162
+ 2119897 (100381610038161003816100381610038161198851015840
119899(119904)10038161003816100381610038161003816
2
+1003816100381610038161003816119885119899
(119904)10038161003816100381610038162
) + 21003816100381610038161003816119891 (119904 0 0 0 0)
10038161003816100381610038162
(29)
Then it follows from Lemma 1 that
E( sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899 (119904)
10038161003816100381610038162
) + Eint119879
119905
11989021205731199041003816100381610038161003816119885119899
(119904)10038161003816100381610038162
119889119904
le 241198722
1198601198902120573119879
E100381610038161003816100381612058510038161003816100381610038162
+12119872
2
119860
120573E
times int
119879
119905
1198902120573119904 10038161003816100381610038161003816
E1015840[119891 (119904 119884
1015840
119899minus1(119904)
1198851015840
119899(119904) 119884
119899minus1(119904) 119885
119899(119904))]
10038161003816100381610038161003816
2
119889119904
le 241198722
1198601198902120573119879
E100381610038161003816100381612058510038161003816100381610038162
+12119872
2
119860
120573E
times int
119879
119905
E1015840[119890
2120573119904 10038161003816100381610038161003816119891 (119904 119884
1015840
119899minus1(119904)
1198851015840
119899(119904) 119884
119899minus1(119904) 119885
119899(119904))
10038161003816100381610038161003816
2
] 119889119904
le 1198621+24119872
2
119860
120573E
times int
119879
119905
E1015840[119890
2120573119904[119887100381610038161003816100381610038161198841015840
119899minus1(119904)10038161003816100381610038161003816
2
+ 1198871003816100381610038161003816119884119899minus1 (119904)
10038161003816100381610038162
+ 119897100381610038161003816100381610038161198851015840
119899(119904)10038161003816100381610038161003816
2
+ 1198971003816100381610038161003816119885119899
(119904)10038161003816100381610038162
]] 119889119904
= 1198621+48119872
2
119860
120573E
times int
119879
119905
1198902120573119904
[1198871003816100381610038161003816119884119899minus1 (119904)
10038161003816100381610038162
+ 1198971003816100381610038161003816119885119899
(119904)10038161003816100381610038162
] 119889119904
(30)
where
1198621= 24119872
2
1198601198902120573119879
E100381610038161003816100381612058510038161003816100381610038162
+24119872
2
119860
120573Eint
119879
119905
1198902120573119904
[2119886 +1003816100381610038161003816119891 (119904 0 0 0 0)
10038161003816100381610038162
] 119889119904 + 1
(31)
If we set 120573 = 961198722
119860max119887 119897 we can obtain
sup119899isinN
E( sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899 (119904)
10038161003816100381610038162
) +1
2int
119879
119905
sup119899isinN
E [11989021205731199041003816100381610038161003816119885119899
(119904)10038161003816100381610038162
] 119889119904
le 1198621+1
2int
119879
119905
sup119899isinN
E [11989021205731199041003816100381610038161003816119884119899minus1 (119904)
10038161003816100381610038162
] 119889119904
le 1198621+1
2int
119879
119905
sup119899isinN
E[ sup119904le119903le119879
11989021205731199031003816100381610038161003816119884119899minus1 (119903)
10038161003816100381610038162
]119889119904
(32)
An application of the Gronwall inequality now implies
sup119899isinN
E( sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899 (119904)
10038161003816100381610038162
) le 21198621exp (119879 minus 119905
2)
le 21198621exp (119879 minus 119905)
(33)
Point (i) of Lemma 6 is now proved
Mathematical Problems in Engineering 7
From formula (32) we know that
int
119879
119905
sup119899isinN
E [11989021205731199041003816100381610038161003816119885119899
(119904)10038161003816100381610038162
] 119889119904
le 21198621+ int
119879
119905
sup119899isinN
E[ sup119904le119903le119879
11989021205731199031003816100381610038161003816119884119899minus1 (119903)
10038161003816100381610038162
]119889119904
le 21198621+ 2119862
1int
119879
119905
exp (119879 minus 119904) 119889119904
= 21198621exp (119879 minus 119905)
(34)
This proves point (ii) of the LemmaTo prove point (iii) we note that
E1015840[10038161003816100381610038161003816119891 (119904 119884
1015840
119899(119904) 119885
1015840
119899+1(119904) 119884
119899(119904) 119885
119899+1(119904))
minus 119891 (119904 1198841015840
119899minus1(119904) 119885
1015840
119899(119904) 119884
119899minus1(119904) 119885
119899(119904))
10038161003816100381610038161003816
2
]
le E1015840[120579 (
100381610038161003816100381610038161198841015840
119899(119904) minus 119884
1015840
119899minus1(119904)10038161003816100381610038161003816
2
) + 119897100381610038161003816100381610038161198851015840
119899+1(119904) minus 119885
1015840
119899(119904)10038161003816100381610038161003816
2
]
+ 120579 (1003816100381610038161003816119884119899 (119904) minus 119884119899minus1 (119904)
10038161003816100381610038162
) + 1198971003816100381610038161003816119885119899+1
(119904) minus 119885119899(119904)10038161003816100381610038162
= E [120579 (1003816100381610038161003816119884119899 (119904) minus 119884119899minus1 (119904)
10038161003816100381610038162
) + 1198971003816100381610038161003816119885119899+1
(119904) minus 119885119899(119904)10038161003816100381610038162
]
+ 120579 (1003816100381610038161003816119884119899 (119904) minus 119884119899minus1 (119904)
10038161003816100381610038162
) + 1198971003816100381610038161003816119885119899+1
(119904) minus 119885119899(119904)10038161003816100381610038162
(35)
By Lemma 1 we have
E( sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899+1 (119904) minus 119884119899 (119904)
10038161003816100381610038162
)
+ Eint119879
119905
11989021205731199041003816100381610038161003816119885119899+1
(119904) minus 119885119899(119904)10038161003816100381610038162
119889119904
le12119872
2
119860
120573E
times int
119879
119905
1198902120573119904 10038161003816100381610038161003816
E1015840[119891 (119904 119884
1015840
119899(119904) 119885
1015840
119899+1(119904) 119884
119899(119904) 119885
119899+1(119904))
minus 119891 (119904 1198841015840
119899minus1(119904) 119885
1015840
119899(119904)
119884119899minus1
(119904) 119885119899(119904))]
10038161003816100381610038161003816
2
119889119904
le12119872
2
119860
120573E
times int
119879
119905
1198902120573119904
E1015840[10038161003816100381610038161003816119891 (119904 119884
1015840
119899(119904) 119885
1015840
119899+1(119904) 119884
119899(119904) 119885
119899+1(119904))
minus 119891 (119904 1198841015840
119899minus1(119904) 119885
1015840
119899(119904)
119884119899minus1
(119904) 119885119899(119904))
10038161003816100381610038161003816
2
] 119889119904
le24119872
2
119860
120573Eint
119879
119905
1198902120573119904
[120579 (1003816100381610038161003816119884119899 (119904) minus 119884119899minus1 (119904)
10038161003816100381610038162
)
+ 1198971003816100381610038161003816119885119899+1
(119904) minus 119885119899(119904)10038161003816100381610038162
] 119889119904
(36)
We can choose 120573 gt 0 sufficiently large such that
(1 minus24119872
2
119860119897
120573)Eint
119879
119905
11989021205731199041003817100381710038171003817119885119899+1
(119904) minus 119885119899(119904)10038171003817100381710038172
119889119904 ge 0 (37)
Then
E( sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899+1 (119904) minus 119884119899 (119904)
10038161003816100381610038162
)
le24119872
2
119860
120573Eint
119879
119905
1198902120573119904120579 (1003816100381610038161003816119884119899 (119904) minus 119884119899minus1 (119904)
10038161003816100381610038162
) 119889119904
le 1198622Eint
119879
119905
120579(E sup119904le119903le119879
11989021205731199031003816100381610038161003816119884119899 (119903) minus 119884119899minus1 (119903)
10038161003816100381610038162
)119889119904
(38)
where we set 1198622= (24119872
2
119860120573)119890
2120573119879
We divide the interval [0 119879] into subintervals 0 = 1205910lt
1205911lt sdot sdot sdot lt 120591
119898= 119879 by setting 120591
119896= 119896120575 119896 = 1 2 3 119898 with
120575 = 119879119898
Lemma 7 For all 119905 isin [120591119896minus1
120591119896] define
1198623= 119862
2120579 (2119862
1exp (119879))
1205931198961(119905) = 119862
3(120591119896minus 119905)
120593119896119899+1
(119905) = 1198622int
120591119896
119905
120579 (120593119896119899(119904)) 119889119904 119899 ge 1
(39)
Then for all 119899 ge 1 the following inequality holds for a suitable120575 gt 0
0 le 120593119896119899(119905) le 120593
119896119899minus1(119905) le sdot sdot sdot le 120593
1198961(119905) (40)
Proof Firstly it needs to be verified that for all 119905 isin [120591119896minus1
120591119896]
the following inequality
1205931198962(119905) = 119862
2int
120591119896
119905
120579 (1205931198961(119904)) 119889119904 = 119862
2int
120591119896
119905
120579 (1198623(120591119896minus 119904)) 119889119904
le 1198623(120591119896minus 119905) = 120593
1198961(119905)
(41)
holds provided 120575 gt 0 is chosen sufficiently smallActually this inequality holds provided that
1198622120579 (119862
3(120591119896minus 119905)) le 119862
3= 119862
2120579 (2119862
1exp (119879)) (42)
8 Mathematical Problems in Engineering
or
1198623(120591119896minus 119905) = 119862
2120579 (2119862
1exp (119879)) (120591
119896minus 119905) le 2119862
1exp (119879)
(43)
Since 1198621gt 1 from 120579(119906) le 119886 + 119887119906 the above inequality holds
if
1198622(119886 + 119887) (120591
119896minus 119905) le 1 (44)
Thus (41) holds for any 119905 isin [120591119896minus1
120591119896] 119896 = 1 2 119898 if 120591
119896minus
120591119896minus1
le 11198622(119886 + 119887) Therefore we can choose a sufficiently
large119898 isin N such that 120575 = 119879119898 le 11198622(119886 + 119887) Clearly such a
120575 only depends on 119886 119887 119897 119879 and119872119860
Now assume that (40) holds for some 119899 ge 2 Then wehave
120593119896119899+1
(119905) = 1198622int
120591119896
119905
120579 (120593119896119899(119904)) 119889119904 le 119862
2int
120591119896
119905
120579 (120593119896119899minus1
(119904)) 119889119904
= 120593119896119899(119905) forall119905 isin [120591
119896minus1 120591
119896]
(45)
This completes the proof
Now we can give the main result of this section
Theorem 8 Assume that (A2) and (A3) hold Then thereexists a unique mild solution (119884 119885) to (11)
Proof Consider the following
Uniqueness To show the uniqueness let both (119884 119885) and( 119885) be solutions of (11) For any 120573 gt 0 similar to the proofof (36) one can obtain
E( sup119905le119904le119879
119890212057311990410038161003816100381610038161003816
119884 (119904) minus (119904)10038161003816100381610038161003816
2
) + Eint119879
119905
119890212057311990410038161003816100381610038161003816
119885 (119904) minus 119885 (119904)10038161003816100381610038161003816
2
119889119904
le24119872
2
119860
120573E
times int
119879
119905
1198902120573119904
[120579 (10038161003816100381610038161003816119884 (119904) minus (119904)
10038161003816100381610038161003816
2
) + 11989710038161003816100381610038161003816119885 (119904) minus 119885 (119904)
10038161003816100381610038161003816
2
] 119889119904
(46)
That is if 120573 is sufficiently large
E( sup119905le119904le119879
119890212057311990410038161003816100381610038161003816
119884 (119904) minus (119904)10038161003816100381610038161003816
2
)
le24119872
2
119860
120573Eint
119879
119905
1198902120573119904
[120579 (10038161003816100381610038161003816119884 (119904) minus (119904)
10038161003816100381610038161003816
2
)] 119889119904
le 1198622Eint
119879
119905
120579(E sup119904le119903le119879
119890212057311990310038161003816100381610038161003816
119884 (119904) minus (119904)10038161003816100381610038161003816
2
)119889119904
(47)
An application of Bihari inequality yields
E( sup119905le119904le119879
119890212057311990410038161003816100381610038161003816
119884 (119904) minus (119904)10038161003816100381610038161003816
2
) = 0 (48)
So 119884(119905) = (119905) for all 119905 isin [0 119879] as It then follows from (46)that 119885(119905) = 119885(119905) for all 119905 isin [0 119879] as as well This establishesthe uniqueness
ExistenceWe claim that the sequence (119884119899 119885
119899) defined by (27)
satisfies
E sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899+1 (119904) minus 119884119899 (119904)
10038161003816100381610038162
997888rarr 0 forall0 le 119905 le 119879 (49)
as 119899 rarr infinIndeed for all 119905 isin [120591
119896minus1 120591
119896] we set 120593
119896119899(119905) =
E sup119904isin[119905120591119896]
1198902120573119904|119884119899+1
(119904) minus 119884119899(119904)|
2 By Lemmas 6 and 7
1205931198961(119905) = E sup
119904isin[119905120591119896]
119890212057311990410038161003816100381610038161198842 (119904) minus 1198841 (119904)
10038161003816100381610038162
le 1198622int
120591119896
119905
120579(E sup119904le119903le120591119896
119890212057311990310038161003816100381610038161198841 (119903) minus 1198840 (119903)
10038161003816100381610038162
)119889119904
le 1198622int
120591119896
119905
120579 (21198621exp (120591
119896minus 119905)) 119889119904
le 1198622120579 (2119862
1exp (119879)) (120591
119896minus 119905) = 119862
3(120591119896minus 119905) = 120593
1198961(119905)
(50)
Suppose that 120593119896119899(119905) le 120593
119896119899(119905) holds for some 119899 ge 1
According to Lemma 6(iii) and Lemma 7 for all 119905 isin [120591119896minus1
120591119896]
we obtain
120593119896119899+1
(119905) = E sup119904isin[119905120591119896]
11989021205731199041003816100381610038161003816119884119899+2 (119904) minus 119884119899+1 (119904)
10038161003816100381610038162
le 1198622int
120591119896
119905
120579(E sup119903isin[119904120591119896]
11989021205731199031003816100381610038161003816119884119899+1 (119903) minus 119884119899 (119903)
10038161003816100381610038162
)119889119904
= 1198622int
120591119896
119905
120579 (120593119896119899(119904)) 119889119904
le 1198622int
120591119896
119905
120579 (120593119896119899(119904)) 119889119904 = 120593
119896119899+1(119905)
(51)
This implies that for all 119899 isin N
120593119896119899(119905) le 120593
119896119899(119905) (52)
By definition 120593119896119899(sdot) is continuous on [120591
119896minus1 120591
119896] Note that
for each 119899 ge 1 120593119896119899(sdot) is decreasing on [120591
119896minus1 120591
119896] and for each
119905 119896 120593119896119899(119905) is a nonincreasing sequence Therefore we define
the function 120593119896(119905) by 120593
119896119899(119905) darr 120593
119896(119905) It is easy to verify that
120593119896(119905) is continuous and nonincreasing on [120591
119896minus1 120591
119896] By the
definitions of 120593119896119899(119905) and 120593
119896(119905) we get
120593119896(119905) = lim
119899rarrinfin
1198622int
120591119896
119905
120579 (120593119896119899(119904)) 119889119904 = 119862
2int
120591119896
119905
120579 (120593119896(119904)) 119889119904
(53)
for all 120591119896minus1
le 119905 le 120591119896 Since int
0+(119889119906120579(119906)) = infin the Bihari
inequality implies
120593119896(119905) = 0 for each 119905 isin [120591
119896minus1 120591
119896] (54)
For each 119896 isin 1 2 119898 (52) and (54) yield
lim119899rarrinfin
120593119896119899(119905) = 0 (55)
Mathematical Problems in Engineering 9
Then
E sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899+1 (119904) minus 119884119899 (119904)
10038161003816100381610038162
le max1le119896le119898
E sup119904isin[120591119896minus1120591119896]
11989021205731199041003816100381610038161003816119884119899+1 (119904) minus 119884119899 (119904)
10038161003816100381610038162
= max1le119896le119898
120593119896119899(119905) 997888rarr 0
(56)
as 119899 rarr infin and this proves the assertion (49)By (36) we obtain
E( sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899+1 (119904) minus 119884119899 (119904)
10038161003816100381610038162
) + (1 minus24119872
2
119860119897
120573)E
times int
119879
119905
11989021205731199041003816100381610038161003816119885119899+1
(119904) minus 119885119899(119904)10038161003816100381610038162
119889119904
le24119872
2
119860
120573Eint
119879
119905
1198902120573119904
[120579 (1003816100381610038161003816119884119899 (119904) minus 119884119899minus1 (119904)
10038161003816100381610038162
)] 119889119904
(57)
Applying (49) to the above formula we see that (119884119899 119885
119899) is
a Cauchy (hence convergent) sequence in S2
F ([0 119879]119867) times
H2
F ([0 119879]L(Γ119867)) denote the limit by (119884 119885) Now letting119899 rarr infin in (27) we obtain that
119884 (119905) = 119890119860(119879minus119905)
120585
+ int
119879
119905
E1015840[119890
119860(119904minus119905)119891 (119904 119884
1015840
(119904) 1198851015840
(119904) 119884 (119904) 119885 (119904))] 119889119904
minus int
119879
119905
119890119860(119904minus119905)
119885 (119904) 119889119882 (119904)
(58)
holds on the entire interval [0 119879]The theorem is nowproved
To illustrate the application of the obtained existenceand uniqueness result we consider the example of backwardstochastic partial differential equations (BSPDEs) of mean-field type
Example 9 Let O be an open bounded domain in R119899
with uniformly 1198622 boundary 120597O let 119861(119905) be a standard 119899-dimensional Brownian motion (equipped with the normalfiltration) and let 120585 O rarr R be an F
119879-measurable
random variable We also let 119871 denote the semielliptic partialdifferential operator on 1198622
(R) of the form
119871 =
119899
sum
119894119895=1
119886119894119895(119909)
1205972
120597119909119894120597119909
119895
+
119899
sum
119894=1
119887119894(119909)
120597
120597119909119894
(59)
The aim is to study the solvability of the following initialboundary value problem
119889119884 (119905 119909)
= (119871119884 (119905 119909) + E1015840
times [119892 (119905 119909 1198841015840
(119905 119909) 1198851015840
(119905 119909) 119884 (119905 119909) 119885 (119905 119909))]) 119889119905
+ 119885 (119905 119909) 119889119861 (119905) ae on (0 119879) times O
119884 (119905 119909) = 0 ae on (0 119879) times 120597O
119884 (119879 119909) = 120585 (119879 119909) ae onO(60)
where
119884 [0 119879] times O 997888rarr R
119885 [0 119879] times O 997888rarr L (R119899 119871
2
(O))
119892 [0 119879] times O timesR timesL (R119899 119871
2
(O))
timesR timesL (R119899 119871
2
(O)) 997888rarr R
(61)
The following assumptions will have to be in force
(H1) 119886119894119895 119887
119894 R119899
rarr R are uniformly continuous andbounded and satisfy the usual uniform ellipticitycondition sum119899
119894119895=1119886119894119895(119909)119908
119894119908119895ge 120582|119908|
2 for some 120582 gt 0
and all 119909 isin O 119908 isin R119899(H2) 119892 is measurable in (119905 119909 119910 119910 119911) and continuous in
( 119911) and there exists 119862 gt 0 such that1003816100381610038161003816119892 (119905 119909 1199101 1 1199101 1199111) minus 119892 (119905 119909 1199102 2 1199102 1199112)
1003816100381610038161003816
le 119862 [10038161003816100381610038161199101 minus 1199102
1003816100381610038161003816 +10038161003816100381610038161 minus 2
1003816100381610038161003816 +10038161003816100381610038161199101 minus 1199102
1003816100381610038161003816 +10038161003816100381610038161199111 minus 1199112
1003816100381610038161003816]
(62)
for all 0 le 119905 le 119879 119909 isin O 1199101 119910
2 119910
1 119910
2isin R
1
2 119911
1 119911
2isin
L(R119899 119871
2(O))
Then we are now in a position of showing existence anduniqueness of the solution of BSPDEs (60)
Theorem 10 If (H1) and (H2) are satisfied then the mean-field BSPDE (60) has a unique mild solution (119884 119885) isin
1198712(0 119879 119871
2(Ω 119871
2(O))) times 1198712F (0 119879 119871
2(R119899
1198712(Ω 119871
2(O))))
Proof Let119867 = 1198712(O) and119870 = R119899 Define the operator 119860 by
119860119884 (119905 sdot) = 119871119884 (119905 sdot) (63)
It is shown in [17] (see Example 21 in [17]) that 119860 generatesa strongly continuous semigroup on 119867 Define the maps 119891
[0 119879] times 119867 timesL(119870119867) times 119867 timesL(119870119867) rarr 119867 by
119891 (119905 1198841015840
(119905) 1198851015840
(119905) 119884 (119905) 119885 (119905)) (119909)
= 119892 (119905 119909 1198841015840
(119905 119909) 1198851015840
(119905 119909) 119884 (119905 119909) 119885 (119905 119909))
(64)
10 Mathematical Problems in Engineering
for all 0 le 119905 le 119879 119909 isin O With these identifications(60) can be written in the form of (11) By (H2) we know119891 satisfy condition (A1) Hence an application of Theorem 4concludes that (60) has a unique mild solution (119884 119885) isin
1198712(0 119879 119871
2(Ω 119871
2(O))) times 1198712F (0 119879 119871
2(R119899
1198712(Ω 119871
2(O))))
4 Mean-Field Stochastic Evolution Equations
Let 119882(119905) 119905 isin [0 119879] be a cylindrical Wiener process withvalues in a Hilbert space Γ defined on a probability space(ΩFP) We fix an interval [119905 119879] sub [0 119879] and considerthe stochastic evolution equations of mean-field type for anunknown process 119883(119904) 119904 isin [119905 119879] with values in a Hilbertspace 119870
119889119883 (119904) = 119861119883 (119904) 119889119904 + E1015840[119887 (119904 119883
1015840
(119904) 119883 (119904))] 119889119904
+ E1015840[120590 (119904 119883
1015840
(119904) 119883 (119904))] 119889119882 (119904)
119883 (119905) = 119909 isin 119870
(65)
where operator 119861 is the generator of a strongly continuoussemigroup 119890
119905119861 119905 ge 0 in the Hilbert space 119870 with 119872119861≜
sup119905isin[0119879]
|119890119905119861|
By a mild solution of (65) we mean an F119904-measurable
process 119883(119904) 119904 isin [119905 119879] with continuous paths in 119870 suchthat P-as
119883 (119904) = 119890119861(119904minus119905)
119909
+ int
119904
119905
119890119861(119904minus120591)
E1015840[119887 (120591 119883
1015840
(120591) 119883 (120591))] 119889120591
+ int
119904
119905
119890119861(119904minus120591)
E1015840[120590 (120591119883
1015840
(120591) 119883 (120591))] 119889119882 (120591)
119904 isin [119905 119879]
(66)
We suppose the following(A4) 119887 [0 119879] times 119870 times 119870 rarr 119870 is a measurable mapping
which satisfies10038161003816100381610038161003816119887 (119905 119909
1015840 119909) minus 119887 (119905 119910
1015840 119910)
10038161003816100381610038161003816
2
le 1198711(100381610038161003816100381610038161199091015840minus 119910
101584010038161003816100381610038161003816
2
+1003816100381610038161003816119909 minus 119910
10038161003816100381610038162
)
119905 isin [0 119879] 1199091015840 119909 119910
1015840 119910 isin 119870
(67)
for some constant 1198711gt 0
(A5) The mapping 120590 [0 119879] times 119870 times 119870 rarr L(Γ 119870) fulfillsthat for every V isin Γ the map 120590V [0 119879] times 119870 times 119870 rarr 119870
is measurable for every 119904 gt 0 119905 isin [0 119879] 1199091015840 1199101015840 119909 119910 isin 119870119890119904119861120590(119905 119909
1015840 119909) isin L(Γ 119870) and
10038161003816100381610038161003816119890119904119861120590 (119905 119909
1015840 119909)
10038161003816100381610038161003816L(Γ119870)le 119871
2119904minus120574(1 +
10038161003816100381610038161003816119909101584010038161003816100381610038161003816+ |119909|)
10038161003816100381610038161003816119890119904119861120590 (119905 119909
1015840 119909) minus 119890
119904119861120590 (119905 119910
1015840 119910)
10038161003816100381610038161003816L(Γ119870)
le 1198712119904minus120574(100381610038161003816100381610038161199091015840minus 119910
101584010038161003816100381610038161003816+1003816100381610038161003816119909 minus 119910
1003816100381610038161003816)
10038161003816100381610038161003816120590 (119905 119909
1015840 119909)
10038161003816100381610038161003816L(Γ119870)le 119871
2(1 +
10038161003816100381610038161003816119909101584010038161003816100381610038161003816+ |119909|)
(68)
for some constants 1198712gt 0 and 120574 isin [0 12)
Theorem 11 Under assumptions (A3) and (A4) (65) has aunique mild solution119883(sdot) isin S2
F ([119905 119879] 119870)
The proof is constructed in two steps like that ofTheorem 4 and it uses standard arguments for stochastic evo-lution equations introduced in the proof of Proposition 32 in[3] Since the proof is straightforward we prefer to omit it
Remark 12 In our paper Lipchitz condtion (A4) is givento get the well-posedness of mean-field stochastic evolutionequations In fact (A4) can be replaced by a weaker conditionsuch as (A3) We just give the condition (A4) for simplicity
From standard arguments we can also get the followingcontinuous dependence theorem
Corollary 13 Assume that for all 120572 in a metric space 119865(119887120572 120590
120572) satisfy (A4) and (A5)with119871
1and119871
2independent of 120572
Also assume that
Eint119879
0
10038161003816100381610038161003816E1015840[119887120572(119904 119883
1015840
(119904) 119883 (119904))]
minus E1015840[1198871205720(119904 119883
1015840
(119904) 119883 (119904))]10038161003816100381610038161003816
2
119889119904 997888rarr 0
Eint119879
0
10038161003816100381610038161003816E1015840[120590
120572(119904 119883
1015840
(119904) 119883 (119904))]
minusE1015840[120590
1205720(119904 119883
1015840
(119904) 119883 (119904))]10038161003816100381610038161003816
2
119889119904 997888rarr 0
(69)
as 120572 rarr 1205720for all119883 isin S2
F ([0 119879] 119870)If we denote by 119883120572
(sdot) the mild solution of mean-field SEE(65) corresponding to the functions (119887
120572 120590
120572) and to the initial
data 119909 then we have
sup119904isin[0119879]
E1003816100381610038161003816119883
120572
(119904) minus 1198831205720 (119904)
10038161003816100381610038162
997888rarr 0 119886119904 120572 997888rarr 1205720 (70)
5 Maximum Principle for BSPDEs ofMean-Field Type
51 Formulation of the Problem Let O isin R119899 be a boundedopen set with smooth boundary 120597O and let 119880 the space ofcontrols be a separable real Hilbert space We denote
U = V (sdot) isin 1198712F(0 119879 119880)
| V119905(120596
1015840 120596) [0 119879] times Ω times Ω
997888rarr 119880 is F otimesF119905-progressively measurable
(71)
Mathematical Problems in Engineering 11
An element ofU is called an admissible controlFor any V isin U we consider the following controlled
BSPDE system in the state space119867 = 1198712(O) (norm | sdot | scalar
product ⟨sdot sdot⟩)
119889119884119905(119909) = minus119860119884
119905(119909) 119889119905
minus E1015840[119891 (119905 119909 (119884
119905(119909))
1015840
(119885119905(119909))
1015840
119884119905(119909) 119885
119905(119909) V
119905)] 119889119905
+ 119885119905(119909) 119889119882 (119905) 119905 isin [0 119879]
119884119879(119909) = 120585 (119909) 119909 isin O
(72)
where 119860 is a partial differential operator 119891 [0 119879] timesO times119867 times
L(Γ119867)times119867timesL(Γ119867)times119880 rarr 119867 and 120585 isin 1198712(ΩF119879P 119867)
The cost functional is given by
119869 (V) = Eint119879
0
intO
E1015840[ℎ (119904 119909 (119884
119904(119909))
1015840
(119885119904(119909))
1015840
119884119904(119909) 119885
119904(119909) V
119904)] 119889119909 119889119904
+E1015840intO
119892 (119909 (1198840(119909))
1015840
1198840(119909)) 119889119909
(73)
where
ℎ [0 119879] times O times 119867 timesL (Γ119867) times 119867 timesL (Γ119867) times 119880 997888rarr R
119892 O times 119867 times119867 997888rarr R
(74)
Our purpose is to minimize the functional 119869(sdot) over Uadsubject to the following state constraint
EintO
Φ(119909 (1198840(119909))
1015840
1198840(119909)) 119889119909 = 0 (75)
where
Φ O times 119867 times119867 997888rarr R (76)
An admissible control 119906 isin Uad that satisfies
119869 (119906) = minVisinUad
119869 (V) (77)
is called optimalThrough what follows the following assumptions will be
in force
(L1) 119860 is a partial differential operator with appropri-ate boundary conditions We assume that 119860 is theinfinitesimal generator of a strongly continuous semi-group 119890119905119860 119905 ge 0 in119867 Moreover for every 119905 isin [0 119879]119890
119905119860119891
1198712(O) le 119872
119860119891
1198712(O) for some constant 119872
119860
independent of 119905 and 119891(L2) 119891 ℎ 119892 and Φ are continuously Gateaux differen-
tiable with respect to (1199101015840 1199111015840 119910 119911) 119891 is continuouslyGateaux differentiable with respect to V and ℎ iscontinuous with respect to V
(L3) The derivatives of 119891 ℎ 119892 and Φ are Lipschitzcontinuous and bounded by
100381610038161003816100381610038161198911199101015840
10038161003816100381610038161003816+10038161003816100381610038161198911199111015840
1003816100381610038161003816 +10038161003816100381610038161003816119891119910
10038161003816100381610038161003816+1003816100381610038161003816119891119911
1003816100381610038161003816 +1003816100381610038161003816119891V
1003816100381610038161003816 +10038161003816100381610038161003816Φ1199101015840
10038161003816100381610038161003816+10038161003816100381610038161003816Φ119910
10038161003816100381610038161003816le 119862
10038161003816100381610038161003816ℎ1199101015840
10038161003816100381610038161003816+1003816100381610038161003816ℎ1199111015840
1003816100381610038161003816 +10038161003816100381610038161003816ℎ119910
10038161003816100381610038161003816+1003816100381610038161003816ℎ119911
1003816100381610038161003816 +100381610038161003816100381610038161198921199101015840
10038161003816100381610038161003816+10038161003816100381610038161003816119892119910
10038161003816100381610038161003816
le 119862 (1 +10038161003816100381610038161003816119910101584010038161003816100381610038161003816+10038161003816100381610038161003816119911101584010038161003816100381610038161003816+10038161003816100381610038161199101003816100381610038161003816 + |119911| + |V|)
(78)
where 119862 is a positive constant
Obviously according to Theorem 4 state equation (72)has a unique mild solution under the above assumptions
Remark 14 We can define the second order differentialoperator
(119860119891) (119909) =
119899
sum
119894119895=1
119886119894119895(119909)
1205972119891
120597119909119894120597119909
119895
(119909) +
119899
sum
119894=1
119887119894(119909)
120597119891
120597119909119894
(119909) (79)
By Example 9 119860 fulfills assumption (L1) if 119886119894119895 119887
119894satisfy
condition (H1)
52 Variation of the Trajectory Let 119906 be an optimal controlwith (119884(sdot) 119885(sdot)) being the corresponding optimal state Let120576 gt 0 and [119903 119903 + 120576] sube [0 119879] For any given V isin Uad weintroduce the spike variation of the control 119906(sdot)
119906120576
119905=
V119905 119905 isin [119903 119903 + 120576]
119906119905 119905 isin [0 119879] [119904 119904 + 120576]
(80)
It is clear that 119906120576(sdot) isin UadLet (119884120576
(sdot) 119885120576(sdot)) be the trajectory corresponding to 119906120576(sdot)
We use the following short notation for brevity
119891 (119905) = 119891 (119905 119909 (119884119905(119909))
1015840
(119885119905(119909))
1015840
119884119905(119909) 119885
119905(119909) 119906
119905)
119891 (119906120576
119905) = 119891 (119905 119909 (119884
119905(119909))
1015840
(119885119905(119909))
1015840
119884119905(119909) 119885
119905(119909) 119906
120576
119905)
(81)
Consider the following equation
119889119870120576
119905(119909) = minus119860119870
120576
119905(119909) 119889119905
minus E1015840[119891
1199101015840 (119905) (119870
120576
119905(119909))
1015840
+ 119891119910(119905) 119870
120576
119905(119909)
+ 1198911199111015840 (119905) (119876
120576
119905(119909))
1015840
+ 119891119911(119905) 119876
120576
119905(119909)
+1
120576(119891 (119906
120576
119905) minus 119891 (119905))] 119889119905 + 119876
120576
119905(119909) 119889119882 (119905)
119870120576
119879(119909) = 0
(82)
Since the coefficients in (82) are bounded it is easy to checkthat there exists a unique mild solution such that
E[ sup119905isin[0119879]
1003816100381610038161003816119870120576
119905
10038161003816100381610038162
+ int
119879
0
1003816100381610038161003816119876120576
119905
10038161003816100381610038162
119889119905] lt infin (83)
We have the following estimate
12 Mathematical Problems in Engineering
Theorem 15 There holds
lim120576rarr0
E[ sup119904isin[119905119879]
1003816100381610038161003816100381610038161003816100381610038161003816
119884120576
119904minus 119884
119904
120576minus 119870
120576
119904
1003816100381610038161003816100381610038161003816100381610038161003816
2
] = 0 forall119905 isin [0 119879]
lim120576rarr0
Eint119879
0
1003816100381610038161003816100381610038161003816100381610038161003816
119885120576
119904minus 119885
119904
120576minus 119876
120576
119904
1003816100381610038161003816100381610038161003816100381610038161003816
2
119889119904 = 0
(84)
Proof We define
120578120576
119904=119884120576
119904minus 119884
119904
120576minus 119870
120576
119904 120577
120576
119904=119885120576
119904minus 119885
119904
120576minus 119876
120576
119904 119904 isin [0 119879]
(85)
For simplicity let us define
Λ120576
119904= ((119884
120576
119904)1015840
(119885120576
119904)1015840
119884120576
119904 119885
120576
119904)
119891 (119904 120582) = 119891 (119904 1198841015840
119904+ 120582(119884
120576
119904minus 119884
119904)1015840
1198851015840
119904+ 120582(119885
120576
119904minus 119885
119904)1015840
119884119904+ 120582 (119884
120576
119904minus 119884
119904)
119885119904+ 120582 (119885
120576
119904minus 119885
119904) 119906
120576
119904)
(86)
By the definition of (119884120576
119904 119885
120576
119904) (119884
119904 119885
119904) and (119870120576
119904 119876
120576
119904) (120578120576
119904 120577
120576
119904)
is the mild solution of
119889120578120576
119904= minus119860120578
120576
119904119889119904 minus E
1015840
[119871 (119904 120576)] 119889119904 + 120577120576
119904119889119882 (119904)
120578120576
119879= 0
(87)
with
119871 (119904 120576) =1
120576(119891 (119904 Λ
120576
119904 119906
120576
119904) minus 119891 (119906
120576
119904))
minus 1198911199101015840 (119904) (119870
120576
119904)1015840
minus 119891119910(119904) 119870
120576
119904minus 119891
1199111015840 (119904) (119876
120576
119904)1015840
minus 119891119911(119904) 119876
120576
119904
= ((120578120576
119904)1015840
+ (119870120576
119904)1015840
) int
1
0
1198911199101015840 (119904 120582) 119889120582 + (120578
120576
119904+ 119870
120576
119904)
times int
1
0
119891119910(119904 120582) 119889120582 minus 119891
1199101015840 (119904) (119870
120576
119904)1015840
minus 119891119910(119904) 119870
120576
119904
+ ((120577120576
119904)1015840
+ (119876120576
119904)1015840
)int
1
0
1198911199111015840 (119904 120582) 119889120582 + (120577
120576
119904+ 119876
120576
119904)
times int
1
0
119891119911(119904 120582) 119889120582 minus 119891
1199111015840 (119904) (119876
120576
119904)1015840
minus 119891119911(119904) 119876
120576
119904
= (120578120576
119904)1015840
int
1
0
1198911199101015840 (119904 120582) 119889120582 + 120578
120576
119904int
1
0
119891119910(119904 120582) 119889120582
+ (120577120576
119904)1015840
int
1
0
1198911199111015840 (119904 120582) 119889120582 + 120577
120576
119904int
1
0
119891119911(119904 120582) 119889120582 + 120574
120576
119904
(88)
where we denote
120574120576
119904= (119870
120576
119904)1015840
int
1
0
(1198911199101015840 (119904 120582) minus 119891
1199101015840 (119904)) 119889120582
+ 119870120576
119904int
1
0
(119891119910(119904 120582) minus 119891
119910(119904)) 119889120582
+ (119876120576
119904)1015840
int
1
0
(1198911199111015840 (119904 120582) minus 119891
1199111015840 (119904)) 119889120582
+ 119876120576
119904int
1
0
(119891119911(119904 120582) minus 119891
119911(119904)) 119889120582
(89)
For any 120573 gt 0 according to Lemma 1 we obtain
E sup119905le119904le119879
11989021205731199041003816100381610038161003816120578
120576
119904
10038161003816100381610038162
+ Eint119879
119905
11989021205731199041003816100381610038161003816120577
120576
119904
10038161003816100381610038162
119889119904
le12119872
2
119860
120573int
119879
119905
1198902120573119904
E [10038161003816100381610038161003816E1015840
[119871 (119904 120576)]10038161003816100381610038161003816
2
] 119889119904
le12119872
2
119860
120573Eint
119879
119905
1198902120573119904
E1015840[|119871 (119904 120576)|
2] 119889119904
(90)
By condition (L3) we have
E [E1015840[|119871 (119904 120576)|
2]]
= E [E1015840[1003816100381610038161003816119871 (119904 120576) minus 120574
120576
119904+ 120574
120576
119904
10038161003816100381610038162
]]
le 81198622E [E
1015840[10038161003816100381610038161003816(120578
120576
119904)101584010038161003816100381610038161003816
2
+1003816100381610038161003816120578120576
119904
10038161003816100381610038162
+10038161003816100381610038161003816(120577
120576
119904)101584010038161003816100381610038161003816
2
+1003816100381610038161003816120577120576
119904
10038161003816100381610038162
]]
+ 2E [E1015840[1003816100381610038161003816120574120576
119904
10038161003816100381610038162
]]
le 161198622E [
1003816100381610038161003816120578120576
119904
10038161003816100381610038162
+1003816100381610038161003816120577120576
119904
10038161003816100381610038162
] + 2E [E1015840[1003816100381610038161003816120574120576
119904
10038161003816100381610038162
]]
(91)
Combined with (91) (90) yields
E sup119905le119904le119879
11989021205731199041003816100381610038161003816120578
120576
119904
10038161003816100381610038162
+ (1 minus192119872
2
1198601198622
120573)Eint
119879
119905
11989021205731199041003816100381610038161003816120577
120576
119904
10038161003816100381610038162
119889119904
le192119872
2
1198601198622
120573Eint
119879
119905
11989021205731199041003816100381610038161003816120578
120576
119904
10038161003816100381610038162
119889119904
+24119872
2
119860
120573Eint
119879
119905
1198902120573119904
[E1015840[1003816100381610038161003816120574120576
119904
10038161003816100381610038162
]] 119889119904
(92)
We claim that
Eint119879
119905
E1015840[1003816100381610038161003816120574120576
119904
10038161003816100381610038162
] 119889119904 997888rarr 0 as 120576 997888rarr 0 (93)
From (89)
120574120576
119904= 119868
1
119904+ 119868
2
119904+ 119868
3
119904+ 119868
4
119904 (94)
Mathematical Problems in Engineering 13
where
1198681
119904= (119870
120576
119904)1015840
int
1
0
(1198911199101015840 (119904 120582) minus 119891
1199101015840 (119904)) 119889120582
1198682
119904= 119870
120576
119904int
1
0
(119891119910(119904 120582) minus 119891
119910(119904)) 119889120582
1198683
119904= (119876
120576
119904)1015840
int
1
0
(1198911199111015840 (119904 120582) minus 119891
1199111015840 (119904)) 119889120582
1198684
119904= 119876
120576
119904int
1
0
(119891119911(119904 120582) minus 119891
119911(119904)) 119889120582
(95)
Then
Eint119879
119905
E1015840[1003816100381610038161003816120574120576
119904
10038161003816100381610038162
] 119889119904
le 4Eint119879
119905
E1015840[100381610038161003816100381610038161198681
119904
10038161003816100381610038161003816
2
+100381610038161003816100381610038161198682
119904
10038161003816100381610038161003816
2
+100381610038161003816100381610038161198683
119904
10038161003816100381610038161003816
2
+100381610038161003816100381610038161198684
119904
10038161003816100381610038161003816
2
] 119889119904
(96)
Take Eint119879119905E1015840[|1198682
119904|2
]119889119904 for example
Eint119879
119905
E1015840[100381610038161003816100381610038161198682
119904
10038161003816100381610038161003816
2
] 119889119904
= Eint119879
119905
E1015840[(119870
120576
119904int
1
0
(119891119910(119904 120582) minus 119891
119910(119904)) 119889120582)
2
]119889119904
le Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119904 120582) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582] 119889119904
le 2Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119904 120582) minus 119891
119910(119906
120576
119905)10038161003816100381610038161003816
2
119889120582] 119889119904
+ 2Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119906
120576
119905) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582] 119889119904
(97)
Note that
Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119906
120576
119905) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582] 119889119904
= Eint119903+120576
119903
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119906
120576
119905) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582] 119889119904
le sup119904isin[119905119879]
E [E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119906
120576
119905) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582]] 120576
997888rarr 0 as 120576 997888rarr 0
(98)
The inequality above holds due to the boundedness of|119870
120576
119904|2
int1
0|119891119910(119906
120576
119905) minus 119891
119910(119904)|
2
119889120582 Indeed Assumption (L3) im-plies the boundedness of 119891
119910(119906
120576
119905) minus 119891
119910(119904) Meanwhile 119870120576
119904is
the solution of mean-field BSEE (82) It can be easy to check119870120576
119904is bounded since the coefficients in (82) are boundedOn the other hand
Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119904 120582) minus 119891
119910(119906
120576
119905)10038161003816100381610038161003816
2
119889120582] 119889119904
le 11986221205822Eint
119879
119905
1003816100381610038161003816119870120576
119904
10038161003816100381610038162
times int
1
0
1198641015840[100381610038161003816100381610038161003816(119884
120576
119904minus 119884
119904)1015840100381610038161003816100381610038161003816
2
+100381610038161003816100381610038161003816(119885
120576
119904minus 119885
119904)1015840100381610038161003816100381610038161003816
2
times10038161003816100381610038161003816119884120576
119904minus 119884
119904
10038161003816100381610038161003816
2
+10038161003816100381610038161003816119885120576
119904minus 119885
119904
10038161003816100381610038161003816
2
] 119889120582 119889119904
(99)
where (119884120576
119904 119885
120576
119904) is the mild solution of the following equation
119889119884120576
119905= minus 119860119884
120576
119905119889119905
minus E1015840[119891 (119905 (119884
120576
119905)1015840
(119885120576
119905)1015840
119884120576
119905 119885
120576
119905 119906
120576
119905)] 119889119905
+ 119885120576
119905119889119882 (119905) 119905 isin [0 119879]
119884120576
119879= 120585
(100)
and (119884119904 119885
119904) is the mild solution of
119889119884119905= minus 119860119884
119905119889119905
minus E1015840[119891 (119905 (119884
119905)1015840
(119885119905)1015840
119884119905 119885
119905 119906
119905)] 119889119905
+ 119885119905119889119882 (119905) 119905 isin [0 119879]
119884119879= 120585
(101)
By the definition of 119906120576119905 according to (L2) we have
E1015840[119891 (119904 119884
1015840
(119904) 1198851015840
(119904) 119884 (119904) 119885 (119904) 119906120576
119905)]
997888rarr E1015840[119891 (119904 119884
1015840
(119904) 1198851015840
(119904) 119884 (119904) 119885 (119904) 119906119905)]
(102)
in 1198712([0 119879]119867) as 120576 rarr 0 Using the continuous dependencetheorem Corollary 5 we obtain
(119884120576
119904 119885
120576
119904) 997888rarr (119884
119904 119885
119904) as 120576 997888rarr 0 (103)
Then
Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119904 120582) minus 119891
119910(119906
120576
119905)10038161003816100381610038161003816
2
119889120582] 119889119904
le 11986221205822Eint
119879
119905
1003816100381610038161003816119870120576
119904
10038161003816100381610038162
times int
1
0
E1015840[100381610038161003816100381610038161003816(119884
120576
119904minus 119884
119904)1015840100381610038161003816100381610038161003816
2
+100381610038161003816100381610038161003816(119885
120576
119904minus 119885
119904)1015840100381610038161003816100381610038161003816
2
times10038161003816100381610038161003816119884120576
119904minus 119884
119904
10038161003816100381610038161003816
2
+10038161003816100381610038161003816119885120576
119904minus 119885
119904
10038161003816100381610038161003816
2
] 119889120582 119889119904
997888rarr 0 as 120576 997888rarr 0
(104)
14 Mathematical Problems in Engineering
Combining (98) with (104) we finally haveEint
119879
119905E1015840[|1198682
119904|2
]119889119904 rarr 0 as 120576 rarr 0The required result (93) follows by using the similar
estimations for 1198681119904 1198683
119904 and 1198684
119904
Note that 1 minus 1921198722
1198601198622120573 gt 0 if 120573 is sufficiently large
Now to prove the desired result (84) it suffices to applyGronwallrsquos lemma and estimate (93) to inequality (92)
To deal with the state constraint (75) we need to recall theEkeland variational principle
Lemma 16 (Ekelandrsquos variational principle see [16 Lemma41]) Let (119878 119889) be a complete metric space and let 119865(sdot) 119878 rarr
R be lower semicontinuous and bounded from below If for 120588 gt0 there exists 119906 isin 119878 such that
119865 (119906) le infVisin119878
119865 (V) + 120588 (105)
then there exists 119906120588 isin 119878 satisfying
(i) 119865 (119906120588) le 119865 (119906)
(ii) 119889 (119906120588 119906) le 120588
(iii) 119865 (119906120588) le 119865 (V) + 120588 sdot 119889 (119906120588 119906) forallV = 119906120588
(106)
Now fix V isin Uad and set
119878 = V (sdot) isin Uad | sup0le119905le119879
E1003816100381610038161003816V11990510038161003816100381610038162
le E100381610038161003816100381611990611990510038161003816100381610038162
+ |V|2
119889 (V (sdot) V (sdot)) = 119898 119905 isin [0 119879] | E1003816100381610038161003816V119905 minus V
119905
10038161003816100381610038162
gt 0
forallV (sdot) V (sdot) isin 119878
(107)
where119898 denotes the Lebesgue measure on RThe following result is proved as Proposition 41 in [16]
Lemma 17 (119878 119889(sdot sdot)) is a complete metric space and 119869120588 is
continuous and bounded on 119878 where
119869120588
(V (sdot)) = (119869 (V (sdot)) minus 119869 (119906 (sdot)) + 120588)2
+
10038161003816100381610038161003816100381610038161003816Eint
O
Φ(119909 (1198840(119909))
1015840
1198840(119909)) 119889119909
10038161003816100381610038161003816100381610038161003816
2
12
forallV (sdot) isin 119878(108)
and (119884 119885) is the mild solution of (72) corresponding to thecontrol V
Now we consider the following free initial state optimalcontrol problem
infV(sdot)isin119878
119869120588
(V (sdot)) (109)
It is easy to check that
0 le infV(sdot)isin119878
119869120588
(V (sdot)) le 119869120588
(119906 (sdot)) = 120588 (110)
According to Ekelandrsquos variational principle there exists a119906120588(sdot) isin 119881 such that
(i) 119869120588 (119906120588 (sdot)) le 120588
(ii) 119889 (119906120588 (sdot) 119906 (sdot)) le 120588
(iii) 119869120588 (119906120588 (sdot)) le 119869120588
(V (sdot)) + 120588119889 (119906120588 (sdot) 119906 (sdot)) forallV (sdot) isin 119878(111)
Using the spike variationmethod we can construct 119906120576120588(sdot) isin 119878as follows
119906120576120588
119905=
V119905 119905 isin [119904 119904 + 120576]
119906120588
119905 119905 isin [0 119879] [119904 119904 + 120576]
(112)
It is clear that 119889(119906120576120588(sdot) 119906120588(sdot)) le 120576 Let (119884120576120588(sdot) 119885
120576120588(sdot)) (resp
(119884120588(sdot) 119885
120588(sdot))) be the solution of (72) with respect to the
control 119906120576120588(sdot) (resp 119906120588(sdot)) Following (82) (119870120576120588
119905 119876
120576120588
119905) is the
mild solution of
119870120576120588
119905= int
119879
119905
119890119860(119904minus119905)
E1015840
times [1198911199101015840 (119904 Λ
120588
119904 119906
120588
119904) (119870
120576120588
119904)1015840
+ 119891119910(119904 Λ
120588
119904 119906
120588
119904)119870
120576120588
119904
+ 1198911199111015840 (119904 Λ
120588
119904 119906
120588
119904) (119876
120576120588
119904)1015840
+ 119891119911(119904 Λ
120588
119904 119906
120588
119904) 119876
120576120588
119904
+1
120576(119891 (119904 Λ
120588
119904 119906
120576120588
119904) minus 119891 (119904 Λ
120588
119904 119906
120588
119904))] 119889119904
minus int
119879
119905
119890119860(119904minus119905)
119876120576120588
119904119889119882 (119904)
(113)
ByTheorem 15 we know that
lim120576rarr0
E[ sup119904isin[119905119879]
100381610038161003816100381610038161003816100381610038161003816
119884120576120588
119904minus 119884
120588
119904
120576minus 119870
120576120588
119904
100381610038161003816100381610038161003816100381610038161003816
2
] = 0 forall119905 isin [0 119879]
lim120576rarr0
Eint119879
0
100381610038161003816100381610038161003816100381610038161003816
119885120576120588
119904minus 119885
120588
119904
120576minus 119876
120576120588
119904
100381610038161003816100381610038161003816100381610038161003816
2
119889119904 = 0
(114)
The proof of the following proposition is technical butbased on the arguments above and we omit it
Proposition 18 One has
1
120576E [Φ ((119884
120576120588
0)1015840
119884120576120588
0) minus Φ((119884
120588
0)1015840
119884120588
0)]
= E [Φ1199101015840 ((119884
120588
0)1015840
119884120588
0) (119870
120576120588
0)1015840
+ Φ119910((119884
120588
0)1015840
119884120588
0)119870
120576120588
0]
+ 119900 (120576)
Mathematical Problems in Engineering 15
1
120576(119869 (119906
120576120588) minus 119869 (119906
120588))
= E [1198921199101015840 ((119884
120588
0)1015840
119884120588
0) (119870
120576120588
0)1015840
+ 119892119910((119884
120588
0)1015840
119884120588
0)119870
120576120588
0]
+ Δ120576+1
120576Eint
119879
0
E1015840[ℎ (119904 Λ
120588
119904 119906
120576120588
119904)
minus ℎ (119904 Λ120588
119904 119906
120588
119904)] 119889119904 + 119900 (120576)
(115)
where
Δ120576= Eint
119879
0
E1015840[ℎ
1199101015840 (119904 Λ
120588
119904 119906
120576120588
119904) (119870
120576120588
119904)1015840
+ ℎ1199111015840 (119904 Λ
120588
119904 119906
120576120588
119904) (119876
120576120588
119904)1015840
+ ℎ119910(119904 Λ
120588
119904 119906
120576120588
119904)119870
120576120588
119904
+ ℎ119911(119904 Λ
120588
119904 119906
120576120588
119904) 119876
120576120588
119904] 119889119904
(116)
53 Variational Inequality and Adjoint Equation In thissubsection the adjoint process is introduced to deduce thevariational inequality
If we set V(sdot) = 119906120576120588(sdot) in (111) and notice that
119889(119906120576120588(sdot) 119906
120588(sdot)) le 120576 we get
minus120588 le1
120576(119869
120588(119906
120576120588
(sdot)) minus 119869120588(119906
120588
(sdot))) (117)
By Lemma 17
1
120576(119869
120588(119906
120576120588
(sdot)) minus 119869120588(119906
120588
(sdot)))
=(119869
120588(119906
120576120588(sdot)))
2
minus (119869120588(119906
120588(sdot)))
2
120576 (119869120588 (119906120576120588 (sdot)) + 119869120588 (119906120588 (sdot)))
=119869 (119906
120576120588(sdot)) + 119869 (119906
120588(sdot)) minus 2119869 (119906 (sdot)) + 2120588
119869120588 (119906120576120588 (sdot)) + 119869120588 (119906120588 (sdot))
times119869 (119906
120576120588(sdot)) minus 119869 (119906
120588(sdot))
120576
+
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] + E [Φ ((119884
120588
0)1015840
119884120588
0)]
119869120588 (119906120576120588 (sdot)) + 119869120588 (119906120588 (sdot))
times
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] minus E [Φ ((119884
120588
0)1015840
119884120588
0)]
120576
997888rarr 119897120588
1
119869 (119906120576120588(sdot)) minus 119869 (119906
120588(sdot))
120576
+ 119897120588
2
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] minus E [Φ ((119884
120588
0)1015840
119884120588
0)]
120576
(118)
where we set
119897120588
1=119869 (119906
120588(sdot)) minus 119869 (119906 (sdot)) + 120588
119869120588 (119906120588 (sdot))
119897120588
2=
E [Φ ((119884120588
0)1015840
119884120588
0)]
119869120588 (119906120588 (sdot))
(119)
and use the limit
119869 (119906120576120588
(sdot)) 997888rarr 119869 (119906120588
(sdot))
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] 997888rarr E [Φ ((119884
120588
0)1015840
119884120588
0)]
(120)
as 120576 rarr 0 according to (115)As |119897120588
1|2
+ |119897120588
2|2
= 1 for all 120588 gt 0 we know that there existsa subsequence of 119897120588
1 119897120588
2 (still denoted by 119897120588
1 119897120588
2) such that
lim120588rarr0
119897120588
1 119897120588
2 = 119897
1 1198972
1003816100381610038161003816119897110038161003816100381610038162
+1003816100381610038161003816119897210038161003816100381610038162
= 1
(121)
Combining (115) (117) with (118) we get
minus120588 le 119897120588
1
119869 (119906120576120588(sdot)) minus 119869 (119906
120588(sdot))
120576
+ 119897120588
2
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] minus E [Φ ((119884
120588
0)1015840
119884120588
0)]
120576
= 119897120588
1Δ120576+ 119897
120588
1E [119892
1199101015840 ((119884
120588
0)1015840
119884120588
0) (119870
120576120588
0)1015840
+119892119910((119884
120588
0)1015840
119884120588
0)119870
120576120588
0]
+119897120588
1
120576Eint
119879
0
E1015840[ℎ (119904 Λ
120588
119904 119906
120576120588
119904) minus ℎ (119904 Λ
120588
119904 119906
120588
119904)] 119889119904
+ 119897120588
2E [Φ
1199101015840 ((119884
120588
0)1015840
119884120588
0) (119870
120576120588
0)1015840
+Φ119910((119884
120588
0)1015840
119884120588
0)119870
120576120588
0] + (119897
120588
1+ 119897
120588
2) 119900 (120576)
(122)
Next we introduce the adjoint equation corresponding tovariational equation (113) whose solution is denoted by119875120588(119905)
119889119875120588
(119905)
= 119860lowast119875120588
(119905) 119889119905
+ E1015840[119891
1199101015840 (119905 Λ
120588
119905 119906
120588
119905) (119875
120588
(119905))1015840
+ 119891119910(119905 Λ
120588
119905 119906
120588
119905) 119875
120588
(119905) + 119897120588
1ℎ1199101015840 (119905 Λ
120588
119905 119906
120588
119905)
+ 119897120588
1ℎ119910(119905 Λ
120588
119905 119906
120588
119905)] 119889119905
16 Mathematical Problems in Engineering
+ E1015840[119891
1199111015840 (119905 Λ
120588
119905 119906
120588
119905) (119875
120588
(119905))1015840
+ 119891119911(119905 Λ
120588
119905 119906
120588
119905) 119875
120588
(119905)
+ 119897120588
1ℎ1199111015840 (119905 Λ
120588
119905 119906
120588
119905) + 119897
120588
1ℎ119911(119905 Λ
120588
119905 119906
120588
119905)] 119889119882 (119905)
119875120588
(0) = 119897120588
1E [119892
1199101015840 ((119884
120588
0(119909))
1015840
119884120588
0(119909))
+ 119892119910((119884
120588
0(119909))
1015840
119884120588
0(119909))]
+ 119897120588
2E [Φ
1199101015840 ((119884
120588
0(119909))
1015840
119884120588
0(119909))
+ Φ119910((119884
120588
0(119909))
1015840
119884120588
0(119909))]
(123)
where 119860lowast is the 1198712(O)-adjoint operator of 119860 Under assump-tions (L1)ndash(L3) this is a linear mean-field SEE with boundedcoefficients An application ofTheorem 11 implies that it has aunique adaptedmild solution such that119875120588(119905) isin S2
F ([0 119879] 119870)When 120588 rarr 0 according to Corollaries 5 and 13 119875120588(119905)
converges to 119875(119905) where
119875 (119905) isin S2
F ([0 119879] 119870) (124)
is the solution of the following equation
119875 (119905) = 1198971E [119892
1199101015840 (119884
1015840
0 119884
0) + 119892
119910(119884
1015840
0 119884
0)]
+ 1198972E [Φ
1199101015840 (119884
1015840
0 119884
0) + Φ
119910(119884
1015840
0 119884
0)]
+ int
119905
0
119890119860lowast(119905minus119904)
E1015840[119891
1199101015840 (119904) 119875
1015840
(119904) + 119891119910(119904) 119875 (119904)
+ 1198971ℎ1199101015840 (119904) + 119897
1ℎ119910(119904)] 119889119904
+ int
119905
0
119890119860lowast(119905minus119904)
E1015840[119891
1199111015840 (119904) 119875
1015840
(119904) + 119891119911(119904) 119875 (119904)
+ 1198971ℎ1199111015840 (119904) + 119897
1ℎ119911(119904)] 119889119882 (119904)
(125)
The following proposition which formally follows fromProposition 18 gives the relation between 119875120588(119905) and119870120576120588
119905
Proposition 19 Consider the following
E [119875120588
(0)119870120576120588
0]
= Eint119879
0
1
120576119875120588
(119904)E1015840[119891 (119904 Λ
120588
119904 119906
120576120588
119904) minus 119891 (119904 Λ
120588
119904 119906
120588
119904)]
minus 119897120588
1119870120576120588
119904E1015840[ℎ
1199101015840 (119904 Λ
120588
119904 119906
120588
119904) + ℎ
119910(119904 Λ
120588
119904 119906
120588
119904)]
minus 119897120588
1119876120576120588
119904E1015840[ℎ
1199111015840 (119904 Λ
120588
119904 119906
120588
119904) + ℎ
119911(119904 Λ
120588
119904 119906
120588
119904)] 119889119904
(126)
The following theorem constitutes the main contributionof this section themaximumprinciple for the BSPDE controlsystem
Theorem 20 Let assumptions (L1)ndash(L3) hold Suppose 119906(sdot) isan optimal control and (119884(sdot) 119885(sdot)) is the corresponding optimalstate trajectory for the BSPDE control systems (72) and (73)with the initial state constraint (75) Then there exists 119875(119905) isinS2
F ([0 119879] 119870) which satisfies (125) such that
H (119905 1198841015840
119905 119885
1015840
119905 119884
119905 119885
119905 V
119905 119875 (119905))
ge H (119905 1198841015840
119905 119885
1015840
119905 119884
119905 119885
119905 119906
119905 119875 (119905))
119886119890 119886119904 forallV isin U119886119889
(127)
whereH [0 119879]times119867timesL(Γ119867)times119867timesL(Γ119867)times119880times119870 rarr R
is the Hamiltonian function defined by
H (119905 119910 119910 119911 V 119901) = 1198971ℎ (119905 119910 119910 119911 V)
+ 119901119891 (119905 119910 119910 119911 V) (128)
Proof By (122) and Proposition 19 we obtain
minus120588 le1
120576Eint
119879
0
119875120588
(119904)E1015840[119891 (119904 Λ
120588
119904 119906
120576120588
119904)
minus 119891 (119904 Λ120588
119904 119906
120588
119904)] 119889119904
+119897120588
1
120576Eint
119879
0
E1015840[ℎ (119904 Λ
120588
119904 119906
120576120588
119904)
minus ℎ (119904 Λ120588
119904 119906
120588
119904)] 119889119904 + (119897
120588
1+ 119897
120588
2) 119900 (120576)
(129)
Letting 120576 rarr 0+ in (129) we derive for ae 120591 isin [0 119879]
minus120588 le 119897120588
1E1015840[ℎ (120591 Λ
120588
120591 V
120591) minus ℎ (120591 Λ
120588
120591 119906
120588
120591)]
+ 119875120588
(119904)E1015840[119891 (120591 Λ
120588
120591 V
120591) minus 119891 (120591 Λ
120588
120591 119906
120588
120591)]
(130)
for all V isin UadFinally taking 120588 rarr 0 in (130) we derive the desired
result
Remark 21 We note that if the coefficients do not dependexplicitly on the marginal law of the underlying diffusionthe result reduces to the classical case that is the SMP forBSPDEs without mean-field term
Remark 22 When we remove the initial state constraint (75)we obtain the general maximum principle for the mean-fieldBSPDEs system (ie without the constraint) with 119897
1= 1
54 Application A Backward Linear Quadratic Control Prob-lem Now we apply our maximum principle to solve an LQproblem For notational simplicity we restrict ourselves tothe free case (ie without the initial state constraint (75)) thegeneral case being handled in a similar way
Mathematical Problems in Engineering 17
Consider the following problem
119869 (119906) =1
2E [(119884 (0))
2] +
1
2Eint
119879
0
119873V2 (119905) 119889119905 997888rarr min
(131)
subject to
119889119884 (119905) = minus119860119884 (119905) 119889119905
minus 119861119884 (119905) + 119861E [119884 (119905)]
+119862V (119905) + 119863119885 (119905) + 119863E [119885 (119905)] 119889119905
+ 119885 (119905) 119889119882 (119905)
119884 (119879) = 120585 119905 isin [0 119879]
(132)
where 119860 is a partial differential operator satisfying condition(L1) and 119861 119861 119862119863119863 and119873 are bounded and deterministicconstantsWe also assume that119873 gt 0 and V isin 1198712F (0 119879 119880) It iseasy to verify that BSPDE (132) admits a uniquemild solution(119884(119905) 119885(119905))
119875(119905) the adjoint process of state equation (132) is thesolution of
119889119875 (119905) = 119860lowast119875 (119905) 119889119905 + 119861119875 (119905) + 119861E [119875 (119905)] 119889119905
+ 119863119875 (119905) + 119863E [119875 (119905)] 119889119882 (119905)
119875 (0) = 119884 (0)
(133)
Let 119906(sdot) be an optimal control and let (119884(sdot) 119885(sdot)) bethe corresponding state process By maximum principle ofTheorem 20 (note that 119897
1= 1 in this problem)
1
2119873V2 (119905) + 119875 (119905) 119862V (119905) ge
1
2119873119906
2
(119905) + 119875 (119905) 119862119906 (119905) (134)
for all V isin 119880ad since the state equation has the form (132)Thisin turn implies
119906 (119905) = minus119862
119873119875 (119905) (135)
It is clear that (131) is a positive quadratic functional of controlbecause of 119873 gt 0 Hence an optimal control exists Thecandidate optimal control (135) is indeed an optimal controlof this LQ problem for it is the only control which satisfies themaximum principle
Next we want to obtain a more explicit representationof the optimal control (135) from the state equation (132)Substituting (135) into state equation (132) yields
119889119884 (119905) = minus119860119884 (119905) 119889119905
minus 119861119884 (119905) + 119861E [119884 (119905)] minus1198622
119873119875 (119905)
+119863119885 (119905) + 119863E [119885 (119905)] 119889119905 + 119885 (119905) 119889119882 (119905)
119884 (119879) = 120585 119905 isin [0 119879]
(136)
Combining the above equation with (133) we obtain thefollowing related feedback control system
119889119884 (119905) = minus119860119884 (119905) 119889119905
minus 119861119884 (119905) + 119861E [119884 (119905)] minus1198622
119873119875 (119905)
+ 119863119885 (119905) + 119863E [119885 (119905)] 119889119905 + 119885 (119905) 119889119882 (119905)
119884 (119879) = 120585
119889119875 (119905) = 119860lowast119875 (119905) 119889119905 + 119861119875 (119905) + 119861E [119875 (119905)] 119889119905
+ 119863119875 (119905) + 119863E [119875 (119905)] 119889119882 (119905)
119875 (0) = 119884 (0)
(137)
Looking at the terminal condition of 119875(119905) in (133) andconsidering the mean-field type of (132) it is reasonable toconjecture that 119875(119905) has the following form
119875 (119905) = 120593 (119905) 119884 (119905) + 120601 (119905)E [119884 (119905)] (138)
where120593(119905)120601(119905) are deterministic differential functionswhichwill be specified below Moreover 120593(0) = 1 120601(0) = 0
Inserting this form into adjoint equation (133) and notic-ing that 119884(119905) satisfies (136) we can compare the coefficientsof 119889119905 and 119889119882(119905) to obtain the following equation
2120593 (119905) 119860119884 (119905) + 2120601 (119905) 119860E [119884 (119905)] + 119861120593 (119905) 119884 (119905)
+ 2 (119861120601 (119905) + 119861120593 (119905) + 119861120601 (119905))E [119884 (119905)]
= minus120593 (119905) 119861119884 (119905) minus1198622
119873120593 (119905) 119884 (119905) + 119863119885 (119905)
+ 119884 (119905)119889120593 (119905)
119889119905+ E [119884 (119905)]
119889120601 (119905)
119889119905
+ (1206012
(119905)1198622
119873+ 2
1198622
119873120593 (119905) 120601 (119905))E [119884 (119905)]
minus (119863120601 (119905) + 119863120601 (119905) + 119863120593 (119905))E [119885 (119905)]
120593 (119905) 119885 (119905) = 119863120593 (119905) 119884 (119905)
+ (119863120601 (119905) + 119863120593 (119905) + 119863120601 (119905))E [119884 (119905)]
(139)
Then subtracting 119885(119905) we have
2120593 (119905) 119860119884 (119905) + 2120601 (119905) 119860E [119884 (119905)] + 119861120593 (119905) 119884 (119905)
+ 2 (119861120601 (119905) + 119861120593 (119905) + 119861120601 (119905))E [119884 (119905)]
18 Mathematical Problems in Engineering
= 120593 (119905) minus119861 +1198622
119873120593 (119905) minus 119863
2119884 (119905)
+ 119884 (119905)119889120593 (119905)
119889119905+ E [119884 (119905)]
119889120601 (119905)
119889119905
+ (1206012
(119905)1198622
119873+ 2
1198622
119873120593 (119905) 120601 (119905))E [119884 (119905)]
minus1
120593 (119905)(119863120601 (119905) + 119863120593 (119905) + 119863120601 (119905))
2
E [119884 (119905)]
minus 2119863 (119863120601 (119905) + 119863120593 (119905) + 119863120601 (119905))E [119884 (119905)]
(140)
Comparing the coefficients of 119884(119905) and E[119884(119905)] respec-tively we get
119889
119889119905120593 (119905) = 2119860
lowast120593 (119905) + 2119861120593 (119905) + 119863
2120593 (119905) minus
1198622
1198731205932
(119905)
120593 (0) = 1
(141)
119889
119889119905120601 (119905) = 2119860
lowast120601 (119905) + (
(119863 + 119863)2
120593 (119905)minus1198622
119873)120601
2
(119905)
+ 2119877 (119905) 120601 (119905) + (1198632+ 2119861 + 2119863119863)120593 (119905)
120601 (0) = 0
(142)
where 119877(119905) = 119861 + 119861 + (119863 + 119863)2
minus (1198622119873)120593(119905)
We solve (141) to get
120593 (119905) = (119890minus(2119860lowast+2119861+119863
2)119905(1 minus
1198622
119873(2119860lowast + 2119861 + 1198632))
+1198622
119873(2119860lowast + 2119861 + 1198632))
minus1
(143)
Then (142) exists a unique solution from the classical Riccatiequation theory
We now conclude the above discussions in the followingresult
Theorem 23 For onersquos linear quadratic stochastic partialdifferential control problem (131)-(132) the unique optimalcontrol 119906(sdot) isin 119880ad is given by
119906 (119905) = minus119862
119873(120593 (119905) 119884 (119905) + 120601 (119905)E [119884 (119905)]) (144)
with 120593(119905) satisfying (143) and 120601(119905) solving (142)
Conflict of Interests
The authors declare that they have no conflict of interestsregarding the publication of this paper
Acknowledgment
This research is partially supported by the Natural ScienceFoundation of China under Grant no 11301310
References
[1] Y Hu and S Peng ldquoAdapted solution of a backward semilinearstochastic evolution equationrdquo Stochastic Analysis and Applica-tions vol 9 no 4 pp 445ndash459 1991
[2] N IMahmudov andMAMcKibben ldquoOnbackward stochasticevolution equations in Hilbert spaces and optimal controlrdquoNonlinear Analysis vol 67 no 4 pp 1260ndash1274 2007
[3] M Fuhrman and G Tessitore ldquoNonlinear Kolmogorov equa-tions in infinite dimensional spaces the backward stochasticdifferential equations approach and applications to optimalcontrolrdquoThe Annals of Probability vol 30 no 3 pp 1397ndash14652002
[4] M Fuhrman and G Tessitore ldquoInfinite horizon backwardstochastic differential equations and elliptic equations inHilbert spacesrdquo Annals of Probability vol 32 no 1B pp 607ndash660 2004
[5] X Mao ldquoAdapted solutions of backward stochastic differentialequations with non-Lipschitz coefficientsrdquo Stochastic Processesand their Applications vol 58 no 2 pp 281ndash292 1995
[6] J Lasry and P Lions ldquoMean field gamesrdquo Japanese Journal ofMathematics vol 2 no 1 pp 229ndash260 2007
[7] R Buckdahn J Li and S Peng ldquoMean-field backward stochas-tic differential equations and related partial differential equa-tionsrdquo Stochastic Processes and their Applications vol 119 no 10pp 3133ndash3154 2009
[8] D Andersson and B Djehiche ldquoAmaximum principle for SDEsofmean-field typerdquoAppliedMathematics andOptimization vol63 no 3 pp 341ndash356 2011
[9] R Buckdahn B Djehiche J Li and S Peng ldquoMean-fieldbackward stochastic differential equations a limit approachrdquoThe Annals of Probability vol 37 no 4 pp 1524ndash1565 2009
[10] R Xu ldquoMean-field backward doubly stochastic differentialequations and related SPDEsrdquo Boundary Value Problems vol2012 article 114 2012
[11] R Buckdahn B Djehiche and J Li ldquoA general stochasticmaximum principle for SDEs of mean-field typerdquo AppliedMathematics and Optimization vol 64 no 2 pp 197ndash216 2011
[12] J Li ldquoStochasticmaximumprinciple in themean-field controlsrdquoAutomatica vol 48 no 2 pp 366ndash373 2012
[13] G Wang C Zhang and W Zhang ldquoStochastic maximumprinciple for mean-field type optimal control under partialinformationrdquo IEEE Transactions on Automatic Control vol 59no 2 pp 522ndash528 2014
[14] M Hafayed ldquoA mean-field necessary and sufficient conditionsfor optimal singular stochastic controlrdquo Communications inMathematics and Statistics vol 1 no 4 pp 417ndash435 2013
[15] M Hafayed ldquoA mean-field maximum principle for optimalcontrol of forward-backward stochastic differential equationswith Poisson jumpprocessesrdquo International Journal ofDynamicsand Control vol 1 no 4 pp 300ndash315 2013
[16] Y Hu and S Peng ldquoMaximum principle for semilinearstochastic evolution control systemsrdquo Stochastics and StochasticsReports vol 33 no 3-4 pp 159ndash180 1990
[17] M Fuhrman Y Hu and G Tessitore ldquoStochastic maximumprinciple for optimal control of SPDEsrdquo Comptes Rendus Math-ematique vol 350 no 13-14 pp 683ndash688 2012
Submit your manuscripts athttpwwwhindawicom
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Mathematical Problems in Engineering
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Differential EquationsInternational Journal of
Volume 2014
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OptimizationJournal of
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International Journal of
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Function Spaces
Abstract and Applied AnalysisHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Discrete Dynamics in Nature and Society
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Decision SciencesAdvances in
Discrete MathematicsJournal of
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Volume 2014 Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Stochastic AnalysisInternational Journal of
Mathematical Problems in Engineering 3
Next we define several classes of stochastic processes withvalues in a Hilbert space119867
(I) H2
F ([0 119879]119867) denotes the set of (classes of 119889P times
119889119905 ae equal) measurable random processes 120595119905 119905 isin
[0 119879] which satisfy
(i) Eint1198790|120595
119905|2119889119905 lt +infin
(ii) 120595119905isF
119905measurable for ae 0 le 119905 le 119879
Evidently H2
F (0 119879119867) is a Banach space en-dowed with the canonical norm
10038171003817100381710038171205951003817100381710038171003817 = Eint
119879
0
100381610038161003816100381612059511990410038161003816100381610038162
119889119904
12
(3)
(II) S2
F ([0 119879]119867) denotes the set of continuous randomprocesses 120595
119905 119905 isin [0 119879] which satisfy
(i) E(sup0le119905le119879
|120595119905|2) lt +infin
(ii) 120595119905isF
119905measurable for ae 0 le 119905 le 119879
(III) 1198710(ΩFP 119867) denotes the space of all 119867 valued F-measurable random variables
(IV) For 1 le 119901 lt infin 119871119901(ΩFP 119867) is the space of allF-measurable random variables such that E[|120585|119901] lt infin
(V) For any 120573 isin R introduce the norm
1003817100381710038171003817(119910 119911)10038171003817100381710038172
120573119905= Eint
119879
119905
1198902120573119904
(1003816100381610038161003816119910 (119904)
10038161003816100381610038162
+ |119911 (119904)|2) 119889119904 (4)
on the Banach space
K120573[119905 119879] = S
2
F ([119905 119879] 119867) timesH2
F ([119905 119879] L (Γ119867)) (5)
For 0 lt 119879 lt infin all the norms sdot 120573119905
with different 120573 isin R areequivalentK[0 119879] = K
0[0 119879] is the Banach space endowed
with the norm
1003817100381710038171003817(119910 119911)10038171003817100381710038172
= Eint119879
0
(1003816100381610038161003816119910 (119904)
10038161003816100381610038162
+ |119911 (119904)|2) 119889119904 (6)
The following result on BSEEs (see Lemma 2 in Mahmu-dov and McKibben [2]) will play a key role in proving thewell-posedness of mean-field BSEEs
Lemma 1 Let 119867 be a Hilbert space and let 119860 119863(119860) sub
119867 rarr 119867 be a linear operator which generates a 1198620-semigroup
119878(119905) 0 le 119905 le 119879 on 119867 For any (120585 119891) isin 1198712(ΩF
119879P 119867) times
H2
F ([0 119879]119867) the following equation
119884 (119905) = 119878 (119879 minus 119905) 120585 + int
119879
119905
119878 (119904 minus 119905) 119891 (119904) 119889119904
+ int
119879
119905
119878 (119904 minus 119905) 119885 (119904) 119889119882 (119904) 119875-119886119904(7)
has a unique solution inK120573[0 119879] moreover
E sup119905le119904le119879
1198902120573119904
|119884 (119904)|2+ Eint
119879
119905
1198902120573119904
|119885 (119904)|2119889119904
le 241198722
119878(119890
2120573119879E100381610038161003816100381612058510038161003816100381610038162
+1
2120573int
119879
119905
1198902120573119903
E1003816100381610038161003816119891 (119903)
10038161003816100381610038162
119889119903)
(8)
where119872119878= sup119878(119905)B(119867) 0 le 119905 le 119879 andB(119867) is the space
of bounded linear operators on119867
3 Mean-Field Backward StochasticEvolution Equations
In this section we study the existence and uniqueness resultof mild solutions to mean-field BSEEs in a Hilbert space 119867To this end we firstly recall some notations introduced byBuckdahn et al [7]
Let (ΩFP) = (Ω times ΩF otimes FP otimes P) be the(noncompleted) product of (ΩFP) with itself and wedefine F = F
119905= F otimes F
119905 0 le 119905 le 119879 on this product
space A random variable 120585 isin 1198710(ΩFP 119867) originally
defined on Ω is extended canonically to Ω 1205851015840(120596
1015840 120596) =
120585(1205961015840) (120596
1015840 120596) isin Ω = Ω times Ω For any 120578 isin 119871
1(ΩFP) the
variable 120578(sdot 120596) Ω rarr 119870 belongs to 1198711(ΩFP)P(119889120596) aswhose expectation is denoted by
E1015840[120578 (sdot 120596)] = int
Ω
120578 (1205961015840 120596)P (119889120596
1015840) (9)
Note that E1015840[120578] = E1015840[120578(sdot 120596)] isin 1198711(ΩFP) and
E [120578] (= intΩ
120578119889P = intΩ
E1015840[120578 (sdot 120596)]P (119889120596)) = E [E
1015840[120578]]
(10)
Themean-field BSEE we consider has the following formfor any given measurable mapping 119891 [0 119879] times119867timesL(Γ119867)times
119867 timesL(Γ119867) rarr 119867 and 120585 isin 1198712(ΩF119879P 119867)
119889119884 (119904) = minus 119860119884 (119904) 119889119904
minus E1015840[119891 (119904 119884
1015840
(119904) 1198851015840
(119904) 119884 (119904) 119885 (119904))] 119889119904
+ 119885 (119904) 119889119882 (119904)
119884 (119879) = 120585 119904 isin [0 119879]
(11)
where 119860 119863(119860) sub 119867 rarr 119867 is the generator of a stronglycontinuous semigroup 119890119905119860 119905 ge 0 in the Hilbert space119867 withthe notation119872
119860≜ sup
119905isin[0119879]|119890119905119860|
4 Mathematical Problems in Engineering
Definition 2 We say that a pair of adapted processes (119884 119885)is a mild solution of mean-field BSEE (11) if (119884 119885) isin
S2
F ([0 119879]119867) timesH2
F ([0 119879]L(Γ119867)) and for all 119905 isin [0 119879]
119884 (119905) = 119890119860(119879minus119905)
120585
+ int
119879
119905
119890119860(119904minus119905)
E1015840[119891 (119904 119884
1015840
(119904) 1198851015840
(119904) 119884 (119904) 119885 (119904))] 119889119904
minus int
119879
119905
119890119860(119904minus119905)
119885 (119904) 119889119882 (119904)
(12)
Remark 3 We emphasize that the coefficient of (11) can beinterpreted as
E1015840[119891 (119904 119884
1015840
(119904) 1198851015840
(119904) 119884 (119904) 119885 (119904))] (120596)
= E1015840[119891 (119904 119884
1015840
(119904) 1198851015840
(119904) 119884 (120596 119904) 119885 (120596 119904))]
= intΩ
119891 (1205961015840 120596 119904 119884 (120596
1015840 119904) 119885 (120596
1015840 119904) 119884 (120596 119904) 119885 (120596 119904))
times P (1198891205961015840)
(13)
31 Lipschitz Case Now we study the existence and unique-ness ofmild solutions tomean-field BSEE (11) under Lipschitzconditions For119891 [0 119879] times 119867 times L(Γ119867) times 119867 times L(Γ119867) rarr
119867 assume the following(A1) There exists an 119871 gt 0 such that
10038161003816100381610038161003816119891 (119905 119910
1015840
1 119911
1015840
1 119910
1 119911
1) minus 119891 (119905 119910
1015840
2 119911
1015840
2 119910
2 119911
2)10038161003816100381610038161003816
2
le 119871 (100381610038161003816100381610038161199101015840
1minus 119910
1015840
2
10038161003816100381610038161003816
2
+100381610038161003816100381610038161199111015840
1minus 119911
1015840
2
10038161003816100381610038161003816
2
+10038161003816100381610038161199101 minus 1199102
10038161003816100381610038162
+10038161003816100381610038161199111 minus 1199112
10038161003816100381610038162
)
(14)
for all 119905 isin [0 119879] 1199101015840119894 119910
119894isin 119867 119911
1015840
119894 119911
119894isin L(Γ119867) (119894 =
1 2)(A2) 119891(sdot 0 0 0 0) isin H2
F ([0 119879]119867)We have the following theorem
Theorem 4 For any random variable 120585 isin 1198712(ΩF
119879P 119867)
under (A1) and (A2) mean-field BSEE (11) admits a uniquemild solution (119884 119885) isin S2
F ([0 119879]119867) timesH2
F ([0 119879]L(Γ119867))
Proof Consider the following
Step 1 For any (119910 119911) isin S2
F ([0 119879]119867) timesH2
F ([0 119879]L(Γ119867))BSEE119884 (119905) = 119890
119860(119879minus119905)120585
+ int
119879
119905
E1015840[119890
119860(119904minus119905)119891 (119904 119910
1015840
(119904) 1199111015840
(119904) 119884 (119904) 119885 (119904))] 119889119904
minus int
119879
119905
119890119860(119904minus119905)
119885 (119904) 119889119882 (119904) 0 le 119905 le 119879
(15)
has a unique solution In order to get this conclusion wedefine
119891(119910119911)
(119904 120583 ]) = E1015840[119891 (119904 119910
1015840
(119904) 1199111015840
(119904) 120583 ])] (16)
Then (15) can be rewritten as
119884 (119905) = 119890119860(119879minus119905)
120585
+ int
119879
119905
119890119860(119904minus119905)
119891(119910119911)
(119884 (119904) 119885 (119904)) 119889119904
minus int
119879
119905
119890119860(119904minus119905)
119885 (119904) 119889119882 (119904)
(17)
Due to (A1) for all (1205831 ]
1) (120583
2 ]
2) isin 119867timesL(Γ119867)119891 satisfies
10038161003816100381610038161003816119891(119910119911)
(1205831 ]
1) minus 119891
(119910119911)(120583
2 ]
2)10038161003816100381610038161003816
2
le 119871 (10038161003816100381610038161205831 minus 1205832
10038161003816100381610038162
+1003816100381610038161003816]1 minus ]
2
10038161003816100381610038162
)
(18)
According to Theorem 31 in [1] BSEE (15) has a uniquesolution
Step 2 FromStep 1 we can define amappingΦ (119884(sdot) 119885(sdot)) =
Φ[(1199101015840(sdot) 119911
1015840(sdot))] K[0 119879] rarr K[0 119879] through
119884 (119905) = 119890119860(119879minus119905)
120585
+ int
119879
119905
E1015840[119890
119860(119904minus119905)119891 (119904 119910
1015840
(119904) 1199111015840
(119904) 119884 (119904) 119885 (119904))] 119889119904
minus int
119879
119905
119890119860(119904minus119905)
119885 (119904) 119889119882 (119904) 0 le 119905 le 119879
(19)
For any (119910119894 119911119894) isin K[0 119879] we set (119884119894 119885
119894) = Φ[(119910
119894 119911
119894)] 119894 =
1 2 (119910 119911) = (1199101minus119910
2 119911
1minus119911
2) and (119884 119885) = (119884
1minus119884
2 119885
1minus119885
2)
Then from Lemma 1 we have
E sup0le119904le119879
119890212057311990410038161003816100381610038161003816
119884 (119904)10038161003816100381610038161003816
2
+ Eint119879
0
119890212057311990410038161003816100381610038161003816
119885 (119904)10038161003816100381610038161003816
2
119889119904
le12119872
2
119860
120573E
Mathematical Problems in Engineering 5
times [int
119879
0
1198902120573119904
100381610038161003816100381610038161003816E1015840[119891 (119904 (119910
1
(119904))1015840
(1199111
(119904))1015840
1198841
(119904) 1198851
(119904))
minus 119891 (119904 (1199102
(119904))1015840
(1199112
(119904))1015840
1198842
(119904) 1198852
(119904))]100381610038161003816100381610038161003816
2
119889119904]
le12119872
2
1198601198712
120573E
times [int
119879
0
1198902120573119904
(E [1003816100381610038161003816119910 (119904)
10038161003816100381610038162
+ |119911 (119904)|2]
+10038161003816100381610038161003816119884 (119904)
10038161003816100381610038161003816
2
+10038161003816100381610038161003816119885 (119904)
10038161003816100381610038161003816
2
) 119889119904]
=12119872
2
1198601198712
120573E
times [int
119879
0
1198902120573119904
(1003816100381610038161003816119910 (119904)
10038161003816100381610038162
+ |119911 (119904)|2
+10038161003816100381610038161003816119884 (119904)
10038161003816100381610038161003816
2
+10038161003816100381610038161003816119885 (119904)
10038161003816100381610038161003816
2
) 119889119904]
(20)
If we set 120573 = 361198722
1198601198712max119879 1 then
Eint119879
0
1198902120573119904
(10038161003816100381610038161003816119884 (119904)
10038161003816100381610038161003816
2
+10038161003816100381610038161003816119885 (119904)
10038161003816100381610038161003816
2
) 119889119904
le 119879 sdot E sup0le119904le119879
119890212057311990410038161003816100381610038161003816
119884 (119904)10038161003816100381610038161003816
2
+ Eint119879
0
119890212057311990410038161003816100381610038161003816
119885 (119904)10038161003816100381610038161003816
2
119889119904
le12119872
2
1198601198712max 119879 1120573
E
times [int
119879
0
1198902120573119904
(1003816100381610038161003816119910 (119904)
10038161003816100381610038162
+ |119911 (119904)|2
+10038161003816100381610038161003816119884 (119904)
10038161003816100381610038161003816
2
+10038161003816100381610038161003816119885 (119904)
10038161003816100381610038161003816
2
) 119889119904]
=1
3E [int
119879
0
1198902120573119904
(1003816100381610038161003816119910 (119904)
10038161003816100381610038162
+ |119911 (119904)|2
+10038161003816100381610038161003816119884 (119904)
10038161003816100381610038161003816
2
+10038161003816100381610038161003816119885 (119904)
10038161003816100381610038161003816
2
) 119889119904]
(21)
That is
Eint119879
0
1198902120573119904
(10038161003816100381610038161003816119884 (119904)
10038161003816100381610038161003816
2
+10038161003816100381610038161003816119885 (119904)
10038161003816100381610038161003816
2
) 119889119904
le1
2Eint
119879
0
1198902120573119904
(1003816100381610038161003816119910 (119904)
10038161003816100381610038162
+ |119911 (119904)|2) 119889119904
(22)
The estimate (22) shows that Φ is a contraction on thespaceK
120573[0 119879] with the norm
(119884 119885)2
120573= Eint
119879
0
1198902120573119904
(|119884 (119904)|2+ |119885 (119904)|
2) 119889119904 (23)
With the contraction mapping theorem there admits aunique fixed point (119884 119885) isin K
120573[0 119879] such that Φ(119884 119885) =
(119884 119885) On the other hand from Step 1 we know thatif Φ(119884 119885) = (119884 119885) then (119884 119885) isin S2
F ([0 119879]119867) times
H2
F ([0 119879]L(Γ119867)) which is the unique mild solution of(11)
Arguing as the previous proof we arrive at the followingassertion in a straightforward way
Corollary 5 Suppose that for all 120572 in a metric space 119865 119891120572is
a given function satisfying (A1) and (A2) with 119871 independenton 120572 Also suppose that
E1015840[119891
120572(119904 119884
1015840
(119904) 1198851015840
(119904) 119884 (119904) 119885 (119904))]
997888rarr E1015840[119891
1205720(119904 119884
1015840
(119904) 1198851015840
(119904) 119884 (119904) 119885 (119904))]
(24)
in 1198712([0 119879]119867) as 120572 rarr 1205720for all (119884 119885) isin S2
F ([0 119879]119867) times
H2
F ([0 119879]L(Γ119867))If we denote by (119884(120585 120572) 119885(120585 120572)) the mild solution of (11)
corresponding to the functions 119891120572and to the final data 120585 isin
1198712(ΩF
119879P 119867) then the map (120572 120585) rarr (119884(120585 120572) 119885(120585 120572))
is continuous from 119865 times 1198712(ΩF
119879P 119867) to S2
F ([0 119879]119867) times
H2
F ([0 119879]L(Γ119867))
32 Non-Lipschitz Case This subsection is devoted to findingsome weaker conditions than the Lipschitz one under whichthe mean-field BSEE has a unique solution To state our mainresult in this section we suppose the following
(A3) For all 119905 isin [0 119879] 1199101015840119894 119910
119894isin 119867 1199111015840
119894 119911
119894isin L(Γ119867) (119894 =
1 2) there exists an 119897 gt 0 such that
10038161003816100381610038161003816119891 (119905 119910
1015840
1 119911
1015840
1 119910
1 119911
1) minus 119891 (119905 119910
1015840
2 119911
1015840
2 119910
2 119911
2)10038161003816100381610038161003816
2
le 120579 (100381610038161003816100381610038161199101015840
1minus 119910
1015840
2
10038161003816100381610038161003816
2
) + 120579 (10038161003816100381610038161199101 minus 1199102
10038161003816100381610038162
)
+ 119897 (100381610038161003816100381610038161199111015840
1minus 119911
1015840
2
10038161003816100381610038161003816
2
+10038161003816100381610038161199111 minus 1199112
10038161003816100381610038162
)
(25)
where 120579 R+rarr R+ is a concave increasing function such
that 120579(0) = 0 120579(119906) gt 0 for 119906 gt 0 and int0+(119889119906120579(119906)) = infin
InMao [5] the author gave three examples of the function120579(sdot) to show the generality of condition (A3) From theseexamples we can see that Lipschitz condition (A1) is a specialcase of the given condition (A3)
Since 120579 is concave and 120579(0) = 0 there exists a pair ofpositive constants 119886 and 119887 such that
120579 (119906) le 119886 + 119887119906 (26)
for all 119906 ge 0 Therefore under assumptions (A2) and(A3) 119891(sdot 1199101015840(sdot) 1199111015840(sdot) 119910(sdot) 119911(sdot)) isin H2
F ([0 119879]119867) whenever
6 Mathematical Problems in Engineering
1199101015840(sdot) 119910(sdot) isin S2
F ([0 119879]119867) and 1199111015840(sdot) 119911(sdot) isin H2
F ([0 119879]L(Γ119867))
By Picard-type iteration we now construct an approxi-mate sequence using which we obtain the desired result Let1198840(119905) equiv 0 and for 119899 isin N let 119884
119899 119885
119899 be a sequence in
S2
F ([0 119879]119867) timesH2
F ([0 119879]L(Γ119867)) defined recursively by
119884119899(119905) = 119890
119860(119879minus119905)120585
+ int
119879
119905
E1015840[119890
119860(119904minus119905)
times119891 (119904 1198841015840
119899minus1(119904) 119885
1015840
119899(119904) 119884
119899minus1(119904) 119885
119899(119904))] 119889119904
minus int
119879
119905
119890119860(119904minus119905)
119885119899(119904) 119889119882 (119904)
(27)
on 0 le 119905 le 119879 From Theorem 4 (27) has a unique mildsolution (119884
119899(119905) 119885
119899(119905))
In order to give the main result we need to prepare thefollowing lemmas about the properties of (119884
119899(119905) 119885
119899(119905)) 119905 isin
[0 119879]
Lemma 6 Under hypotheses (A2) and (A3) there existpositive constants 119862
1and 119862
2such that
(i)E( sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899 (119904)
10038161003816100381610038162
) le 21198621exp (119879 minus 119905)
(ii)Eint119879
119905
11989021205731199041003816100381610038161003816119885119899
(119904)10038161003816100381610038162
119889119904 le 21198621exp (119879 minus 119905)
(iii)E( sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899+1 (119904) minus 119884119899 (119904)
10038161003816100381610038162
)
le 1198622int
119879
119905
120579(E sup119904le119903le119879
11989021205731199031003816100381610038161003816119884119899 (119903) minus 119884119899minus1 (119903)
10038161003816100381610038162
)119889119904
(28)
for all 119905 isin [0 119879] and 119899 ge 1
Proof Using the hypotheses (A2) and (A3)with 120579(119906) le 119886 + 119887119906
yields
10038161003816100381610038161003816119891 (119904 119884
1015840
119899minus1(119904) 119885
1015840
119899(119904) 119884
119899minus1(119904) 119885
119899(119904))
10038161003816100381610038161003816
2
le 2120579 (100381610038161003816100381610038161198841015840
119899minus1(119904)10038161003816100381610038161003816
2
) + 2120579 (1003816100381610038161003816119884119899minus1 (119904)
10038161003816100381610038162
)
+ 2119897 (100381610038161003816100381610038161198851015840
119899(119904)10038161003816100381610038161003816
2
+1003816100381610038161003816119885119899
(119904)10038161003816100381610038162
) + 21003816100381610038161003816119891 (119904 0 0 0 0)
10038161003816100381610038162
le 4119886 + 2119887100381610038161003816100381610038161198841015840
119899minus1(119904)10038161003816100381610038161003816
2
+ 21198871003816100381610038161003816119884119899minus1 (119904)
10038161003816100381610038162
+ 2119897 (100381610038161003816100381610038161198851015840
119899(119904)10038161003816100381610038161003816
2
+1003816100381610038161003816119885119899
(119904)10038161003816100381610038162
) + 21003816100381610038161003816119891 (119904 0 0 0 0)
10038161003816100381610038162
(29)
Then it follows from Lemma 1 that
E( sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899 (119904)
10038161003816100381610038162
) + Eint119879
119905
11989021205731199041003816100381610038161003816119885119899
(119904)10038161003816100381610038162
119889119904
le 241198722
1198601198902120573119879
E100381610038161003816100381612058510038161003816100381610038162
+12119872
2
119860
120573E
times int
119879
119905
1198902120573119904 10038161003816100381610038161003816
E1015840[119891 (119904 119884
1015840
119899minus1(119904)
1198851015840
119899(119904) 119884
119899minus1(119904) 119885
119899(119904))]
10038161003816100381610038161003816
2
119889119904
le 241198722
1198601198902120573119879
E100381610038161003816100381612058510038161003816100381610038162
+12119872
2
119860
120573E
times int
119879
119905
E1015840[119890
2120573119904 10038161003816100381610038161003816119891 (119904 119884
1015840
119899minus1(119904)
1198851015840
119899(119904) 119884
119899minus1(119904) 119885
119899(119904))
10038161003816100381610038161003816
2
] 119889119904
le 1198621+24119872
2
119860
120573E
times int
119879
119905
E1015840[119890
2120573119904[119887100381610038161003816100381610038161198841015840
119899minus1(119904)10038161003816100381610038161003816
2
+ 1198871003816100381610038161003816119884119899minus1 (119904)
10038161003816100381610038162
+ 119897100381610038161003816100381610038161198851015840
119899(119904)10038161003816100381610038161003816
2
+ 1198971003816100381610038161003816119885119899
(119904)10038161003816100381610038162
]] 119889119904
= 1198621+48119872
2
119860
120573E
times int
119879
119905
1198902120573119904
[1198871003816100381610038161003816119884119899minus1 (119904)
10038161003816100381610038162
+ 1198971003816100381610038161003816119885119899
(119904)10038161003816100381610038162
] 119889119904
(30)
where
1198621= 24119872
2
1198601198902120573119879
E100381610038161003816100381612058510038161003816100381610038162
+24119872
2
119860
120573Eint
119879
119905
1198902120573119904
[2119886 +1003816100381610038161003816119891 (119904 0 0 0 0)
10038161003816100381610038162
] 119889119904 + 1
(31)
If we set 120573 = 961198722
119860max119887 119897 we can obtain
sup119899isinN
E( sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899 (119904)
10038161003816100381610038162
) +1
2int
119879
119905
sup119899isinN
E [11989021205731199041003816100381610038161003816119885119899
(119904)10038161003816100381610038162
] 119889119904
le 1198621+1
2int
119879
119905
sup119899isinN
E [11989021205731199041003816100381610038161003816119884119899minus1 (119904)
10038161003816100381610038162
] 119889119904
le 1198621+1
2int
119879
119905
sup119899isinN
E[ sup119904le119903le119879
11989021205731199031003816100381610038161003816119884119899minus1 (119903)
10038161003816100381610038162
]119889119904
(32)
An application of the Gronwall inequality now implies
sup119899isinN
E( sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899 (119904)
10038161003816100381610038162
) le 21198621exp (119879 minus 119905
2)
le 21198621exp (119879 minus 119905)
(33)
Point (i) of Lemma 6 is now proved
Mathematical Problems in Engineering 7
From formula (32) we know that
int
119879
119905
sup119899isinN
E [11989021205731199041003816100381610038161003816119885119899
(119904)10038161003816100381610038162
] 119889119904
le 21198621+ int
119879
119905
sup119899isinN
E[ sup119904le119903le119879
11989021205731199031003816100381610038161003816119884119899minus1 (119903)
10038161003816100381610038162
]119889119904
le 21198621+ 2119862
1int
119879
119905
exp (119879 minus 119904) 119889119904
= 21198621exp (119879 minus 119905)
(34)
This proves point (ii) of the LemmaTo prove point (iii) we note that
E1015840[10038161003816100381610038161003816119891 (119904 119884
1015840
119899(119904) 119885
1015840
119899+1(119904) 119884
119899(119904) 119885
119899+1(119904))
minus 119891 (119904 1198841015840
119899minus1(119904) 119885
1015840
119899(119904) 119884
119899minus1(119904) 119885
119899(119904))
10038161003816100381610038161003816
2
]
le E1015840[120579 (
100381610038161003816100381610038161198841015840
119899(119904) minus 119884
1015840
119899minus1(119904)10038161003816100381610038161003816
2
) + 119897100381610038161003816100381610038161198851015840
119899+1(119904) minus 119885
1015840
119899(119904)10038161003816100381610038161003816
2
]
+ 120579 (1003816100381610038161003816119884119899 (119904) minus 119884119899minus1 (119904)
10038161003816100381610038162
) + 1198971003816100381610038161003816119885119899+1
(119904) minus 119885119899(119904)10038161003816100381610038162
= E [120579 (1003816100381610038161003816119884119899 (119904) minus 119884119899minus1 (119904)
10038161003816100381610038162
) + 1198971003816100381610038161003816119885119899+1
(119904) minus 119885119899(119904)10038161003816100381610038162
]
+ 120579 (1003816100381610038161003816119884119899 (119904) minus 119884119899minus1 (119904)
10038161003816100381610038162
) + 1198971003816100381610038161003816119885119899+1
(119904) minus 119885119899(119904)10038161003816100381610038162
(35)
By Lemma 1 we have
E( sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899+1 (119904) minus 119884119899 (119904)
10038161003816100381610038162
)
+ Eint119879
119905
11989021205731199041003816100381610038161003816119885119899+1
(119904) minus 119885119899(119904)10038161003816100381610038162
119889119904
le12119872
2
119860
120573E
times int
119879
119905
1198902120573119904 10038161003816100381610038161003816
E1015840[119891 (119904 119884
1015840
119899(119904) 119885
1015840
119899+1(119904) 119884
119899(119904) 119885
119899+1(119904))
minus 119891 (119904 1198841015840
119899minus1(119904) 119885
1015840
119899(119904)
119884119899minus1
(119904) 119885119899(119904))]
10038161003816100381610038161003816
2
119889119904
le12119872
2
119860
120573E
times int
119879
119905
1198902120573119904
E1015840[10038161003816100381610038161003816119891 (119904 119884
1015840
119899(119904) 119885
1015840
119899+1(119904) 119884
119899(119904) 119885
119899+1(119904))
minus 119891 (119904 1198841015840
119899minus1(119904) 119885
1015840
119899(119904)
119884119899minus1
(119904) 119885119899(119904))
10038161003816100381610038161003816
2
] 119889119904
le24119872
2
119860
120573Eint
119879
119905
1198902120573119904
[120579 (1003816100381610038161003816119884119899 (119904) minus 119884119899minus1 (119904)
10038161003816100381610038162
)
+ 1198971003816100381610038161003816119885119899+1
(119904) minus 119885119899(119904)10038161003816100381610038162
] 119889119904
(36)
We can choose 120573 gt 0 sufficiently large such that
(1 minus24119872
2
119860119897
120573)Eint
119879
119905
11989021205731199041003817100381710038171003817119885119899+1
(119904) minus 119885119899(119904)10038171003817100381710038172
119889119904 ge 0 (37)
Then
E( sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899+1 (119904) minus 119884119899 (119904)
10038161003816100381610038162
)
le24119872
2
119860
120573Eint
119879
119905
1198902120573119904120579 (1003816100381610038161003816119884119899 (119904) minus 119884119899minus1 (119904)
10038161003816100381610038162
) 119889119904
le 1198622Eint
119879
119905
120579(E sup119904le119903le119879
11989021205731199031003816100381610038161003816119884119899 (119903) minus 119884119899minus1 (119903)
10038161003816100381610038162
)119889119904
(38)
where we set 1198622= (24119872
2
119860120573)119890
2120573119879
We divide the interval [0 119879] into subintervals 0 = 1205910lt
1205911lt sdot sdot sdot lt 120591
119898= 119879 by setting 120591
119896= 119896120575 119896 = 1 2 3 119898 with
120575 = 119879119898
Lemma 7 For all 119905 isin [120591119896minus1
120591119896] define
1198623= 119862
2120579 (2119862
1exp (119879))
1205931198961(119905) = 119862
3(120591119896minus 119905)
120593119896119899+1
(119905) = 1198622int
120591119896
119905
120579 (120593119896119899(119904)) 119889119904 119899 ge 1
(39)
Then for all 119899 ge 1 the following inequality holds for a suitable120575 gt 0
0 le 120593119896119899(119905) le 120593
119896119899minus1(119905) le sdot sdot sdot le 120593
1198961(119905) (40)
Proof Firstly it needs to be verified that for all 119905 isin [120591119896minus1
120591119896]
the following inequality
1205931198962(119905) = 119862
2int
120591119896
119905
120579 (1205931198961(119904)) 119889119904 = 119862
2int
120591119896
119905
120579 (1198623(120591119896minus 119904)) 119889119904
le 1198623(120591119896minus 119905) = 120593
1198961(119905)
(41)
holds provided 120575 gt 0 is chosen sufficiently smallActually this inequality holds provided that
1198622120579 (119862
3(120591119896minus 119905)) le 119862
3= 119862
2120579 (2119862
1exp (119879)) (42)
8 Mathematical Problems in Engineering
or
1198623(120591119896minus 119905) = 119862
2120579 (2119862
1exp (119879)) (120591
119896minus 119905) le 2119862
1exp (119879)
(43)
Since 1198621gt 1 from 120579(119906) le 119886 + 119887119906 the above inequality holds
if
1198622(119886 + 119887) (120591
119896minus 119905) le 1 (44)
Thus (41) holds for any 119905 isin [120591119896minus1
120591119896] 119896 = 1 2 119898 if 120591
119896minus
120591119896minus1
le 11198622(119886 + 119887) Therefore we can choose a sufficiently
large119898 isin N such that 120575 = 119879119898 le 11198622(119886 + 119887) Clearly such a
120575 only depends on 119886 119887 119897 119879 and119872119860
Now assume that (40) holds for some 119899 ge 2 Then wehave
120593119896119899+1
(119905) = 1198622int
120591119896
119905
120579 (120593119896119899(119904)) 119889119904 le 119862
2int
120591119896
119905
120579 (120593119896119899minus1
(119904)) 119889119904
= 120593119896119899(119905) forall119905 isin [120591
119896minus1 120591
119896]
(45)
This completes the proof
Now we can give the main result of this section
Theorem 8 Assume that (A2) and (A3) hold Then thereexists a unique mild solution (119884 119885) to (11)
Proof Consider the following
Uniqueness To show the uniqueness let both (119884 119885) and( 119885) be solutions of (11) For any 120573 gt 0 similar to the proofof (36) one can obtain
E( sup119905le119904le119879
119890212057311990410038161003816100381610038161003816
119884 (119904) minus (119904)10038161003816100381610038161003816
2
) + Eint119879
119905
119890212057311990410038161003816100381610038161003816
119885 (119904) minus 119885 (119904)10038161003816100381610038161003816
2
119889119904
le24119872
2
119860
120573E
times int
119879
119905
1198902120573119904
[120579 (10038161003816100381610038161003816119884 (119904) minus (119904)
10038161003816100381610038161003816
2
) + 11989710038161003816100381610038161003816119885 (119904) minus 119885 (119904)
10038161003816100381610038161003816
2
] 119889119904
(46)
That is if 120573 is sufficiently large
E( sup119905le119904le119879
119890212057311990410038161003816100381610038161003816
119884 (119904) minus (119904)10038161003816100381610038161003816
2
)
le24119872
2
119860
120573Eint
119879
119905
1198902120573119904
[120579 (10038161003816100381610038161003816119884 (119904) minus (119904)
10038161003816100381610038161003816
2
)] 119889119904
le 1198622Eint
119879
119905
120579(E sup119904le119903le119879
119890212057311990310038161003816100381610038161003816
119884 (119904) minus (119904)10038161003816100381610038161003816
2
)119889119904
(47)
An application of Bihari inequality yields
E( sup119905le119904le119879
119890212057311990410038161003816100381610038161003816
119884 (119904) minus (119904)10038161003816100381610038161003816
2
) = 0 (48)
So 119884(119905) = (119905) for all 119905 isin [0 119879] as It then follows from (46)that 119885(119905) = 119885(119905) for all 119905 isin [0 119879] as as well This establishesthe uniqueness
ExistenceWe claim that the sequence (119884119899 119885
119899) defined by (27)
satisfies
E sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899+1 (119904) minus 119884119899 (119904)
10038161003816100381610038162
997888rarr 0 forall0 le 119905 le 119879 (49)
as 119899 rarr infinIndeed for all 119905 isin [120591
119896minus1 120591
119896] we set 120593
119896119899(119905) =
E sup119904isin[119905120591119896]
1198902120573119904|119884119899+1
(119904) minus 119884119899(119904)|
2 By Lemmas 6 and 7
1205931198961(119905) = E sup
119904isin[119905120591119896]
119890212057311990410038161003816100381610038161198842 (119904) minus 1198841 (119904)
10038161003816100381610038162
le 1198622int
120591119896
119905
120579(E sup119904le119903le120591119896
119890212057311990310038161003816100381610038161198841 (119903) minus 1198840 (119903)
10038161003816100381610038162
)119889119904
le 1198622int
120591119896
119905
120579 (21198621exp (120591
119896minus 119905)) 119889119904
le 1198622120579 (2119862
1exp (119879)) (120591
119896minus 119905) = 119862
3(120591119896minus 119905) = 120593
1198961(119905)
(50)
Suppose that 120593119896119899(119905) le 120593
119896119899(119905) holds for some 119899 ge 1
According to Lemma 6(iii) and Lemma 7 for all 119905 isin [120591119896minus1
120591119896]
we obtain
120593119896119899+1
(119905) = E sup119904isin[119905120591119896]
11989021205731199041003816100381610038161003816119884119899+2 (119904) minus 119884119899+1 (119904)
10038161003816100381610038162
le 1198622int
120591119896
119905
120579(E sup119903isin[119904120591119896]
11989021205731199031003816100381610038161003816119884119899+1 (119903) minus 119884119899 (119903)
10038161003816100381610038162
)119889119904
= 1198622int
120591119896
119905
120579 (120593119896119899(119904)) 119889119904
le 1198622int
120591119896
119905
120579 (120593119896119899(119904)) 119889119904 = 120593
119896119899+1(119905)
(51)
This implies that for all 119899 isin N
120593119896119899(119905) le 120593
119896119899(119905) (52)
By definition 120593119896119899(sdot) is continuous on [120591
119896minus1 120591
119896] Note that
for each 119899 ge 1 120593119896119899(sdot) is decreasing on [120591
119896minus1 120591
119896] and for each
119905 119896 120593119896119899(119905) is a nonincreasing sequence Therefore we define
the function 120593119896(119905) by 120593
119896119899(119905) darr 120593
119896(119905) It is easy to verify that
120593119896(119905) is continuous and nonincreasing on [120591
119896minus1 120591
119896] By the
definitions of 120593119896119899(119905) and 120593
119896(119905) we get
120593119896(119905) = lim
119899rarrinfin
1198622int
120591119896
119905
120579 (120593119896119899(119904)) 119889119904 = 119862
2int
120591119896
119905
120579 (120593119896(119904)) 119889119904
(53)
for all 120591119896minus1
le 119905 le 120591119896 Since int
0+(119889119906120579(119906)) = infin the Bihari
inequality implies
120593119896(119905) = 0 for each 119905 isin [120591
119896minus1 120591
119896] (54)
For each 119896 isin 1 2 119898 (52) and (54) yield
lim119899rarrinfin
120593119896119899(119905) = 0 (55)
Mathematical Problems in Engineering 9
Then
E sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899+1 (119904) minus 119884119899 (119904)
10038161003816100381610038162
le max1le119896le119898
E sup119904isin[120591119896minus1120591119896]
11989021205731199041003816100381610038161003816119884119899+1 (119904) minus 119884119899 (119904)
10038161003816100381610038162
= max1le119896le119898
120593119896119899(119905) 997888rarr 0
(56)
as 119899 rarr infin and this proves the assertion (49)By (36) we obtain
E( sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899+1 (119904) minus 119884119899 (119904)
10038161003816100381610038162
) + (1 minus24119872
2
119860119897
120573)E
times int
119879
119905
11989021205731199041003816100381610038161003816119885119899+1
(119904) minus 119885119899(119904)10038161003816100381610038162
119889119904
le24119872
2
119860
120573Eint
119879
119905
1198902120573119904
[120579 (1003816100381610038161003816119884119899 (119904) minus 119884119899minus1 (119904)
10038161003816100381610038162
)] 119889119904
(57)
Applying (49) to the above formula we see that (119884119899 119885
119899) is
a Cauchy (hence convergent) sequence in S2
F ([0 119879]119867) times
H2
F ([0 119879]L(Γ119867)) denote the limit by (119884 119885) Now letting119899 rarr infin in (27) we obtain that
119884 (119905) = 119890119860(119879minus119905)
120585
+ int
119879
119905
E1015840[119890
119860(119904minus119905)119891 (119904 119884
1015840
(119904) 1198851015840
(119904) 119884 (119904) 119885 (119904))] 119889119904
minus int
119879
119905
119890119860(119904minus119905)
119885 (119904) 119889119882 (119904)
(58)
holds on the entire interval [0 119879]The theorem is nowproved
To illustrate the application of the obtained existenceand uniqueness result we consider the example of backwardstochastic partial differential equations (BSPDEs) of mean-field type
Example 9 Let O be an open bounded domain in R119899
with uniformly 1198622 boundary 120597O let 119861(119905) be a standard 119899-dimensional Brownian motion (equipped with the normalfiltration) and let 120585 O rarr R be an F
119879-measurable
random variable We also let 119871 denote the semielliptic partialdifferential operator on 1198622
(R) of the form
119871 =
119899
sum
119894119895=1
119886119894119895(119909)
1205972
120597119909119894120597119909
119895
+
119899
sum
119894=1
119887119894(119909)
120597
120597119909119894
(59)
The aim is to study the solvability of the following initialboundary value problem
119889119884 (119905 119909)
= (119871119884 (119905 119909) + E1015840
times [119892 (119905 119909 1198841015840
(119905 119909) 1198851015840
(119905 119909) 119884 (119905 119909) 119885 (119905 119909))]) 119889119905
+ 119885 (119905 119909) 119889119861 (119905) ae on (0 119879) times O
119884 (119905 119909) = 0 ae on (0 119879) times 120597O
119884 (119879 119909) = 120585 (119879 119909) ae onO(60)
where
119884 [0 119879] times O 997888rarr R
119885 [0 119879] times O 997888rarr L (R119899 119871
2
(O))
119892 [0 119879] times O timesR timesL (R119899 119871
2
(O))
timesR timesL (R119899 119871
2
(O)) 997888rarr R
(61)
The following assumptions will have to be in force
(H1) 119886119894119895 119887
119894 R119899
rarr R are uniformly continuous andbounded and satisfy the usual uniform ellipticitycondition sum119899
119894119895=1119886119894119895(119909)119908
119894119908119895ge 120582|119908|
2 for some 120582 gt 0
and all 119909 isin O 119908 isin R119899(H2) 119892 is measurable in (119905 119909 119910 119910 119911) and continuous in
( 119911) and there exists 119862 gt 0 such that1003816100381610038161003816119892 (119905 119909 1199101 1 1199101 1199111) minus 119892 (119905 119909 1199102 2 1199102 1199112)
1003816100381610038161003816
le 119862 [10038161003816100381610038161199101 minus 1199102
1003816100381610038161003816 +10038161003816100381610038161 minus 2
1003816100381610038161003816 +10038161003816100381610038161199101 minus 1199102
1003816100381610038161003816 +10038161003816100381610038161199111 minus 1199112
1003816100381610038161003816]
(62)
for all 0 le 119905 le 119879 119909 isin O 1199101 119910
2 119910
1 119910
2isin R
1
2 119911
1 119911
2isin
L(R119899 119871
2(O))
Then we are now in a position of showing existence anduniqueness of the solution of BSPDEs (60)
Theorem 10 If (H1) and (H2) are satisfied then the mean-field BSPDE (60) has a unique mild solution (119884 119885) isin
1198712(0 119879 119871
2(Ω 119871
2(O))) times 1198712F (0 119879 119871
2(R119899
1198712(Ω 119871
2(O))))
Proof Let119867 = 1198712(O) and119870 = R119899 Define the operator 119860 by
119860119884 (119905 sdot) = 119871119884 (119905 sdot) (63)
It is shown in [17] (see Example 21 in [17]) that 119860 generatesa strongly continuous semigroup on 119867 Define the maps 119891
[0 119879] times 119867 timesL(119870119867) times 119867 timesL(119870119867) rarr 119867 by
119891 (119905 1198841015840
(119905) 1198851015840
(119905) 119884 (119905) 119885 (119905)) (119909)
= 119892 (119905 119909 1198841015840
(119905 119909) 1198851015840
(119905 119909) 119884 (119905 119909) 119885 (119905 119909))
(64)
10 Mathematical Problems in Engineering
for all 0 le 119905 le 119879 119909 isin O With these identifications(60) can be written in the form of (11) By (H2) we know119891 satisfy condition (A1) Hence an application of Theorem 4concludes that (60) has a unique mild solution (119884 119885) isin
1198712(0 119879 119871
2(Ω 119871
2(O))) times 1198712F (0 119879 119871
2(R119899
1198712(Ω 119871
2(O))))
4 Mean-Field Stochastic Evolution Equations
Let 119882(119905) 119905 isin [0 119879] be a cylindrical Wiener process withvalues in a Hilbert space Γ defined on a probability space(ΩFP) We fix an interval [119905 119879] sub [0 119879] and considerthe stochastic evolution equations of mean-field type for anunknown process 119883(119904) 119904 isin [119905 119879] with values in a Hilbertspace 119870
119889119883 (119904) = 119861119883 (119904) 119889119904 + E1015840[119887 (119904 119883
1015840
(119904) 119883 (119904))] 119889119904
+ E1015840[120590 (119904 119883
1015840
(119904) 119883 (119904))] 119889119882 (119904)
119883 (119905) = 119909 isin 119870
(65)
where operator 119861 is the generator of a strongly continuoussemigroup 119890
119905119861 119905 ge 0 in the Hilbert space 119870 with 119872119861≜
sup119905isin[0119879]
|119890119905119861|
By a mild solution of (65) we mean an F119904-measurable
process 119883(119904) 119904 isin [119905 119879] with continuous paths in 119870 suchthat P-as
119883 (119904) = 119890119861(119904minus119905)
119909
+ int
119904
119905
119890119861(119904minus120591)
E1015840[119887 (120591 119883
1015840
(120591) 119883 (120591))] 119889120591
+ int
119904
119905
119890119861(119904minus120591)
E1015840[120590 (120591119883
1015840
(120591) 119883 (120591))] 119889119882 (120591)
119904 isin [119905 119879]
(66)
We suppose the following(A4) 119887 [0 119879] times 119870 times 119870 rarr 119870 is a measurable mapping
which satisfies10038161003816100381610038161003816119887 (119905 119909
1015840 119909) minus 119887 (119905 119910
1015840 119910)
10038161003816100381610038161003816
2
le 1198711(100381610038161003816100381610038161199091015840minus 119910
101584010038161003816100381610038161003816
2
+1003816100381610038161003816119909 minus 119910
10038161003816100381610038162
)
119905 isin [0 119879] 1199091015840 119909 119910
1015840 119910 isin 119870
(67)
for some constant 1198711gt 0
(A5) The mapping 120590 [0 119879] times 119870 times 119870 rarr L(Γ 119870) fulfillsthat for every V isin Γ the map 120590V [0 119879] times 119870 times 119870 rarr 119870
is measurable for every 119904 gt 0 119905 isin [0 119879] 1199091015840 1199101015840 119909 119910 isin 119870119890119904119861120590(119905 119909
1015840 119909) isin L(Γ 119870) and
10038161003816100381610038161003816119890119904119861120590 (119905 119909
1015840 119909)
10038161003816100381610038161003816L(Γ119870)le 119871
2119904minus120574(1 +
10038161003816100381610038161003816119909101584010038161003816100381610038161003816+ |119909|)
10038161003816100381610038161003816119890119904119861120590 (119905 119909
1015840 119909) minus 119890
119904119861120590 (119905 119910
1015840 119910)
10038161003816100381610038161003816L(Γ119870)
le 1198712119904minus120574(100381610038161003816100381610038161199091015840minus 119910
101584010038161003816100381610038161003816+1003816100381610038161003816119909 minus 119910
1003816100381610038161003816)
10038161003816100381610038161003816120590 (119905 119909
1015840 119909)
10038161003816100381610038161003816L(Γ119870)le 119871
2(1 +
10038161003816100381610038161003816119909101584010038161003816100381610038161003816+ |119909|)
(68)
for some constants 1198712gt 0 and 120574 isin [0 12)
Theorem 11 Under assumptions (A3) and (A4) (65) has aunique mild solution119883(sdot) isin S2
F ([119905 119879] 119870)
The proof is constructed in two steps like that ofTheorem 4 and it uses standard arguments for stochastic evo-lution equations introduced in the proof of Proposition 32 in[3] Since the proof is straightforward we prefer to omit it
Remark 12 In our paper Lipchitz condtion (A4) is givento get the well-posedness of mean-field stochastic evolutionequations In fact (A4) can be replaced by a weaker conditionsuch as (A3) We just give the condition (A4) for simplicity
From standard arguments we can also get the followingcontinuous dependence theorem
Corollary 13 Assume that for all 120572 in a metric space 119865(119887120572 120590
120572) satisfy (A4) and (A5)with119871
1and119871
2independent of 120572
Also assume that
Eint119879
0
10038161003816100381610038161003816E1015840[119887120572(119904 119883
1015840
(119904) 119883 (119904))]
minus E1015840[1198871205720(119904 119883
1015840
(119904) 119883 (119904))]10038161003816100381610038161003816
2
119889119904 997888rarr 0
Eint119879
0
10038161003816100381610038161003816E1015840[120590
120572(119904 119883
1015840
(119904) 119883 (119904))]
minusE1015840[120590
1205720(119904 119883
1015840
(119904) 119883 (119904))]10038161003816100381610038161003816
2
119889119904 997888rarr 0
(69)
as 120572 rarr 1205720for all119883 isin S2
F ([0 119879] 119870)If we denote by 119883120572
(sdot) the mild solution of mean-field SEE(65) corresponding to the functions (119887
120572 120590
120572) and to the initial
data 119909 then we have
sup119904isin[0119879]
E1003816100381610038161003816119883
120572
(119904) minus 1198831205720 (119904)
10038161003816100381610038162
997888rarr 0 119886119904 120572 997888rarr 1205720 (70)
5 Maximum Principle for BSPDEs ofMean-Field Type
51 Formulation of the Problem Let O isin R119899 be a boundedopen set with smooth boundary 120597O and let 119880 the space ofcontrols be a separable real Hilbert space We denote
U = V (sdot) isin 1198712F(0 119879 119880)
| V119905(120596
1015840 120596) [0 119879] times Ω times Ω
997888rarr 119880 is F otimesF119905-progressively measurable
(71)
Mathematical Problems in Engineering 11
An element ofU is called an admissible controlFor any V isin U we consider the following controlled
BSPDE system in the state space119867 = 1198712(O) (norm | sdot | scalar
product ⟨sdot sdot⟩)
119889119884119905(119909) = minus119860119884
119905(119909) 119889119905
minus E1015840[119891 (119905 119909 (119884
119905(119909))
1015840
(119885119905(119909))
1015840
119884119905(119909) 119885
119905(119909) V
119905)] 119889119905
+ 119885119905(119909) 119889119882 (119905) 119905 isin [0 119879]
119884119879(119909) = 120585 (119909) 119909 isin O
(72)
where 119860 is a partial differential operator 119891 [0 119879] timesO times119867 times
L(Γ119867)times119867timesL(Γ119867)times119880 rarr 119867 and 120585 isin 1198712(ΩF119879P 119867)
The cost functional is given by
119869 (V) = Eint119879
0
intO
E1015840[ℎ (119904 119909 (119884
119904(119909))
1015840
(119885119904(119909))
1015840
119884119904(119909) 119885
119904(119909) V
119904)] 119889119909 119889119904
+E1015840intO
119892 (119909 (1198840(119909))
1015840
1198840(119909)) 119889119909
(73)
where
ℎ [0 119879] times O times 119867 timesL (Γ119867) times 119867 timesL (Γ119867) times 119880 997888rarr R
119892 O times 119867 times119867 997888rarr R
(74)
Our purpose is to minimize the functional 119869(sdot) over Uadsubject to the following state constraint
EintO
Φ(119909 (1198840(119909))
1015840
1198840(119909)) 119889119909 = 0 (75)
where
Φ O times 119867 times119867 997888rarr R (76)
An admissible control 119906 isin Uad that satisfies
119869 (119906) = minVisinUad
119869 (V) (77)
is called optimalThrough what follows the following assumptions will be
in force
(L1) 119860 is a partial differential operator with appropri-ate boundary conditions We assume that 119860 is theinfinitesimal generator of a strongly continuous semi-group 119890119905119860 119905 ge 0 in119867 Moreover for every 119905 isin [0 119879]119890
119905119860119891
1198712(O) le 119872
119860119891
1198712(O) for some constant 119872
119860
independent of 119905 and 119891(L2) 119891 ℎ 119892 and Φ are continuously Gateaux differen-
tiable with respect to (1199101015840 1199111015840 119910 119911) 119891 is continuouslyGateaux differentiable with respect to V and ℎ iscontinuous with respect to V
(L3) The derivatives of 119891 ℎ 119892 and Φ are Lipschitzcontinuous and bounded by
100381610038161003816100381610038161198911199101015840
10038161003816100381610038161003816+10038161003816100381610038161198911199111015840
1003816100381610038161003816 +10038161003816100381610038161003816119891119910
10038161003816100381610038161003816+1003816100381610038161003816119891119911
1003816100381610038161003816 +1003816100381610038161003816119891V
1003816100381610038161003816 +10038161003816100381610038161003816Φ1199101015840
10038161003816100381610038161003816+10038161003816100381610038161003816Φ119910
10038161003816100381610038161003816le 119862
10038161003816100381610038161003816ℎ1199101015840
10038161003816100381610038161003816+1003816100381610038161003816ℎ1199111015840
1003816100381610038161003816 +10038161003816100381610038161003816ℎ119910
10038161003816100381610038161003816+1003816100381610038161003816ℎ119911
1003816100381610038161003816 +100381610038161003816100381610038161198921199101015840
10038161003816100381610038161003816+10038161003816100381610038161003816119892119910
10038161003816100381610038161003816
le 119862 (1 +10038161003816100381610038161003816119910101584010038161003816100381610038161003816+10038161003816100381610038161003816119911101584010038161003816100381610038161003816+10038161003816100381610038161199101003816100381610038161003816 + |119911| + |V|)
(78)
where 119862 is a positive constant
Obviously according to Theorem 4 state equation (72)has a unique mild solution under the above assumptions
Remark 14 We can define the second order differentialoperator
(119860119891) (119909) =
119899
sum
119894119895=1
119886119894119895(119909)
1205972119891
120597119909119894120597119909
119895
(119909) +
119899
sum
119894=1
119887119894(119909)
120597119891
120597119909119894
(119909) (79)
By Example 9 119860 fulfills assumption (L1) if 119886119894119895 119887
119894satisfy
condition (H1)
52 Variation of the Trajectory Let 119906 be an optimal controlwith (119884(sdot) 119885(sdot)) being the corresponding optimal state Let120576 gt 0 and [119903 119903 + 120576] sube [0 119879] For any given V isin Uad weintroduce the spike variation of the control 119906(sdot)
119906120576
119905=
V119905 119905 isin [119903 119903 + 120576]
119906119905 119905 isin [0 119879] [119904 119904 + 120576]
(80)
It is clear that 119906120576(sdot) isin UadLet (119884120576
(sdot) 119885120576(sdot)) be the trajectory corresponding to 119906120576(sdot)
We use the following short notation for brevity
119891 (119905) = 119891 (119905 119909 (119884119905(119909))
1015840
(119885119905(119909))
1015840
119884119905(119909) 119885
119905(119909) 119906
119905)
119891 (119906120576
119905) = 119891 (119905 119909 (119884
119905(119909))
1015840
(119885119905(119909))
1015840
119884119905(119909) 119885
119905(119909) 119906
120576
119905)
(81)
Consider the following equation
119889119870120576
119905(119909) = minus119860119870
120576
119905(119909) 119889119905
minus E1015840[119891
1199101015840 (119905) (119870
120576
119905(119909))
1015840
+ 119891119910(119905) 119870
120576
119905(119909)
+ 1198911199111015840 (119905) (119876
120576
119905(119909))
1015840
+ 119891119911(119905) 119876
120576
119905(119909)
+1
120576(119891 (119906
120576
119905) minus 119891 (119905))] 119889119905 + 119876
120576
119905(119909) 119889119882 (119905)
119870120576
119879(119909) = 0
(82)
Since the coefficients in (82) are bounded it is easy to checkthat there exists a unique mild solution such that
E[ sup119905isin[0119879]
1003816100381610038161003816119870120576
119905
10038161003816100381610038162
+ int
119879
0
1003816100381610038161003816119876120576
119905
10038161003816100381610038162
119889119905] lt infin (83)
We have the following estimate
12 Mathematical Problems in Engineering
Theorem 15 There holds
lim120576rarr0
E[ sup119904isin[119905119879]
1003816100381610038161003816100381610038161003816100381610038161003816
119884120576
119904minus 119884
119904
120576minus 119870
120576
119904
1003816100381610038161003816100381610038161003816100381610038161003816
2
] = 0 forall119905 isin [0 119879]
lim120576rarr0
Eint119879
0
1003816100381610038161003816100381610038161003816100381610038161003816
119885120576
119904minus 119885
119904
120576minus 119876
120576
119904
1003816100381610038161003816100381610038161003816100381610038161003816
2
119889119904 = 0
(84)
Proof We define
120578120576
119904=119884120576
119904minus 119884
119904
120576minus 119870
120576
119904 120577
120576
119904=119885120576
119904minus 119885
119904
120576minus 119876
120576
119904 119904 isin [0 119879]
(85)
For simplicity let us define
Λ120576
119904= ((119884
120576
119904)1015840
(119885120576
119904)1015840
119884120576
119904 119885
120576
119904)
119891 (119904 120582) = 119891 (119904 1198841015840
119904+ 120582(119884
120576
119904minus 119884
119904)1015840
1198851015840
119904+ 120582(119885
120576
119904minus 119885
119904)1015840
119884119904+ 120582 (119884
120576
119904minus 119884
119904)
119885119904+ 120582 (119885
120576
119904minus 119885
119904) 119906
120576
119904)
(86)
By the definition of (119884120576
119904 119885
120576
119904) (119884
119904 119885
119904) and (119870120576
119904 119876
120576
119904) (120578120576
119904 120577
120576
119904)
is the mild solution of
119889120578120576
119904= minus119860120578
120576
119904119889119904 minus E
1015840
[119871 (119904 120576)] 119889119904 + 120577120576
119904119889119882 (119904)
120578120576
119879= 0
(87)
with
119871 (119904 120576) =1
120576(119891 (119904 Λ
120576
119904 119906
120576
119904) minus 119891 (119906
120576
119904))
minus 1198911199101015840 (119904) (119870
120576
119904)1015840
minus 119891119910(119904) 119870
120576
119904minus 119891
1199111015840 (119904) (119876
120576
119904)1015840
minus 119891119911(119904) 119876
120576
119904
= ((120578120576
119904)1015840
+ (119870120576
119904)1015840
) int
1
0
1198911199101015840 (119904 120582) 119889120582 + (120578
120576
119904+ 119870
120576
119904)
times int
1
0
119891119910(119904 120582) 119889120582 minus 119891
1199101015840 (119904) (119870
120576
119904)1015840
minus 119891119910(119904) 119870
120576
119904
+ ((120577120576
119904)1015840
+ (119876120576
119904)1015840
)int
1
0
1198911199111015840 (119904 120582) 119889120582 + (120577
120576
119904+ 119876
120576
119904)
times int
1
0
119891119911(119904 120582) 119889120582 minus 119891
1199111015840 (119904) (119876
120576
119904)1015840
minus 119891119911(119904) 119876
120576
119904
= (120578120576
119904)1015840
int
1
0
1198911199101015840 (119904 120582) 119889120582 + 120578
120576
119904int
1
0
119891119910(119904 120582) 119889120582
+ (120577120576
119904)1015840
int
1
0
1198911199111015840 (119904 120582) 119889120582 + 120577
120576
119904int
1
0
119891119911(119904 120582) 119889120582 + 120574
120576
119904
(88)
where we denote
120574120576
119904= (119870
120576
119904)1015840
int
1
0
(1198911199101015840 (119904 120582) minus 119891
1199101015840 (119904)) 119889120582
+ 119870120576
119904int
1
0
(119891119910(119904 120582) minus 119891
119910(119904)) 119889120582
+ (119876120576
119904)1015840
int
1
0
(1198911199111015840 (119904 120582) minus 119891
1199111015840 (119904)) 119889120582
+ 119876120576
119904int
1
0
(119891119911(119904 120582) minus 119891
119911(119904)) 119889120582
(89)
For any 120573 gt 0 according to Lemma 1 we obtain
E sup119905le119904le119879
11989021205731199041003816100381610038161003816120578
120576
119904
10038161003816100381610038162
+ Eint119879
119905
11989021205731199041003816100381610038161003816120577
120576
119904
10038161003816100381610038162
119889119904
le12119872
2
119860
120573int
119879
119905
1198902120573119904
E [10038161003816100381610038161003816E1015840
[119871 (119904 120576)]10038161003816100381610038161003816
2
] 119889119904
le12119872
2
119860
120573Eint
119879
119905
1198902120573119904
E1015840[|119871 (119904 120576)|
2] 119889119904
(90)
By condition (L3) we have
E [E1015840[|119871 (119904 120576)|
2]]
= E [E1015840[1003816100381610038161003816119871 (119904 120576) minus 120574
120576
119904+ 120574
120576
119904
10038161003816100381610038162
]]
le 81198622E [E
1015840[10038161003816100381610038161003816(120578
120576
119904)101584010038161003816100381610038161003816
2
+1003816100381610038161003816120578120576
119904
10038161003816100381610038162
+10038161003816100381610038161003816(120577
120576
119904)101584010038161003816100381610038161003816
2
+1003816100381610038161003816120577120576
119904
10038161003816100381610038162
]]
+ 2E [E1015840[1003816100381610038161003816120574120576
119904
10038161003816100381610038162
]]
le 161198622E [
1003816100381610038161003816120578120576
119904
10038161003816100381610038162
+1003816100381610038161003816120577120576
119904
10038161003816100381610038162
] + 2E [E1015840[1003816100381610038161003816120574120576
119904
10038161003816100381610038162
]]
(91)
Combined with (91) (90) yields
E sup119905le119904le119879
11989021205731199041003816100381610038161003816120578
120576
119904
10038161003816100381610038162
+ (1 minus192119872
2
1198601198622
120573)Eint
119879
119905
11989021205731199041003816100381610038161003816120577
120576
119904
10038161003816100381610038162
119889119904
le192119872
2
1198601198622
120573Eint
119879
119905
11989021205731199041003816100381610038161003816120578
120576
119904
10038161003816100381610038162
119889119904
+24119872
2
119860
120573Eint
119879
119905
1198902120573119904
[E1015840[1003816100381610038161003816120574120576
119904
10038161003816100381610038162
]] 119889119904
(92)
We claim that
Eint119879
119905
E1015840[1003816100381610038161003816120574120576
119904
10038161003816100381610038162
] 119889119904 997888rarr 0 as 120576 997888rarr 0 (93)
From (89)
120574120576
119904= 119868
1
119904+ 119868
2
119904+ 119868
3
119904+ 119868
4
119904 (94)
Mathematical Problems in Engineering 13
where
1198681
119904= (119870
120576
119904)1015840
int
1
0
(1198911199101015840 (119904 120582) minus 119891
1199101015840 (119904)) 119889120582
1198682
119904= 119870
120576
119904int
1
0
(119891119910(119904 120582) minus 119891
119910(119904)) 119889120582
1198683
119904= (119876
120576
119904)1015840
int
1
0
(1198911199111015840 (119904 120582) minus 119891
1199111015840 (119904)) 119889120582
1198684
119904= 119876
120576
119904int
1
0
(119891119911(119904 120582) minus 119891
119911(119904)) 119889120582
(95)
Then
Eint119879
119905
E1015840[1003816100381610038161003816120574120576
119904
10038161003816100381610038162
] 119889119904
le 4Eint119879
119905
E1015840[100381610038161003816100381610038161198681
119904
10038161003816100381610038161003816
2
+100381610038161003816100381610038161198682
119904
10038161003816100381610038161003816
2
+100381610038161003816100381610038161198683
119904
10038161003816100381610038161003816
2
+100381610038161003816100381610038161198684
119904
10038161003816100381610038161003816
2
] 119889119904
(96)
Take Eint119879119905E1015840[|1198682
119904|2
]119889119904 for example
Eint119879
119905
E1015840[100381610038161003816100381610038161198682
119904
10038161003816100381610038161003816
2
] 119889119904
= Eint119879
119905
E1015840[(119870
120576
119904int
1
0
(119891119910(119904 120582) minus 119891
119910(119904)) 119889120582)
2
]119889119904
le Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119904 120582) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582] 119889119904
le 2Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119904 120582) minus 119891
119910(119906
120576
119905)10038161003816100381610038161003816
2
119889120582] 119889119904
+ 2Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119906
120576
119905) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582] 119889119904
(97)
Note that
Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119906
120576
119905) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582] 119889119904
= Eint119903+120576
119903
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119906
120576
119905) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582] 119889119904
le sup119904isin[119905119879]
E [E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119906
120576
119905) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582]] 120576
997888rarr 0 as 120576 997888rarr 0
(98)
The inequality above holds due to the boundedness of|119870
120576
119904|2
int1
0|119891119910(119906
120576
119905) minus 119891
119910(119904)|
2
119889120582 Indeed Assumption (L3) im-plies the boundedness of 119891
119910(119906
120576
119905) minus 119891
119910(119904) Meanwhile 119870120576
119904is
the solution of mean-field BSEE (82) It can be easy to check119870120576
119904is bounded since the coefficients in (82) are boundedOn the other hand
Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119904 120582) minus 119891
119910(119906
120576
119905)10038161003816100381610038161003816
2
119889120582] 119889119904
le 11986221205822Eint
119879
119905
1003816100381610038161003816119870120576
119904
10038161003816100381610038162
times int
1
0
1198641015840[100381610038161003816100381610038161003816(119884
120576
119904minus 119884
119904)1015840100381610038161003816100381610038161003816
2
+100381610038161003816100381610038161003816(119885
120576
119904minus 119885
119904)1015840100381610038161003816100381610038161003816
2
times10038161003816100381610038161003816119884120576
119904minus 119884
119904
10038161003816100381610038161003816
2
+10038161003816100381610038161003816119885120576
119904minus 119885
119904
10038161003816100381610038161003816
2
] 119889120582 119889119904
(99)
where (119884120576
119904 119885
120576
119904) is the mild solution of the following equation
119889119884120576
119905= minus 119860119884
120576
119905119889119905
minus E1015840[119891 (119905 (119884
120576
119905)1015840
(119885120576
119905)1015840
119884120576
119905 119885
120576
119905 119906
120576
119905)] 119889119905
+ 119885120576
119905119889119882 (119905) 119905 isin [0 119879]
119884120576
119879= 120585
(100)
and (119884119904 119885
119904) is the mild solution of
119889119884119905= minus 119860119884
119905119889119905
minus E1015840[119891 (119905 (119884
119905)1015840
(119885119905)1015840
119884119905 119885
119905 119906
119905)] 119889119905
+ 119885119905119889119882 (119905) 119905 isin [0 119879]
119884119879= 120585
(101)
By the definition of 119906120576119905 according to (L2) we have
E1015840[119891 (119904 119884
1015840
(119904) 1198851015840
(119904) 119884 (119904) 119885 (119904) 119906120576
119905)]
997888rarr E1015840[119891 (119904 119884
1015840
(119904) 1198851015840
(119904) 119884 (119904) 119885 (119904) 119906119905)]
(102)
in 1198712([0 119879]119867) as 120576 rarr 0 Using the continuous dependencetheorem Corollary 5 we obtain
(119884120576
119904 119885
120576
119904) 997888rarr (119884
119904 119885
119904) as 120576 997888rarr 0 (103)
Then
Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119904 120582) minus 119891
119910(119906
120576
119905)10038161003816100381610038161003816
2
119889120582] 119889119904
le 11986221205822Eint
119879
119905
1003816100381610038161003816119870120576
119904
10038161003816100381610038162
times int
1
0
E1015840[100381610038161003816100381610038161003816(119884
120576
119904minus 119884
119904)1015840100381610038161003816100381610038161003816
2
+100381610038161003816100381610038161003816(119885
120576
119904minus 119885
119904)1015840100381610038161003816100381610038161003816
2
times10038161003816100381610038161003816119884120576
119904minus 119884
119904
10038161003816100381610038161003816
2
+10038161003816100381610038161003816119885120576
119904minus 119885
119904
10038161003816100381610038161003816
2
] 119889120582 119889119904
997888rarr 0 as 120576 997888rarr 0
(104)
14 Mathematical Problems in Engineering
Combining (98) with (104) we finally haveEint
119879
119905E1015840[|1198682
119904|2
]119889119904 rarr 0 as 120576 rarr 0The required result (93) follows by using the similar
estimations for 1198681119904 1198683
119904 and 1198684
119904
Note that 1 minus 1921198722
1198601198622120573 gt 0 if 120573 is sufficiently large
Now to prove the desired result (84) it suffices to applyGronwallrsquos lemma and estimate (93) to inequality (92)
To deal with the state constraint (75) we need to recall theEkeland variational principle
Lemma 16 (Ekelandrsquos variational principle see [16 Lemma41]) Let (119878 119889) be a complete metric space and let 119865(sdot) 119878 rarr
R be lower semicontinuous and bounded from below If for 120588 gt0 there exists 119906 isin 119878 such that
119865 (119906) le infVisin119878
119865 (V) + 120588 (105)
then there exists 119906120588 isin 119878 satisfying
(i) 119865 (119906120588) le 119865 (119906)
(ii) 119889 (119906120588 119906) le 120588
(iii) 119865 (119906120588) le 119865 (V) + 120588 sdot 119889 (119906120588 119906) forallV = 119906120588
(106)
Now fix V isin Uad and set
119878 = V (sdot) isin Uad | sup0le119905le119879
E1003816100381610038161003816V11990510038161003816100381610038162
le E100381610038161003816100381611990611990510038161003816100381610038162
+ |V|2
119889 (V (sdot) V (sdot)) = 119898 119905 isin [0 119879] | E1003816100381610038161003816V119905 minus V
119905
10038161003816100381610038162
gt 0
forallV (sdot) V (sdot) isin 119878
(107)
where119898 denotes the Lebesgue measure on RThe following result is proved as Proposition 41 in [16]
Lemma 17 (119878 119889(sdot sdot)) is a complete metric space and 119869120588 is
continuous and bounded on 119878 where
119869120588
(V (sdot)) = (119869 (V (sdot)) minus 119869 (119906 (sdot)) + 120588)2
+
10038161003816100381610038161003816100381610038161003816Eint
O
Φ(119909 (1198840(119909))
1015840
1198840(119909)) 119889119909
10038161003816100381610038161003816100381610038161003816
2
12
forallV (sdot) isin 119878(108)
and (119884 119885) is the mild solution of (72) corresponding to thecontrol V
Now we consider the following free initial state optimalcontrol problem
infV(sdot)isin119878
119869120588
(V (sdot)) (109)
It is easy to check that
0 le infV(sdot)isin119878
119869120588
(V (sdot)) le 119869120588
(119906 (sdot)) = 120588 (110)
According to Ekelandrsquos variational principle there exists a119906120588(sdot) isin 119881 such that
(i) 119869120588 (119906120588 (sdot)) le 120588
(ii) 119889 (119906120588 (sdot) 119906 (sdot)) le 120588
(iii) 119869120588 (119906120588 (sdot)) le 119869120588
(V (sdot)) + 120588119889 (119906120588 (sdot) 119906 (sdot)) forallV (sdot) isin 119878(111)
Using the spike variationmethod we can construct 119906120576120588(sdot) isin 119878as follows
119906120576120588
119905=
V119905 119905 isin [119904 119904 + 120576]
119906120588
119905 119905 isin [0 119879] [119904 119904 + 120576]
(112)
It is clear that 119889(119906120576120588(sdot) 119906120588(sdot)) le 120576 Let (119884120576120588(sdot) 119885
120576120588(sdot)) (resp
(119884120588(sdot) 119885
120588(sdot))) be the solution of (72) with respect to the
control 119906120576120588(sdot) (resp 119906120588(sdot)) Following (82) (119870120576120588
119905 119876
120576120588
119905) is the
mild solution of
119870120576120588
119905= int
119879
119905
119890119860(119904minus119905)
E1015840
times [1198911199101015840 (119904 Λ
120588
119904 119906
120588
119904) (119870
120576120588
119904)1015840
+ 119891119910(119904 Λ
120588
119904 119906
120588
119904)119870
120576120588
119904
+ 1198911199111015840 (119904 Λ
120588
119904 119906
120588
119904) (119876
120576120588
119904)1015840
+ 119891119911(119904 Λ
120588
119904 119906
120588
119904) 119876
120576120588
119904
+1
120576(119891 (119904 Λ
120588
119904 119906
120576120588
119904) minus 119891 (119904 Λ
120588
119904 119906
120588
119904))] 119889119904
minus int
119879
119905
119890119860(119904minus119905)
119876120576120588
119904119889119882 (119904)
(113)
ByTheorem 15 we know that
lim120576rarr0
E[ sup119904isin[119905119879]
100381610038161003816100381610038161003816100381610038161003816
119884120576120588
119904minus 119884
120588
119904
120576minus 119870
120576120588
119904
100381610038161003816100381610038161003816100381610038161003816
2
] = 0 forall119905 isin [0 119879]
lim120576rarr0
Eint119879
0
100381610038161003816100381610038161003816100381610038161003816
119885120576120588
119904minus 119885
120588
119904
120576minus 119876
120576120588
119904
100381610038161003816100381610038161003816100381610038161003816
2
119889119904 = 0
(114)
The proof of the following proposition is technical butbased on the arguments above and we omit it
Proposition 18 One has
1
120576E [Φ ((119884
120576120588
0)1015840
119884120576120588
0) minus Φ((119884
120588
0)1015840
119884120588
0)]
= E [Φ1199101015840 ((119884
120588
0)1015840
119884120588
0) (119870
120576120588
0)1015840
+ Φ119910((119884
120588
0)1015840
119884120588
0)119870
120576120588
0]
+ 119900 (120576)
Mathematical Problems in Engineering 15
1
120576(119869 (119906
120576120588) minus 119869 (119906
120588))
= E [1198921199101015840 ((119884
120588
0)1015840
119884120588
0) (119870
120576120588
0)1015840
+ 119892119910((119884
120588
0)1015840
119884120588
0)119870
120576120588
0]
+ Δ120576+1
120576Eint
119879
0
E1015840[ℎ (119904 Λ
120588
119904 119906
120576120588
119904)
minus ℎ (119904 Λ120588
119904 119906
120588
119904)] 119889119904 + 119900 (120576)
(115)
where
Δ120576= Eint
119879
0
E1015840[ℎ
1199101015840 (119904 Λ
120588
119904 119906
120576120588
119904) (119870
120576120588
119904)1015840
+ ℎ1199111015840 (119904 Λ
120588
119904 119906
120576120588
119904) (119876
120576120588
119904)1015840
+ ℎ119910(119904 Λ
120588
119904 119906
120576120588
119904)119870
120576120588
119904
+ ℎ119911(119904 Λ
120588
119904 119906
120576120588
119904) 119876
120576120588
119904] 119889119904
(116)
53 Variational Inequality and Adjoint Equation In thissubsection the adjoint process is introduced to deduce thevariational inequality
If we set V(sdot) = 119906120576120588(sdot) in (111) and notice that
119889(119906120576120588(sdot) 119906
120588(sdot)) le 120576 we get
minus120588 le1
120576(119869
120588(119906
120576120588
(sdot)) minus 119869120588(119906
120588
(sdot))) (117)
By Lemma 17
1
120576(119869
120588(119906
120576120588
(sdot)) minus 119869120588(119906
120588
(sdot)))
=(119869
120588(119906
120576120588(sdot)))
2
minus (119869120588(119906
120588(sdot)))
2
120576 (119869120588 (119906120576120588 (sdot)) + 119869120588 (119906120588 (sdot)))
=119869 (119906
120576120588(sdot)) + 119869 (119906
120588(sdot)) minus 2119869 (119906 (sdot)) + 2120588
119869120588 (119906120576120588 (sdot)) + 119869120588 (119906120588 (sdot))
times119869 (119906
120576120588(sdot)) minus 119869 (119906
120588(sdot))
120576
+
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] + E [Φ ((119884
120588
0)1015840
119884120588
0)]
119869120588 (119906120576120588 (sdot)) + 119869120588 (119906120588 (sdot))
times
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] minus E [Φ ((119884
120588
0)1015840
119884120588
0)]
120576
997888rarr 119897120588
1
119869 (119906120576120588(sdot)) minus 119869 (119906
120588(sdot))
120576
+ 119897120588
2
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] minus E [Φ ((119884
120588
0)1015840
119884120588
0)]
120576
(118)
where we set
119897120588
1=119869 (119906
120588(sdot)) minus 119869 (119906 (sdot)) + 120588
119869120588 (119906120588 (sdot))
119897120588
2=
E [Φ ((119884120588
0)1015840
119884120588
0)]
119869120588 (119906120588 (sdot))
(119)
and use the limit
119869 (119906120576120588
(sdot)) 997888rarr 119869 (119906120588
(sdot))
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] 997888rarr E [Φ ((119884
120588
0)1015840
119884120588
0)]
(120)
as 120576 rarr 0 according to (115)As |119897120588
1|2
+ |119897120588
2|2
= 1 for all 120588 gt 0 we know that there existsa subsequence of 119897120588
1 119897120588
2 (still denoted by 119897120588
1 119897120588
2) such that
lim120588rarr0
119897120588
1 119897120588
2 = 119897
1 1198972
1003816100381610038161003816119897110038161003816100381610038162
+1003816100381610038161003816119897210038161003816100381610038162
= 1
(121)
Combining (115) (117) with (118) we get
minus120588 le 119897120588
1
119869 (119906120576120588(sdot)) minus 119869 (119906
120588(sdot))
120576
+ 119897120588
2
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] minus E [Φ ((119884
120588
0)1015840
119884120588
0)]
120576
= 119897120588
1Δ120576+ 119897
120588
1E [119892
1199101015840 ((119884
120588
0)1015840
119884120588
0) (119870
120576120588
0)1015840
+119892119910((119884
120588
0)1015840
119884120588
0)119870
120576120588
0]
+119897120588
1
120576Eint
119879
0
E1015840[ℎ (119904 Λ
120588
119904 119906
120576120588
119904) minus ℎ (119904 Λ
120588
119904 119906
120588
119904)] 119889119904
+ 119897120588
2E [Φ
1199101015840 ((119884
120588
0)1015840
119884120588
0) (119870
120576120588
0)1015840
+Φ119910((119884
120588
0)1015840
119884120588
0)119870
120576120588
0] + (119897
120588
1+ 119897
120588
2) 119900 (120576)
(122)
Next we introduce the adjoint equation corresponding tovariational equation (113) whose solution is denoted by119875120588(119905)
119889119875120588
(119905)
= 119860lowast119875120588
(119905) 119889119905
+ E1015840[119891
1199101015840 (119905 Λ
120588
119905 119906
120588
119905) (119875
120588
(119905))1015840
+ 119891119910(119905 Λ
120588
119905 119906
120588
119905) 119875
120588
(119905) + 119897120588
1ℎ1199101015840 (119905 Λ
120588
119905 119906
120588
119905)
+ 119897120588
1ℎ119910(119905 Λ
120588
119905 119906
120588
119905)] 119889119905
16 Mathematical Problems in Engineering
+ E1015840[119891
1199111015840 (119905 Λ
120588
119905 119906
120588
119905) (119875
120588
(119905))1015840
+ 119891119911(119905 Λ
120588
119905 119906
120588
119905) 119875
120588
(119905)
+ 119897120588
1ℎ1199111015840 (119905 Λ
120588
119905 119906
120588
119905) + 119897
120588
1ℎ119911(119905 Λ
120588
119905 119906
120588
119905)] 119889119882 (119905)
119875120588
(0) = 119897120588
1E [119892
1199101015840 ((119884
120588
0(119909))
1015840
119884120588
0(119909))
+ 119892119910((119884
120588
0(119909))
1015840
119884120588
0(119909))]
+ 119897120588
2E [Φ
1199101015840 ((119884
120588
0(119909))
1015840
119884120588
0(119909))
+ Φ119910((119884
120588
0(119909))
1015840
119884120588
0(119909))]
(123)
where 119860lowast is the 1198712(O)-adjoint operator of 119860 Under assump-tions (L1)ndash(L3) this is a linear mean-field SEE with boundedcoefficients An application ofTheorem 11 implies that it has aunique adaptedmild solution such that119875120588(119905) isin S2
F ([0 119879] 119870)When 120588 rarr 0 according to Corollaries 5 and 13 119875120588(119905)
converges to 119875(119905) where
119875 (119905) isin S2
F ([0 119879] 119870) (124)
is the solution of the following equation
119875 (119905) = 1198971E [119892
1199101015840 (119884
1015840
0 119884
0) + 119892
119910(119884
1015840
0 119884
0)]
+ 1198972E [Φ
1199101015840 (119884
1015840
0 119884
0) + Φ
119910(119884
1015840
0 119884
0)]
+ int
119905
0
119890119860lowast(119905minus119904)
E1015840[119891
1199101015840 (119904) 119875
1015840
(119904) + 119891119910(119904) 119875 (119904)
+ 1198971ℎ1199101015840 (119904) + 119897
1ℎ119910(119904)] 119889119904
+ int
119905
0
119890119860lowast(119905minus119904)
E1015840[119891
1199111015840 (119904) 119875
1015840
(119904) + 119891119911(119904) 119875 (119904)
+ 1198971ℎ1199111015840 (119904) + 119897
1ℎ119911(119904)] 119889119882 (119904)
(125)
The following proposition which formally follows fromProposition 18 gives the relation between 119875120588(119905) and119870120576120588
119905
Proposition 19 Consider the following
E [119875120588
(0)119870120576120588
0]
= Eint119879
0
1
120576119875120588
(119904)E1015840[119891 (119904 Λ
120588
119904 119906
120576120588
119904) minus 119891 (119904 Λ
120588
119904 119906
120588
119904)]
minus 119897120588
1119870120576120588
119904E1015840[ℎ
1199101015840 (119904 Λ
120588
119904 119906
120588
119904) + ℎ
119910(119904 Λ
120588
119904 119906
120588
119904)]
minus 119897120588
1119876120576120588
119904E1015840[ℎ
1199111015840 (119904 Λ
120588
119904 119906
120588
119904) + ℎ
119911(119904 Λ
120588
119904 119906
120588
119904)] 119889119904
(126)
The following theorem constitutes the main contributionof this section themaximumprinciple for the BSPDE controlsystem
Theorem 20 Let assumptions (L1)ndash(L3) hold Suppose 119906(sdot) isan optimal control and (119884(sdot) 119885(sdot)) is the corresponding optimalstate trajectory for the BSPDE control systems (72) and (73)with the initial state constraint (75) Then there exists 119875(119905) isinS2
F ([0 119879] 119870) which satisfies (125) such that
H (119905 1198841015840
119905 119885
1015840
119905 119884
119905 119885
119905 V
119905 119875 (119905))
ge H (119905 1198841015840
119905 119885
1015840
119905 119884
119905 119885
119905 119906
119905 119875 (119905))
119886119890 119886119904 forallV isin U119886119889
(127)
whereH [0 119879]times119867timesL(Γ119867)times119867timesL(Γ119867)times119880times119870 rarr R
is the Hamiltonian function defined by
H (119905 119910 119910 119911 V 119901) = 1198971ℎ (119905 119910 119910 119911 V)
+ 119901119891 (119905 119910 119910 119911 V) (128)
Proof By (122) and Proposition 19 we obtain
minus120588 le1
120576Eint
119879
0
119875120588
(119904)E1015840[119891 (119904 Λ
120588
119904 119906
120576120588
119904)
minus 119891 (119904 Λ120588
119904 119906
120588
119904)] 119889119904
+119897120588
1
120576Eint
119879
0
E1015840[ℎ (119904 Λ
120588
119904 119906
120576120588
119904)
minus ℎ (119904 Λ120588
119904 119906
120588
119904)] 119889119904 + (119897
120588
1+ 119897
120588
2) 119900 (120576)
(129)
Letting 120576 rarr 0+ in (129) we derive for ae 120591 isin [0 119879]
minus120588 le 119897120588
1E1015840[ℎ (120591 Λ
120588
120591 V
120591) minus ℎ (120591 Λ
120588
120591 119906
120588
120591)]
+ 119875120588
(119904)E1015840[119891 (120591 Λ
120588
120591 V
120591) minus 119891 (120591 Λ
120588
120591 119906
120588
120591)]
(130)
for all V isin UadFinally taking 120588 rarr 0 in (130) we derive the desired
result
Remark 21 We note that if the coefficients do not dependexplicitly on the marginal law of the underlying diffusionthe result reduces to the classical case that is the SMP forBSPDEs without mean-field term
Remark 22 When we remove the initial state constraint (75)we obtain the general maximum principle for the mean-fieldBSPDEs system (ie without the constraint) with 119897
1= 1
54 Application A Backward Linear Quadratic Control Prob-lem Now we apply our maximum principle to solve an LQproblem For notational simplicity we restrict ourselves tothe free case (ie without the initial state constraint (75)) thegeneral case being handled in a similar way
Mathematical Problems in Engineering 17
Consider the following problem
119869 (119906) =1
2E [(119884 (0))
2] +
1
2Eint
119879
0
119873V2 (119905) 119889119905 997888rarr min
(131)
subject to
119889119884 (119905) = minus119860119884 (119905) 119889119905
minus 119861119884 (119905) + 119861E [119884 (119905)]
+119862V (119905) + 119863119885 (119905) + 119863E [119885 (119905)] 119889119905
+ 119885 (119905) 119889119882 (119905)
119884 (119879) = 120585 119905 isin [0 119879]
(132)
where 119860 is a partial differential operator satisfying condition(L1) and 119861 119861 119862119863119863 and119873 are bounded and deterministicconstantsWe also assume that119873 gt 0 and V isin 1198712F (0 119879 119880) It iseasy to verify that BSPDE (132) admits a uniquemild solution(119884(119905) 119885(119905))
119875(119905) the adjoint process of state equation (132) is thesolution of
119889119875 (119905) = 119860lowast119875 (119905) 119889119905 + 119861119875 (119905) + 119861E [119875 (119905)] 119889119905
+ 119863119875 (119905) + 119863E [119875 (119905)] 119889119882 (119905)
119875 (0) = 119884 (0)
(133)
Let 119906(sdot) be an optimal control and let (119884(sdot) 119885(sdot)) bethe corresponding state process By maximum principle ofTheorem 20 (note that 119897
1= 1 in this problem)
1
2119873V2 (119905) + 119875 (119905) 119862V (119905) ge
1
2119873119906
2
(119905) + 119875 (119905) 119862119906 (119905) (134)
for all V isin 119880ad since the state equation has the form (132)Thisin turn implies
119906 (119905) = minus119862
119873119875 (119905) (135)
It is clear that (131) is a positive quadratic functional of controlbecause of 119873 gt 0 Hence an optimal control exists Thecandidate optimal control (135) is indeed an optimal controlof this LQ problem for it is the only control which satisfies themaximum principle
Next we want to obtain a more explicit representationof the optimal control (135) from the state equation (132)Substituting (135) into state equation (132) yields
119889119884 (119905) = minus119860119884 (119905) 119889119905
minus 119861119884 (119905) + 119861E [119884 (119905)] minus1198622
119873119875 (119905)
+119863119885 (119905) + 119863E [119885 (119905)] 119889119905 + 119885 (119905) 119889119882 (119905)
119884 (119879) = 120585 119905 isin [0 119879]
(136)
Combining the above equation with (133) we obtain thefollowing related feedback control system
119889119884 (119905) = minus119860119884 (119905) 119889119905
minus 119861119884 (119905) + 119861E [119884 (119905)] minus1198622
119873119875 (119905)
+ 119863119885 (119905) + 119863E [119885 (119905)] 119889119905 + 119885 (119905) 119889119882 (119905)
119884 (119879) = 120585
119889119875 (119905) = 119860lowast119875 (119905) 119889119905 + 119861119875 (119905) + 119861E [119875 (119905)] 119889119905
+ 119863119875 (119905) + 119863E [119875 (119905)] 119889119882 (119905)
119875 (0) = 119884 (0)
(137)
Looking at the terminal condition of 119875(119905) in (133) andconsidering the mean-field type of (132) it is reasonable toconjecture that 119875(119905) has the following form
119875 (119905) = 120593 (119905) 119884 (119905) + 120601 (119905)E [119884 (119905)] (138)
where120593(119905)120601(119905) are deterministic differential functionswhichwill be specified below Moreover 120593(0) = 1 120601(0) = 0
Inserting this form into adjoint equation (133) and notic-ing that 119884(119905) satisfies (136) we can compare the coefficientsof 119889119905 and 119889119882(119905) to obtain the following equation
2120593 (119905) 119860119884 (119905) + 2120601 (119905) 119860E [119884 (119905)] + 119861120593 (119905) 119884 (119905)
+ 2 (119861120601 (119905) + 119861120593 (119905) + 119861120601 (119905))E [119884 (119905)]
= minus120593 (119905) 119861119884 (119905) minus1198622
119873120593 (119905) 119884 (119905) + 119863119885 (119905)
+ 119884 (119905)119889120593 (119905)
119889119905+ E [119884 (119905)]
119889120601 (119905)
119889119905
+ (1206012
(119905)1198622
119873+ 2
1198622
119873120593 (119905) 120601 (119905))E [119884 (119905)]
minus (119863120601 (119905) + 119863120601 (119905) + 119863120593 (119905))E [119885 (119905)]
120593 (119905) 119885 (119905) = 119863120593 (119905) 119884 (119905)
+ (119863120601 (119905) + 119863120593 (119905) + 119863120601 (119905))E [119884 (119905)]
(139)
Then subtracting 119885(119905) we have
2120593 (119905) 119860119884 (119905) + 2120601 (119905) 119860E [119884 (119905)] + 119861120593 (119905) 119884 (119905)
+ 2 (119861120601 (119905) + 119861120593 (119905) + 119861120601 (119905))E [119884 (119905)]
18 Mathematical Problems in Engineering
= 120593 (119905) minus119861 +1198622
119873120593 (119905) minus 119863
2119884 (119905)
+ 119884 (119905)119889120593 (119905)
119889119905+ E [119884 (119905)]
119889120601 (119905)
119889119905
+ (1206012
(119905)1198622
119873+ 2
1198622
119873120593 (119905) 120601 (119905))E [119884 (119905)]
minus1
120593 (119905)(119863120601 (119905) + 119863120593 (119905) + 119863120601 (119905))
2
E [119884 (119905)]
minus 2119863 (119863120601 (119905) + 119863120593 (119905) + 119863120601 (119905))E [119884 (119905)]
(140)
Comparing the coefficients of 119884(119905) and E[119884(119905)] respec-tively we get
119889
119889119905120593 (119905) = 2119860
lowast120593 (119905) + 2119861120593 (119905) + 119863
2120593 (119905) minus
1198622
1198731205932
(119905)
120593 (0) = 1
(141)
119889
119889119905120601 (119905) = 2119860
lowast120601 (119905) + (
(119863 + 119863)2
120593 (119905)minus1198622
119873)120601
2
(119905)
+ 2119877 (119905) 120601 (119905) + (1198632+ 2119861 + 2119863119863)120593 (119905)
120601 (0) = 0
(142)
where 119877(119905) = 119861 + 119861 + (119863 + 119863)2
minus (1198622119873)120593(119905)
We solve (141) to get
120593 (119905) = (119890minus(2119860lowast+2119861+119863
2)119905(1 minus
1198622
119873(2119860lowast + 2119861 + 1198632))
+1198622
119873(2119860lowast + 2119861 + 1198632))
minus1
(143)
Then (142) exists a unique solution from the classical Riccatiequation theory
We now conclude the above discussions in the followingresult
Theorem 23 For onersquos linear quadratic stochastic partialdifferential control problem (131)-(132) the unique optimalcontrol 119906(sdot) isin 119880ad is given by
119906 (119905) = minus119862
119873(120593 (119905) 119884 (119905) + 120601 (119905)E [119884 (119905)]) (144)
with 120593(119905) satisfying (143) and 120601(119905) solving (142)
Conflict of Interests
The authors declare that they have no conflict of interestsregarding the publication of this paper
Acknowledgment
This research is partially supported by the Natural ScienceFoundation of China under Grant no 11301310
References
[1] Y Hu and S Peng ldquoAdapted solution of a backward semilinearstochastic evolution equationrdquo Stochastic Analysis and Applica-tions vol 9 no 4 pp 445ndash459 1991
[2] N IMahmudov andMAMcKibben ldquoOnbackward stochasticevolution equations in Hilbert spaces and optimal controlrdquoNonlinear Analysis vol 67 no 4 pp 1260ndash1274 2007
[3] M Fuhrman and G Tessitore ldquoNonlinear Kolmogorov equa-tions in infinite dimensional spaces the backward stochasticdifferential equations approach and applications to optimalcontrolrdquoThe Annals of Probability vol 30 no 3 pp 1397ndash14652002
[4] M Fuhrman and G Tessitore ldquoInfinite horizon backwardstochastic differential equations and elliptic equations inHilbert spacesrdquo Annals of Probability vol 32 no 1B pp 607ndash660 2004
[5] X Mao ldquoAdapted solutions of backward stochastic differentialequations with non-Lipschitz coefficientsrdquo Stochastic Processesand their Applications vol 58 no 2 pp 281ndash292 1995
[6] J Lasry and P Lions ldquoMean field gamesrdquo Japanese Journal ofMathematics vol 2 no 1 pp 229ndash260 2007
[7] R Buckdahn J Li and S Peng ldquoMean-field backward stochas-tic differential equations and related partial differential equa-tionsrdquo Stochastic Processes and their Applications vol 119 no 10pp 3133ndash3154 2009
[8] D Andersson and B Djehiche ldquoAmaximum principle for SDEsofmean-field typerdquoAppliedMathematics andOptimization vol63 no 3 pp 341ndash356 2011
[9] R Buckdahn B Djehiche J Li and S Peng ldquoMean-fieldbackward stochastic differential equations a limit approachrdquoThe Annals of Probability vol 37 no 4 pp 1524ndash1565 2009
[10] R Xu ldquoMean-field backward doubly stochastic differentialequations and related SPDEsrdquo Boundary Value Problems vol2012 article 114 2012
[11] R Buckdahn B Djehiche and J Li ldquoA general stochasticmaximum principle for SDEs of mean-field typerdquo AppliedMathematics and Optimization vol 64 no 2 pp 197ndash216 2011
[12] J Li ldquoStochasticmaximumprinciple in themean-field controlsrdquoAutomatica vol 48 no 2 pp 366ndash373 2012
[13] G Wang C Zhang and W Zhang ldquoStochastic maximumprinciple for mean-field type optimal control under partialinformationrdquo IEEE Transactions on Automatic Control vol 59no 2 pp 522ndash528 2014
[14] M Hafayed ldquoA mean-field necessary and sufficient conditionsfor optimal singular stochastic controlrdquo Communications inMathematics and Statistics vol 1 no 4 pp 417ndash435 2013
[15] M Hafayed ldquoA mean-field maximum principle for optimalcontrol of forward-backward stochastic differential equationswith Poisson jumpprocessesrdquo International Journal ofDynamicsand Control vol 1 no 4 pp 300ndash315 2013
[16] Y Hu and S Peng ldquoMaximum principle for semilinearstochastic evolution control systemsrdquo Stochastics and StochasticsReports vol 33 no 3-4 pp 159ndash180 1990
[17] M Fuhrman Y Hu and G Tessitore ldquoStochastic maximumprinciple for optimal control of SPDEsrdquo Comptes Rendus Math-ematique vol 350 no 13-14 pp 683ndash688 2012
Submit your manuscripts athttpwwwhindawicom
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Mathematical Problems in Engineering
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Differential EquationsInternational Journal of
Volume 2014
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Function Spaces
Abstract and Applied AnalysisHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Algebra
Discrete Dynamics in Nature and Society
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Decision SciencesAdvances in
Discrete MathematicsJournal of
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Volume 2014 Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Stochastic AnalysisInternational Journal of
4 Mathematical Problems in Engineering
Definition 2 We say that a pair of adapted processes (119884 119885)is a mild solution of mean-field BSEE (11) if (119884 119885) isin
S2
F ([0 119879]119867) timesH2
F ([0 119879]L(Γ119867)) and for all 119905 isin [0 119879]
119884 (119905) = 119890119860(119879minus119905)
120585
+ int
119879
119905
119890119860(119904minus119905)
E1015840[119891 (119904 119884
1015840
(119904) 1198851015840
(119904) 119884 (119904) 119885 (119904))] 119889119904
minus int
119879
119905
119890119860(119904minus119905)
119885 (119904) 119889119882 (119904)
(12)
Remark 3 We emphasize that the coefficient of (11) can beinterpreted as
E1015840[119891 (119904 119884
1015840
(119904) 1198851015840
(119904) 119884 (119904) 119885 (119904))] (120596)
= E1015840[119891 (119904 119884
1015840
(119904) 1198851015840
(119904) 119884 (120596 119904) 119885 (120596 119904))]
= intΩ
119891 (1205961015840 120596 119904 119884 (120596
1015840 119904) 119885 (120596
1015840 119904) 119884 (120596 119904) 119885 (120596 119904))
times P (1198891205961015840)
(13)
31 Lipschitz Case Now we study the existence and unique-ness ofmild solutions tomean-field BSEE (11) under Lipschitzconditions For119891 [0 119879] times 119867 times L(Γ119867) times 119867 times L(Γ119867) rarr
119867 assume the following(A1) There exists an 119871 gt 0 such that
10038161003816100381610038161003816119891 (119905 119910
1015840
1 119911
1015840
1 119910
1 119911
1) minus 119891 (119905 119910
1015840
2 119911
1015840
2 119910
2 119911
2)10038161003816100381610038161003816
2
le 119871 (100381610038161003816100381610038161199101015840
1minus 119910
1015840
2
10038161003816100381610038161003816
2
+100381610038161003816100381610038161199111015840
1minus 119911
1015840
2
10038161003816100381610038161003816
2
+10038161003816100381610038161199101 minus 1199102
10038161003816100381610038162
+10038161003816100381610038161199111 minus 1199112
10038161003816100381610038162
)
(14)
for all 119905 isin [0 119879] 1199101015840119894 119910
119894isin 119867 119911
1015840
119894 119911
119894isin L(Γ119867) (119894 =
1 2)(A2) 119891(sdot 0 0 0 0) isin H2
F ([0 119879]119867)We have the following theorem
Theorem 4 For any random variable 120585 isin 1198712(ΩF
119879P 119867)
under (A1) and (A2) mean-field BSEE (11) admits a uniquemild solution (119884 119885) isin S2
F ([0 119879]119867) timesH2
F ([0 119879]L(Γ119867))
Proof Consider the following
Step 1 For any (119910 119911) isin S2
F ([0 119879]119867) timesH2
F ([0 119879]L(Γ119867))BSEE119884 (119905) = 119890
119860(119879minus119905)120585
+ int
119879
119905
E1015840[119890
119860(119904minus119905)119891 (119904 119910
1015840
(119904) 1199111015840
(119904) 119884 (119904) 119885 (119904))] 119889119904
minus int
119879
119905
119890119860(119904minus119905)
119885 (119904) 119889119882 (119904) 0 le 119905 le 119879
(15)
has a unique solution In order to get this conclusion wedefine
119891(119910119911)
(119904 120583 ]) = E1015840[119891 (119904 119910
1015840
(119904) 1199111015840
(119904) 120583 ])] (16)
Then (15) can be rewritten as
119884 (119905) = 119890119860(119879minus119905)
120585
+ int
119879
119905
119890119860(119904minus119905)
119891(119910119911)
(119884 (119904) 119885 (119904)) 119889119904
minus int
119879
119905
119890119860(119904minus119905)
119885 (119904) 119889119882 (119904)
(17)
Due to (A1) for all (1205831 ]
1) (120583
2 ]
2) isin 119867timesL(Γ119867)119891 satisfies
10038161003816100381610038161003816119891(119910119911)
(1205831 ]
1) minus 119891
(119910119911)(120583
2 ]
2)10038161003816100381610038161003816
2
le 119871 (10038161003816100381610038161205831 minus 1205832
10038161003816100381610038162
+1003816100381610038161003816]1 minus ]
2
10038161003816100381610038162
)
(18)
According to Theorem 31 in [1] BSEE (15) has a uniquesolution
Step 2 FromStep 1 we can define amappingΦ (119884(sdot) 119885(sdot)) =
Φ[(1199101015840(sdot) 119911
1015840(sdot))] K[0 119879] rarr K[0 119879] through
119884 (119905) = 119890119860(119879minus119905)
120585
+ int
119879
119905
E1015840[119890
119860(119904minus119905)119891 (119904 119910
1015840
(119904) 1199111015840
(119904) 119884 (119904) 119885 (119904))] 119889119904
minus int
119879
119905
119890119860(119904minus119905)
119885 (119904) 119889119882 (119904) 0 le 119905 le 119879
(19)
For any (119910119894 119911119894) isin K[0 119879] we set (119884119894 119885
119894) = Φ[(119910
119894 119911
119894)] 119894 =
1 2 (119910 119911) = (1199101minus119910
2 119911
1minus119911
2) and (119884 119885) = (119884
1minus119884
2 119885
1minus119885
2)
Then from Lemma 1 we have
E sup0le119904le119879
119890212057311990410038161003816100381610038161003816
119884 (119904)10038161003816100381610038161003816
2
+ Eint119879
0
119890212057311990410038161003816100381610038161003816
119885 (119904)10038161003816100381610038161003816
2
119889119904
le12119872
2
119860
120573E
Mathematical Problems in Engineering 5
times [int
119879
0
1198902120573119904
100381610038161003816100381610038161003816E1015840[119891 (119904 (119910
1
(119904))1015840
(1199111
(119904))1015840
1198841
(119904) 1198851
(119904))
minus 119891 (119904 (1199102
(119904))1015840
(1199112
(119904))1015840
1198842
(119904) 1198852
(119904))]100381610038161003816100381610038161003816
2
119889119904]
le12119872
2
1198601198712
120573E
times [int
119879
0
1198902120573119904
(E [1003816100381610038161003816119910 (119904)
10038161003816100381610038162
+ |119911 (119904)|2]
+10038161003816100381610038161003816119884 (119904)
10038161003816100381610038161003816
2
+10038161003816100381610038161003816119885 (119904)
10038161003816100381610038161003816
2
) 119889119904]
=12119872
2
1198601198712
120573E
times [int
119879
0
1198902120573119904
(1003816100381610038161003816119910 (119904)
10038161003816100381610038162
+ |119911 (119904)|2
+10038161003816100381610038161003816119884 (119904)
10038161003816100381610038161003816
2
+10038161003816100381610038161003816119885 (119904)
10038161003816100381610038161003816
2
) 119889119904]
(20)
If we set 120573 = 361198722
1198601198712max119879 1 then
Eint119879
0
1198902120573119904
(10038161003816100381610038161003816119884 (119904)
10038161003816100381610038161003816
2
+10038161003816100381610038161003816119885 (119904)
10038161003816100381610038161003816
2
) 119889119904
le 119879 sdot E sup0le119904le119879
119890212057311990410038161003816100381610038161003816
119884 (119904)10038161003816100381610038161003816
2
+ Eint119879
0
119890212057311990410038161003816100381610038161003816
119885 (119904)10038161003816100381610038161003816
2
119889119904
le12119872
2
1198601198712max 119879 1120573
E
times [int
119879
0
1198902120573119904
(1003816100381610038161003816119910 (119904)
10038161003816100381610038162
+ |119911 (119904)|2
+10038161003816100381610038161003816119884 (119904)
10038161003816100381610038161003816
2
+10038161003816100381610038161003816119885 (119904)
10038161003816100381610038161003816
2
) 119889119904]
=1
3E [int
119879
0
1198902120573119904
(1003816100381610038161003816119910 (119904)
10038161003816100381610038162
+ |119911 (119904)|2
+10038161003816100381610038161003816119884 (119904)
10038161003816100381610038161003816
2
+10038161003816100381610038161003816119885 (119904)
10038161003816100381610038161003816
2
) 119889119904]
(21)
That is
Eint119879
0
1198902120573119904
(10038161003816100381610038161003816119884 (119904)
10038161003816100381610038161003816
2
+10038161003816100381610038161003816119885 (119904)
10038161003816100381610038161003816
2
) 119889119904
le1
2Eint
119879
0
1198902120573119904
(1003816100381610038161003816119910 (119904)
10038161003816100381610038162
+ |119911 (119904)|2) 119889119904
(22)
The estimate (22) shows that Φ is a contraction on thespaceK
120573[0 119879] with the norm
(119884 119885)2
120573= Eint
119879
0
1198902120573119904
(|119884 (119904)|2+ |119885 (119904)|
2) 119889119904 (23)
With the contraction mapping theorem there admits aunique fixed point (119884 119885) isin K
120573[0 119879] such that Φ(119884 119885) =
(119884 119885) On the other hand from Step 1 we know thatif Φ(119884 119885) = (119884 119885) then (119884 119885) isin S2
F ([0 119879]119867) times
H2
F ([0 119879]L(Γ119867)) which is the unique mild solution of(11)
Arguing as the previous proof we arrive at the followingassertion in a straightforward way
Corollary 5 Suppose that for all 120572 in a metric space 119865 119891120572is
a given function satisfying (A1) and (A2) with 119871 independenton 120572 Also suppose that
E1015840[119891
120572(119904 119884
1015840
(119904) 1198851015840
(119904) 119884 (119904) 119885 (119904))]
997888rarr E1015840[119891
1205720(119904 119884
1015840
(119904) 1198851015840
(119904) 119884 (119904) 119885 (119904))]
(24)
in 1198712([0 119879]119867) as 120572 rarr 1205720for all (119884 119885) isin S2
F ([0 119879]119867) times
H2
F ([0 119879]L(Γ119867))If we denote by (119884(120585 120572) 119885(120585 120572)) the mild solution of (11)
corresponding to the functions 119891120572and to the final data 120585 isin
1198712(ΩF
119879P 119867) then the map (120572 120585) rarr (119884(120585 120572) 119885(120585 120572))
is continuous from 119865 times 1198712(ΩF
119879P 119867) to S2
F ([0 119879]119867) times
H2
F ([0 119879]L(Γ119867))
32 Non-Lipschitz Case This subsection is devoted to findingsome weaker conditions than the Lipschitz one under whichthe mean-field BSEE has a unique solution To state our mainresult in this section we suppose the following
(A3) For all 119905 isin [0 119879] 1199101015840119894 119910
119894isin 119867 1199111015840
119894 119911
119894isin L(Γ119867) (119894 =
1 2) there exists an 119897 gt 0 such that
10038161003816100381610038161003816119891 (119905 119910
1015840
1 119911
1015840
1 119910
1 119911
1) minus 119891 (119905 119910
1015840
2 119911
1015840
2 119910
2 119911
2)10038161003816100381610038161003816
2
le 120579 (100381610038161003816100381610038161199101015840
1minus 119910
1015840
2
10038161003816100381610038161003816
2
) + 120579 (10038161003816100381610038161199101 minus 1199102
10038161003816100381610038162
)
+ 119897 (100381610038161003816100381610038161199111015840
1minus 119911
1015840
2
10038161003816100381610038161003816
2
+10038161003816100381610038161199111 minus 1199112
10038161003816100381610038162
)
(25)
where 120579 R+rarr R+ is a concave increasing function such
that 120579(0) = 0 120579(119906) gt 0 for 119906 gt 0 and int0+(119889119906120579(119906)) = infin
InMao [5] the author gave three examples of the function120579(sdot) to show the generality of condition (A3) From theseexamples we can see that Lipschitz condition (A1) is a specialcase of the given condition (A3)
Since 120579 is concave and 120579(0) = 0 there exists a pair ofpositive constants 119886 and 119887 such that
120579 (119906) le 119886 + 119887119906 (26)
for all 119906 ge 0 Therefore under assumptions (A2) and(A3) 119891(sdot 1199101015840(sdot) 1199111015840(sdot) 119910(sdot) 119911(sdot)) isin H2
F ([0 119879]119867) whenever
6 Mathematical Problems in Engineering
1199101015840(sdot) 119910(sdot) isin S2
F ([0 119879]119867) and 1199111015840(sdot) 119911(sdot) isin H2
F ([0 119879]L(Γ119867))
By Picard-type iteration we now construct an approxi-mate sequence using which we obtain the desired result Let1198840(119905) equiv 0 and for 119899 isin N let 119884
119899 119885
119899 be a sequence in
S2
F ([0 119879]119867) timesH2
F ([0 119879]L(Γ119867)) defined recursively by
119884119899(119905) = 119890
119860(119879minus119905)120585
+ int
119879
119905
E1015840[119890
119860(119904minus119905)
times119891 (119904 1198841015840
119899minus1(119904) 119885
1015840
119899(119904) 119884
119899minus1(119904) 119885
119899(119904))] 119889119904
minus int
119879
119905
119890119860(119904minus119905)
119885119899(119904) 119889119882 (119904)
(27)
on 0 le 119905 le 119879 From Theorem 4 (27) has a unique mildsolution (119884
119899(119905) 119885
119899(119905))
In order to give the main result we need to prepare thefollowing lemmas about the properties of (119884
119899(119905) 119885
119899(119905)) 119905 isin
[0 119879]
Lemma 6 Under hypotheses (A2) and (A3) there existpositive constants 119862
1and 119862
2such that
(i)E( sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899 (119904)
10038161003816100381610038162
) le 21198621exp (119879 minus 119905)
(ii)Eint119879
119905
11989021205731199041003816100381610038161003816119885119899
(119904)10038161003816100381610038162
119889119904 le 21198621exp (119879 minus 119905)
(iii)E( sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899+1 (119904) minus 119884119899 (119904)
10038161003816100381610038162
)
le 1198622int
119879
119905
120579(E sup119904le119903le119879
11989021205731199031003816100381610038161003816119884119899 (119903) minus 119884119899minus1 (119903)
10038161003816100381610038162
)119889119904
(28)
for all 119905 isin [0 119879] and 119899 ge 1
Proof Using the hypotheses (A2) and (A3)with 120579(119906) le 119886 + 119887119906
yields
10038161003816100381610038161003816119891 (119904 119884
1015840
119899minus1(119904) 119885
1015840
119899(119904) 119884
119899minus1(119904) 119885
119899(119904))
10038161003816100381610038161003816
2
le 2120579 (100381610038161003816100381610038161198841015840
119899minus1(119904)10038161003816100381610038161003816
2
) + 2120579 (1003816100381610038161003816119884119899minus1 (119904)
10038161003816100381610038162
)
+ 2119897 (100381610038161003816100381610038161198851015840
119899(119904)10038161003816100381610038161003816
2
+1003816100381610038161003816119885119899
(119904)10038161003816100381610038162
) + 21003816100381610038161003816119891 (119904 0 0 0 0)
10038161003816100381610038162
le 4119886 + 2119887100381610038161003816100381610038161198841015840
119899minus1(119904)10038161003816100381610038161003816
2
+ 21198871003816100381610038161003816119884119899minus1 (119904)
10038161003816100381610038162
+ 2119897 (100381610038161003816100381610038161198851015840
119899(119904)10038161003816100381610038161003816
2
+1003816100381610038161003816119885119899
(119904)10038161003816100381610038162
) + 21003816100381610038161003816119891 (119904 0 0 0 0)
10038161003816100381610038162
(29)
Then it follows from Lemma 1 that
E( sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899 (119904)
10038161003816100381610038162
) + Eint119879
119905
11989021205731199041003816100381610038161003816119885119899
(119904)10038161003816100381610038162
119889119904
le 241198722
1198601198902120573119879
E100381610038161003816100381612058510038161003816100381610038162
+12119872
2
119860
120573E
times int
119879
119905
1198902120573119904 10038161003816100381610038161003816
E1015840[119891 (119904 119884
1015840
119899minus1(119904)
1198851015840
119899(119904) 119884
119899minus1(119904) 119885
119899(119904))]
10038161003816100381610038161003816
2
119889119904
le 241198722
1198601198902120573119879
E100381610038161003816100381612058510038161003816100381610038162
+12119872
2
119860
120573E
times int
119879
119905
E1015840[119890
2120573119904 10038161003816100381610038161003816119891 (119904 119884
1015840
119899minus1(119904)
1198851015840
119899(119904) 119884
119899minus1(119904) 119885
119899(119904))
10038161003816100381610038161003816
2
] 119889119904
le 1198621+24119872
2
119860
120573E
times int
119879
119905
E1015840[119890
2120573119904[119887100381610038161003816100381610038161198841015840
119899minus1(119904)10038161003816100381610038161003816
2
+ 1198871003816100381610038161003816119884119899minus1 (119904)
10038161003816100381610038162
+ 119897100381610038161003816100381610038161198851015840
119899(119904)10038161003816100381610038161003816
2
+ 1198971003816100381610038161003816119885119899
(119904)10038161003816100381610038162
]] 119889119904
= 1198621+48119872
2
119860
120573E
times int
119879
119905
1198902120573119904
[1198871003816100381610038161003816119884119899minus1 (119904)
10038161003816100381610038162
+ 1198971003816100381610038161003816119885119899
(119904)10038161003816100381610038162
] 119889119904
(30)
where
1198621= 24119872
2
1198601198902120573119879
E100381610038161003816100381612058510038161003816100381610038162
+24119872
2
119860
120573Eint
119879
119905
1198902120573119904
[2119886 +1003816100381610038161003816119891 (119904 0 0 0 0)
10038161003816100381610038162
] 119889119904 + 1
(31)
If we set 120573 = 961198722
119860max119887 119897 we can obtain
sup119899isinN
E( sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899 (119904)
10038161003816100381610038162
) +1
2int
119879
119905
sup119899isinN
E [11989021205731199041003816100381610038161003816119885119899
(119904)10038161003816100381610038162
] 119889119904
le 1198621+1
2int
119879
119905
sup119899isinN
E [11989021205731199041003816100381610038161003816119884119899minus1 (119904)
10038161003816100381610038162
] 119889119904
le 1198621+1
2int
119879
119905
sup119899isinN
E[ sup119904le119903le119879
11989021205731199031003816100381610038161003816119884119899minus1 (119903)
10038161003816100381610038162
]119889119904
(32)
An application of the Gronwall inequality now implies
sup119899isinN
E( sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899 (119904)
10038161003816100381610038162
) le 21198621exp (119879 minus 119905
2)
le 21198621exp (119879 minus 119905)
(33)
Point (i) of Lemma 6 is now proved
Mathematical Problems in Engineering 7
From formula (32) we know that
int
119879
119905
sup119899isinN
E [11989021205731199041003816100381610038161003816119885119899
(119904)10038161003816100381610038162
] 119889119904
le 21198621+ int
119879
119905
sup119899isinN
E[ sup119904le119903le119879
11989021205731199031003816100381610038161003816119884119899minus1 (119903)
10038161003816100381610038162
]119889119904
le 21198621+ 2119862
1int
119879
119905
exp (119879 minus 119904) 119889119904
= 21198621exp (119879 minus 119905)
(34)
This proves point (ii) of the LemmaTo prove point (iii) we note that
E1015840[10038161003816100381610038161003816119891 (119904 119884
1015840
119899(119904) 119885
1015840
119899+1(119904) 119884
119899(119904) 119885
119899+1(119904))
minus 119891 (119904 1198841015840
119899minus1(119904) 119885
1015840
119899(119904) 119884
119899minus1(119904) 119885
119899(119904))
10038161003816100381610038161003816
2
]
le E1015840[120579 (
100381610038161003816100381610038161198841015840
119899(119904) minus 119884
1015840
119899minus1(119904)10038161003816100381610038161003816
2
) + 119897100381610038161003816100381610038161198851015840
119899+1(119904) minus 119885
1015840
119899(119904)10038161003816100381610038161003816
2
]
+ 120579 (1003816100381610038161003816119884119899 (119904) minus 119884119899minus1 (119904)
10038161003816100381610038162
) + 1198971003816100381610038161003816119885119899+1
(119904) minus 119885119899(119904)10038161003816100381610038162
= E [120579 (1003816100381610038161003816119884119899 (119904) minus 119884119899minus1 (119904)
10038161003816100381610038162
) + 1198971003816100381610038161003816119885119899+1
(119904) minus 119885119899(119904)10038161003816100381610038162
]
+ 120579 (1003816100381610038161003816119884119899 (119904) minus 119884119899minus1 (119904)
10038161003816100381610038162
) + 1198971003816100381610038161003816119885119899+1
(119904) minus 119885119899(119904)10038161003816100381610038162
(35)
By Lemma 1 we have
E( sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899+1 (119904) minus 119884119899 (119904)
10038161003816100381610038162
)
+ Eint119879
119905
11989021205731199041003816100381610038161003816119885119899+1
(119904) minus 119885119899(119904)10038161003816100381610038162
119889119904
le12119872
2
119860
120573E
times int
119879
119905
1198902120573119904 10038161003816100381610038161003816
E1015840[119891 (119904 119884
1015840
119899(119904) 119885
1015840
119899+1(119904) 119884
119899(119904) 119885
119899+1(119904))
minus 119891 (119904 1198841015840
119899minus1(119904) 119885
1015840
119899(119904)
119884119899minus1
(119904) 119885119899(119904))]
10038161003816100381610038161003816
2
119889119904
le12119872
2
119860
120573E
times int
119879
119905
1198902120573119904
E1015840[10038161003816100381610038161003816119891 (119904 119884
1015840
119899(119904) 119885
1015840
119899+1(119904) 119884
119899(119904) 119885
119899+1(119904))
minus 119891 (119904 1198841015840
119899minus1(119904) 119885
1015840
119899(119904)
119884119899minus1
(119904) 119885119899(119904))
10038161003816100381610038161003816
2
] 119889119904
le24119872
2
119860
120573Eint
119879
119905
1198902120573119904
[120579 (1003816100381610038161003816119884119899 (119904) minus 119884119899minus1 (119904)
10038161003816100381610038162
)
+ 1198971003816100381610038161003816119885119899+1
(119904) minus 119885119899(119904)10038161003816100381610038162
] 119889119904
(36)
We can choose 120573 gt 0 sufficiently large such that
(1 minus24119872
2
119860119897
120573)Eint
119879
119905
11989021205731199041003817100381710038171003817119885119899+1
(119904) minus 119885119899(119904)10038171003817100381710038172
119889119904 ge 0 (37)
Then
E( sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899+1 (119904) minus 119884119899 (119904)
10038161003816100381610038162
)
le24119872
2
119860
120573Eint
119879
119905
1198902120573119904120579 (1003816100381610038161003816119884119899 (119904) minus 119884119899minus1 (119904)
10038161003816100381610038162
) 119889119904
le 1198622Eint
119879
119905
120579(E sup119904le119903le119879
11989021205731199031003816100381610038161003816119884119899 (119903) minus 119884119899minus1 (119903)
10038161003816100381610038162
)119889119904
(38)
where we set 1198622= (24119872
2
119860120573)119890
2120573119879
We divide the interval [0 119879] into subintervals 0 = 1205910lt
1205911lt sdot sdot sdot lt 120591
119898= 119879 by setting 120591
119896= 119896120575 119896 = 1 2 3 119898 with
120575 = 119879119898
Lemma 7 For all 119905 isin [120591119896minus1
120591119896] define
1198623= 119862
2120579 (2119862
1exp (119879))
1205931198961(119905) = 119862
3(120591119896minus 119905)
120593119896119899+1
(119905) = 1198622int
120591119896
119905
120579 (120593119896119899(119904)) 119889119904 119899 ge 1
(39)
Then for all 119899 ge 1 the following inequality holds for a suitable120575 gt 0
0 le 120593119896119899(119905) le 120593
119896119899minus1(119905) le sdot sdot sdot le 120593
1198961(119905) (40)
Proof Firstly it needs to be verified that for all 119905 isin [120591119896minus1
120591119896]
the following inequality
1205931198962(119905) = 119862
2int
120591119896
119905
120579 (1205931198961(119904)) 119889119904 = 119862
2int
120591119896
119905
120579 (1198623(120591119896minus 119904)) 119889119904
le 1198623(120591119896minus 119905) = 120593
1198961(119905)
(41)
holds provided 120575 gt 0 is chosen sufficiently smallActually this inequality holds provided that
1198622120579 (119862
3(120591119896minus 119905)) le 119862
3= 119862
2120579 (2119862
1exp (119879)) (42)
8 Mathematical Problems in Engineering
or
1198623(120591119896minus 119905) = 119862
2120579 (2119862
1exp (119879)) (120591
119896minus 119905) le 2119862
1exp (119879)
(43)
Since 1198621gt 1 from 120579(119906) le 119886 + 119887119906 the above inequality holds
if
1198622(119886 + 119887) (120591
119896minus 119905) le 1 (44)
Thus (41) holds for any 119905 isin [120591119896minus1
120591119896] 119896 = 1 2 119898 if 120591
119896minus
120591119896minus1
le 11198622(119886 + 119887) Therefore we can choose a sufficiently
large119898 isin N such that 120575 = 119879119898 le 11198622(119886 + 119887) Clearly such a
120575 only depends on 119886 119887 119897 119879 and119872119860
Now assume that (40) holds for some 119899 ge 2 Then wehave
120593119896119899+1
(119905) = 1198622int
120591119896
119905
120579 (120593119896119899(119904)) 119889119904 le 119862
2int
120591119896
119905
120579 (120593119896119899minus1
(119904)) 119889119904
= 120593119896119899(119905) forall119905 isin [120591
119896minus1 120591
119896]
(45)
This completes the proof
Now we can give the main result of this section
Theorem 8 Assume that (A2) and (A3) hold Then thereexists a unique mild solution (119884 119885) to (11)
Proof Consider the following
Uniqueness To show the uniqueness let both (119884 119885) and( 119885) be solutions of (11) For any 120573 gt 0 similar to the proofof (36) one can obtain
E( sup119905le119904le119879
119890212057311990410038161003816100381610038161003816
119884 (119904) minus (119904)10038161003816100381610038161003816
2
) + Eint119879
119905
119890212057311990410038161003816100381610038161003816
119885 (119904) minus 119885 (119904)10038161003816100381610038161003816
2
119889119904
le24119872
2
119860
120573E
times int
119879
119905
1198902120573119904
[120579 (10038161003816100381610038161003816119884 (119904) minus (119904)
10038161003816100381610038161003816
2
) + 11989710038161003816100381610038161003816119885 (119904) minus 119885 (119904)
10038161003816100381610038161003816
2
] 119889119904
(46)
That is if 120573 is sufficiently large
E( sup119905le119904le119879
119890212057311990410038161003816100381610038161003816
119884 (119904) minus (119904)10038161003816100381610038161003816
2
)
le24119872
2
119860
120573Eint
119879
119905
1198902120573119904
[120579 (10038161003816100381610038161003816119884 (119904) minus (119904)
10038161003816100381610038161003816
2
)] 119889119904
le 1198622Eint
119879
119905
120579(E sup119904le119903le119879
119890212057311990310038161003816100381610038161003816
119884 (119904) minus (119904)10038161003816100381610038161003816
2
)119889119904
(47)
An application of Bihari inequality yields
E( sup119905le119904le119879
119890212057311990410038161003816100381610038161003816
119884 (119904) minus (119904)10038161003816100381610038161003816
2
) = 0 (48)
So 119884(119905) = (119905) for all 119905 isin [0 119879] as It then follows from (46)that 119885(119905) = 119885(119905) for all 119905 isin [0 119879] as as well This establishesthe uniqueness
ExistenceWe claim that the sequence (119884119899 119885
119899) defined by (27)
satisfies
E sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899+1 (119904) minus 119884119899 (119904)
10038161003816100381610038162
997888rarr 0 forall0 le 119905 le 119879 (49)
as 119899 rarr infinIndeed for all 119905 isin [120591
119896minus1 120591
119896] we set 120593
119896119899(119905) =
E sup119904isin[119905120591119896]
1198902120573119904|119884119899+1
(119904) minus 119884119899(119904)|
2 By Lemmas 6 and 7
1205931198961(119905) = E sup
119904isin[119905120591119896]
119890212057311990410038161003816100381610038161198842 (119904) minus 1198841 (119904)
10038161003816100381610038162
le 1198622int
120591119896
119905
120579(E sup119904le119903le120591119896
119890212057311990310038161003816100381610038161198841 (119903) minus 1198840 (119903)
10038161003816100381610038162
)119889119904
le 1198622int
120591119896
119905
120579 (21198621exp (120591
119896minus 119905)) 119889119904
le 1198622120579 (2119862
1exp (119879)) (120591
119896minus 119905) = 119862
3(120591119896minus 119905) = 120593
1198961(119905)
(50)
Suppose that 120593119896119899(119905) le 120593
119896119899(119905) holds for some 119899 ge 1
According to Lemma 6(iii) and Lemma 7 for all 119905 isin [120591119896minus1
120591119896]
we obtain
120593119896119899+1
(119905) = E sup119904isin[119905120591119896]
11989021205731199041003816100381610038161003816119884119899+2 (119904) minus 119884119899+1 (119904)
10038161003816100381610038162
le 1198622int
120591119896
119905
120579(E sup119903isin[119904120591119896]
11989021205731199031003816100381610038161003816119884119899+1 (119903) minus 119884119899 (119903)
10038161003816100381610038162
)119889119904
= 1198622int
120591119896
119905
120579 (120593119896119899(119904)) 119889119904
le 1198622int
120591119896
119905
120579 (120593119896119899(119904)) 119889119904 = 120593
119896119899+1(119905)
(51)
This implies that for all 119899 isin N
120593119896119899(119905) le 120593
119896119899(119905) (52)
By definition 120593119896119899(sdot) is continuous on [120591
119896minus1 120591
119896] Note that
for each 119899 ge 1 120593119896119899(sdot) is decreasing on [120591
119896minus1 120591
119896] and for each
119905 119896 120593119896119899(119905) is a nonincreasing sequence Therefore we define
the function 120593119896(119905) by 120593
119896119899(119905) darr 120593
119896(119905) It is easy to verify that
120593119896(119905) is continuous and nonincreasing on [120591
119896minus1 120591
119896] By the
definitions of 120593119896119899(119905) and 120593
119896(119905) we get
120593119896(119905) = lim
119899rarrinfin
1198622int
120591119896
119905
120579 (120593119896119899(119904)) 119889119904 = 119862
2int
120591119896
119905
120579 (120593119896(119904)) 119889119904
(53)
for all 120591119896minus1
le 119905 le 120591119896 Since int
0+(119889119906120579(119906)) = infin the Bihari
inequality implies
120593119896(119905) = 0 for each 119905 isin [120591
119896minus1 120591
119896] (54)
For each 119896 isin 1 2 119898 (52) and (54) yield
lim119899rarrinfin
120593119896119899(119905) = 0 (55)
Mathematical Problems in Engineering 9
Then
E sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899+1 (119904) minus 119884119899 (119904)
10038161003816100381610038162
le max1le119896le119898
E sup119904isin[120591119896minus1120591119896]
11989021205731199041003816100381610038161003816119884119899+1 (119904) minus 119884119899 (119904)
10038161003816100381610038162
= max1le119896le119898
120593119896119899(119905) 997888rarr 0
(56)
as 119899 rarr infin and this proves the assertion (49)By (36) we obtain
E( sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899+1 (119904) minus 119884119899 (119904)
10038161003816100381610038162
) + (1 minus24119872
2
119860119897
120573)E
times int
119879
119905
11989021205731199041003816100381610038161003816119885119899+1
(119904) minus 119885119899(119904)10038161003816100381610038162
119889119904
le24119872
2
119860
120573Eint
119879
119905
1198902120573119904
[120579 (1003816100381610038161003816119884119899 (119904) minus 119884119899minus1 (119904)
10038161003816100381610038162
)] 119889119904
(57)
Applying (49) to the above formula we see that (119884119899 119885
119899) is
a Cauchy (hence convergent) sequence in S2
F ([0 119879]119867) times
H2
F ([0 119879]L(Γ119867)) denote the limit by (119884 119885) Now letting119899 rarr infin in (27) we obtain that
119884 (119905) = 119890119860(119879minus119905)
120585
+ int
119879
119905
E1015840[119890
119860(119904minus119905)119891 (119904 119884
1015840
(119904) 1198851015840
(119904) 119884 (119904) 119885 (119904))] 119889119904
minus int
119879
119905
119890119860(119904minus119905)
119885 (119904) 119889119882 (119904)
(58)
holds on the entire interval [0 119879]The theorem is nowproved
To illustrate the application of the obtained existenceand uniqueness result we consider the example of backwardstochastic partial differential equations (BSPDEs) of mean-field type
Example 9 Let O be an open bounded domain in R119899
with uniformly 1198622 boundary 120597O let 119861(119905) be a standard 119899-dimensional Brownian motion (equipped with the normalfiltration) and let 120585 O rarr R be an F
119879-measurable
random variable We also let 119871 denote the semielliptic partialdifferential operator on 1198622
(R) of the form
119871 =
119899
sum
119894119895=1
119886119894119895(119909)
1205972
120597119909119894120597119909
119895
+
119899
sum
119894=1
119887119894(119909)
120597
120597119909119894
(59)
The aim is to study the solvability of the following initialboundary value problem
119889119884 (119905 119909)
= (119871119884 (119905 119909) + E1015840
times [119892 (119905 119909 1198841015840
(119905 119909) 1198851015840
(119905 119909) 119884 (119905 119909) 119885 (119905 119909))]) 119889119905
+ 119885 (119905 119909) 119889119861 (119905) ae on (0 119879) times O
119884 (119905 119909) = 0 ae on (0 119879) times 120597O
119884 (119879 119909) = 120585 (119879 119909) ae onO(60)
where
119884 [0 119879] times O 997888rarr R
119885 [0 119879] times O 997888rarr L (R119899 119871
2
(O))
119892 [0 119879] times O timesR timesL (R119899 119871
2
(O))
timesR timesL (R119899 119871
2
(O)) 997888rarr R
(61)
The following assumptions will have to be in force
(H1) 119886119894119895 119887
119894 R119899
rarr R are uniformly continuous andbounded and satisfy the usual uniform ellipticitycondition sum119899
119894119895=1119886119894119895(119909)119908
119894119908119895ge 120582|119908|
2 for some 120582 gt 0
and all 119909 isin O 119908 isin R119899(H2) 119892 is measurable in (119905 119909 119910 119910 119911) and continuous in
( 119911) and there exists 119862 gt 0 such that1003816100381610038161003816119892 (119905 119909 1199101 1 1199101 1199111) minus 119892 (119905 119909 1199102 2 1199102 1199112)
1003816100381610038161003816
le 119862 [10038161003816100381610038161199101 minus 1199102
1003816100381610038161003816 +10038161003816100381610038161 minus 2
1003816100381610038161003816 +10038161003816100381610038161199101 minus 1199102
1003816100381610038161003816 +10038161003816100381610038161199111 minus 1199112
1003816100381610038161003816]
(62)
for all 0 le 119905 le 119879 119909 isin O 1199101 119910
2 119910
1 119910
2isin R
1
2 119911
1 119911
2isin
L(R119899 119871
2(O))
Then we are now in a position of showing existence anduniqueness of the solution of BSPDEs (60)
Theorem 10 If (H1) and (H2) are satisfied then the mean-field BSPDE (60) has a unique mild solution (119884 119885) isin
1198712(0 119879 119871
2(Ω 119871
2(O))) times 1198712F (0 119879 119871
2(R119899
1198712(Ω 119871
2(O))))
Proof Let119867 = 1198712(O) and119870 = R119899 Define the operator 119860 by
119860119884 (119905 sdot) = 119871119884 (119905 sdot) (63)
It is shown in [17] (see Example 21 in [17]) that 119860 generatesa strongly continuous semigroup on 119867 Define the maps 119891
[0 119879] times 119867 timesL(119870119867) times 119867 timesL(119870119867) rarr 119867 by
119891 (119905 1198841015840
(119905) 1198851015840
(119905) 119884 (119905) 119885 (119905)) (119909)
= 119892 (119905 119909 1198841015840
(119905 119909) 1198851015840
(119905 119909) 119884 (119905 119909) 119885 (119905 119909))
(64)
10 Mathematical Problems in Engineering
for all 0 le 119905 le 119879 119909 isin O With these identifications(60) can be written in the form of (11) By (H2) we know119891 satisfy condition (A1) Hence an application of Theorem 4concludes that (60) has a unique mild solution (119884 119885) isin
1198712(0 119879 119871
2(Ω 119871
2(O))) times 1198712F (0 119879 119871
2(R119899
1198712(Ω 119871
2(O))))
4 Mean-Field Stochastic Evolution Equations
Let 119882(119905) 119905 isin [0 119879] be a cylindrical Wiener process withvalues in a Hilbert space Γ defined on a probability space(ΩFP) We fix an interval [119905 119879] sub [0 119879] and considerthe stochastic evolution equations of mean-field type for anunknown process 119883(119904) 119904 isin [119905 119879] with values in a Hilbertspace 119870
119889119883 (119904) = 119861119883 (119904) 119889119904 + E1015840[119887 (119904 119883
1015840
(119904) 119883 (119904))] 119889119904
+ E1015840[120590 (119904 119883
1015840
(119904) 119883 (119904))] 119889119882 (119904)
119883 (119905) = 119909 isin 119870
(65)
where operator 119861 is the generator of a strongly continuoussemigroup 119890
119905119861 119905 ge 0 in the Hilbert space 119870 with 119872119861≜
sup119905isin[0119879]
|119890119905119861|
By a mild solution of (65) we mean an F119904-measurable
process 119883(119904) 119904 isin [119905 119879] with continuous paths in 119870 suchthat P-as
119883 (119904) = 119890119861(119904minus119905)
119909
+ int
119904
119905
119890119861(119904minus120591)
E1015840[119887 (120591 119883
1015840
(120591) 119883 (120591))] 119889120591
+ int
119904
119905
119890119861(119904minus120591)
E1015840[120590 (120591119883
1015840
(120591) 119883 (120591))] 119889119882 (120591)
119904 isin [119905 119879]
(66)
We suppose the following(A4) 119887 [0 119879] times 119870 times 119870 rarr 119870 is a measurable mapping
which satisfies10038161003816100381610038161003816119887 (119905 119909
1015840 119909) minus 119887 (119905 119910
1015840 119910)
10038161003816100381610038161003816
2
le 1198711(100381610038161003816100381610038161199091015840minus 119910
101584010038161003816100381610038161003816
2
+1003816100381610038161003816119909 minus 119910
10038161003816100381610038162
)
119905 isin [0 119879] 1199091015840 119909 119910
1015840 119910 isin 119870
(67)
for some constant 1198711gt 0
(A5) The mapping 120590 [0 119879] times 119870 times 119870 rarr L(Γ 119870) fulfillsthat for every V isin Γ the map 120590V [0 119879] times 119870 times 119870 rarr 119870
is measurable for every 119904 gt 0 119905 isin [0 119879] 1199091015840 1199101015840 119909 119910 isin 119870119890119904119861120590(119905 119909
1015840 119909) isin L(Γ 119870) and
10038161003816100381610038161003816119890119904119861120590 (119905 119909
1015840 119909)
10038161003816100381610038161003816L(Γ119870)le 119871
2119904minus120574(1 +
10038161003816100381610038161003816119909101584010038161003816100381610038161003816+ |119909|)
10038161003816100381610038161003816119890119904119861120590 (119905 119909
1015840 119909) minus 119890
119904119861120590 (119905 119910
1015840 119910)
10038161003816100381610038161003816L(Γ119870)
le 1198712119904minus120574(100381610038161003816100381610038161199091015840minus 119910
101584010038161003816100381610038161003816+1003816100381610038161003816119909 minus 119910
1003816100381610038161003816)
10038161003816100381610038161003816120590 (119905 119909
1015840 119909)
10038161003816100381610038161003816L(Γ119870)le 119871
2(1 +
10038161003816100381610038161003816119909101584010038161003816100381610038161003816+ |119909|)
(68)
for some constants 1198712gt 0 and 120574 isin [0 12)
Theorem 11 Under assumptions (A3) and (A4) (65) has aunique mild solution119883(sdot) isin S2
F ([119905 119879] 119870)
The proof is constructed in two steps like that ofTheorem 4 and it uses standard arguments for stochastic evo-lution equations introduced in the proof of Proposition 32 in[3] Since the proof is straightforward we prefer to omit it
Remark 12 In our paper Lipchitz condtion (A4) is givento get the well-posedness of mean-field stochastic evolutionequations In fact (A4) can be replaced by a weaker conditionsuch as (A3) We just give the condition (A4) for simplicity
From standard arguments we can also get the followingcontinuous dependence theorem
Corollary 13 Assume that for all 120572 in a metric space 119865(119887120572 120590
120572) satisfy (A4) and (A5)with119871
1and119871
2independent of 120572
Also assume that
Eint119879
0
10038161003816100381610038161003816E1015840[119887120572(119904 119883
1015840
(119904) 119883 (119904))]
minus E1015840[1198871205720(119904 119883
1015840
(119904) 119883 (119904))]10038161003816100381610038161003816
2
119889119904 997888rarr 0
Eint119879
0
10038161003816100381610038161003816E1015840[120590
120572(119904 119883
1015840
(119904) 119883 (119904))]
minusE1015840[120590
1205720(119904 119883
1015840
(119904) 119883 (119904))]10038161003816100381610038161003816
2
119889119904 997888rarr 0
(69)
as 120572 rarr 1205720for all119883 isin S2
F ([0 119879] 119870)If we denote by 119883120572
(sdot) the mild solution of mean-field SEE(65) corresponding to the functions (119887
120572 120590
120572) and to the initial
data 119909 then we have
sup119904isin[0119879]
E1003816100381610038161003816119883
120572
(119904) minus 1198831205720 (119904)
10038161003816100381610038162
997888rarr 0 119886119904 120572 997888rarr 1205720 (70)
5 Maximum Principle for BSPDEs ofMean-Field Type
51 Formulation of the Problem Let O isin R119899 be a boundedopen set with smooth boundary 120597O and let 119880 the space ofcontrols be a separable real Hilbert space We denote
U = V (sdot) isin 1198712F(0 119879 119880)
| V119905(120596
1015840 120596) [0 119879] times Ω times Ω
997888rarr 119880 is F otimesF119905-progressively measurable
(71)
Mathematical Problems in Engineering 11
An element ofU is called an admissible controlFor any V isin U we consider the following controlled
BSPDE system in the state space119867 = 1198712(O) (norm | sdot | scalar
product ⟨sdot sdot⟩)
119889119884119905(119909) = minus119860119884
119905(119909) 119889119905
minus E1015840[119891 (119905 119909 (119884
119905(119909))
1015840
(119885119905(119909))
1015840
119884119905(119909) 119885
119905(119909) V
119905)] 119889119905
+ 119885119905(119909) 119889119882 (119905) 119905 isin [0 119879]
119884119879(119909) = 120585 (119909) 119909 isin O
(72)
where 119860 is a partial differential operator 119891 [0 119879] timesO times119867 times
L(Γ119867)times119867timesL(Γ119867)times119880 rarr 119867 and 120585 isin 1198712(ΩF119879P 119867)
The cost functional is given by
119869 (V) = Eint119879
0
intO
E1015840[ℎ (119904 119909 (119884
119904(119909))
1015840
(119885119904(119909))
1015840
119884119904(119909) 119885
119904(119909) V
119904)] 119889119909 119889119904
+E1015840intO
119892 (119909 (1198840(119909))
1015840
1198840(119909)) 119889119909
(73)
where
ℎ [0 119879] times O times 119867 timesL (Γ119867) times 119867 timesL (Γ119867) times 119880 997888rarr R
119892 O times 119867 times119867 997888rarr R
(74)
Our purpose is to minimize the functional 119869(sdot) over Uadsubject to the following state constraint
EintO
Φ(119909 (1198840(119909))
1015840
1198840(119909)) 119889119909 = 0 (75)
where
Φ O times 119867 times119867 997888rarr R (76)
An admissible control 119906 isin Uad that satisfies
119869 (119906) = minVisinUad
119869 (V) (77)
is called optimalThrough what follows the following assumptions will be
in force
(L1) 119860 is a partial differential operator with appropri-ate boundary conditions We assume that 119860 is theinfinitesimal generator of a strongly continuous semi-group 119890119905119860 119905 ge 0 in119867 Moreover for every 119905 isin [0 119879]119890
119905119860119891
1198712(O) le 119872
119860119891
1198712(O) for some constant 119872
119860
independent of 119905 and 119891(L2) 119891 ℎ 119892 and Φ are continuously Gateaux differen-
tiable with respect to (1199101015840 1199111015840 119910 119911) 119891 is continuouslyGateaux differentiable with respect to V and ℎ iscontinuous with respect to V
(L3) The derivatives of 119891 ℎ 119892 and Φ are Lipschitzcontinuous and bounded by
100381610038161003816100381610038161198911199101015840
10038161003816100381610038161003816+10038161003816100381610038161198911199111015840
1003816100381610038161003816 +10038161003816100381610038161003816119891119910
10038161003816100381610038161003816+1003816100381610038161003816119891119911
1003816100381610038161003816 +1003816100381610038161003816119891V
1003816100381610038161003816 +10038161003816100381610038161003816Φ1199101015840
10038161003816100381610038161003816+10038161003816100381610038161003816Φ119910
10038161003816100381610038161003816le 119862
10038161003816100381610038161003816ℎ1199101015840
10038161003816100381610038161003816+1003816100381610038161003816ℎ1199111015840
1003816100381610038161003816 +10038161003816100381610038161003816ℎ119910
10038161003816100381610038161003816+1003816100381610038161003816ℎ119911
1003816100381610038161003816 +100381610038161003816100381610038161198921199101015840
10038161003816100381610038161003816+10038161003816100381610038161003816119892119910
10038161003816100381610038161003816
le 119862 (1 +10038161003816100381610038161003816119910101584010038161003816100381610038161003816+10038161003816100381610038161003816119911101584010038161003816100381610038161003816+10038161003816100381610038161199101003816100381610038161003816 + |119911| + |V|)
(78)
where 119862 is a positive constant
Obviously according to Theorem 4 state equation (72)has a unique mild solution under the above assumptions
Remark 14 We can define the second order differentialoperator
(119860119891) (119909) =
119899
sum
119894119895=1
119886119894119895(119909)
1205972119891
120597119909119894120597119909
119895
(119909) +
119899
sum
119894=1
119887119894(119909)
120597119891
120597119909119894
(119909) (79)
By Example 9 119860 fulfills assumption (L1) if 119886119894119895 119887
119894satisfy
condition (H1)
52 Variation of the Trajectory Let 119906 be an optimal controlwith (119884(sdot) 119885(sdot)) being the corresponding optimal state Let120576 gt 0 and [119903 119903 + 120576] sube [0 119879] For any given V isin Uad weintroduce the spike variation of the control 119906(sdot)
119906120576
119905=
V119905 119905 isin [119903 119903 + 120576]
119906119905 119905 isin [0 119879] [119904 119904 + 120576]
(80)
It is clear that 119906120576(sdot) isin UadLet (119884120576
(sdot) 119885120576(sdot)) be the trajectory corresponding to 119906120576(sdot)
We use the following short notation for brevity
119891 (119905) = 119891 (119905 119909 (119884119905(119909))
1015840
(119885119905(119909))
1015840
119884119905(119909) 119885
119905(119909) 119906
119905)
119891 (119906120576
119905) = 119891 (119905 119909 (119884
119905(119909))
1015840
(119885119905(119909))
1015840
119884119905(119909) 119885
119905(119909) 119906
120576
119905)
(81)
Consider the following equation
119889119870120576
119905(119909) = minus119860119870
120576
119905(119909) 119889119905
minus E1015840[119891
1199101015840 (119905) (119870
120576
119905(119909))
1015840
+ 119891119910(119905) 119870
120576
119905(119909)
+ 1198911199111015840 (119905) (119876
120576
119905(119909))
1015840
+ 119891119911(119905) 119876
120576
119905(119909)
+1
120576(119891 (119906
120576
119905) minus 119891 (119905))] 119889119905 + 119876
120576
119905(119909) 119889119882 (119905)
119870120576
119879(119909) = 0
(82)
Since the coefficients in (82) are bounded it is easy to checkthat there exists a unique mild solution such that
E[ sup119905isin[0119879]
1003816100381610038161003816119870120576
119905
10038161003816100381610038162
+ int
119879
0
1003816100381610038161003816119876120576
119905
10038161003816100381610038162
119889119905] lt infin (83)
We have the following estimate
12 Mathematical Problems in Engineering
Theorem 15 There holds
lim120576rarr0
E[ sup119904isin[119905119879]
1003816100381610038161003816100381610038161003816100381610038161003816
119884120576
119904minus 119884
119904
120576minus 119870
120576
119904
1003816100381610038161003816100381610038161003816100381610038161003816
2
] = 0 forall119905 isin [0 119879]
lim120576rarr0
Eint119879
0
1003816100381610038161003816100381610038161003816100381610038161003816
119885120576
119904minus 119885
119904
120576minus 119876
120576
119904
1003816100381610038161003816100381610038161003816100381610038161003816
2
119889119904 = 0
(84)
Proof We define
120578120576
119904=119884120576
119904minus 119884
119904
120576minus 119870
120576
119904 120577
120576
119904=119885120576
119904minus 119885
119904
120576minus 119876
120576
119904 119904 isin [0 119879]
(85)
For simplicity let us define
Λ120576
119904= ((119884
120576
119904)1015840
(119885120576
119904)1015840
119884120576
119904 119885
120576
119904)
119891 (119904 120582) = 119891 (119904 1198841015840
119904+ 120582(119884
120576
119904minus 119884
119904)1015840
1198851015840
119904+ 120582(119885
120576
119904minus 119885
119904)1015840
119884119904+ 120582 (119884
120576
119904minus 119884
119904)
119885119904+ 120582 (119885
120576
119904minus 119885
119904) 119906
120576
119904)
(86)
By the definition of (119884120576
119904 119885
120576
119904) (119884
119904 119885
119904) and (119870120576
119904 119876
120576
119904) (120578120576
119904 120577
120576
119904)
is the mild solution of
119889120578120576
119904= minus119860120578
120576
119904119889119904 minus E
1015840
[119871 (119904 120576)] 119889119904 + 120577120576
119904119889119882 (119904)
120578120576
119879= 0
(87)
with
119871 (119904 120576) =1
120576(119891 (119904 Λ
120576
119904 119906
120576
119904) minus 119891 (119906
120576
119904))
minus 1198911199101015840 (119904) (119870
120576
119904)1015840
minus 119891119910(119904) 119870
120576
119904minus 119891
1199111015840 (119904) (119876
120576
119904)1015840
minus 119891119911(119904) 119876
120576
119904
= ((120578120576
119904)1015840
+ (119870120576
119904)1015840
) int
1
0
1198911199101015840 (119904 120582) 119889120582 + (120578
120576
119904+ 119870
120576
119904)
times int
1
0
119891119910(119904 120582) 119889120582 minus 119891
1199101015840 (119904) (119870
120576
119904)1015840
minus 119891119910(119904) 119870
120576
119904
+ ((120577120576
119904)1015840
+ (119876120576
119904)1015840
)int
1
0
1198911199111015840 (119904 120582) 119889120582 + (120577
120576
119904+ 119876
120576
119904)
times int
1
0
119891119911(119904 120582) 119889120582 minus 119891
1199111015840 (119904) (119876
120576
119904)1015840
minus 119891119911(119904) 119876
120576
119904
= (120578120576
119904)1015840
int
1
0
1198911199101015840 (119904 120582) 119889120582 + 120578
120576
119904int
1
0
119891119910(119904 120582) 119889120582
+ (120577120576
119904)1015840
int
1
0
1198911199111015840 (119904 120582) 119889120582 + 120577
120576
119904int
1
0
119891119911(119904 120582) 119889120582 + 120574
120576
119904
(88)
where we denote
120574120576
119904= (119870
120576
119904)1015840
int
1
0
(1198911199101015840 (119904 120582) minus 119891
1199101015840 (119904)) 119889120582
+ 119870120576
119904int
1
0
(119891119910(119904 120582) minus 119891
119910(119904)) 119889120582
+ (119876120576
119904)1015840
int
1
0
(1198911199111015840 (119904 120582) minus 119891
1199111015840 (119904)) 119889120582
+ 119876120576
119904int
1
0
(119891119911(119904 120582) minus 119891
119911(119904)) 119889120582
(89)
For any 120573 gt 0 according to Lemma 1 we obtain
E sup119905le119904le119879
11989021205731199041003816100381610038161003816120578
120576
119904
10038161003816100381610038162
+ Eint119879
119905
11989021205731199041003816100381610038161003816120577
120576
119904
10038161003816100381610038162
119889119904
le12119872
2
119860
120573int
119879
119905
1198902120573119904
E [10038161003816100381610038161003816E1015840
[119871 (119904 120576)]10038161003816100381610038161003816
2
] 119889119904
le12119872
2
119860
120573Eint
119879
119905
1198902120573119904
E1015840[|119871 (119904 120576)|
2] 119889119904
(90)
By condition (L3) we have
E [E1015840[|119871 (119904 120576)|
2]]
= E [E1015840[1003816100381610038161003816119871 (119904 120576) minus 120574
120576
119904+ 120574
120576
119904
10038161003816100381610038162
]]
le 81198622E [E
1015840[10038161003816100381610038161003816(120578
120576
119904)101584010038161003816100381610038161003816
2
+1003816100381610038161003816120578120576
119904
10038161003816100381610038162
+10038161003816100381610038161003816(120577
120576
119904)101584010038161003816100381610038161003816
2
+1003816100381610038161003816120577120576
119904
10038161003816100381610038162
]]
+ 2E [E1015840[1003816100381610038161003816120574120576
119904
10038161003816100381610038162
]]
le 161198622E [
1003816100381610038161003816120578120576
119904
10038161003816100381610038162
+1003816100381610038161003816120577120576
119904
10038161003816100381610038162
] + 2E [E1015840[1003816100381610038161003816120574120576
119904
10038161003816100381610038162
]]
(91)
Combined with (91) (90) yields
E sup119905le119904le119879
11989021205731199041003816100381610038161003816120578
120576
119904
10038161003816100381610038162
+ (1 minus192119872
2
1198601198622
120573)Eint
119879
119905
11989021205731199041003816100381610038161003816120577
120576
119904
10038161003816100381610038162
119889119904
le192119872
2
1198601198622
120573Eint
119879
119905
11989021205731199041003816100381610038161003816120578
120576
119904
10038161003816100381610038162
119889119904
+24119872
2
119860
120573Eint
119879
119905
1198902120573119904
[E1015840[1003816100381610038161003816120574120576
119904
10038161003816100381610038162
]] 119889119904
(92)
We claim that
Eint119879
119905
E1015840[1003816100381610038161003816120574120576
119904
10038161003816100381610038162
] 119889119904 997888rarr 0 as 120576 997888rarr 0 (93)
From (89)
120574120576
119904= 119868
1
119904+ 119868
2
119904+ 119868
3
119904+ 119868
4
119904 (94)
Mathematical Problems in Engineering 13
where
1198681
119904= (119870
120576
119904)1015840
int
1
0
(1198911199101015840 (119904 120582) minus 119891
1199101015840 (119904)) 119889120582
1198682
119904= 119870
120576
119904int
1
0
(119891119910(119904 120582) minus 119891
119910(119904)) 119889120582
1198683
119904= (119876
120576
119904)1015840
int
1
0
(1198911199111015840 (119904 120582) minus 119891
1199111015840 (119904)) 119889120582
1198684
119904= 119876
120576
119904int
1
0
(119891119911(119904 120582) minus 119891
119911(119904)) 119889120582
(95)
Then
Eint119879
119905
E1015840[1003816100381610038161003816120574120576
119904
10038161003816100381610038162
] 119889119904
le 4Eint119879
119905
E1015840[100381610038161003816100381610038161198681
119904
10038161003816100381610038161003816
2
+100381610038161003816100381610038161198682
119904
10038161003816100381610038161003816
2
+100381610038161003816100381610038161198683
119904
10038161003816100381610038161003816
2
+100381610038161003816100381610038161198684
119904
10038161003816100381610038161003816
2
] 119889119904
(96)
Take Eint119879119905E1015840[|1198682
119904|2
]119889119904 for example
Eint119879
119905
E1015840[100381610038161003816100381610038161198682
119904
10038161003816100381610038161003816
2
] 119889119904
= Eint119879
119905
E1015840[(119870
120576
119904int
1
0
(119891119910(119904 120582) minus 119891
119910(119904)) 119889120582)
2
]119889119904
le Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119904 120582) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582] 119889119904
le 2Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119904 120582) minus 119891
119910(119906
120576
119905)10038161003816100381610038161003816
2
119889120582] 119889119904
+ 2Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119906
120576
119905) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582] 119889119904
(97)
Note that
Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119906
120576
119905) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582] 119889119904
= Eint119903+120576
119903
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119906
120576
119905) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582] 119889119904
le sup119904isin[119905119879]
E [E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119906
120576
119905) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582]] 120576
997888rarr 0 as 120576 997888rarr 0
(98)
The inequality above holds due to the boundedness of|119870
120576
119904|2
int1
0|119891119910(119906
120576
119905) minus 119891
119910(119904)|
2
119889120582 Indeed Assumption (L3) im-plies the boundedness of 119891
119910(119906
120576
119905) minus 119891
119910(119904) Meanwhile 119870120576
119904is
the solution of mean-field BSEE (82) It can be easy to check119870120576
119904is bounded since the coefficients in (82) are boundedOn the other hand
Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119904 120582) minus 119891
119910(119906
120576
119905)10038161003816100381610038161003816
2
119889120582] 119889119904
le 11986221205822Eint
119879
119905
1003816100381610038161003816119870120576
119904
10038161003816100381610038162
times int
1
0
1198641015840[100381610038161003816100381610038161003816(119884
120576
119904minus 119884
119904)1015840100381610038161003816100381610038161003816
2
+100381610038161003816100381610038161003816(119885
120576
119904minus 119885
119904)1015840100381610038161003816100381610038161003816
2
times10038161003816100381610038161003816119884120576
119904minus 119884
119904
10038161003816100381610038161003816
2
+10038161003816100381610038161003816119885120576
119904minus 119885
119904
10038161003816100381610038161003816
2
] 119889120582 119889119904
(99)
where (119884120576
119904 119885
120576
119904) is the mild solution of the following equation
119889119884120576
119905= minus 119860119884
120576
119905119889119905
minus E1015840[119891 (119905 (119884
120576
119905)1015840
(119885120576
119905)1015840
119884120576
119905 119885
120576
119905 119906
120576
119905)] 119889119905
+ 119885120576
119905119889119882 (119905) 119905 isin [0 119879]
119884120576
119879= 120585
(100)
and (119884119904 119885
119904) is the mild solution of
119889119884119905= minus 119860119884
119905119889119905
minus E1015840[119891 (119905 (119884
119905)1015840
(119885119905)1015840
119884119905 119885
119905 119906
119905)] 119889119905
+ 119885119905119889119882 (119905) 119905 isin [0 119879]
119884119879= 120585
(101)
By the definition of 119906120576119905 according to (L2) we have
E1015840[119891 (119904 119884
1015840
(119904) 1198851015840
(119904) 119884 (119904) 119885 (119904) 119906120576
119905)]
997888rarr E1015840[119891 (119904 119884
1015840
(119904) 1198851015840
(119904) 119884 (119904) 119885 (119904) 119906119905)]
(102)
in 1198712([0 119879]119867) as 120576 rarr 0 Using the continuous dependencetheorem Corollary 5 we obtain
(119884120576
119904 119885
120576
119904) 997888rarr (119884
119904 119885
119904) as 120576 997888rarr 0 (103)
Then
Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119904 120582) minus 119891
119910(119906
120576
119905)10038161003816100381610038161003816
2
119889120582] 119889119904
le 11986221205822Eint
119879
119905
1003816100381610038161003816119870120576
119904
10038161003816100381610038162
times int
1
0
E1015840[100381610038161003816100381610038161003816(119884
120576
119904minus 119884
119904)1015840100381610038161003816100381610038161003816
2
+100381610038161003816100381610038161003816(119885
120576
119904minus 119885
119904)1015840100381610038161003816100381610038161003816
2
times10038161003816100381610038161003816119884120576
119904minus 119884
119904
10038161003816100381610038161003816
2
+10038161003816100381610038161003816119885120576
119904minus 119885
119904
10038161003816100381610038161003816
2
] 119889120582 119889119904
997888rarr 0 as 120576 997888rarr 0
(104)
14 Mathematical Problems in Engineering
Combining (98) with (104) we finally haveEint
119879
119905E1015840[|1198682
119904|2
]119889119904 rarr 0 as 120576 rarr 0The required result (93) follows by using the similar
estimations for 1198681119904 1198683
119904 and 1198684
119904
Note that 1 minus 1921198722
1198601198622120573 gt 0 if 120573 is sufficiently large
Now to prove the desired result (84) it suffices to applyGronwallrsquos lemma and estimate (93) to inequality (92)
To deal with the state constraint (75) we need to recall theEkeland variational principle
Lemma 16 (Ekelandrsquos variational principle see [16 Lemma41]) Let (119878 119889) be a complete metric space and let 119865(sdot) 119878 rarr
R be lower semicontinuous and bounded from below If for 120588 gt0 there exists 119906 isin 119878 such that
119865 (119906) le infVisin119878
119865 (V) + 120588 (105)
then there exists 119906120588 isin 119878 satisfying
(i) 119865 (119906120588) le 119865 (119906)
(ii) 119889 (119906120588 119906) le 120588
(iii) 119865 (119906120588) le 119865 (V) + 120588 sdot 119889 (119906120588 119906) forallV = 119906120588
(106)
Now fix V isin Uad and set
119878 = V (sdot) isin Uad | sup0le119905le119879
E1003816100381610038161003816V11990510038161003816100381610038162
le E100381610038161003816100381611990611990510038161003816100381610038162
+ |V|2
119889 (V (sdot) V (sdot)) = 119898 119905 isin [0 119879] | E1003816100381610038161003816V119905 minus V
119905
10038161003816100381610038162
gt 0
forallV (sdot) V (sdot) isin 119878
(107)
where119898 denotes the Lebesgue measure on RThe following result is proved as Proposition 41 in [16]
Lemma 17 (119878 119889(sdot sdot)) is a complete metric space and 119869120588 is
continuous and bounded on 119878 where
119869120588
(V (sdot)) = (119869 (V (sdot)) minus 119869 (119906 (sdot)) + 120588)2
+
10038161003816100381610038161003816100381610038161003816Eint
O
Φ(119909 (1198840(119909))
1015840
1198840(119909)) 119889119909
10038161003816100381610038161003816100381610038161003816
2
12
forallV (sdot) isin 119878(108)
and (119884 119885) is the mild solution of (72) corresponding to thecontrol V
Now we consider the following free initial state optimalcontrol problem
infV(sdot)isin119878
119869120588
(V (sdot)) (109)
It is easy to check that
0 le infV(sdot)isin119878
119869120588
(V (sdot)) le 119869120588
(119906 (sdot)) = 120588 (110)
According to Ekelandrsquos variational principle there exists a119906120588(sdot) isin 119881 such that
(i) 119869120588 (119906120588 (sdot)) le 120588
(ii) 119889 (119906120588 (sdot) 119906 (sdot)) le 120588
(iii) 119869120588 (119906120588 (sdot)) le 119869120588
(V (sdot)) + 120588119889 (119906120588 (sdot) 119906 (sdot)) forallV (sdot) isin 119878(111)
Using the spike variationmethod we can construct 119906120576120588(sdot) isin 119878as follows
119906120576120588
119905=
V119905 119905 isin [119904 119904 + 120576]
119906120588
119905 119905 isin [0 119879] [119904 119904 + 120576]
(112)
It is clear that 119889(119906120576120588(sdot) 119906120588(sdot)) le 120576 Let (119884120576120588(sdot) 119885
120576120588(sdot)) (resp
(119884120588(sdot) 119885
120588(sdot))) be the solution of (72) with respect to the
control 119906120576120588(sdot) (resp 119906120588(sdot)) Following (82) (119870120576120588
119905 119876
120576120588
119905) is the
mild solution of
119870120576120588
119905= int
119879
119905
119890119860(119904minus119905)
E1015840
times [1198911199101015840 (119904 Λ
120588
119904 119906
120588
119904) (119870
120576120588
119904)1015840
+ 119891119910(119904 Λ
120588
119904 119906
120588
119904)119870
120576120588
119904
+ 1198911199111015840 (119904 Λ
120588
119904 119906
120588
119904) (119876
120576120588
119904)1015840
+ 119891119911(119904 Λ
120588
119904 119906
120588
119904) 119876
120576120588
119904
+1
120576(119891 (119904 Λ
120588
119904 119906
120576120588
119904) minus 119891 (119904 Λ
120588
119904 119906
120588
119904))] 119889119904
minus int
119879
119905
119890119860(119904minus119905)
119876120576120588
119904119889119882 (119904)
(113)
ByTheorem 15 we know that
lim120576rarr0
E[ sup119904isin[119905119879]
100381610038161003816100381610038161003816100381610038161003816
119884120576120588
119904minus 119884
120588
119904
120576minus 119870
120576120588
119904
100381610038161003816100381610038161003816100381610038161003816
2
] = 0 forall119905 isin [0 119879]
lim120576rarr0
Eint119879
0
100381610038161003816100381610038161003816100381610038161003816
119885120576120588
119904minus 119885
120588
119904
120576minus 119876
120576120588
119904
100381610038161003816100381610038161003816100381610038161003816
2
119889119904 = 0
(114)
The proof of the following proposition is technical butbased on the arguments above and we omit it
Proposition 18 One has
1
120576E [Φ ((119884
120576120588
0)1015840
119884120576120588
0) minus Φ((119884
120588
0)1015840
119884120588
0)]
= E [Φ1199101015840 ((119884
120588
0)1015840
119884120588
0) (119870
120576120588
0)1015840
+ Φ119910((119884
120588
0)1015840
119884120588
0)119870
120576120588
0]
+ 119900 (120576)
Mathematical Problems in Engineering 15
1
120576(119869 (119906
120576120588) minus 119869 (119906
120588))
= E [1198921199101015840 ((119884
120588
0)1015840
119884120588
0) (119870
120576120588
0)1015840
+ 119892119910((119884
120588
0)1015840
119884120588
0)119870
120576120588
0]
+ Δ120576+1
120576Eint
119879
0
E1015840[ℎ (119904 Λ
120588
119904 119906
120576120588
119904)
minus ℎ (119904 Λ120588
119904 119906
120588
119904)] 119889119904 + 119900 (120576)
(115)
where
Δ120576= Eint
119879
0
E1015840[ℎ
1199101015840 (119904 Λ
120588
119904 119906
120576120588
119904) (119870
120576120588
119904)1015840
+ ℎ1199111015840 (119904 Λ
120588
119904 119906
120576120588
119904) (119876
120576120588
119904)1015840
+ ℎ119910(119904 Λ
120588
119904 119906
120576120588
119904)119870
120576120588
119904
+ ℎ119911(119904 Λ
120588
119904 119906
120576120588
119904) 119876
120576120588
119904] 119889119904
(116)
53 Variational Inequality and Adjoint Equation In thissubsection the adjoint process is introduced to deduce thevariational inequality
If we set V(sdot) = 119906120576120588(sdot) in (111) and notice that
119889(119906120576120588(sdot) 119906
120588(sdot)) le 120576 we get
minus120588 le1
120576(119869
120588(119906
120576120588
(sdot)) minus 119869120588(119906
120588
(sdot))) (117)
By Lemma 17
1
120576(119869
120588(119906
120576120588
(sdot)) minus 119869120588(119906
120588
(sdot)))
=(119869
120588(119906
120576120588(sdot)))
2
minus (119869120588(119906
120588(sdot)))
2
120576 (119869120588 (119906120576120588 (sdot)) + 119869120588 (119906120588 (sdot)))
=119869 (119906
120576120588(sdot)) + 119869 (119906
120588(sdot)) minus 2119869 (119906 (sdot)) + 2120588
119869120588 (119906120576120588 (sdot)) + 119869120588 (119906120588 (sdot))
times119869 (119906
120576120588(sdot)) minus 119869 (119906
120588(sdot))
120576
+
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] + E [Φ ((119884
120588
0)1015840
119884120588
0)]
119869120588 (119906120576120588 (sdot)) + 119869120588 (119906120588 (sdot))
times
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] minus E [Φ ((119884
120588
0)1015840
119884120588
0)]
120576
997888rarr 119897120588
1
119869 (119906120576120588(sdot)) minus 119869 (119906
120588(sdot))
120576
+ 119897120588
2
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] minus E [Φ ((119884
120588
0)1015840
119884120588
0)]
120576
(118)
where we set
119897120588
1=119869 (119906
120588(sdot)) minus 119869 (119906 (sdot)) + 120588
119869120588 (119906120588 (sdot))
119897120588
2=
E [Φ ((119884120588
0)1015840
119884120588
0)]
119869120588 (119906120588 (sdot))
(119)
and use the limit
119869 (119906120576120588
(sdot)) 997888rarr 119869 (119906120588
(sdot))
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] 997888rarr E [Φ ((119884
120588
0)1015840
119884120588
0)]
(120)
as 120576 rarr 0 according to (115)As |119897120588
1|2
+ |119897120588
2|2
= 1 for all 120588 gt 0 we know that there existsa subsequence of 119897120588
1 119897120588
2 (still denoted by 119897120588
1 119897120588
2) such that
lim120588rarr0
119897120588
1 119897120588
2 = 119897
1 1198972
1003816100381610038161003816119897110038161003816100381610038162
+1003816100381610038161003816119897210038161003816100381610038162
= 1
(121)
Combining (115) (117) with (118) we get
minus120588 le 119897120588
1
119869 (119906120576120588(sdot)) minus 119869 (119906
120588(sdot))
120576
+ 119897120588
2
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] minus E [Φ ((119884
120588
0)1015840
119884120588
0)]
120576
= 119897120588
1Δ120576+ 119897
120588
1E [119892
1199101015840 ((119884
120588
0)1015840
119884120588
0) (119870
120576120588
0)1015840
+119892119910((119884
120588
0)1015840
119884120588
0)119870
120576120588
0]
+119897120588
1
120576Eint
119879
0
E1015840[ℎ (119904 Λ
120588
119904 119906
120576120588
119904) minus ℎ (119904 Λ
120588
119904 119906
120588
119904)] 119889119904
+ 119897120588
2E [Φ
1199101015840 ((119884
120588
0)1015840
119884120588
0) (119870
120576120588
0)1015840
+Φ119910((119884
120588
0)1015840
119884120588
0)119870
120576120588
0] + (119897
120588
1+ 119897
120588
2) 119900 (120576)
(122)
Next we introduce the adjoint equation corresponding tovariational equation (113) whose solution is denoted by119875120588(119905)
119889119875120588
(119905)
= 119860lowast119875120588
(119905) 119889119905
+ E1015840[119891
1199101015840 (119905 Λ
120588
119905 119906
120588
119905) (119875
120588
(119905))1015840
+ 119891119910(119905 Λ
120588
119905 119906
120588
119905) 119875
120588
(119905) + 119897120588
1ℎ1199101015840 (119905 Λ
120588
119905 119906
120588
119905)
+ 119897120588
1ℎ119910(119905 Λ
120588
119905 119906
120588
119905)] 119889119905
16 Mathematical Problems in Engineering
+ E1015840[119891
1199111015840 (119905 Λ
120588
119905 119906
120588
119905) (119875
120588
(119905))1015840
+ 119891119911(119905 Λ
120588
119905 119906
120588
119905) 119875
120588
(119905)
+ 119897120588
1ℎ1199111015840 (119905 Λ
120588
119905 119906
120588
119905) + 119897
120588
1ℎ119911(119905 Λ
120588
119905 119906
120588
119905)] 119889119882 (119905)
119875120588
(0) = 119897120588
1E [119892
1199101015840 ((119884
120588
0(119909))
1015840
119884120588
0(119909))
+ 119892119910((119884
120588
0(119909))
1015840
119884120588
0(119909))]
+ 119897120588
2E [Φ
1199101015840 ((119884
120588
0(119909))
1015840
119884120588
0(119909))
+ Φ119910((119884
120588
0(119909))
1015840
119884120588
0(119909))]
(123)
where 119860lowast is the 1198712(O)-adjoint operator of 119860 Under assump-tions (L1)ndash(L3) this is a linear mean-field SEE with boundedcoefficients An application ofTheorem 11 implies that it has aunique adaptedmild solution such that119875120588(119905) isin S2
F ([0 119879] 119870)When 120588 rarr 0 according to Corollaries 5 and 13 119875120588(119905)
converges to 119875(119905) where
119875 (119905) isin S2
F ([0 119879] 119870) (124)
is the solution of the following equation
119875 (119905) = 1198971E [119892
1199101015840 (119884
1015840
0 119884
0) + 119892
119910(119884
1015840
0 119884
0)]
+ 1198972E [Φ
1199101015840 (119884
1015840
0 119884
0) + Φ
119910(119884
1015840
0 119884
0)]
+ int
119905
0
119890119860lowast(119905minus119904)
E1015840[119891
1199101015840 (119904) 119875
1015840
(119904) + 119891119910(119904) 119875 (119904)
+ 1198971ℎ1199101015840 (119904) + 119897
1ℎ119910(119904)] 119889119904
+ int
119905
0
119890119860lowast(119905minus119904)
E1015840[119891
1199111015840 (119904) 119875
1015840
(119904) + 119891119911(119904) 119875 (119904)
+ 1198971ℎ1199111015840 (119904) + 119897
1ℎ119911(119904)] 119889119882 (119904)
(125)
The following proposition which formally follows fromProposition 18 gives the relation between 119875120588(119905) and119870120576120588
119905
Proposition 19 Consider the following
E [119875120588
(0)119870120576120588
0]
= Eint119879
0
1
120576119875120588
(119904)E1015840[119891 (119904 Λ
120588
119904 119906
120576120588
119904) minus 119891 (119904 Λ
120588
119904 119906
120588
119904)]
minus 119897120588
1119870120576120588
119904E1015840[ℎ
1199101015840 (119904 Λ
120588
119904 119906
120588
119904) + ℎ
119910(119904 Λ
120588
119904 119906
120588
119904)]
minus 119897120588
1119876120576120588
119904E1015840[ℎ
1199111015840 (119904 Λ
120588
119904 119906
120588
119904) + ℎ
119911(119904 Λ
120588
119904 119906
120588
119904)] 119889119904
(126)
The following theorem constitutes the main contributionof this section themaximumprinciple for the BSPDE controlsystem
Theorem 20 Let assumptions (L1)ndash(L3) hold Suppose 119906(sdot) isan optimal control and (119884(sdot) 119885(sdot)) is the corresponding optimalstate trajectory for the BSPDE control systems (72) and (73)with the initial state constraint (75) Then there exists 119875(119905) isinS2
F ([0 119879] 119870) which satisfies (125) such that
H (119905 1198841015840
119905 119885
1015840
119905 119884
119905 119885
119905 V
119905 119875 (119905))
ge H (119905 1198841015840
119905 119885
1015840
119905 119884
119905 119885
119905 119906
119905 119875 (119905))
119886119890 119886119904 forallV isin U119886119889
(127)
whereH [0 119879]times119867timesL(Γ119867)times119867timesL(Γ119867)times119880times119870 rarr R
is the Hamiltonian function defined by
H (119905 119910 119910 119911 V 119901) = 1198971ℎ (119905 119910 119910 119911 V)
+ 119901119891 (119905 119910 119910 119911 V) (128)
Proof By (122) and Proposition 19 we obtain
minus120588 le1
120576Eint
119879
0
119875120588
(119904)E1015840[119891 (119904 Λ
120588
119904 119906
120576120588
119904)
minus 119891 (119904 Λ120588
119904 119906
120588
119904)] 119889119904
+119897120588
1
120576Eint
119879
0
E1015840[ℎ (119904 Λ
120588
119904 119906
120576120588
119904)
minus ℎ (119904 Λ120588
119904 119906
120588
119904)] 119889119904 + (119897
120588
1+ 119897
120588
2) 119900 (120576)
(129)
Letting 120576 rarr 0+ in (129) we derive for ae 120591 isin [0 119879]
minus120588 le 119897120588
1E1015840[ℎ (120591 Λ
120588
120591 V
120591) minus ℎ (120591 Λ
120588
120591 119906
120588
120591)]
+ 119875120588
(119904)E1015840[119891 (120591 Λ
120588
120591 V
120591) minus 119891 (120591 Λ
120588
120591 119906
120588
120591)]
(130)
for all V isin UadFinally taking 120588 rarr 0 in (130) we derive the desired
result
Remark 21 We note that if the coefficients do not dependexplicitly on the marginal law of the underlying diffusionthe result reduces to the classical case that is the SMP forBSPDEs without mean-field term
Remark 22 When we remove the initial state constraint (75)we obtain the general maximum principle for the mean-fieldBSPDEs system (ie without the constraint) with 119897
1= 1
54 Application A Backward Linear Quadratic Control Prob-lem Now we apply our maximum principle to solve an LQproblem For notational simplicity we restrict ourselves tothe free case (ie without the initial state constraint (75)) thegeneral case being handled in a similar way
Mathematical Problems in Engineering 17
Consider the following problem
119869 (119906) =1
2E [(119884 (0))
2] +
1
2Eint
119879
0
119873V2 (119905) 119889119905 997888rarr min
(131)
subject to
119889119884 (119905) = minus119860119884 (119905) 119889119905
minus 119861119884 (119905) + 119861E [119884 (119905)]
+119862V (119905) + 119863119885 (119905) + 119863E [119885 (119905)] 119889119905
+ 119885 (119905) 119889119882 (119905)
119884 (119879) = 120585 119905 isin [0 119879]
(132)
where 119860 is a partial differential operator satisfying condition(L1) and 119861 119861 119862119863119863 and119873 are bounded and deterministicconstantsWe also assume that119873 gt 0 and V isin 1198712F (0 119879 119880) It iseasy to verify that BSPDE (132) admits a uniquemild solution(119884(119905) 119885(119905))
119875(119905) the adjoint process of state equation (132) is thesolution of
119889119875 (119905) = 119860lowast119875 (119905) 119889119905 + 119861119875 (119905) + 119861E [119875 (119905)] 119889119905
+ 119863119875 (119905) + 119863E [119875 (119905)] 119889119882 (119905)
119875 (0) = 119884 (0)
(133)
Let 119906(sdot) be an optimal control and let (119884(sdot) 119885(sdot)) bethe corresponding state process By maximum principle ofTheorem 20 (note that 119897
1= 1 in this problem)
1
2119873V2 (119905) + 119875 (119905) 119862V (119905) ge
1
2119873119906
2
(119905) + 119875 (119905) 119862119906 (119905) (134)
for all V isin 119880ad since the state equation has the form (132)Thisin turn implies
119906 (119905) = minus119862
119873119875 (119905) (135)
It is clear that (131) is a positive quadratic functional of controlbecause of 119873 gt 0 Hence an optimal control exists Thecandidate optimal control (135) is indeed an optimal controlof this LQ problem for it is the only control which satisfies themaximum principle
Next we want to obtain a more explicit representationof the optimal control (135) from the state equation (132)Substituting (135) into state equation (132) yields
119889119884 (119905) = minus119860119884 (119905) 119889119905
minus 119861119884 (119905) + 119861E [119884 (119905)] minus1198622
119873119875 (119905)
+119863119885 (119905) + 119863E [119885 (119905)] 119889119905 + 119885 (119905) 119889119882 (119905)
119884 (119879) = 120585 119905 isin [0 119879]
(136)
Combining the above equation with (133) we obtain thefollowing related feedback control system
119889119884 (119905) = minus119860119884 (119905) 119889119905
minus 119861119884 (119905) + 119861E [119884 (119905)] minus1198622
119873119875 (119905)
+ 119863119885 (119905) + 119863E [119885 (119905)] 119889119905 + 119885 (119905) 119889119882 (119905)
119884 (119879) = 120585
119889119875 (119905) = 119860lowast119875 (119905) 119889119905 + 119861119875 (119905) + 119861E [119875 (119905)] 119889119905
+ 119863119875 (119905) + 119863E [119875 (119905)] 119889119882 (119905)
119875 (0) = 119884 (0)
(137)
Looking at the terminal condition of 119875(119905) in (133) andconsidering the mean-field type of (132) it is reasonable toconjecture that 119875(119905) has the following form
119875 (119905) = 120593 (119905) 119884 (119905) + 120601 (119905)E [119884 (119905)] (138)
where120593(119905)120601(119905) are deterministic differential functionswhichwill be specified below Moreover 120593(0) = 1 120601(0) = 0
Inserting this form into adjoint equation (133) and notic-ing that 119884(119905) satisfies (136) we can compare the coefficientsof 119889119905 and 119889119882(119905) to obtain the following equation
2120593 (119905) 119860119884 (119905) + 2120601 (119905) 119860E [119884 (119905)] + 119861120593 (119905) 119884 (119905)
+ 2 (119861120601 (119905) + 119861120593 (119905) + 119861120601 (119905))E [119884 (119905)]
= minus120593 (119905) 119861119884 (119905) minus1198622
119873120593 (119905) 119884 (119905) + 119863119885 (119905)
+ 119884 (119905)119889120593 (119905)
119889119905+ E [119884 (119905)]
119889120601 (119905)
119889119905
+ (1206012
(119905)1198622
119873+ 2
1198622
119873120593 (119905) 120601 (119905))E [119884 (119905)]
minus (119863120601 (119905) + 119863120601 (119905) + 119863120593 (119905))E [119885 (119905)]
120593 (119905) 119885 (119905) = 119863120593 (119905) 119884 (119905)
+ (119863120601 (119905) + 119863120593 (119905) + 119863120601 (119905))E [119884 (119905)]
(139)
Then subtracting 119885(119905) we have
2120593 (119905) 119860119884 (119905) + 2120601 (119905) 119860E [119884 (119905)] + 119861120593 (119905) 119884 (119905)
+ 2 (119861120601 (119905) + 119861120593 (119905) + 119861120601 (119905))E [119884 (119905)]
18 Mathematical Problems in Engineering
= 120593 (119905) minus119861 +1198622
119873120593 (119905) minus 119863
2119884 (119905)
+ 119884 (119905)119889120593 (119905)
119889119905+ E [119884 (119905)]
119889120601 (119905)
119889119905
+ (1206012
(119905)1198622
119873+ 2
1198622
119873120593 (119905) 120601 (119905))E [119884 (119905)]
minus1
120593 (119905)(119863120601 (119905) + 119863120593 (119905) + 119863120601 (119905))
2
E [119884 (119905)]
minus 2119863 (119863120601 (119905) + 119863120593 (119905) + 119863120601 (119905))E [119884 (119905)]
(140)
Comparing the coefficients of 119884(119905) and E[119884(119905)] respec-tively we get
119889
119889119905120593 (119905) = 2119860
lowast120593 (119905) + 2119861120593 (119905) + 119863
2120593 (119905) minus
1198622
1198731205932
(119905)
120593 (0) = 1
(141)
119889
119889119905120601 (119905) = 2119860
lowast120601 (119905) + (
(119863 + 119863)2
120593 (119905)minus1198622
119873)120601
2
(119905)
+ 2119877 (119905) 120601 (119905) + (1198632+ 2119861 + 2119863119863)120593 (119905)
120601 (0) = 0
(142)
where 119877(119905) = 119861 + 119861 + (119863 + 119863)2
minus (1198622119873)120593(119905)
We solve (141) to get
120593 (119905) = (119890minus(2119860lowast+2119861+119863
2)119905(1 minus
1198622
119873(2119860lowast + 2119861 + 1198632))
+1198622
119873(2119860lowast + 2119861 + 1198632))
minus1
(143)
Then (142) exists a unique solution from the classical Riccatiequation theory
We now conclude the above discussions in the followingresult
Theorem 23 For onersquos linear quadratic stochastic partialdifferential control problem (131)-(132) the unique optimalcontrol 119906(sdot) isin 119880ad is given by
119906 (119905) = minus119862
119873(120593 (119905) 119884 (119905) + 120601 (119905)E [119884 (119905)]) (144)
with 120593(119905) satisfying (143) and 120601(119905) solving (142)
Conflict of Interests
The authors declare that they have no conflict of interestsregarding the publication of this paper
Acknowledgment
This research is partially supported by the Natural ScienceFoundation of China under Grant no 11301310
References
[1] Y Hu and S Peng ldquoAdapted solution of a backward semilinearstochastic evolution equationrdquo Stochastic Analysis and Applica-tions vol 9 no 4 pp 445ndash459 1991
[2] N IMahmudov andMAMcKibben ldquoOnbackward stochasticevolution equations in Hilbert spaces and optimal controlrdquoNonlinear Analysis vol 67 no 4 pp 1260ndash1274 2007
[3] M Fuhrman and G Tessitore ldquoNonlinear Kolmogorov equa-tions in infinite dimensional spaces the backward stochasticdifferential equations approach and applications to optimalcontrolrdquoThe Annals of Probability vol 30 no 3 pp 1397ndash14652002
[4] M Fuhrman and G Tessitore ldquoInfinite horizon backwardstochastic differential equations and elliptic equations inHilbert spacesrdquo Annals of Probability vol 32 no 1B pp 607ndash660 2004
[5] X Mao ldquoAdapted solutions of backward stochastic differentialequations with non-Lipschitz coefficientsrdquo Stochastic Processesand their Applications vol 58 no 2 pp 281ndash292 1995
[6] J Lasry and P Lions ldquoMean field gamesrdquo Japanese Journal ofMathematics vol 2 no 1 pp 229ndash260 2007
[7] R Buckdahn J Li and S Peng ldquoMean-field backward stochas-tic differential equations and related partial differential equa-tionsrdquo Stochastic Processes and their Applications vol 119 no 10pp 3133ndash3154 2009
[8] D Andersson and B Djehiche ldquoAmaximum principle for SDEsofmean-field typerdquoAppliedMathematics andOptimization vol63 no 3 pp 341ndash356 2011
[9] R Buckdahn B Djehiche J Li and S Peng ldquoMean-fieldbackward stochastic differential equations a limit approachrdquoThe Annals of Probability vol 37 no 4 pp 1524ndash1565 2009
[10] R Xu ldquoMean-field backward doubly stochastic differentialequations and related SPDEsrdquo Boundary Value Problems vol2012 article 114 2012
[11] R Buckdahn B Djehiche and J Li ldquoA general stochasticmaximum principle for SDEs of mean-field typerdquo AppliedMathematics and Optimization vol 64 no 2 pp 197ndash216 2011
[12] J Li ldquoStochasticmaximumprinciple in themean-field controlsrdquoAutomatica vol 48 no 2 pp 366ndash373 2012
[13] G Wang C Zhang and W Zhang ldquoStochastic maximumprinciple for mean-field type optimal control under partialinformationrdquo IEEE Transactions on Automatic Control vol 59no 2 pp 522ndash528 2014
[14] M Hafayed ldquoA mean-field necessary and sufficient conditionsfor optimal singular stochastic controlrdquo Communications inMathematics and Statistics vol 1 no 4 pp 417ndash435 2013
[15] M Hafayed ldquoA mean-field maximum principle for optimalcontrol of forward-backward stochastic differential equationswith Poisson jumpprocessesrdquo International Journal ofDynamicsand Control vol 1 no 4 pp 300ndash315 2013
[16] Y Hu and S Peng ldquoMaximum principle for semilinearstochastic evolution control systemsrdquo Stochastics and StochasticsReports vol 33 no 3-4 pp 159ndash180 1990
[17] M Fuhrman Y Hu and G Tessitore ldquoStochastic maximumprinciple for optimal control of SPDEsrdquo Comptes Rendus Math-ematique vol 350 no 13-14 pp 683ndash688 2012
Submit your manuscripts athttpwwwhindawicom
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
MathematicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Mathematical Problems in Engineering
Hindawi Publishing Corporationhttpwwwhindawicom
Differential EquationsInternational Journal of
Volume 2014
Applied MathematicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Probability and StatisticsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Mathematical PhysicsAdvances in
Complex AnalysisJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
OptimizationJournal of
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CombinatoricsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Operations ResearchAdvances in
Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Function Spaces
Abstract and Applied AnalysisHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of Mathematics and Mathematical Sciences
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Algebra
Discrete Dynamics in Nature and Society
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Decision SciencesAdvances in
Discrete MathematicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom
Volume 2014 Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Stochastic AnalysisInternational Journal of
Mathematical Problems in Engineering 5
times [int
119879
0
1198902120573119904
100381610038161003816100381610038161003816E1015840[119891 (119904 (119910
1
(119904))1015840
(1199111
(119904))1015840
1198841
(119904) 1198851
(119904))
minus 119891 (119904 (1199102
(119904))1015840
(1199112
(119904))1015840
1198842
(119904) 1198852
(119904))]100381610038161003816100381610038161003816
2
119889119904]
le12119872
2
1198601198712
120573E
times [int
119879
0
1198902120573119904
(E [1003816100381610038161003816119910 (119904)
10038161003816100381610038162
+ |119911 (119904)|2]
+10038161003816100381610038161003816119884 (119904)
10038161003816100381610038161003816
2
+10038161003816100381610038161003816119885 (119904)
10038161003816100381610038161003816
2
) 119889119904]
=12119872
2
1198601198712
120573E
times [int
119879
0
1198902120573119904
(1003816100381610038161003816119910 (119904)
10038161003816100381610038162
+ |119911 (119904)|2
+10038161003816100381610038161003816119884 (119904)
10038161003816100381610038161003816
2
+10038161003816100381610038161003816119885 (119904)
10038161003816100381610038161003816
2
) 119889119904]
(20)
If we set 120573 = 361198722
1198601198712max119879 1 then
Eint119879
0
1198902120573119904
(10038161003816100381610038161003816119884 (119904)
10038161003816100381610038161003816
2
+10038161003816100381610038161003816119885 (119904)
10038161003816100381610038161003816
2
) 119889119904
le 119879 sdot E sup0le119904le119879
119890212057311990410038161003816100381610038161003816
119884 (119904)10038161003816100381610038161003816
2
+ Eint119879
0
119890212057311990410038161003816100381610038161003816
119885 (119904)10038161003816100381610038161003816
2
119889119904
le12119872
2
1198601198712max 119879 1120573
E
times [int
119879
0
1198902120573119904
(1003816100381610038161003816119910 (119904)
10038161003816100381610038162
+ |119911 (119904)|2
+10038161003816100381610038161003816119884 (119904)
10038161003816100381610038161003816
2
+10038161003816100381610038161003816119885 (119904)
10038161003816100381610038161003816
2
) 119889119904]
=1
3E [int
119879
0
1198902120573119904
(1003816100381610038161003816119910 (119904)
10038161003816100381610038162
+ |119911 (119904)|2
+10038161003816100381610038161003816119884 (119904)
10038161003816100381610038161003816
2
+10038161003816100381610038161003816119885 (119904)
10038161003816100381610038161003816
2
) 119889119904]
(21)
That is
Eint119879
0
1198902120573119904
(10038161003816100381610038161003816119884 (119904)
10038161003816100381610038161003816
2
+10038161003816100381610038161003816119885 (119904)
10038161003816100381610038161003816
2
) 119889119904
le1
2Eint
119879
0
1198902120573119904
(1003816100381610038161003816119910 (119904)
10038161003816100381610038162
+ |119911 (119904)|2) 119889119904
(22)
The estimate (22) shows that Φ is a contraction on thespaceK
120573[0 119879] with the norm
(119884 119885)2
120573= Eint
119879
0
1198902120573119904
(|119884 (119904)|2+ |119885 (119904)|
2) 119889119904 (23)
With the contraction mapping theorem there admits aunique fixed point (119884 119885) isin K
120573[0 119879] such that Φ(119884 119885) =
(119884 119885) On the other hand from Step 1 we know thatif Φ(119884 119885) = (119884 119885) then (119884 119885) isin S2
F ([0 119879]119867) times
H2
F ([0 119879]L(Γ119867)) which is the unique mild solution of(11)
Arguing as the previous proof we arrive at the followingassertion in a straightforward way
Corollary 5 Suppose that for all 120572 in a metric space 119865 119891120572is
a given function satisfying (A1) and (A2) with 119871 independenton 120572 Also suppose that
E1015840[119891
120572(119904 119884
1015840
(119904) 1198851015840
(119904) 119884 (119904) 119885 (119904))]
997888rarr E1015840[119891
1205720(119904 119884
1015840
(119904) 1198851015840
(119904) 119884 (119904) 119885 (119904))]
(24)
in 1198712([0 119879]119867) as 120572 rarr 1205720for all (119884 119885) isin S2
F ([0 119879]119867) times
H2
F ([0 119879]L(Γ119867))If we denote by (119884(120585 120572) 119885(120585 120572)) the mild solution of (11)
corresponding to the functions 119891120572and to the final data 120585 isin
1198712(ΩF
119879P 119867) then the map (120572 120585) rarr (119884(120585 120572) 119885(120585 120572))
is continuous from 119865 times 1198712(ΩF
119879P 119867) to S2
F ([0 119879]119867) times
H2
F ([0 119879]L(Γ119867))
32 Non-Lipschitz Case This subsection is devoted to findingsome weaker conditions than the Lipschitz one under whichthe mean-field BSEE has a unique solution To state our mainresult in this section we suppose the following
(A3) For all 119905 isin [0 119879] 1199101015840119894 119910
119894isin 119867 1199111015840
119894 119911
119894isin L(Γ119867) (119894 =
1 2) there exists an 119897 gt 0 such that
10038161003816100381610038161003816119891 (119905 119910
1015840
1 119911
1015840
1 119910
1 119911
1) minus 119891 (119905 119910
1015840
2 119911
1015840
2 119910
2 119911
2)10038161003816100381610038161003816
2
le 120579 (100381610038161003816100381610038161199101015840
1minus 119910
1015840
2
10038161003816100381610038161003816
2
) + 120579 (10038161003816100381610038161199101 minus 1199102
10038161003816100381610038162
)
+ 119897 (100381610038161003816100381610038161199111015840
1minus 119911
1015840
2
10038161003816100381610038161003816
2
+10038161003816100381610038161199111 minus 1199112
10038161003816100381610038162
)
(25)
where 120579 R+rarr R+ is a concave increasing function such
that 120579(0) = 0 120579(119906) gt 0 for 119906 gt 0 and int0+(119889119906120579(119906)) = infin
InMao [5] the author gave three examples of the function120579(sdot) to show the generality of condition (A3) From theseexamples we can see that Lipschitz condition (A1) is a specialcase of the given condition (A3)
Since 120579 is concave and 120579(0) = 0 there exists a pair ofpositive constants 119886 and 119887 such that
120579 (119906) le 119886 + 119887119906 (26)
for all 119906 ge 0 Therefore under assumptions (A2) and(A3) 119891(sdot 1199101015840(sdot) 1199111015840(sdot) 119910(sdot) 119911(sdot)) isin H2
F ([0 119879]119867) whenever
6 Mathematical Problems in Engineering
1199101015840(sdot) 119910(sdot) isin S2
F ([0 119879]119867) and 1199111015840(sdot) 119911(sdot) isin H2
F ([0 119879]L(Γ119867))
By Picard-type iteration we now construct an approxi-mate sequence using which we obtain the desired result Let1198840(119905) equiv 0 and for 119899 isin N let 119884
119899 119885
119899 be a sequence in
S2
F ([0 119879]119867) timesH2
F ([0 119879]L(Γ119867)) defined recursively by
119884119899(119905) = 119890
119860(119879minus119905)120585
+ int
119879
119905
E1015840[119890
119860(119904minus119905)
times119891 (119904 1198841015840
119899minus1(119904) 119885
1015840
119899(119904) 119884
119899minus1(119904) 119885
119899(119904))] 119889119904
minus int
119879
119905
119890119860(119904minus119905)
119885119899(119904) 119889119882 (119904)
(27)
on 0 le 119905 le 119879 From Theorem 4 (27) has a unique mildsolution (119884
119899(119905) 119885
119899(119905))
In order to give the main result we need to prepare thefollowing lemmas about the properties of (119884
119899(119905) 119885
119899(119905)) 119905 isin
[0 119879]
Lemma 6 Under hypotheses (A2) and (A3) there existpositive constants 119862
1and 119862
2such that
(i)E( sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899 (119904)
10038161003816100381610038162
) le 21198621exp (119879 minus 119905)
(ii)Eint119879
119905
11989021205731199041003816100381610038161003816119885119899
(119904)10038161003816100381610038162
119889119904 le 21198621exp (119879 minus 119905)
(iii)E( sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899+1 (119904) minus 119884119899 (119904)
10038161003816100381610038162
)
le 1198622int
119879
119905
120579(E sup119904le119903le119879
11989021205731199031003816100381610038161003816119884119899 (119903) minus 119884119899minus1 (119903)
10038161003816100381610038162
)119889119904
(28)
for all 119905 isin [0 119879] and 119899 ge 1
Proof Using the hypotheses (A2) and (A3)with 120579(119906) le 119886 + 119887119906
yields
10038161003816100381610038161003816119891 (119904 119884
1015840
119899minus1(119904) 119885
1015840
119899(119904) 119884
119899minus1(119904) 119885
119899(119904))
10038161003816100381610038161003816
2
le 2120579 (100381610038161003816100381610038161198841015840
119899minus1(119904)10038161003816100381610038161003816
2
) + 2120579 (1003816100381610038161003816119884119899minus1 (119904)
10038161003816100381610038162
)
+ 2119897 (100381610038161003816100381610038161198851015840
119899(119904)10038161003816100381610038161003816
2
+1003816100381610038161003816119885119899
(119904)10038161003816100381610038162
) + 21003816100381610038161003816119891 (119904 0 0 0 0)
10038161003816100381610038162
le 4119886 + 2119887100381610038161003816100381610038161198841015840
119899minus1(119904)10038161003816100381610038161003816
2
+ 21198871003816100381610038161003816119884119899minus1 (119904)
10038161003816100381610038162
+ 2119897 (100381610038161003816100381610038161198851015840
119899(119904)10038161003816100381610038161003816
2
+1003816100381610038161003816119885119899
(119904)10038161003816100381610038162
) + 21003816100381610038161003816119891 (119904 0 0 0 0)
10038161003816100381610038162
(29)
Then it follows from Lemma 1 that
E( sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899 (119904)
10038161003816100381610038162
) + Eint119879
119905
11989021205731199041003816100381610038161003816119885119899
(119904)10038161003816100381610038162
119889119904
le 241198722
1198601198902120573119879
E100381610038161003816100381612058510038161003816100381610038162
+12119872
2
119860
120573E
times int
119879
119905
1198902120573119904 10038161003816100381610038161003816
E1015840[119891 (119904 119884
1015840
119899minus1(119904)
1198851015840
119899(119904) 119884
119899minus1(119904) 119885
119899(119904))]
10038161003816100381610038161003816
2
119889119904
le 241198722
1198601198902120573119879
E100381610038161003816100381612058510038161003816100381610038162
+12119872
2
119860
120573E
times int
119879
119905
E1015840[119890
2120573119904 10038161003816100381610038161003816119891 (119904 119884
1015840
119899minus1(119904)
1198851015840
119899(119904) 119884
119899minus1(119904) 119885
119899(119904))
10038161003816100381610038161003816
2
] 119889119904
le 1198621+24119872
2
119860
120573E
times int
119879
119905
E1015840[119890
2120573119904[119887100381610038161003816100381610038161198841015840
119899minus1(119904)10038161003816100381610038161003816
2
+ 1198871003816100381610038161003816119884119899minus1 (119904)
10038161003816100381610038162
+ 119897100381610038161003816100381610038161198851015840
119899(119904)10038161003816100381610038161003816
2
+ 1198971003816100381610038161003816119885119899
(119904)10038161003816100381610038162
]] 119889119904
= 1198621+48119872
2
119860
120573E
times int
119879
119905
1198902120573119904
[1198871003816100381610038161003816119884119899minus1 (119904)
10038161003816100381610038162
+ 1198971003816100381610038161003816119885119899
(119904)10038161003816100381610038162
] 119889119904
(30)
where
1198621= 24119872
2
1198601198902120573119879
E100381610038161003816100381612058510038161003816100381610038162
+24119872
2
119860
120573Eint
119879
119905
1198902120573119904
[2119886 +1003816100381610038161003816119891 (119904 0 0 0 0)
10038161003816100381610038162
] 119889119904 + 1
(31)
If we set 120573 = 961198722
119860max119887 119897 we can obtain
sup119899isinN
E( sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899 (119904)
10038161003816100381610038162
) +1
2int
119879
119905
sup119899isinN
E [11989021205731199041003816100381610038161003816119885119899
(119904)10038161003816100381610038162
] 119889119904
le 1198621+1
2int
119879
119905
sup119899isinN
E [11989021205731199041003816100381610038161003816119884119899minus1 (119904)
10038161003816100381610038162
] 119889119904
le 1198621+1
2int
119879
119905
sup119899isinN
E[ sup119904le119903le119879
11989021205731199031003816100381610038161003816119884119899minus1 (119903)
10038161003816100381610038162
]119889119904
(32)
An application of the Gronwall inequality now implies
sup119899isinN
E( sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899 (119904)
10038161003816100381610038162
) le 21198621exp (119879 minus 119905
2)
le 21198621exp (119879 minus 119905)
(33)
Point (i) of Lemma 6 is now proved
Mathematical Problems in Engineering 7
From formula (32) we know that
int
119879
119905
sup119899isinN
E [11989021205731199041003816100381610038161003816119885119899
(119904)10038161003816100381610038162
] 119889119904
le 21198621+ int
119879
119905
sup119899isinN
E[ sup119904le119903le119879
11989021205731199031003816100381610038161003816119884119899minus1 (119903)
10038161003816100381610038162
]119889119904
le 21198621+ 2119862
1int
119879
119905
exp (119879 minus 119904) 119889119904
= 21198621exp (119879 minus 119905)
(34)
This proves point (ii) of the LemmaTo prove point (iii) we note that
E1015840[10038161003816100381610038161003816119891 (119904 119884
1015840
119899(119904) 119885
1015840
119899+1(119904) 119884
119899(119904) 119885
119899+1(119904))
minus 119891 (119904 1198841015840
119899minus1(119904) 119885
1015840
119899(119904) 119884
119899minus1(119904) 119885
119899(119904))
10038161003816100381610038161003816
2
]
le E1015840[120579 (
100381610038161003816100381610038161198841015840
119899(119904) minus 119884
1015840
119899minus1(119904)10038161003816100381610038161003816
2
) + 119897100381610038161003816100381610038161198851015840
119899+1(119904) minus 119885
1015840
119899(119904)10038161003816100381610038161003816
2
]
+ 120579 (1003816100381610038161003816119884119899 (119904) minus 119884119899minus1 (119904)
10038161003816100381610038162
) + 1198971003816100381610038161003816119885119899+1
(119904) minus 119885119899(119904)10038161003816100381610038162
= E [120579 (1003816100381610038161003816119884119899 (119904) minus 119884119899minus1 (119904)
10038161003816100381610038162
) + 1198971003816100381610038161003816119885119899+1
(119904) minus 119885119899(119904)10038161003816100381610038162
]
+ 120579 (1003816100381610038161003816119884119899 (119904) minus 119884119899minus1 (119904)
10038161003816100381610038162
) + 1198971003816100381610038161003816119885119899+1
(119904) minus 119885119899(119904)10038161003816100381610038162
(35)
By Lemma 1 we have
E( sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899+1 (119904) minus 119884119899 (119904)
10038161003816100381610038162
)
+ Eint119879
119905
11989021205731199041003816100381610038161003816119885119899+1
(119904) minus 119885119899(119904)10038161003816100381610038162
119889119904
le12119872
2
119860
120573E
times int
119879
119905
1198902120573119904 10038161003816100381610038161003816
E1015840[119891 (119904 119884
1015840
119899(119904) 119885
1015840
119899+1(119904) 119884
119899(119904) 119885
119899+1(119904))
minus 119891 (119904 1198841015840
119899minus1(119904) 119885
1015840
119899(119904)
119884119899minus1
(119904) 119885119899(119904))]
10038161003816100381610038161003816
2
119889119904
le12119872
2
119860
120573E
times int
119879
119905
1198902120573119904
E1015840[10038161003816100381610038161003816119891 (119904 119884
1015840
119899(119904) 119885
1015840
119899+1(119904) 119884
119899(119904) 119885
119899+1(119904))
minus 119891 (119904 1198841015840
119899minus1(119904) 119885
1015840
119899(119904)
119884119899minus1
(119904) 119885119899(119904))
10038161003816100381610038161003816
2
] 119889119904
le24119872
2
119860
120573Eint
119879
119905
1198902120573119904
[120579 (1003816100381610038161003816119884119899 (119904) minus 119884119899minus1 (119904)
10038161003816100381610038162
)
+ 1198971003816100381610038161003816119885119899+1
(119904) minus 119885119899(119904)10038161003816100381610038162
] 119889119904
(36)
We can choose 120573 gt 0 sufficiently large such that
(1 minus24119872
2
119860119897
120573)Eint
119879
119905
11989021205731199041003817100381710038171003817119885119899+1
(119904) minus 119885119899(119904)10038171003817100381710038172
119889119904 ge 0 (37)
Then
E( sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899+1 (119904) minus 119884119899 (119904)
10038161003816100381610038162
)
le24119872
2
119860
120573Eint
119879
119905
1198902120573119904120579 (1003816100381610038161003816119884119899 (119904) minus 119884119899minus1 (119904)
10038161003816100381610038162
) 119889119904
le 1198622Eint
119879
119905
120579(E sup119904le119903le119879
11989021205731199031003816100381610038161003816119884119899 (119903) minus 119884119899minus1 (119903)
10038161003816100381610038162
)119889119904
(38)
where we set 1198622= (24119872
2
119860120573)119890
2120573119879
We divide the interval [0 119879] into subintervals 0 = 1205910lt
1205911lt sdot sdot sdot lt 120591
119898= 119879 by setting 120591
119896= 119896120575 119896 = 1 2 3 119898 with
120575 = 119879119898
Lemma 7 For all 119905 isin [120591119896minus1
120591119896] define
1198623= 119862
2120579 (2119862
1exp (119879))
1205931198961(119905) = 119862
3(120591119896minus 119905)
120593119896119899+1
(119905) = 1198622int
120591119896
119905
120579 (120593119896119899(119904)) 119889119904 119899 ge 1
(39)
Then for all 119899 ge 1 the following inequality holds for a suitable120575 gt 0
0 le 120593119896119899(119905) le 120593
119896119899minus1(119905) le sdot sdot sdot le 120593
1198961(119905) (40)
Proof Firstly it needs to be verified that for all 119905 isin [120591119896minus1
120591119896]
the following inequality
1205931198962(119905) = 119862
2int
120591119896
119905
120579 (1205931198961(119904)) 119889119904 = 119862
2int
120591119896
119905
120579 (1198623(120591119896minus 119904)) 119889119904
le 1198623(120591119896minus 119905) = 120593
1198961(119905)
(41)
holds provided 120575 gt 0 is chosen sufficiently smallActually this inequality holds provided that
1198622120579 (119862
3(120591119896minus 119905)) le 119862
3= 119862
2120579 (2119862
1exp (119879)) (42)
8 Mathematical Problems in Engineering
or
1198623(120591119896minus 119905) = 119862
2120579 (2119862
1exp (119879)) (120591
119896minus 119905) le 2119862
1exp (119879)
(43)
Since 1198621gt 1 from 120579(119906) le 119886 + 119887119906 the above inequality holds
if
1198622(119886 + 119887) (120591
119896minus 119905) le 1 (44)
Thus (41) holds for any 119905 isin [120591119896minus1
120591119896] 119896 = 1 2 119898 if 120591
119896minus
120591119896minus1
le 11198622(119886 + 119887) Therefore we can choose a sufficiently
large119898 isin N such that 120575 = 119879119898 le 11198622(119886 + 119887) Clearly such a
120575 only depends on 119886 119887 119897 119879 and119872119860
Now assume that (40) holds for some 119899 ge 2 Then wehave
120593119896119899+1
(119905) = 1198622int
120591119896
119905
120579 (120593119896119899(119904)) 119889119904 le 119862
2int
120591119896
119905
120579 (120593119896119899minus1
(119904)) 119889119904
= 120593119896119899(119905) forall119905 isin [120591
119896minus1 120591
119896]
(45)
This completes the proof
Now we can give the main result of this section
Theorem 8 Assume that (A2) and (A3) hold Then thereexists a unique mild solution (119884 119885) to (11)
Proof Consider the following
Uniqueness To show the uniqueness let both (119884 119885) and( 119885) be solutions of (11) For any 120573 gt 0 similar to the proofof (36) one can obtain
E( sup119905le119904le119879
119890212057311990410038161003816100381610038161003816
119884 (119904) minus (119904)10038161003816100381610038161003816
2
) + Eint119879
119905
119890212057311990410038161003816100381610038161003816
119885 (119904) minus 119885 (119904)10038161003816100381610038161003816
2
119889119904
le24119872
2
119860
120573E
times int
119879
119905
1198902120573119904
[120579 (10038161003816100381610038161003816119884 (119904) minus (119904)
10038161003816100381610038161003816
2
) + 11989710038161003816100381610038161003816119885 (119904) minus 119885 (119904)
10038161003816100381610038161003816
2
] 119889119904
(46)
That is if 120573 is sufficiently large
E( sup119905le119904le119879
119890212057311990410038161003816100381610038161003816
119884 (119904) minus (119904)10038161003816100381610038161003816
2
)
le24119872
2
119860
120573Eint
119879
119905
1198902120573119904
[120579 (10038161003816100381610038161003816119884 (119904) minus (119904)
10038161003816100381610038161003816
2
)] 119889119904
le 1198622Eint
119879
119905
120579(E sup119904le119903le119879
119890212057311990310038161003816100381610038161003816
119884 (119904) minus (119904)10038161003816100381610038161003816
2
)119889119904
(47)
An application of Bihari inequality yields
E( sup119905le119904le119879
119890212057311990410038161003816100381610038161003816
119884 (119904) minus (119904)10038161003816100381610038161003816
2
) = 0 (48)
So 119884(119905) = (119905) for all 119905 isin [0 119879] as It then follows from (46)that 119885(119905) = 119885(119905) for all 119905 isin [0 119879] as as well This establishesthe uniqueness
ExistenceWe claim that the sequence (119884119899 119885
119899) defined by (27)
satisfies
E sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899+1 (119904) minus 119884119899 (119904)
10038161003816100381610038162
997888rarr 0 forall0 le 119905 le 119879 (49)
as 119899 rarr infinIndeed for all 119905 isin [120591
119896minus1 120591
119896] we set 120593
119896119899(119905) =
E sup119904isin[119905120591119896]
1198902120573119904|119884119899+1
(119904) minus 119884119899(119904)|
2 By Lemmas 6 and 7
1205931198961(119905) = E sup
119904isin[119905120591119896]
119890212057311990410038161003816100381610038161198842 (119904) minus 1198841 (119904)
10038161003816100381610038162
le 1198622int
120591119896
119905
120579(E sup119904le119903le120591119896
119890212057311990310038161003816100381610038161198841 (119903) minus 1198840 (119903)
10038161003816100381610038162
)119889119904
le 1198622int
120591119896
119905
120579 (21198621exp (120591
119896minus 119905)) 119889119904
le 1198622120579 (2119862
1exp (119879)) (120591
119896minus 119905) = 119862
3(120591119896minus 119905) = 120593
1198961(119905)
(50)
Suppose that 120593119896119899(119905) le 120593
119896119899(119905) holds for some 119899 ge 1
According to Lemma 6(iii) and Lemma 7 for all 119905 isin [120591119896minus1
120591119896]
we obtain
120593119896119899+1
(119905) = E sup119904isin[119905120591119896]
11989021205731199041003816100381610038161003816119884119899+2 (119904) minus 119884119899+1 (119904)
10038161003816100381610038162
le 1198622int
120591119896
119905
120579(E sup119903isin[119904120591119896]
11989021205731199031003816100381610038161003816119884119899+1 (119903) minus 119884119899 (119903)
10038161003816100381610038162
)119889119904
= 1198622int
120591119896
119905
120579 (120593119896119899(119904)) 119889119904
le 1198622int
120591119896
119905
120579 (120593119896119899(119904)) 119889119904 = 120593
119896119899+1(119905)
(51)
This implies that for all 119899 isin N
120593119896119899(119905) le 120593
119896119899(119905) (52)
By definition 120593119896119899(sdot) is continuous on [120591
119896minus1 120591
119896] Note that
for each 119899 ge 1 120593119896119899(sdot) is decreasing on [120591
119896minus1 120591
119896] and for each
119905 119896 120593119896119899(119905) is a nonincreasing sequence Therefore we define
the function 120593119896(119905) by 120593
119896119899(119905) darr 120593
119896(119905) It is easy to verify that
120593119896(119905) is continuous and nonincreasing on [120591
119896minus1 120591
119896] By the
definitions of 120593119896119899(119905) and 120593
119896(119905) we get
120593119896(119905) = lim
119899rarrinfin
1198622int
120591119896
119905
120579 (120593119896119899(119904)) 119889119904 = 119862
2int
120591119896
119905
120579 (120593119896(119904)) 119889119904
(53)
for all 120591119896minus1
le 119905 le 120591119896 Since int
0+(119889119906120579(119906)) = infin the Bihari
inequality implies
120593119896(119905) = 0 for each 119905 isin [120591
119896minus1 120591
119896] (54)
For each 119896 isin 1 2 119898 (52) and (54) yield
lim119899rarrinfin
120593119896119899(119905) = 0 (55)
Mathematical Problems in Engineering 9
Then
E sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899+1 (119904) minus 119884119899 (119904)
10038161003816100381610038162
le max1le119896le119898
E sup119904isin[120591119896minus1120591119896]
11989021205731199041003816100381610038161003816119884119899+1 (119904) minus 119884119899 (119904)
10038161003816100381610038162
= max1le119896le119898
120593119896119899(119905) 997888rarr 0
(56)
as 119899 rarr infin and this proves the assertion (49)By (36) we obtain
E( sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899+1 (119904) minus 119884119899 (119904)
10038161003816100381610038162
) + (1 minus24119872
2
119860119897
120573)E
times int
119879
119905
11989021205731199041003816100381610038161003816119885119899+1
(119904) minus 119885119899(119904)10038161003816100381610038162
119889119904
le24119872
2
119860
120573Eint
119879
119905
1198902120573119904
[120579 (1003816100381610038161003816119884119899 (119904) minus 119884119899minus1 (119904)
10038161003816100381610038162
)] 119889119904
(57)
Applying (49) to the above formula we see that (119884119899 119885
119899) is
a Cauchy (hence convergent) sequence in S2
F ([0 119879]119867) times
H2
F ([0 119879]L(Γ119867)) denote the limit by (119884 119885) Now letting119899 rarr infin in (27) we obtain that
119884 (119905) = 119890119860(119879minus119905)
120585
+ int
119879
119905
E1015840[119890
119860(119904minus119905)119891 (119904 119884
1015840
(119904) 1198851015840
(119904) 119884 (119904) 119885 (119904))] 119889119904
minus int
119879
119905
119890119860(119904minus119905)
119885 (119904) 119889119882 (119904)
(58)
holds on the entire interval [0 119879]The theorem is nowproved
To illustrate the application of the obtained existenceand uniqueness result we consider the example of backwardstochastic partial differential equations (BSPDEs) of mean-field type
Example 9 Let O be an open bounded domain in R119899
with uniformly 1198622 boundary 120597O let 119861(119905) be a standard 119899-dimensional Brownian motion (equipped with the normalfiltration) and let 120585 O rarr R be an F
119879-measurable
random variable We also let 119871 denote the semielliptic partialdifferential operator on 1198622
(R) of the form
119871 =
119899
sum
119894119895=1
119886119894119895(119909)
1205972
120597119909119894120597119909
119895
+
119899
sum
119894=1
119887119894(119909)
120597
120597119909119894
(59)
The aim is to study the solvability of the following initialboundary value problem
119889119884 (119905 119909)
= (119871119884 (119905 119909) + E1015840
times [119892 (119905 119909 1198841015840
(119905 119909) 1198851015840
(119905 119909) 119884 (119905 119909) 119885 (119905 119909))]) 119889119905
+ 119885 (119905 119909) 119889119861 (119905) ae on (0 119879) times O
119884 (119905 119909) = 0 ae on (0 119879) times 120597O
119884 (119879 119909) = 120585 (119879 119909) ae onO(60)
where
119884 [0 119879] times O 997888rarr R
119885 [0 119879] times O 997888rarr L (R119899 119871
2
(O))
119892 [0 119879] times O timesR timesL (R119899 119871
2
(O))
timesR timesL (R119899 119871
2
(O)) 997888rarr R
(61)
The following assumptions will have to be in force
(H1) 119886119894119895 119887
119894 R119899
rarr R are uniformly continuous andbounded and satisfy the usual uniform ellipticitycondition sum119899
119894119895=1119886119894119895(119909)119908
119894119908119895ge 120582|119908|
2 for some 120582 gt 0
and all 119909 isin O 119908 isin R119899(H2) 119892 is measurable in (119905 119909 119910 119910 119911) and continuous in
( 119911) and there exists 119862 gt 0 such that1003816100381610038161003816119892 (119905 119909 1199101 1 1199101 1199111) minus 119892 (119905 119909 1199102 2 1199102 1199112)
1003816100381610038161003816
le 119862 [10038161003816100381610038161199101 minus 1199102
1003816100381610038161003816 +10038161003816100381610038161 minus 2
1003816100381610038161003816 +10038161003816100381610038161199101 minus 1199102
1003816100381610038161003816 +10038161003816100381610038161199111 minus 1199112
1003816100381610038161003816]
(62)
for all 0 le 119905 le 119879 119909 isin O 1199101 119910
2 119910
1 119910
2isin R
1
2 119911
1 119911
2isin
L(R119899 119871
2(O))
Then we are now in a position of showing existence anduniqueness of the solution of BSPDEs (60)
Theorem 10 If (H1) and (H2) are satisfied then the mean-field BSPDE (60) has a unique mild solution (119884 119885) isin
1198712(0 119879 119871
2(Ω 119871
2(O))) times 1198712F (0 119879 119871
2(R119899
1198712(Ω 119871
2(O))))
Proof Let119867 = 1198712(O) and119870 = R119899 Define the operator 119860 by
119860119884 (119905 sdot) = 119871119884 (119905 sdot) (63)
It is shown in [17] (see Example 21 in [17]) that 119860 generatesa strongly continuous semigroup on 119867 Define the maps 119891
[0 119879] times 119867 timesL(119870119867) times 119867 timesL(119870119867) rarr 119867 by
119891 (119905 1198841015840
(119905) 1198851015840
(119905) 119884 (119905) 119885 (119905)) (119909)
= 119892 (119905 119909 1198841015840
(119905 119909) 1198851015840
(119905 119909) 119884 (119905 119909) 119885 (119905 119909))
(64)
10 Mathematical Problems in Engineering
for all 0 le 119905 le 119879 119909 isin O With these identifications(60) can be written in the form of (11) By (H2) we know119891 satisfy condition (A1) Hence an application of Theorem 4concludes that (60) has a unique mild solution (119884 119885) isin
1198712(0 119879 119871
2(Ω 119871
2(O))) times 1198712F (0 119879 119871
2(R119899
1198712(Ω 119871
2(O))))
4 Mean-Field Stochastic Evolution Equations
Let 119882(119905) 119905 isin [0 119879] be a cylindrical Wiener process withvalues in a Hilbert space Γ defined on a probability space(ΩFP) We fix an interval [119905 119879] sub [0 119879] and considerthe stochastic evolution equations of mean-field type for anunknown process 119883(119904) 119904 isin [119905 119879] with values in a Hilbertspace 119870
119889119883 (119904) = 119861119883 (119904) 119889119904 + E1015840[119887 (119904 119883
1015840
(119904) 119883 (119904))] 119889119904
+ E1015840[120590 (119904 119883
1015840
(119904) 119883 (119904))] 119889119882 (119904)
119883 (119905) = 119909 isin 119870
(65)
where operator 119861 is the generator of a strongly continuoussemigroup 119890
119905119861 119905 ge 0 in the Hilbert space 119870 with 119872119861≜
sup119905isin[0119879]
|119890119905119861|
By a mild solution of (65) we mean an F119904-measurable
process 119883(119904) 119904 isin [119905 119879] with continuous paths in 119870 suchthat P-as
119883 (119904) = 119890119861(119904minus119905)
119909
+ int
119904
119905
119890119861(119904minus120591)
E1015840[119887 (120591 119883
1015840
(120591) 119883 (120591))] 119889120591
+ int
119904
119905
119890119861(119904minus120591)
E1015840[120590 (120591119883
1015840
(120591) 119883 (120591))] 119889119882 (120591)
119904 isin [119905 119879]
(66)
We suppose the following(A4) 119887 [0 119879] times 119870 times 119870 rarr 119870 is a measurable mapping
which satisfies10038161003816100381610038161003816119887 (119905 119909
1015840 119909) minus 119887 (119905 119910
1015840 119910)
10038161003816100381610038161003816
2
le 1198711(100381610038161003816100381610038161199091015840minus 119910
101584010038161003816100381610038161003816
2
+1003816100381610038161003816119909 minus 119910
10038161003816100381610038162
)
119905 isin [0 119879] 1199091015840 119909 119910
1015840 119910 isin 119870
(67)
for some constant 1198711gt 0
(A5) The mapping 120590 [0 119879] times 119870 times 119870 rarr L(Γ 119870) fulfillsthat for every V isin Γ the map 120590V [0 119879] times 119870 times 119870 rarr 119870
is measurable for every 119904 gt 0 119905 isin [0 119879] 1199091015840 1199101015840 119909 119910 isin 119870119890119904119861120590(119905 119909
1015840 119909) isin L(Γ 119870) and
10038161003816100381610038161003816119890119904119861120590 (119905 119909
1015840 119909)
10038161003816100381610038161003816L(Γ119870)le 119871
2119904minus120574(1 +
10038161003816100381610038161003816119909101584010038161003816100381610038161003816+ |119909|)
10038161003816100381610038161003816119890119904119861120590 (119905 119909
1015840 119909) minus 119890
119904119861120590 (119905 119910
1015840 119910)
10038161003816100381610038161003816L(Γ119870)
le 1198712119904minus120574(100381610038161003816100381610038161199091015840minus 119910
101584010038161003816100381610038161003816+1003816100381610038161003816119909 minus 119910
1003816100381610038161003816)
10038161003816100381610038161003816120590 (119905 119909
1015840 119909)
10038161003816100381610038161003816L(Γ119870)le 119871
2(1 +
10038161003816100381610038161003816119909101584010038161003816100381610038161003816+ |119909|)
(68)
for some constants 1198712gt 0 and 120574 isin [0 12)
Theorem 11 Under assumptions (A3) and (A4) (65) has aunique mild solution119883(sdot) isin S2
F ([119905 119879] 119870)
The proof is constructed in two steps like that ofTheorem 4 and it uses standard arguments for stochastic evo-lution equations introduced in the proof of Proposition 32 in[3] Since the proof is straightforward we prefer to omit it
Remark 12 In our paper Lipchitz condtion (A4) is givento get the well-posedness of mean-field stochastic evolutionequations In fact (A4) can be replaced by a weaker conditionsuch as (A3) We just give the condition (A4) for simplicity
From standard arguments we can also get the followingcontinuous dependence theorem
Corollary 13 Assume that for all 120572 in a metric space 119865(119887120572 120590
120572) satisfy (A4) and (A5)with119871
1and119871
2independent of 120572
Also assume that
Eint119879
0
10038161003816100381610038161003816E1015840[119887120572(119904 119883
1015840
(119904) 119883 (119904))]
minus E1015840[1198871205720(119904 119883
1015840
(119904) 119883 (119904))]10038161003816100381610038161003816
2
119889119904 997888rarr 0
Eint119879
0
10038161003816100381610038161003816E1015840[120590
120572(119904 119883
1015840
(119904) 119883 (119904))]
minusE1015840[120590
1205720(119904 119883
1015840
(119904) 119883 (119904))]10038161003816100381610038161003816
2
119889119904 997888rarr 0
(69)
as 120572 rarr 1205720for all119883 isin S2
F ([0 119879] 119870)If we denote by 119883120572
(sdot) the mild solution of mean-field SEE(65) corresponding to the functions (119887
120572 120590
120572) and to the initial
data 119909 then we have
sup119904isin[0119879]
E1003816100381610038161003816119883
120572
(119904) minus 1198831205720 (119904)
10038161003816100381610038162
997888rarr 0 119886119904 120572 997888rarr 1205720 (70)
5 Maximum Principle for BSPDEs ofMean-Field Type
51 Formulation of the Problem Let O isin R119899 be a boundedopen set with smooth boundary 120597O and let 119880 the space ofcontrols be a separable real Hilbert space We denote
U = V (sdot) isin 1198712F(0 119879 119880)
| V119905(120596
1015840 120596) [0 119879] times Ω times Ω
997888rarr 119880 is F otimesF119905-progressively measurable
(71)
Mathematical Problems in Engineering 11
An element ofU is called an admissible controlFor any V isin U we consider the following controlled
BSPDE system in the state space119867 = 1198712(O) (norm | sdot | scalar
product ⟨sdot sdot⟩)
119889119884119905(119909) = minus119860119884
119905(119909) 119889119905
minus E1015840[119891 (119905 119909 (119884
119905(119909))
1015840
(119885119905(119909))
1015840
119884119905(119909) 119885
119905(119909) V
119905)] 119889119905
+ 119885119905(119909) 119889119882 (119905) 119905 isin [0 119879]
119884119879(119909) = 120585 (119909) 119909 isin O
(72)
where 119860 is a partial differential operator 119891 [0 119879] timesO times119867 times
L(Γ119867)times119867timesL(Γ119867)times119880 rarr 119867 and 120585 isin 1198712(ΩF119879P 119867)
The cost functional is given by
119869 (V) = Eint119879
0
intO
E1015840[ℎ (119904 119909 (119884
119904(119909))
1015840
(119885119904(119909))
1015840
119884119904(119909) 119885
119904(119909) V
119904)] 119889119909 119889119904
+E1015840intO
119892 (119909 (1198840(119909))
1015840
1198840(119909)) 119889119909
(73)
where
ℎ [0 119879] times O times 119867 timesL (Γ119867) times 119867 timesL (Γ119867) times 119880 997888rarr R
119892 O times 119867 times119867 997888rarr R
(74)
Our purpose is to minimize the functional 119869(sdot) over Uadsubject to the following state constraint
EintO
Φ(119909 (1198840(119909))
1015840
1198840(119909)) 119889119909 = 0 (75)
where
Φ O times 119867 times119867 997888rarr R (76)
An admissible control 119906 isin Uad that satisfies
119869 (119906) = minVisinUad
119869 (V) (77)
is called optimalThrough what follows the following assumptions will be
in force
(L1) 119860 is a partial differential operator with appropri-ate boundary conditions We assume that 119860 is theinfinitesimal generator of a strongly continuous semi-group 119890119905119860 119905 ge 0 in119867 Moreover for every 119905 isin [0 119879]119890
119905119860119891
1198712(O) le 119872
119860119891
1198712(O) for some constant 119872
119860
independent of 119905 and 119891(L2) 119891 ℎ 119892 and Φ are continuously Gateaux differen-
tiable with respect to (1199101015840 1199111015840 119910 119911) 119891 is continuouslyGateaux differentiable with respect to V and ℎ iscontinuous with respect to V
(L3) The derivatives of 119891 ℎ 119892 and Φ are Lipschitzcontinuous and bounded by
100381610038161003816100381610038161198911199101015840
10038161003816100381610038161003816+10038161003816100381610038161198911199111015840
1003816100381610038161003816 +10038161003816100381610038161003816119891119910
10038161003816100381610038161003816+1003816100381610038161003816119891119911
1003816100381610038161003816 +1003816100381610038161003816119891V
1003816100381610038161003816 +10038161003816100381610038161003816Φ1199101015840
10038161003816100381610038161003816+10038161003816100381610038161003816Φ119910
10038161003816100381610038161003816le 119862
10038161003816100381610038161003816ℎ1199101015840
10038161003816100381610038161003816+1003816100381610038161003816ℎ1199111015840
1003816100381610038161003816 +10038161003816100381610038161003816ℎ119910
10038161003816100381610038161003816+1003816100381610038161003816ℎ119911
1003816100381610038161003816 +100381610038161003816100381610038161198921199101015840
10038161003816100381610038161003816+10038161003816100381610038161003816119892119910
10038161003816100381610038161003816
le 119862 (1 +10038161003816100381610038161003816119910101584010038161003816100381610038161003816+10038161003816100381610038161003816119911101584010038161003816100381610038161003816+10038161003816100381610038161199101003816100381610038161003816 + |119911| + |V|)
(78)
where 119862 is a positive constant
Obviously according to Theorem 4 state equation (72)has a unique mild solution under the above assumptions
Remark 14 We can define the second order differentialoperator
(119860119891) (119909) =
119899
sum
119894119895=1
119886119894119895(119909)
1205972119891
120597119909119894120597119909
119895
(119909) +
119899
sum
119894=1
119887119894(119909)
120597119891
120597119909119894
(119909) (79)
By Example 9 119860 fulfills assumption (L1) if 119886119894119895 119887
119894satisfy
condition (H1)
52 Variation of the Trajectory Let 119906 be an optimal controlwith (119884(sdot) 119885(sdot)) being the corresponding optimal state Let120576 gt 0 and [119903 119903 + 120576] sube [0 119879] For any given V isin Uad weintroduce the spike variation of the control 119906(sdot)
119906120576
119905=
V119905 119905 isin [119903 119903 + 120576]
119906119905 119905 isin [0 119879] [119904 119904 + 120576]
(80)
It is clear that 119906120576(sdot) isin UadLet (119884120576
(sdot) 119885120576(sdot)) be the trajectory corresponding to 119906120576(sdot)
We use the following short notation for brevity
119891 (119905) = 119891 (119905 119909 (119884119905(119909))
1015840
(119885119905(119909))
1015840
119884119905(119909) 119885
119905(119909) 119906
119905)
119891 (119906120576
119905) = 119891 (119905 119909 (119884
119905(119909))
1015840
(119885119905(119909))
1015840
119884119905(119909) 119885
119905(119909) 119906
120576
119905)
(81)
Consider the following equation
119889119870120576
119905(119909) = minus119860119870
120576
119905(119909) 119889119905
minus E1015840[119891
1199101015840 (119905) (119870
120576
119905(119909))
1015840
+ 119891119910(119905) 119870
120576
119905(119909)
+ 1198911199111015840 (119905) (119876
120576
119905(119909))
1015840
+ 119891119911(119905) 119876
120576
119905(119909)
+1
120576(119891 (119906
120576
119905) minus 119891 (119905))] 119889119905 + 119876
120576
119905(119909) 119889119882 (119905)
119870120576
119879(119909) = 0
(82)
Since the coefficients in (82) are bounded it is easy to checkthat there exists a unique mild solution such that
E[ sup119905isin[0119879]
1003816100381610038161003816119870120576
119905
10038161003816100381610038162
+ int
119879
0
1003816100381610038161003816119876120576
119905
10038161003816100381610038162
119889119905] lt infin (83)
We have the following estimate
12 Mathematical Problems in Engineering
Theorem 15 There holds
lim120576rarr0
E[ sup119904isin[119905119879]
1003816100381610038161003816100381610038161003816100381610038161003816
119884120576
119904minus 119884
119904
120576minus 119870
120576
119904
1003816100381610038161003816100381610038161003816100381610038161003816
2
] = 0 forall119905 isin [0 119879]
lim120576rarr0
Eint119879
0
1003816100381610038161003816100381610038161003816100381610038161003816
119885120576
119904minus 119885
119904
120576minus 119876
120576
119904
1003816100381610038161003816100381610038161003816100381610038161003816
2
119889119904 = 0
(84)
Proof We define
120578120576
119904=119884120576
119904minus 119884
119904
120576minus 119870
120576
119904 120577
120576
119904=119885120576
119904minus 119885
119904
120576minus 119876
120576
119904 119904 isin [0 119879]
(85)
For simplicity let us define
Λ120576
119904= ((119884
120576
119904)1015840
(119885120576
119904)1015840
119884120576
119904 119885
120576
119904)
119891 (119904 120582) = 119891 (119904 1198841015840
119904+ 120582(119884
120576
119904minus 119884
119904)1015840
1198851015840
119904+ 120582(119885
120576
119904minus 119885
119904)1015840
119884119904+ 120582 (119884
120576
119904minus 119884
119904)
119885119904+ 120582 (119885
120576
119904minus 119885
119904) 119906
120576
119904)
(86)
By the definition of (119884120576
119904 119885
120576
119904) (119884
119904 119885
119904) and (119870120576
119904 119876
120576
119904) (120578120576
119904 120577
120576
119904)
is the mild solution of
119889120578120576
119904= minus119860120578
120576
119904119889119904 minus E
1015840
[119871 (119904 120576)] 119889119904 + 120577120576
119904119889119882 (119904)
120578120576
119879= 0
(87)
with
119871 (119904 120576) =1
120576(119891 (119904 Λ
120576
119904 119906
120576
119904) minus 119891 (119906
120576
119904))
minus 1198911199101015840 (119904) (119870
120576
119904)1015840
minus 119891119910(119904) 119870
120576
119904minus 119891
1199111015840 (119904) (119876
120576
119904)1015840
minus 119891119911(119904) 119876
120576
119904
= ((120578120576
119904)1015840
+ (119870120576
119904)1015840
) int
1
0
1198911199101015840 (119904 120582) 119889120582 + (120578
120576
119904+ 119870
120576
119904)
times int
1
0
119891119910(119904 120582) 119889120582 minus 119891
1199101015840 (119904) (119870
120576
119904)1015840
minus 119891119910(119904) 119870
120576
119904
+ ((120577120576
119904)1015840
+ (119876120576
119904)1015840
)int
1
0
1198911199111015840 (119904 120582) 119889120582 + (120577
120576
119904+ 119876
120576
119904)
times int
1
0
119891119911(119904 120582) 119889120582 minus 119891
1199111015840 (119904) (119876
120576
119904)1015840
minus 119891119911(119904) 119876
120576
119904
= (120578120576
119904)1015840
int
1
0
1198911199101015840 (119904 120582) 119889120582 + 120578
120576
119904int
1
0
119891119910(119904 120582) 119889120582
+ (120577120576
119904)1015840
int
1
0
1198911199111015840 (119904 120582) 119889120582 + 120577
120576
119904int
1
0
119891119911(119904 120582) 119889120582 + 120574
120576
119904
(88)
where we denote
120574120576
119904= (119870
120576
119904)1015840
int
1
0
(1198911199101015840 (119904 120582) minus 119891
1199101015840 (119904)) 119889120582
+ 119870120576
119904int
1
0
(119891119910(119904 120582) minus 119891
119910(119904)) 119889120582
+ (119876120576
119904)1015840
int
1
0
(1198911199111015840 (119904 120582) minus 119891
1199111015840 (119904)) 119889120582
+ 119876120576
119904int
1
0
(119891119911(119904 120582) minus 119891
119911(119904)) 119889120582
(89)
For any 120573 gt 0 according to Lemma 1 we obtain
E sup119905le119904le119879
11989021205731199041003816100381610038161003816120578
120576
119904
10038161003816100381610038162
+ Eint119879
119905
11989021205731199041003816100381610038161003816120577
120576
119904
10038161003816100381610038162
119889119904
le12119872
2
119860
120573int
119879
119905
1198902120573119904
E [10038161003816100381610038161003816E1015840
[119871 (119904 120576)]10038161003816100381610038161003816
2
] 119889119904
le12119872
2
119860
120573Eint
119879
119905
1198902120573119904
E1015840[|119871 (119904 120576)|
2] 119889119904
(90)
By condition (L3) we have
E [E1015840[|119871 (119904 120576)|
2]]
= E [E1015840[1003816100381610038161003816119871 (119904 120576) minus 120574
120576
119904+ 120574
120576
119904
10038161003816100381610038162
]]
le 81198622E [E
1015840[10038161003816100381610038161003816(120578
120576
119904)101584010038161003816100381610038161003816
2
+1003816100381610038161003816120578120576
119904
10038161003816100381610038162
+10038161003816100381610038161003816(120577
120576
119904)101584010038161003816100381610038161003816
2
+1003816100381610038161003816120577120576
119904
10038161003816100381610038162
]]
+ 2E [E1015840[1003816100381610038161003816120574120576
119904
10038161003816100381610038162
]]
le 161198622E [
1003816100381610038161003816120578120576
119904
10038161003816100381610038162
+1003816100381610038161003816120577120576
119904
10038161003816100381610038162
] + 2E [E1015840[1003816100381610038161003816120574120576
119904
10038161003816100381610038162
]]
(91)
Combined with (91) (90) yields
E sup119905le119904le119879
11989021205731199041003816100381610038161003816120578
120576
119904
10038161003816100381610038162
+ (1 minus192119872
2
1198601198622
120573)Eint
119879
119905
11989021205731199041003816100381610038161003816120577
120576
119904
10038161003816100381610038162
119889119904
le192119872
2
1198601198622
120573Eint
119879
119905
11989021205731199041003816100381610038161003816120578
120576
119904
10038161003816100381610038162
119889119904
+24119872
2
119860
120573Eint
119879
119905
1198902120573119904
[E1015840[1003816100381610038161003816120574120576
119904
10038161003816100381610038162
]] 119889119904
(92)
We claim that
Eint119879
119905
E1015840[1003816100381610038161003816120574120576
119904
10038161003816100381610038162
] 119889119904 997888rarr 0 as 120576 997888rarr 0 (93)
From (89)
120574120576
119904= 119868
1
119904+ 119868
2
119904+ 119868
3
119904+ 119868
4
119904 (94)
Mathematical Problems in Engineering 13
where
1198681
119904= (119870
120576
119904)1015840
int
1
0
(1198911199101015840 (119904 120582) minus 119891
1199101015840 (119904)) 119889120582
1198682
119904= 119870
120576
119904int
1
0
(119891119910(119904 120582) minus 119891
119910(119904)) 119889120582
1198683
119904= (119876
120576
119904)1015840
int
1
0
(1198911199111015840 (119904 120582) minus 119891
1199111015840 (119904)) 119889120582
1198684
119904= 119876
120576
119904int
1
0
(119891119911(119904 120582) minus 119891
119911(119904)) 119889120582
(95)
Then
Eint119879
119905
E1015840[1003816100381610038161003816120574120576
119904
10038161003816100381610038162
] 119889119904
le 4Eint119879
119905
E1015840[100381610038161003816100381610038161198681
119904
10038161003816100381610038161003816
2
+100381610038161003816100381610038161198682
119904
10038161003816100381610038161003816
2
+100381610038161003816100381610038161198683
119904
10038161003816100381610038161003816
2
+100381610038161003816100381610038161198684
119904
10038161003816100381610038161003816
2
] 119889119904
(96)
Take Eint119879119905E1015840[|1198682
119904|2
]119889119904 for example
Eint119879
119905
E1015840[100381610038161003816100381610038161198682
119904
10038161003816100381610038161003816
2
] 119889119904
= Eint119879
119905
E1015840[(119870
120576
119904int
1
0
(119891119910(119904 120582) minus 119891
119910(119904)) 119889120582)
2
]119889119904
le Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119904 120582) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582] 119889119904
le 2Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119904 120582) minus 119891
119910(119906
120576
119905)10038161003816100381610038161003816
2
119889120582] 119889119904
+ 2Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119906
120576
119905) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582] 119889119904
(97)
Note that
Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119906
120576
119905) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582] 119889119904
= Eint119903+120576
119903
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119906
120576
119905) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582] 119889119904
le sup119904isin[119905119879]
E [E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119906
120576
119905) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582]] 120576
997888rarr 0 as 120576 997888rarr 0
(98)
The inequality above holds due to the boundedness of|119870
120576
119904|2
int1
0|119891119910(119906
120576
119905) minus 119891
119910(119904)|
2
119889120582 Indeed Assumption (L3) im-plies the boundedness of 119891
119910(119906
120576
119905) minus 119891
119910(119904) Meanwhile 119870120576
119904is
the solution of mean-field BSEE (82) It can be easy to check119870120576
119904is bounded since the coefficients in (82) are boundedOn the other hand
Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119904 120582) minus 119891
119910(119906
120576
119905)10038161003816100381610038161003816
2
119889120582] 119889119904
le 11986221205822Eint
119879
119905
1003816100381610038161003816119870120576
119904
10038161003816100381610038162
times int
1
0
1198641015840[100381610038161003816100381610038161003816(119884
120576
119904minus 119884
119904)1015840100381610038161003816100381610038161003816
2
+100381610038161003816100381610038161003816(119885
120576
119904minus 119885
119904)1015840100381610038161003816100381610038161003816
2
times10038161003816100381610038161003816119884120576
119904minus 119884
119904
10038161003816100381610038161003816
2
+10038161003816100381610038161003816119885120576
119904minus 119885
119904
10038161003816100381610038161003816
2
] 119889120582 119889119904
(99)
where (119884120576
119904 119885
120576
119904) is the mild solution of the following equation
119889119884120576
119905= minus 119860119884
120576
119905119889119905
minus E1015840[119891 (119905 (119884
120576
119905)1015840
(119885120576
119905)1015840
119884120576
119905 119885
120576
119905 119906
120576
119905)] 119889119905
+ 119885120576
119905119889119882 (119905) 119905 isin [0 119879]
119884120576
119879= 120585
(100)
and (119884119904 119885
119904) is the mild solution of
119889119884119905= minus 119860119884
119905119889119905
minus E1015840[119891 (119905 (119884
119905)1015840
(119885119905)1015840
119884119905 119885
119905 119906
119905)] 119889119905
+ 119885119905119889119882 (119905) 119905 isin [0 119879]
119884119879= 120585
(101)
By the definition of 119906120576119905 according to (L2) we have
E1015840[119891 (119904 119884
1015840
(119904) 1198851015840
(119904) 119884 (119904) 119885 (119904) 119906120576
119905)]
997888rarr E1015840[119891 (119904 119884
1015840
(119904) 1198851015840
(119904) 119884 (119904) 119885 (119904) 119906119905)]
(102)
in 1198712([0 119879]119867) as 120576 rarr 0 Using the continuous dependencetheorem Corollary 5 we obtain
(119884120576
119904 119885
120576
119904) 997888rarr (119884
119904 119885
119904) as 120576 997888rarr 0 (103)
Then
Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119904 120582) minus 119891
119910(119906
120576
119905)10038161003816100381610038161003816
2
119889120582] 119889119904
le 11986221205822Eint
119879
119905
1003816100381610038161003816119870120576
119904
10038161003816100381610038162
times int
1
0
E1015840[100381610038161003816100381610038161003816(119884
120576
119904minus 119884
119904)1015840100381610038161003816100381610038161003816
2
+100381610038161003816100381610038161003816(119885
120576
119904minus 119885
119904)1015840100381610038161003816100381610038161003816
2
times10038161003816100381610038161003816119884120576
119904minus 119884
119904
10038161003816100381610038161003816
2
+10038161003816100381610038161003816119885120576
119904minus 119885
119904
10038161003816100381610038161003816
2
] 119889120582 119889119904
997888rarr 0 as 120576 997888rarr 0
(104)
14 Mathematical Problems in Engineering
Combining (98) with (104) we finally haveEint
119879
119905E1015840[|1198682
119904|2
]119889119904 rarr 0 as 120576 rarr 0The required result (93) follows by using the similar
estimations for 1198681119904 1198683
119904 and 1198684
119904
Note that 1 minus 1921198722
1198601198622120573 gt 0 if 120573 is sufficiently large
Now to prove the desired result (84) it suffices to applyGronwallrsquos lemma and estimate (93) to inequality (92)
To deal with the state constraint (75) we need to recall theEkeland variational principle
Lemma 16 (Ekelandrsquos variational principle see [16 Lemma41]) Let (119878 119889) be a complete metric space and let 119865(sdot) 119878 rarr
R be lower semicontinuous and bounded from below If for 120588 gt0 there exists 119906 isin 119878 such that
119865 (119906) le infVisin119878
119865 (V) + 120588 (105)
then there exists 119906120588 isin 119878 satisfying
(i) 119865 (119906120588) le 119865 (119906)
(ii) 119889 (119906120588 119906) le 120588
(iii) 119865 (119906120588) le 119865 (V) + 120588 sdot 119889 (119906120588 119906) forallV = 119906120588
(106)
Now fix V isin Uad and set
119878 = V (sdot) isin Uad | sup0le119905le119879
E1003816100381610038161003816V11990510038161003816100381610038162
le E100381610038161003816100381611990611990510038161003816100381610038162
+ |V|2
119889 (V (sdot) V (sdot)) = 119898 119905 isin [0 119879] | E1003816100381610038161003816V119905 minus V
119905
10038161003816100381610038162
gt 0
forallV (sdot) V (sdot) isin 119878
(107)
where119898 denotes the Lebesgue measure on RThe following result is proved as Proposition 41 in [16]
Lemma 17 (119878 119889(sdot sdot)) is a complete metric space and 119869120588 is
continuous and bounded on 119878 where
119869120588
(V (sdot)) = (119869 (V (sdot)) minus 119869 (119906 (sdot)) + 120588)2
+
10038161003816100381610038161003816100381610038161003816Eint
O
Φ(119909 (1198840(119909))
1015840
1198840(119909)) 119889119909
10038161003816100381610038161003816100381610038161003816
2
12
forallV (sdot) isin 119878(108)
and (119884 119885) is the mild solution of (72) corresponding to thecontrol V
Now we consider the following free initial state optimalcontrol problem
infV(sdot)isin119878
119869120588
(V (sdot)) (109)
It is easy to check that
0 le infV(sdot)isin119878
119869120588
(V (sdot)) le 119869120588
(119906 (sdot)) = 120588 (110)
According to Ekelandrsquos variational principle there exists a119906120588(sdot) isin 119881 such that
(i) 119869120588 (119906120588 (sdot)) le 120588
(ii) 119889 (119906120588 (sdot) 119906 (sdot)) le 120588
(iii) 119869120588 (119906120588 (sdot)) le 119869120588
(V (sdot)) + 120588119889 (119906120588 (sdot) 119906 (sdot)) forallV (sdot) isin 119878(111)
Using the spike variationmethod we can construct 119906120576120588(sdot) isin 119878as follows
119906120576120588
119905=
V119905 119905 isin [119904 119904 + 120576]
119906120588
119905 119905 isin [0 119879] [119904 119904 + 120576]
(112)
It is clear that 119889(119906120576120588(sdot) 119906120588(sdot)) le 120576 Let (119884120576120588(sdot) 119885
120576120588(sdot)) (resp
(119884120588(sdot) 119885
120588(sdot))) be the solution of (72) with respect to the
control 119906120576120588(sdot) (resp 119906120588(sdot)) Following (82) (119870120576120588
119905 119876
120576120588
119905) is the
mild solution of
119870120576120588
119905= int
119879
119905
119890119860(119904minus119905)
E1015840
times [1198911199101015840 (119904 Λ
120588
119904 119906
120588
119904) (119870
120576120588
119904)1015840
+ 119891119910(119904 Λ
120588
119904 119906
120588
119904)119870
120576120588
119904
+ 1198911199111015840 (119904 Λ
120588
119904 119906
120588
119904) (119876
120576120588
119904)1015840
+ 119891119911(119904 Λ
120588
119904 119906
120588
119904) 119876
120576120588
119904
+1
120576(119891 (119904 Λ
120588
119904 119906
120576120588
119904) minus 119891 (119904 Λ
120588
119904 119906
120588
119904))] 119889119904
minus int
119879
119905
119890119860(119904minus119905)
119876120576120588
119904119889119882 (119904)
(113)
ByTheorem 15 we know that
lim120576rarr0
E[ sup119904isin[119905119879]
100381610038161003816100381610038161003816100381610038161003816
119884120576120588
119904minus 119884
120588
119904
120576minus 119870
120576120588
119904
100381610038161003816100381610038161003816100381610038161003816
2
] = 0 forall119905 isin [0 119879]
lim120576rarr0
Eint119879
0
100381610038161003816100381610038161003816100381610038161003816
119885120576120588
119904minus 119885
120588
119904
120576minus 119876
120576120588
119904
100381610038161003816100381610038161003816100381610038161003816
2
119889119904 = 0
(114)
The proof of the following proposition is technical butbased on the arguments above and we omit it
Proposition 18 One has
1
120576E [Φ ((119884
120576120588
0)1015840
119884120576120588
0) minus Φ((119884
120588
0)1015840
119884120588
0)]
= E [Φ1199101015840 ((119884
120588
0)1015840
119884120588
0) (119870
120576120588
0)1015840
+ Φ119910((119884
120588
0)1015840
119884120588
0)119870
120576120588
0]
+ 119900 (120576)
Mathematical Problems in Engineering 15
1
120576(119869 (119906
120576120588) minus 119869 (119906
120588))
= E [1198921199101015840 ((119884
120588
0)1015840
119884120588
0) (119870
120576120588
0)1015840
+ 119892119910((119884
120588
0)1015840
119884120588
0)119870
120576120588
0]
+ Δ120576+1
120576Eint
119879
0
E1015840[ℎ (119904 Λ
120588
119904 119906
120576120588
119904)
minus ℎ (119904 Λ120588
119904 119906
120588
119904)] 119889119904 + 119900 (120576)
(115)
where
Δ120576= Eint
119879
0
E1015840[ℎ
1199101015840 (119904 Λ
120588
119904 119906
120576120588
119904) (119870
120576120588
119904)1015840
+ ℎ1199111015840 (119904 Λ
120588
119904 119906
120576120588
119904) (119876
120576120588
119904)1015840
+ ℎ119910(119904 Λ
120588
119904 119906
120576120588
119904)119870
120576120588
119904
+ ℎ119911(119904 Λ
120588
119904 119906
120576120588
119904) 119876
120576120588
119904] 119889119904
(116)
53 Variational Inequality and Adjoint Equation In thissubsection the adjoint process is introduced to deduce thevariational inequality
If we set V(sdot) = 119906120576120588(sdot) in (111) and notice that
119889(119906120576120588(sdot) 119906
120588(sdot)) le 120576 we get
minus120588 le1
120576(119869
120588(119906
120576120588
(sdot)) minus 119869120588(119906
120588
(sdot))) (117)
By Lemma 17
1
120576(119869
120588(119906
120576120588
(sdot)) minus 119869120588(119906
120588
(sdot)))
=(119869
120588(119906
120576120588(sdot)))
2
minus (119869120588(119906
120588(sdot)))
2
120576 (119869120588 (119906120576120588 (sdot)) + 119869120588 (119906120588 (sdot)))
=119869 (119906
120576120588(sdot)) + 119869 (119906
120588(sdot)) minus 2119869 (119906 (sdot)) + 2120588
119869120588 (119906120576120588 (sdot)) + 119869120588 (119906120588 (sdot))
times119869 (119906
120576120588(sdot)) minus 119869 (119906
120588(sdot))
120576
+
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] + E [Φ ((119884
120588
0)1015840
119884120588
0)]
119869120588 (119906120576120588 (sdot)) + 119869120588 (119906120588 (sdot))
times
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] minus E [Φ ((119884
120588
0)1015840
119884120588
0)]
120576
997888rarr 119897120588
1
119869 (119906120576120588(sdot)) minus 119869 (119906
120588(sdot))
120576
+ 119897120588
2
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] minus E [Φ ((119884
120588
0)1015840
119884120588
0)]
120576
(118)
where we set
119897120588
1=119869 (119906
120588(sdot)) minus 119869 (119906 (sdot)) + 120588
119869120588 (119906120588 (sdot))
119897120588
2=
E [Φ ((119884120588
0)1015840
119884120588
0)]
119869120588 (119906120588 (sdot))
(119)
and use the limit
119869 (119906120576120588
(sdot)) 997888rarr 119869 (119906120588
(sdot))
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] 997888rarr E [Φ ((119884
120588
0)1015840
119884120588
0)]
(120)
as 120576 rarr 0 according to (115)As |119897120588
1|2
+ |119897120588
2|2
= 1 for all 120588 gt 0 we know that there existsa subsequence of 119897120588
1 119897120588
2 (still denoted by 119897120588
1 119897120588
2) such that
lim120588rarr0
119897120588
1 119897120588
2 = 119897
1 1198972
1003816100381610038161003816119897110038161003816100381610038162
+1003816100381610038161003816119897210038161003816100381610038162
= 1
(121)
Combining (115) (117) with (118) we get
minus120588 le 119897120588
1
119869 (119906120576120588(sdot)) minus 119869 (119906
120588(sdot))
120576
+ 119897120588
2
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] minus E [Φ ((119884
120588
0)1015840
119884120588
0)]
120576
= 119897120588
1Δ120576+ 119897
120588
1E [119892
1199101015840 ((119884
120588
0)1015840
119884120588
0) (119870
120576120588
0)1015840
+119892119910((119884
120588
0)1015840
119884120588
0)119870
120576120588
0]
+119897120588
1
120576Eint
119879
0
E1015840[ℎ (119904 Λ
120588
119904 119906
120576120588
119904) minus ℎ (119904 Λ
120588
119904 119906
120588
119904)] 119889119904
+ 119897120588
2E [Φ
1199101015840 ((119884
120588
0)1015840
119884120588
0) (119870
120576120588
0)1015840
+Φ119910((119884
120588
0)1015840
119884120588
0)119870
120576120588
0] + (119897
120588
1+ 119897
120588
2) 119900 (120576)
(122)
Next we introduce the adjoint equation corresponding tovariational equation (113) whose solution is denoted by119875120588(119905)
119889119875120588
(119905)
= 119860lowast119875120588
(119905) 119889119905
+ E1015840[119891
1199101015840 (119905 Λ
120588
119905 119906
120588
119905) (119875
120588
(119905))1015840
+ 119891119910(119905 Λ
120588
119905 119906
120588
119905) 119875
120588
(119905) + 119897120588
1ℎ1199101015840 (119905 Λ
120588
119905 119906
120588
119905)
+ 119897120588
1ℎ119910(119905 Λ
120588
119905 119906
120588
119905)] 119889119905
16 Mathematical Problems in Engineering
+ E1015840[119891
1199111015840 (119905 Λ
120588
119905 119906
120588
119905) (119875
120588
(119905))1015840
+ 119891119911(119905 Λ
120588
119905 119906
120588
119905) 119875
120588
(119905)
+ 119897120588
1ℎ1199111015840 (119905 Λ
120588
119905 119906
120588
119905) + 119897
120588
1ℎ119911(119905 Λ
120588
119905 119906
120588
119905)] 119889119882 (119905)
119875120588
(0) = 119897120588
1E [119892
1199101015840 ((119884
120588
0(119909))
1015840
119884120588
0(119909))
+ 119892119910((119884
120588
0(119909))
1015840
119884120588
0(119909))]
+ 119897120588
2E [Φ
1199101015840 ((119884
120588
0(119909))
1015840
119884120588
0(119909))
+ Φ119910((119884
120588
0(119909))
1015840
119884120588
0(119909))]
(123)
where 119860lowast is the 1198712(O)-adjoint operator of 119860 Under assump-tions (L1)ndash(L3) this is a linear mean-field SEE with boundedcoefficients An application ofTheorem 11 implies that it has aunique adaptedmild solution such that119875120588(119905) isin S2
F ([0 119879] 119870)When 120588 rarr 0 according to Corollaries 5 and 13 119875120588(119905)
converges to 119875(119905) where
119875 (119905) isin S2
F ([0 119879] 119870) (124)
is the solution of the following equation
119875 (119905) = 1198971E [119892
1199101015840 (119884
1015840
0 119884
0) + 119892
119910(119884
1015840
0 119884
0)]
+ 1198972E [Φ
1199101015840 (119884
1015840
0 119884
0) + Φ
119910(119884
1015840
0 119884
0)]
+ int
119905
0
119890119860lowast(119905minus119904)
E1015840[119891
1199101015840 (119904) 119875
1015840
(119904) + 119891119910(119904) 119875 (119904)
+ 1198971ℎ1199101015840 (119904) + 119897
1ℎ119910(119904)] 119889119904
+ int
119905
0
119890119860lowast(119905minus119904)
E1015840[119891
1199111015840 (119904) 119875
1015840
(119904) + 119891119911(119904) 119875 (119904)
+ 1198971ℎ1199111015840 (119904) + 119897
1ℎ119911(119904)] 119889119882 (119904)
(125)
The following proposition which formally follows fromProposition 18 gives the relation between 119875120588(119905) and119870120576120588
119905
Proposition 19 Consider the following
E [119875120588
(0)119870120576120588
0]
= Eint119879
0
1
120576119875120588
(119904)E1015840[119891 (119904 Λ
120588
119904 119906
120576120588
119904) minus 119891 (119904 Λ
120588
119904 119906
120588
119904)]
minus 119897120588
1119870120576120588
119904E1015840[ℎ
1199101015840 (119904 Λ
120588
119904 119906
120588
119904) + ℎ
119910(119904 Λ
120588
119904 119906
120588
119904)]
minus 119897120588
1119876120576120588
119904E1015840[ℎ
1199111015840 (119904 Λ
120588
119904 119906
120588
119904) + ℎ
119911(119904 Λ
120588
119904 119906
120588
119904)] 119889119904
(126)
The following theorem constitutes the main contributionof this section themaximumprinciple for the BSPDE controlsystem
Theorem 20 Let assumptions (L1)ndash(L3) hold Suppose 119906(sdot) isan optimal control and (119884(sdot) 119885(sdot)) is the corresponding optimalstate trajectory for the BSPDE control systems (72) and (73)with the initial state constraint (75) Then there exists 119875(119905) isinS2
F ([0 119879] 119870) which satisfies (125) such that
H (119905 1198841015840
119905 119885
1015840
119905 119884
119905 119885
119905 V
119905 119875 (119905))
ge H (119905 1198841015840
119905 119885
1015840
119905 119884
119905 119885
119905 119906
119905 119875 (119905))
119886119890 119886119904 forallV isin U119886119889
(127)
whereH [0 119879]times119867timesL(Γ119867)times119867timesL(Γ119867)times119880times119870 rarr R
is the Hamiltonian function defined by
H (119905 119910 119910 119911 V 119901) = 1198971ℎ (119905 119910 119910 119911 V)
+ 119901119891 (119905 119910 119910 119911 V) (128)
Proof By (122) and Proposition 19 we obtain
minus120588 le1
120576Eint
119879
0
119875120588
(119904)E1015840[119891 (119904 Λ
120588
119904 119906
120576120588
119904)
minus 119891 (119904 Λ120588
119904 119906
120588
119904)] 119889119904
+119897120588
1
120576Eint
119879
0
E1015840[ℎ (119904 Λ
120588
119904 119906
120576120588
119904)
minus ℎ (119904 Λ120588
119904 119906
120588
119904)] 119889119904 + (119897
120588
1+ 119897
120588
2) 119900 (120576)
(129)
Letting 120576 rarr 0+ in (129) we derive for ae 120591 isin [0 119879]
minus120588 le 119897120588
1E1015840[ℎ (120591 Λ
120588
120591 V
120591) minus ℎ (120591 Λ
120588
120591 119906
120588
120591)]
+ 119875120588
(119904)E1015840[119891 (120591 Λ
120588
120591 V
120591) minus 119891 (120591 Λ
120588
120591 119906
120588
120591)]
(130)
for all V isin UadFinally taking 120588 rarr 0 in (130) we derive the desired
result
Remark 21 We note that if the coefficients do not dependexplicitly on the marginal law of the underlying diffusionthe result reduces to the classical case that is the SMP forBSPDEs without mean-field term
Remark 22 When we remove the initial state constraint (75)we obtain the general maximum principle for the mean-fieldBSPDEs system (ie without the constraint) with 119897
1= 1
54 Application A Backward Linear Quadratic Control Prob-lem Now we apply our maximum principle to solve an LQproblem For notational simplicity we restrict ourselves tothe free case (ie without the initial state constraint (75)) thegeneral case being handled in a similar way
Mathematical Problems in Engineering 17
Consider the following problem
119869 (119906) =1
2E [(119884 (0))
2] +
1
2Eint
119879
0
119873V2 (119905) 119889119905 997888rarr min
(131)
subject to
119889119884 (119905) = minus119860119884 (119905) 119889119905
minus 119861119884 (119905) + 119861E [119884 (119905)]
+119862V (119905) + 119863119885 (119905) + 119863E [119885 (119905)] 119889119905
+ 119885 (119905) 119889119882 (119905)
119884 (119879) = 120585 119905 isin [0 119879]
(132)
where 119860 is a partial differential operator satisfying condition(L1) and 119861 119861 119862119863119863 and119873 are bounded and deterministicconstantsWe also assume that119873 gt 0 and V isin 1198712F (0 119879 119880) It iseasy to verify that BSPDE (132) admits a uniquemild solution(119884(119905) 119885(119905))
119875(119905) the adjoint process of state equation (132) is thesolution of
119889119875 (119905) = 119860lowast119875 (119905) 119889119905 + 119861119875 (119905) + 119861E [119875 (119905)] 119889119905
+ 119863119875 (119905) + 119863E [119875 (119905)] 119889119882 (119905)
119875 (0) = 119884 (0)
(133)
Let 119906(sdot) be an optimal control and let (119884(sdot) 119885(sdot)) bethe corresponding state process By maximum principle ofTheorem 20 (note that 119897
1= 1 in this problem)
1
2119873V2 (119905) + 119875 (119905) 119862V (119905) ge
1
2119873119906
2
(119905) + 119875 (119905) 119862119906 (119905) (134)
for all V isin 119880ad since the state equation has the form (132)Thisin turn implies
119906 (119905) = minus119862
119873119875 (119905) (135)
It is clear that (131) is a positive quadratic functional of controlbecause of 119873 gt 0 Hence an optimal control exists Thecandidate optimal control (135) is indeed an optimal controlof this LQ problem for it is the only control which satisfies themaximum principle
Next we want to obtain a more explicit representationof the optimal control (135) from the state equation (132)Substituting (135) into state equation (132) yields
119889119884 (119905) = minus119860119884 (119905) 119889119905
minus 119861119884 (119905) + 119861E [119884 (119905)] minus1198622
119873119875 (119905)
+119863119885 (119905) + 119863E [119885 (119905)] 119889119905 + 119885 (119905) 119889119882 (119905)
119884 (119879) = 120585 119905 isin [0 119879]
(136)
Combining the above equation with (133) we obtain thefollowing related feedback control system
119889119884 (119905) = minus119860119884 (119905) 119889119905
minus 119861119884 (119905) + 119861E [119884 (119905)] minus1198622
119873119875 (119905)
+ 119863119885 (119905) + 119863E [119885 (119905)] 119889119905 + 119885 (119905) 119889119882 (119905)
119884 (119879) = 120585
119889119875 (119905) = 119860lowast119875 (119905) 119889119905 + 119861119875 (119905) + 119861E [119875 (119905)] 119889119905
+ 119863119875 (119905) + 119863E [119875 (119905)] 119889119882 (119905)
119875 (0) = 119884 (0)
(137)
Looking at the terminal condition of 119875(119905) in (133) andconsidering the mean-field type of (132) it is reasonable toconjecture that 119875(119905) has the following form
119875 (119905) = 120593 (119905) 119884 (119905) + 120601 (119905)E [119884 (119905)] (138)
where120593(119905)120601(119905) are deterministic differential functionswhichwill be specified below Moreover 120593(0) = 1 120601(0) = 0
Inserting this form into adjoint equation (133) and notic-ing that 119884(119905) satisfies (136) we can compare the coefficientsof 119889119905 and 119889119882(119905) to obtain the following equation
2120593 (119905) 119860119884 (119905) + 2120601 (119905) 119860E [119884 (119905)] + 119861120593 (119905) 119884 (119905)
+ 2 (119861120601 (119905) + 119861120593 (119905) + 119861120601 (119905))E [119884 (119905)]
= minus120593 (119905) 119861119884 (119905) minus1198622
119873120593 (119905) 119884 (119905) + 119863119885 (119905)
+ 119884 (119905)119889120593 (119905)
119889119905+ E [119884 (119905)]
119889120601 (119905)
119889119905
+ (1206012
(119905)1198622
119873+ 2
1198622
119873120593 (119905) 120601 (119905))E [119884 (119905)]
minus (119863120601 (119905) + 119863120601 (119905) + 119863120593 (119905))E [119885 (119905)]
120593 (119905) 119885 (119905) = 119863120593 (119905) 119884 (119905)
+ (119863120601 (119905) + 119863120593 (119905) + 119863120601 (119905))E [119884 (119905)]
(139)
Then subtracting 119885(119905) we have
2120593 (119905) 119860119884 (119905) + 2120601 (119905) 119860E [119884 (119905)] + 119861120593 (119905) 119884 (119905)
+ 2 (119861120601 (119905) + 119861120593 (119905) + 119861120601 (119905))E [119884 (119905)]
18 Mathematical Problems in Engineering
= 120593 (119905) minus119861 +1198622
119873120593 (119905) minus 119863
2119884 (119905)
+ 119884 (119905)119889120593 (119905)
119889119905+ E [119884 (119905)]
119889120601 (119905)
119889119905
+ (1206012
(119905)1198622
119873+ 2
1198622
119873120593 (119905) 120601 (119905))E [119884 (119905)]
minus1
120593 (119905)(119863120601 (119905) + 119863120593 (119905) + 119863120601 (119905))
2
E [119884 (119905)]
minus 2119863 (119863120601 (119905) + 119863120593 (119905) + 119863120601 (119905))E [119884 (119905)]
(140)
Comparing the coefficients of 119884(119905) and E[119884(119905)] respec-tively we get
119889
119889119905120593 (119905) = 2119860
lowast120593 (119905) + 2119861120593 (119905) + 119863
2120593 (119905) minus
1198622
1198731205932
(119905)
120593 (0) = 1
(141)
119889
119889119905120601 (119905) = 2119860
lowast120601 (119905) + (
(119863 + 119863)2
120593 (119905)minus1198622
119873)120601
2
(119905)
+ 2119877 (119905) 120601 (119905) + (1198632+ 2119861 + 2119863119863)120593 (119905)
120601 (0) = 0
(142)
where 119877(119905) = 119861 + 119861 + (119863 + 119863)2
minus (1198622119873)120593(119905)
We solve (141) to get
120593 (119905) = (119890minus(2119860lowast+2119861+119863
2)119905(1 minus
1198622
119873(2119860lowast + 2119861 + 1198632))
+1198622
119873(2119860lowast + 2119861 + 1198632))
minus1
(143)
Then (142) exists a unique solution from the classical Riccatiequation theory
We now conclude the above discussions in the followingresult
Theorem 23 For onersquos linear quadratic stochastic partialdifferential control problem (131)-(132) the unique optimalcontrol 119906(sdot) isin 119880ad is given by
119906 (119905) = minus119862
119873(120593 (119905) 119884 (119905) + 120601 (119905)E [119884 (119905)]) (144)
with 120593(119905) satisfying (143) and 120601(119905) solving (142)
Conflict of Interests
The authors declare that they have no conflict of interestsregarding the publication of this paper
Acknowledgment
This research is partially supported by the Natural ScienceFoundation of China under Grant no 11301310
References
[1] Y Hu and S Peng ldquoAdapted solution of a backward semilinearstochastic evolution equationrdquo Stochastic Analysis and Applica-tions vol 9 no 4 pp 445ndash459 1991
[2] N IMahmudov andMAMcKibben ldquoOnbackward stochasticevolution equations in Hilbert spaces and optimal controlrdquoNonlinear Analysis vol 67 no 4 pp 1260ndash1274 2007
[3] M Fuhrman and G Tessitore ldquoNonlinear Kolmogorov equa-tions in infinite dimensional spaces the backward stochasticdifferential equations approach and applications to optimalcontrolrdquoThe Annals of Probability vol 30 no 3 pp 1397ndash14652002
[4] M Fuhrman and G Tessitore ldquoInfinite horizon backwardstochastic differential equations and elliptic equations inHilbert spacesrdquo Annals of Probability vol 32 no 1B pp 607ndash660 2004
[5] X Mao ldquoAdapted solutions of backward stochastic differentialequations with non-Lipschitz coefficientsrdquo Stochastic Processesand their Applications vol 58 no 2 pp 281ndash292 1995
[6] J Lasry and P Lions ldquoMean field gamesrdquo Japanese Journal ofMathematics vol 2 no 1 pp 229ndash260 2007
[7] R Buckdahn J Li and S Peng ldquoMean-field backward stochas-tic differential equations and related partial differential equa-tionsrdquo Stochastic Processes and their Applications vol 119 no 10pp 3133ndash3154 2009
[8] D Andersson and B Djehiche ldquoAmaximum principle for SDEsofmean-field typerdquoAppliedMathematics andOptimization vol63 no 3 pp 341ndash356 2011
[9] R Buckdahn B Djehiche J Li and S Peng ldquoMean-fieldbackward stochastic differential equations a limit approachrdquoThe Annals of Probability vol 37 no 4 pp 1524ndash1565 2009
[10] R Xu ldquoMean-field backward doubly stochastic differentialequations and related SPDEsrdquo Boundary Value Problems vol2012 article 114 2012
[11] R Buckdahn B Djehiche and J Li ldquoA general stochasticmaximum principle for SDEs of mean-field typerdquo AppliedMathematics and Optimization vol 64 no 2 pp 197ndash216 2011
[12] J Li ldquoStochasticmaximumprinciple in themean-field controlsrdquoAutomatica vol 48 no 2 pp 366ndash373 2012
[13] G Wang C Zhang and W Zhang ldquoStochastic maximumprinciple for mean-field type optimal control under partialinformationrdquo IEEE Transactions on Automatic Control vol 59no 2 pp 522ndash528 2014
[14] M Hafayed ldquoA mean-field necessary and sufficient conditionsfor optimal singular stochastic controlrdquo Communications inMathematics and Statistics vol 1 no 4 pp 417ndash435 2013
[15] M Hafayed ldquoA mean-field maximum principle for optimalcontrol of forward-backward stochastic differential equationswith Poisson jumpprocessesrdquo International Journal ofDynamicsand Control vol 1 no 4 pp 300ndash315 2013
[16] Y Hu and S Peng ldquoMaximum principle for semilinearstochastic evolution control systemsrdquo Stochastics and StochasticsReports vol 33 no 3-4 pp 159ndash180 1990
[17] M Fuhrman Y Hu and G Tessitore ldquoStochastic maximumprinciple for optimal control of SPDEsrdquo Comptes Rendus Math-ematique vol 350 no 13-14 pp 683ndash688 2012
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Mathematical Problems in Engineering
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Volume 2014 Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Stochastic AnalysisInternational Journal of
6 Mathematical Problems in Engineering
1199101015840(sdot) 119910(sdot) isin S2
F ([0 119879]119867) and 1199111015840(sdot) 119911(sdot) isin H2
F ([0 119879]L(Γ119867))
By Picard-type iteration we now construct an approxi-mate sequence using which we obtain the desired result Let1198840(119905) equiv 0 and for 119899 isin N let 119884
119899 119885
119899 be a sequence in
S2
F ([0 119879]119867) timesH2
F ([0 119879]L(Γ119867)) defined recursively by
119884119899(119905) = 119890
119860(119879minus119905)120585
+ int
119879
119905
E1015840[119890
119860(119904minus119905)
times119891 (119904 1198841015840
119899minus1(119904) 119885
1015840
119899(119904) 119884
119899minus1(119904) 119885
119899(119904))] 119889119904
minus int
119879
119905
119890119860(119904minus119905)
119885119899(119904) 119889119882 (119904)
(27)
on 0 le 119905 le 119879 From Theorem 4 (27) has a unique mildsolution (119884
119899(119905) 119885
119899(119905))
In order to give the main result we need to prepare thefollowing lemmas about the properties of (119884
119899(119905) 119885
119899(119905)) 119905 isin
[0 119879]
Lemma 6 Under hypotheses (A2) and (A3) there existpositive constants 119862
1and 119862
2such that
(i)E( sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899 (119904)
10038161003816100381610038162
) le 21198621exp (119879 minus 119905)
(ii)Eint119879
119905
11989021205731199041003816100381610038161003816119885119899
(119904)10038161003816100381610038162
119889119904 le 21198621exp (119879 minus 119905)
(iii)E( sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899+1 (119904) minus 119884119899 (119904)
10038161003816100381610038162
)
le 1198622int
119879
119905
120579(E sup119904le119903le119879
11989021205731199031003816100381610038161003816119884119899 (119903) minus 119884119899minus1 (119903)
10038161003816100381610038162
)119889119904
(28)
for all 119905 isin [0 119879] and 119899 ge 1
Proof Using the hypotheses (A2) and (A3)with 120579(119906) le 119886 + 119887119906
yields
10038161003816100381610038161003816119891 (119904 119884
1015840
119899minus1(119904) 119885
1015840
119899(119904) 119884
119899minus1(119904) 119885
119899(119904))
10038161003816100381610038161003816
2
le 2120579 (100381610038161003816100381610038161198841015840
119899minus1(119904)10038161003816100381610038161003816
2
) + 2120579 (1003816100381610038161003816119884119899minus1 (119904)
10038161003816100381610038162
)
+ 2119897 (100381610038161003816100381610038161198851015840
119899(119904)10038161003816100381610038161003816
2
+1003816100381610038161003816119885119899
(119904)10038161003816100381610038162
) + 21003816100381610038161003816119891 (119904 0 0 0 0)
10038161003816100381610038162
le 4119886 + 2119887100381610038161003816100381610038161198841015840
119899minus1(119904)10038161003816100381610038161003816
2
+ 21198871003816100381610038161003816119884119899minus1 (119904)
10038161003816100381610038162
+ 2119897 (100381610038161003816100381610038161198851015840
119899(119904)10038161003816100381610038161003816
2
+1003816100381610038161003816119885119899
(119904)10038161003816100381610038162
) + 21003816100381610038161003816119891 (119904 0 0 0 0)
10038161003816100381610038162
(29)
Then it follows from Lemma 1 that
E( sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899 (119904)
10038161003816100381610038162
) + Eint119879
119905
11989021205731199041003816100381610038161003816119885119899
(119904)10038161003816100381610038162
119889119904
le 241198722
1198601198902120573119879
E100381610038161003816100381612058510038161003816100381610038162
+12119872
2
119860
120573E
times int
119879
119905
1198902120573119904 10038161003816100381610038161003816
E1015840[119891 (119904 119884
1015840
119899minus1(119904)
1198851015840
119899(119904) 119884
119899minus1(119904) 119885
119899(119904))]
10038161003816100381610038161003816
2
119889119904
le 241198722
1198601198902120573119879
E100381610038161003816100381612058510038161003816100381610038162
+12119872
2
119860
120573E
times int
119879
119905
E1015840[119890
2120573119904 10038161003816100381610038161003816119891 (119904 119884
1015840
119899minus1(119904)
1198851015840
119899(119904) 119884
119899minus1(119904) 119885
119899(119904))
10038161003816100381610038161003816
2
] 119889119904
le 1198621+24119872
2
119860
120573E
times int
119879
119905
E1015840[119890
2120573119904[119887100381610038161003816100381610038161198841015840
119899minus1(119904)10038161003816100381610038161003816
2
+ 1198871003816100381610038161003816119884119899minus1 (119904)
10038161003816100381610038162
+ 119897100381610038161003816100381610038161198851015840
119899(119904)10038161003816100381610038161003816
2
+ 1198971003816100381610038161003816119885119899
(119904)10038161003816100381610038162
]] 119889119904
= 1198621+48119872
2
119860
120573E
times int
119879
119905
1198902120573119904
[1198871003816100381610038161003816119884119899minus1 (119904)
10038161003816100381610038162
+ 1198971003816100381610038161003816119885119899
(119904)10038161003816100381610038162
] 119889119904
(30)
where
1198621= 24119872
2
1198601198902120573119879
E100381610038161003816100381612058510038161003816100381610038162
+24119872
2
119860
120573Eint
119879
119905
1198902120573119904
[2119886 +1003816100381610038161003816119891 (119904 0 0 0 0)
10038161003816100381610038162
] 119889119904 + 1
(31)
If we set 120573 = 961198722
119860max119887 119897 we can obtain
sup119899isinN
E( sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899 (119904)
10038161003816100381610038162
) +1
2int
119879
119905
sup119899isinN
E [11989021205731199041003816100381610038161003816119885119899
(119904)10038161003816100381610038162
] 119889119904
le 1198621+1
2int
119879
119905
sup119899isinN
E [11989021205731199041003816100381610038161003816119884119899minus1 (119904)
10038161003816100381610038162
] 119889119904
le 1198621+1
2int
119879
119905
sup119899isinN
E[ sup119904le119903le119879
11989021205731199031003816100381610038161003816119884119899minus1 (119903)
10038161003816100381610038162
]119889119904
(32)
An application of the Gronwall inequality now implies
sup119899isinN
E( sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899 (119904)
10038161003816100381610038162
) le 21198621exp (119879 minus 119905
2)
le 21198621exp (119879 minus 119905)
(33)
Point (i) of Lemma 6 is now proved
Mathematical Problems in Engineering 7
From formula (32) we know that
int
119879
119905
sup119899isinN
E [11989021205731199041003816100381610038161003816119885119899
(119904)10038161003816100381610038162
] 119889119904
le 21198621+ int
119879
119905
sup119899isinN
E[ sup119904le119903le119879
11989021205731199031003816100381610038161003816119884119899minus1 (119903)
10038161003816100381610038162
]119889119904
le 21198621+ 2119862
1int
119879
119905
exp (119879 minus 119904) 119889119904
= 21198621exp (119879 minus 119905)
(34)
This proves point (ii) of the LemmaTo prove point (iii) we note that
E1015840[10038161003816100381610038161003816119891 (119904 119884
1015840
119899(119904) 119885
1015840
119899+1(119904) 119884
119899(119904) 119885
119899+1(119904))
minus 119891 (119904 1198841015840
119899minus1(119904) 119885
1015840
119899(119904) 119884
119899minus1(119904) 119885
119899(119904))
10038161003816100381610038161003816
2
]
le E1015840[120579 (
100381610038161003816100381610038161198841015840
119899(119904) minus 119884
1015840
119899minus1(119904)10038161003816100381610038161003816
2
) + 119897100381610038161003816100381610038161198851015840
119899+1(119904) minus 119885
1015840
119899(119904)10038161003816100381610038161003816
2
]
+ 120579 (1003816100381610038161003816119884119899 (119904) minus 119884119899minus1 (119904)
10038161003816100381610038162
) + 1198971003816100381610038161003816119885119899+1
(119904) minus 119885119899(119904)10038161003816100381610038162
= E [120579 (1003816100381610038161003816119884119899 (119904) minus 119884119899minus1 (119904)
10038161003816100381610038162
) + 1198971003816100381610038161003816119885119899+1
(119904) minus 119885119899(119904)10038161003816100381610038162
]
+ 120579 (1003816100381610038161003816119884119899 (119904) minus 119884119899minus1 (119904)
10038161003816100381610038162
) + 1198971003816100381610038161003816119885119899+1
(119904) minus 119885119899(119904)10038161003816100381610038162
(35)
By Lemma 1 we have
E( sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899+1 (119904) minus 119884119899 (119904)
10038161003816100381610038162
)
+ Eint119879
119905
11989021205731199041003816100381610038161003816119885119899+1
(119904) minus 119885119899(119904)10038161003816100381610038162
119889119904
le12119872
2
119860
120573E
times int
119879
119905
1198902120573119904 10038161003816100381610038161003816
E1015840[119891 (119904 119884
1015840
119899(119904) 119885
1015840
119899+1(119904) 119884
119899(119904) 119885
119899+1(119904))
minus 119891 (119904 1198841015840
119899minus1(119904) 119885
1015840
119899(119904)
119884119899minus1
(119904) 119885119899(119904))]
10038161003816100381610038161003816
2
119889119904
le12119872
2
119860
120573E
times int
119879
119905
1198902120573119904
E1015840[10038161003816100381610038161003816119891 (119904 119884
1015840
119899(119904) 119885
1015840
119899+1(119904) 119884
119899(119904) 119885
119899+1(119904))
minus 119891 (119904 1198841015840
119899minus1(119904) 119885
1015840
119899(119904)
119884119899minus1
(119904) 119885119899(119904))
10038161003816100381610038161003816
2
] 119889119904
le24119872
2
119860
120573Eint
119879
119905
1198902120573119904
[120579 (1003816100381610038161003816119884119899 (119904) minus 119884119899minus1 (119904)
10038161003816100381610038162
)
+ 1198971003816100381610038161003816119885119899+1
(119904) minus 119885119899(119904)10038161003816100381610038162
] 119889119904
(36)
We can choose 120573 gt 0 sufficiently large such that
(1 minus24119872
2
119860119897
120573)Eint
119879
119905
11989021205731199041003817100381710038171003817119885119899+1
(119904) minus 119885119899(119904)10038171003817100381710038172
119889119904 ge 0 (37)
Then
E( sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899+1 (119904) minus 119884119899 (119904)
10038161003816100381610038162
)
le24119872
2
119860
120573Eint
119879
119905
1198902120573119904120579 (1003816100381610038161003816119884119899 (119904) minus 119884119899minus1 (119904)
10038161003816100381610038162
) 119889119904
le 1198622Eint
119879
119905
120579(E sup119904le119903le119879
11989021205731199031003816100381610038161003816119884119899 (119903) minus 119884119899minus1 (119903)
10038161003816100381610038162
)119889119904
(38)
where we set 1198622= (24119872
2
119860120573)119890
2120573119879
We divide the interval [0 119879] into subintervals 0 = 1205910lt
1205911lt sdot sdot sdot lt 120591
119898= 119879 by setting 120591
119896= 119896120575 119896 = 1 2 3 119898 with
120575 = 119879119898
Lemma 7 For all 119905 isin [120591119896minus1
120591119896] define
1198623= 119862
2120579 (2119862
1exp (119879))
1205931198961(119905) = 119862
3(120591119896minus 119905)
120593119896119899+1
(119905) = 1198622int
120591119896
119905
120579 (120593119896119899(119904)) 119889119904 119899 ge 1
(39)
Then for all 119899 ge 1 the following inequality holds for a suitable120575 gt 0
0 le 120593119896119899(119905) le 120593
119896119899minus1(119905) le sdot sdot sdot le 120593
1198961(119905) (40)
Proof Firstly it needs to be verified that for all 119905 isin [120591119896minus1
120591119896]
the following inequality
1205931198962(119905) = 119862
2int
120591119896
119905
120579 (1205931198961(119904)) 119889119904 = 119862
2int
120591119896
119905
120579 (1198623(120591119896minus 119904)) 119889119904
le 1198623(120591119896minus 119905) = 120593
1198961(119905)
(41)
holds provided 120575 gt 0 is chosen sufficiently smallActually this inequality holds provided that
1198622120579 (119862
3(120591119896minus 119905)) le 119862
3= 119862
2120579 (2119862
1exp (119879)) (42)
8 Mathematical Problems in Engineering
or
1198623(120591119896minus 119905) = 119862
2120579 (2119862
1exp (119879)) (120591
119896minus 119905) le 2119862
1exp (119879)
(43)
Since 1198621gt 1 from 120579(119906) le 119886 + 119887119906 the above inequality holds
if
1198622(119886 + 119887) (120591
119896minus 119905) le 1 (44)
Thus (41) holds for any 119905 isin [120591119896minus1
120591119896] 119896 = 1 2 119898 if 120591
119896minus
120591119896minus1
le 11198622(119886 + 119887) Therefore we can choose a sufficiently
large119898 isin N such that 120575 = 119879119898 le 11198622(119886 + 119887) Clearly such a
120575 only depends on 119886 119887 119897 119879 and119872119860
Now assume that (40) holds for some 119899 ge 2 Then wehave
120593119896119899+1
(119905) = 1198622int
120591119896
119905
120579 (120593119896119899(119904)) 119889119904 le 119862
2int
120591119896
119905
120579 (120593119896119899minus1
(119904)) 119889119904
= 120593119896119899(119905) forall119905 isin [120591
119896minus1 120591
119896]
(45)
This completes the proof
Now we can give the main result of this section
Theorem 8 Assume that (A2) and (A3) hold Then thereexists a unique mild solution (119884 119885) to (11)
Proof Consider the following
Uniqueness To show the uniqueness let both (119884 119885) and( 119885) be solutions of (11) For any 120573 gt 0 similar to the proofof (36) one can obtain
E( sup119905le119904le119879
119890212057311990410038161003816100381610038161003816
119884 (119904) minus (119904)10038161003816100381610038161003816
2
) + Eint119879
119905
119890212057311990410038161003816100381610038161003816
119885 (119904) minus 119885 (119904)10038161003816100381610038161003816
2
119889119904
le24119872
2
119860
120573E
times int
119879
119905
1198902120573119904
[120579 (10038161003816100381610038161003816119884 (119904) minus (119904)
10038161003816100381610038161003816
2
) + 11989710038161003816100381610038161003816119885 (119904) minus 119885 (119904)
10038161003816100381610038161003816
2
] 119889119904
(46)
That is if 120573 is sufficiently large
E( sup119905le119904le119879
119890212057311990410038161003816100381610038161003816
119884 (119904) minus (119904)10038161003816100381610038161003816
2
)
le24119872
2
119860
120573Eint
119879
119905
1198902120573119904
[120579 (10038161003816100381610038161003816119884 (119904) minus (119904)
10038161003816100381610038161003816
2
)] 119889119904
le 1198622Eint
119879
119905
120579(E sup119904le119903le119879
119890212057311990310038161003816100381610038161003816
119884 (119904) minus (119904)10038161003816100381610038161003816
2
)119889119904
(47)
An application of Bihari inequality yields
E( sup119905le119904le119879
119890212057311990410038161003816100381610038161003816
119884 (119904) minus (119904)10038161003816100381610038161003816
2
) = 0 (48)
So 119884(119905) = (119905) for all 119905 isin [0 119879] as It then follows from (46)that 119885(119905) = 119885(119905) for all 119905 isin [0 119879] as as well This establishesthe uniqueness
ExistenceWe claim that the sequence (119884119899 119885
119899) defined by (27)
satisfies
E sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899+1 (119904) minus 119884119899 (119904)
10038161003816100381610038162
997888rarr 0 forall0 le 119905 le 119879 (49)
as 119899 rarr infinIndeed for all 119905 isin [120591
119896minus1 120591
119896] we set 120593
119896119899(119905) =
E sup119904isin[119905120591119896]
1198902120573119904|119884119899+1
(119904) minus 119884119899(119904)|
2 By Lemmas 6 and 7
1205931198961(119905) = E sup
119904isin[119905120591119896]
119890212057311990410038161003816100381610038161198842 (119904) minus 1198841 (119904)
10038161003816100381610038162
le 1198622int
120591119896
119905
120579(E sup119904le119903le120591119896
119890212057311990310038161003816100381610038161198841 (119903) minus 1198840 (119903)
10038161003816100381610038162
)119889119904
le 1198622int
120591119896
119905
120579 (21198621exp (120591
119896minus 119905)) 119889119904
le 1198622120579 (2119862
1exp (119879)) (120591
119896minus 119905) = 119862
3(120591119896minus 119905) = 120593
1198961(119905)
(50)
Suppose that 120593119896119899(119905) le 120593
119896119899(119905) holds for some 119899 ge 1
According to Lemma 6(iii) and Lemma 7 for all 119905 isin [120591119896minus1
120591119896]
we obtain
120593119896119899+1
(119905) = E sup119904isin[119905120591119896]
11989021205731199041003816100381610038161003816119884119899+2 (119904) minus 119884119899+1 (119904)
10038161003816100381610038162
le 1198622int
120591119896
119905
120579(E sup119903isin[119904120591119896]
11989021205731199031003816100381610038161003816119884119899+1 (119903) minus 119884119899 (119903)
10038161003816100381610038162
)119889119904
= 1198622int
120591119896
119905
120579 (120593119896119899(119904)) 119889119904
le 1198622int
120591119896
119905
120579 (120593119896119899(119904)) 119889119904 = 120593
119896119899+1(119905)
(51)
This implies that for all 119899 isin N
120593119896119899(119905) le 120593
119896119899(119905) (52)
By definition 120593119896119899(sdot) is continuous on [120591
119896minus1 120591
119896] Note that
for each 119899 ge 1 120593119896119899(sdot) is decreasing on [120591
119896minus1 120591
119896] and for each
119905 119896 120593119896119899(119905) is a nonincreasing sequence Therefore we define
the function 120593119896(119905) by 120593
119896119899(119905) darr 120593
119896(119905) It is easy to verify that
120593119896(119905) is continuous and nonincreasing on [120591
119896minus1 120591
119896] By the
definitions of 120593119896119899(119905) and 120593
119896(119905) we get
120593119896(119905) = lim
119899rarrinfin
1198622int
120591119896
119905
120579 (120593119896119899(119904)) 119889119904 = 119862
2int
120591119896
119905
120579 (120593119896(119904)) 119889119904
(53)
for all 120591119896minus1
le 119905 le 120591119896 Since int
0+(119889119906120579(119906)) = infin the Bihari
inequality implies
120593119896(119905) = 0 for each 119905 isin [120591
119896minus1 120591
119896] (54)
For each 119896 isin 1 2 119898 (52) and (54) yield
lim119899rarrinfin
120593119896119899(119905) = 0 (55)
Mathematical Problems in Engineering 9
Then
E sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899+1 (119904) minus 119884119899 (119904)
10038161003816100381610038162
le max1le119896le119898
E sup119904isin[120591119896minus1120591119896]
11989021205731199041003816100381610038161003816119884119899+1 (119904) minus 119884119899 (119904)
10038161003816100381610038162
= max1le119896le119898
120593119896119899(119905) 997888rarr 0
(56)
as 119899 rarr infin and this proves the assertion (49)By (36) we obtain
E( sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899+1 (119904) minus 119884119899 (119904)
10038161003816100381610038162
) + (1 minus24119872
2
119860119897
120573)E
times int
119879
119905
11989021205731199041003816100381610038161003816119885119899+1
(119904) minus 119885119899(119904)10038161003816100381610038162
119889119904
le24119872
2
119860
120573Eint
119879
119905
1198902120573119904
[120579 (1003816100381610038161003816119884119899 (119904) minus 119884119899minus1 (119904)
10038161003816100381610038162
)] 119889119904
(57)
Applying (49) to the above formula we see that (119884119899 119885
119899) is
a Cauchy (hence convergent) sequence in S2
F ([0 119879]119867) times
H2
F ([0 119879]L(Γ119867)) denote the limit by (119884 119885) Now letting119899 rarr infin in (27) we obtain that
119884 (119905) = 119890119860(119879minus119905)
120585
+ int
119879
119905
E1015840[119890
119860(119904minus119905)119891 (119904 119884
1015840
(119904) 1198851015840
(119904) 119884 (119904) 119885 (119904))] 119889119904
minus int
119879
119905
119890119860(119904minus119905)
119885 (119904) 119889119882 (119904)
(58)
holds on the entire interval [0 119879]The theorem is nowproved
To illustrate the application of the obtained existenceand uniqueness result we consider the example of backwardstochastic partial differential equations (BSPDEs) of mean-field type
Example 9 Let O be an open bounded domain in R119899
with uniformly 1198622 boundary 120597O let 119861(119905) be a standard 119899-dimensional Brownian motion (equipped with the normalfiltration) and let 120585 O rarr R be an F
119879-measurable
random variable We also let 119871 denote the semielliptic partialdifferential operator on 1198622
(R) of the form
119871 =
119899
sum
119894119895=1
119886119894119895(119909)
1205972
120597119909119894120597119909
119895
+
119899
sum
119894=1
119887119894(119909)
120597
120597119909119894
(59)
The aim is to study the solvability of the following initialboundary value problem
119889119884 (119905 119909)
= (119871119884 (119905 119909) + E1015840
times [119892 (119905 119909 1198841015840
(119905 119909) 1198851015840
(119905 119909) 119884 (119905 119909) 119885 (119905 119909))]) 119889119905
+ 119885 (119905 119909) 119889119861 (119905) ae on (0 119879) times O
119884 (119905 119909) = 0 ae on (0 119879) times 120597O
119884 (119879 119909) = 120585 (119879 119909) ae onO(60)
where
119884 [0 119879] times O 997888rarr R
119885 [0 119879] times O 997888rarr L (R119899 119871
2
(O))
119892 [0 119879] times O timesR timesL (R119899 119871
2
(O))
timesR timesL (R119899 119871
2
(O)) 997888rarr R
(61)
The following assumptions will have to be in force
(H1) 119886119894119895 119887
119894 R119899
rarr R are uniformly continuous andbounded and satisfy the usual uniform ellipticitycondition sum119899
119894119895=1119886119894119895(119909)119908
119894119908119895ge 120582|119908|
2 for some 120582 gt 0
and all 119909 isin O 119908 isin R119899(H2) 119892 is measurable in (119905 119909 119910 119910 119911) and continuous in
( 119911) and there exists 119862 gt 0 such that1003816100381610038161003816119892 (119905 119909 1199101 1 1199101 1199111) minus 119892 (119905 119909 1199102 2 1199102 1199112)
1003816100381610038161003816
le 119862 [10038161003816100381610038161199101 minus 1199102
1003816100381610038161003816 +10038161003816100381610038161 minus 2
1003816100381610038161003816 +10038161003816100381610038161199101 minus 1199102
1003816100381610038161003816 +10038161003816100381610038161199111 minus 1199112
1003816100381610038161003816]
(62)
for all 0 le 119905 le 119879 119909 isin O 1199101 119910
2 119910
1 119910
2isin R
1
2 119911
1 119911
2isin
L(R119899 119871
2(O))
Then we are now in a position of showing existence anduniqueness of the solution of BSPDEs (60)
Theorem 10 If (H1) and (H2) are satisfied then the mean-field BSPDE (60) has a unique mild solution (119884 119885) isin
1198712(0 119879 119871
2(Ω 119871
2(O))) times 1198712F (0 119879 119871
2(R119899
1198712(Ω 119871
2(O))))
Proof Let119867 = 1198712(O) and119870 = R119899 Define the operator 119860 by
119860119884 (119905 sdot) = 119871119884 (119905 sdot) (63)
It is shown in [17] (see Example 21 in [17]) that 119860 generatesa strongly continuous semigroup on 119867 Define the maps 119891
[0 119879] times 119867 timesL(119870119867) times 119867 timesL(119870119867) rarr 119867 by
119891 (119905 1198841015840
(119905) 1198851015840
(119905) 119884 (119905) 119885 (119905)) (119909)
= 119892 (119905 119909 1198841015840
(119905 119909) 1198851015840
(119905 119909) 119884 (119905 119909) 119885 (119905 119909))
(64)
10 Mathematical Problems in Engineering
for all 0 le 119905 le 119879 119909 isin O With these identifications(60) can be written in the form of (11) By (H2) we know119891 satisfy condition (A1) Hence an application of Theorem 4concludes that (60) has a unique mild solution (119884 119885) isin
1198712(0 119879 119871
2(Ω 119871
2(O))) times 1198712F (0 119879 119871
2(R119899
1198712(Ω 119871
2(O))))
4 Mean-Field Stochastic Evolution Equations
Let 119882(119905) 119905 isin [0 119879] be a cylindrical Wiener process withvalues in a Hilbert space Γ defined on a probability space(ΩFP) We fix an interval [119905 119879] sub [0 119879] and considerthe stochastic evolution equations of mean-field type for anunknown process 119883(119904) 119904 isin [119905 119879] with values in a Hilbertspace 119870
119889119883 (119904) = 119861119883 (119904) 119889119904 + E1015840[119887 (119904 119883
1015840
(119904) 119883 (119904))] 119889119904
+ E1015840[120590 (119904 119883
1015840
(119904) 119883 (119904))] 119889119882 (119904)
119883 (119905) = 119909 isin 119870
(65)
where operator 119861 is the generator of a strongly continuoussemigroup 119890
119905119861 119905 ge 0 in the Hilbert space 119870 with 119872119861≜
sup119905isin[0119879]
|119890119905119861|
By a mild solution of (65) we mean an F119904-measurable
process 119883(119904) 119904 isin [119905 119879] with continuous paths in 119870 suchthat P-as
119883 (119904) = 119890119861(119904minus119905)
119909
+ int
119904
119905
119890119861(119904minus120591)
E1015840[119887 (120591 119883
1015840
(120591) 119883 (120591))] 119889120591
+ int
119904
119905
119890119861(119904minus120591)
E1015840[120590 (120591119883
1015840
(120591) 119883 (120591))] 119889119882 (120591)
119904 isin [119905 119879]
(66)
We suppose the following(A4) 119887 [0 119879] times 119870 times 119870 rarr 119870 is a measurable mapping
which satisfies10038161003816100381610038161003816119887 (119905 119909
1015840 119909) minus 119887 (119905 119910
1015840 119910)
10038161003816100381610038161003816
2
le 1198711(100381610038161003816100381610038161199091015840minus 119910
101584010038161003816100381610038161003816
2
+1003816100381610038161003816119909 minus 119910
10038161003816100381610038162
)
119905 isin [0 119879] 1199091015840 119909 119910
1015840 119910 isin 119870
(67)
for some constant 1198711gt 0
(A5) The mapping 120590 [0 119879] times 119870 times 119870 rarr L(Γ 119870) fulfillsthat for every V isin Γ the map 120590V [0 119879] times 119870 times 119870 rarr 119870
is measurable for every 119904 gt 0 119905 isin [0 119879] 1199091015840 1199101015840 119909 119910 isin 119870119890119904119861120590(119905 119909
1015840 119909) isin L(Γ 119870) and
10038161003816100381610038161003816119890119904119861120590 (119905 119909
1015840 119909)
10038161003816100381610038161003816L(Γ119870)le 119871
2119904minus120574(1 +
10038161003816100381610038161003816119909101584010038161003816100381610038161003816+ |119909|)
10038161003816100381610038161003816119890119904119861120590 (119905 119909
1015840 119909) minus 119890
119904119861120590 (119905 119910
1015840 119910)
10038161003816100381610038161003816L(Γ119870)
le 1198712119904minus120574(100381610038161003816100381610038161199091015840minus 119910
101584010038161003816100381610038161003816+1003816100381610038161003816119909 minus 119910
1003816100381610038161003816)
10038161003816100381610038161003816120590 (119905 119909
1015840 119909)
10038161003816100381610038161003816L(Γ119870)le 119871
2(1 +
10038161003816100381610038161003816119909101584010038161003816100381610038161003816+ |119909|)
(68)
for some constants 1198712gt 0 and 120574 isin [0 12)
Theorem 11 Under assumptions (A3) and (A4) (65) has aunique mild solution119883(sdot) isin S2
F ([119905 119879] 119870)
The proof is constructed in two steps like that ofTheorem 4 and it uses standard arguments for stochastic evo-lution equations introduced in the proof of Proposition 32 in[3] Since the proof is straightforward we prefer to omit it
Remark 12 In our paper Lipchitz condtion (A4) is givento get the well-posedness of mean-field stochastic evolutionequations In fact (A4) can be replaced by a weaker conditionsuch as (A3) We just give the condition (A4) for simplicity
From standard arguments we can also get the followingcontinuous dependence theorem
Corollary 13 Assume that for all 120572 in a metric space 119865(119887120572 120590
120572) satisfy (A4) and (A5)with119871
1and119871
2independent of 120572
Also assume that
Eint119879
0
10038161003816100381610038161003816E1015840[119887120572(119904 119883
1015840
(119904) 119883 (119904))]
minus E1015840[1198871205720(119904 119883
1015840
(119904) 119883 (119904))]10038161003816100381610038161003816
2
119889119904 997888rarr 0
Eint119879
0
10038161003816100381610038161003816E1015840[120590
120572(119904 119883
1015840
(119904) 119883 (119904))]
minusE1015840[120590
1205720(119904 119883
1015840
(119904) 119883 (119904))]10038161003816100381610038161003816
2
119889119904 997888rarr 0
(69)
as 120572 rarr 1205720for all119883 isin S2
F ([0 119879] 119870)If we denote by 119883120572
(sdot) the mild solution of mean-field SEE(65) corresponding to the functions (119887
120572 120590
120572) and to the initial
data 119909 then we have
sup119904isin[0119879]
E1003816100381610038161003816119883
120572
(119904) minus 1198831205720 (119904)
10038161003816100381610038162
997888rarr 0 119886119904 120572 997888rarr 1205720 (70)
5 Maximum Principle for BSPDEs ofMean-Field Type
51 Formulation of the Problem Let O isin R119899 be a boundedopen set with smooth boundary 120597O and let 119880 the space ofcontrols be a separable real Hilbert space We denote
U = V (sdot) isin 1198712F(0 119879 119880)
| V119905(120596
1015840 120596) [0 119879] times Ω times Ω
997888rarr 119880 is F otimesF119905-progressively measurable
(71)
Mathematical Problems in Engineering 11
An element ofU is called an admissible controlFor any V isin U we consider the following controlled
BSPDE system in the state space119867 = 1198712(O) (norm | sdot | scalar
product ⟨sdot sdot⟩)
119889119884119905(119909) = minus119860119884
119905(119909) 119889119905
minus E1015840[119891 (119905 119909 (119884
119905(119909))
1015840
(119885119905(119909))
1015840
119884119905(119909) 119885
119905(119909) V
119905)] 119889119905
+ 119885119905(119909) 119889119882 (119905) 119905 isin [0 119879]
119884119879(119909) = 120585 (119909) 119909 isin O
(72)
where 119860 is a partial differential operator 119891 [0 119879] timesO times119867 times
L(Γ119867)times119867timesL(Γ119867)times119880 rarr 119867 and 120585 isin 1198712(ΩF119879P 119867)
The cost functional is given by
119869 (V) = Eint119879
0
intO
E1015840[ℎ (119904 119909 (119884
119904(119909))
1015840
(119885119904(119909))
1015840
119884119904(119909) 119885
119904(119909) V
119904)] 119889119909 119889119904
+E1015840intO
119892 (119909 (1198840(119909))
1015840
1198840(119909)) 119889119909
(73)
where
ℎ [0 119879] times O times 119867 timesL (Γ119867) times 119867 timesL (Γ119867) times 119880 997888rarr R
119892 O times 119867 times119867 997888rarr R
(74)
Our purpose is to minimize the functional 119869(sdot) over Uadsubject to the following state constraint
EintO
Φ(119909 (1198840(119909))
1015840
1198840(119909)) 119889119909 = 0 (75)
where
Φ O times 119867 times119867 997888rarr R (76)
An admissible control 119906 isin Uad that satisfies
119869 (119906) = minVisinUad
119869 (V) (77)
is called optimalThrough what follows the following assumptions will be
in force
(L1) 119860 is a partial differential operator with appropri-ate boundary conditions We assume that 119860 is theinfinitesimal generator of a strongly continuous semi-group 119890119905119860 119905 ge 0 in119867 Moreover for every 119905 isin [0 119879]119890
119905119860119891
1198712(O) le 119872
119860119891
1198712(O) for some constant 119872
119860
independent of 119905 and 119891(L2) 119891 ℎ 119892 and Φ are continuously Gateaux differen-
tiable with respect to (1199101015840 1199111015840 119910 119911) 119891 is continuouslyGateaux differentiable with respect to V and ℎ iscontinuous with respect to V
(L3) The derivatives of 119891 ℎ 119892 and Φ are Lipschitzcontinuous and bounded by
100381610038161003816100381610038161198911199101015840
10038161003816100381610038161003816+10038161003816100381610038161198911199111015840
1003816100381610038161003816 +10038161003816100381610038161003816119891119910
10038161003816100381610038161003816+1003816100381610038161003816119891119911
1003816100381610038161003816 +1003816100381610038161003816119891V
1003816100381610038161003816 +10038161003816100381610038161003816Φ1199101015840
10038161003816100381610038161003816+10038161003816100381610038161003816Φ119910
10038161003816100381610038161003816le 119862
10038161003816100381610038161003816ℎ1199101015840
10038161003816100381610038161003816+1003816100381610038161003816ℎ1199111015840
1003816100381610038161003816 +10038161003816100381610038161003816ℎ119910
10038161003816100381610038161003816+1003816100381610038161003816ℎ119911
1003816100381610038161003816 +100381610038161003816100381610038161198921199101015840
10038161003816100381610038161003816+10038161003816100381610038161003816119892119910
10038161003816100381610038161003816
le 119862 (1 +10038161003816100381610038161003816119910101584010038161003816100381610038161003816+10038161003816100381610038161003816119911101584010038161003816100381610038161003816+10038161003816100381610038161199101003816100381610038161003816 + |119911| + |V|)
(78)
where 119862 is a positive constant
Obviously according to Theorem 4 state equation (72)has a unique mild solution under the above assumptions
Remark 14 We can define the second order differentialoperator
(119860119891) (119909) =
119899
sum
119894119895=1
119886119894119895(119909)
1205972119891
120597119909119894120597119909
119895
(119909) +
119899
sum
119894=1
119887119894(119909)
120597119891
120597119909119894
(119909) (79)
By Example 9 119860 fulfills assumption (L1) if 119886119894119895 119887
119894satisfy
condition (H1)
52 Variation of the Trajectory Let 119906 be an optimal controlwith (119884(sdot) 119885(sdot)) being the corresponding optimal state Let120576 gt 0 and [119903 119903 + 120576] sube [0 119879] For any given V isin Uad weintroduce the spike variation of the control 119906(sdot)
119906120576
119905=
V119905 119905 isin [119903 119903 + 120576]
119906119905 119905 isin [0 119879] [119904 119904 + 120576]
(80)
It is clear that 119906120576(sdot) isin UadLet (119884120576
(sdot) 119885120576(sdot)) be the trajectory corresponding to 119906120576(sdot)
We use the following short notation for brevity
119891 (119905) = 119891 (119905 119909 (119884119905(119909))
1015840
(119885119905(119909))
1015840
119884119905(119909) 119885
119905(119909) 119906
119905)
119891 (119906120576
119905) = 119891 (119905 119909 (119884
119905(119909))
1015840
(119885119905(119909))
1015840
119884119905(119909) 119885
119905(119909) 119906
120576
119905)
(81)
Consider the following equation
119889119870120576
119905(119909) = minus119860119870
120576
119905(119909) 119889119905
minus E1015840[119891
1199101015840 (119905) (119870
120576
119905(119909))
1015840
+ 119891119910(119905) 119870
120576
119905(119909)
+ 1198911199111015840 (119905) (119876
120576
119905(119909))
1015840
+ 119891119911(119905) 119876
120576
119905(119909)
+1
120576(119891 (119906
120576
119905) minus 119891 (119905))] 119889119905 + 119876
120576
119905(119909) 119889119882 (119905)
119870120576
119879(119909) = 0
(82)
Since the coefficients in (82) are bounded it is easy to checkthat there exists a unique mild solution such that
E[ sup119905isin[0119879]
1003816100381610038161003816119870120576
119905
10038161003816100381610038162
+ int
119879
0
1003816100381610038161003816119876120576
119905
10038161003816100381610038162
119889119905] lt infin (83)
We have the following estimate
12 Mathematical Problems in Engineering
Theorem 15 There holds
lim120576rarr0
E[ sup119904isin[119905119879]
1003816100381610038161003816100381610038161003816100381610038161003816
119884120576
119904minus 119884
119904
120576minus 119870
120576
119904
1003816100381610038161003816100381610038161003816100381610038161003816
2
] = 0 forall119905 isin [0 119879]
lim120576rarr0
Eint119879
0
1003816100381610038161003816100381610038161003816100381610038161003816
119885120576
119904minus 119885
119904
120576minus 119876
120576
119904
1003816100381610038161003816100381610038161003816100381610038161003816
2
119889119904 = 0
(84)
Proof We define
120578120576
119904=119884120576
119904minus 119884
119904
120576minus 119870
120576
119904 120577
120576
119904=119885120576
119904minus 119885
119904
120576minus 119876
120576
119904 119904 isin [0 119879]
(85)
For simplicity let us define
Λ120576
119904= ((119884
120576
119904)1015840
(119885120576
119904)1015840
119884120576
119904 119885
120576
119904)
119891 (119904 120582) = 119891 (119904 1198841015840
119904+ 120582(119884
120576
119904minus 119884
119904)1015840
1198851015840
119904+ 120582(119885
120576
119904minus 119885
119904)1015840
119884119904+ 120582 (119884
120576
119904minus 119884
119904)
119885119904+ 120582 (119885
120576
119904minus 119885
119904) 119906
120576
119904)
(86)
By the definition of (119884120576
119904 119885
120576
119904) (119884
119904 119885
119904) and (119870120576
119904 119876
120576
119904) (120578120576
119904 120577
120576
119904)
is the mild solution of
119889120578120576
119904= minus119860120578
120576
119904119889119904 minus E
1015840
[119871 (119904 120576)] 119889119904 + 120577120576
119904119889119882 (119904)
120578120576
119879= 0
(87)
with
119871 (119904 120576) =1
120576(119891 (119904 Λ
120576
119904 119906
120576
119904) minus 119891 (119906
120576
119904))
minus 1198911199101015840 (119904) (119870
120576
119904)1015840
minus 119891119910(119904) 119870
120576
119904minus 119891
1199111015840 (119904) (119876
120576
119904)1015840
minus 119891119911(119904) 119876
120576
119904
= ((120578120576
119904)1015840
+ (119870120576
119904)1015840
) int
1
0
1198911199101015840 (119904 120582) 119889120582 + (120578
120576
119904+ 119870
120576
119904)
times int
1
0
119891119910(119904 120582) 119889120582 minus 119891
1199101015840 (119904) (119870
120576
119904)1015840
minus 119891119910(119904) 119870
120576
119904
+ ((120577120576
119904)1015840
+ (119876120576
119904)1015840
)int
1
0
1198911199111015840 (119904 120582) 119889120582 + (120577
120576
119904+ 119876
120576
119904)
times int
1
0
119891119911(119904 120582) 119889120582 minus 119891
1199111015840 (119904) (119876
120576
119904)1015840
minus 119891119911(119904) 119876
120576
119904
= (120578120576
119904)1015840
int
1
0
1198911199101015840 (119904 120582) 119889120582 + 120578
120576
119904int
1
0
119891119910(119904 120582) 119889120582
+ (120577120576
119904)1015840
int
1
0
1198911199111015840 (119904 120582) 119889120582 + 120577
120576
119904int
1
0
119891119911(119904 120582) 119889120582 + 120574
120576
119904
(88)
where we denote
120574120576
119904= (119870
120576
119904)1015840
int
1
0
(1198911199101015840 (119904 120582) minus 119891
1199101015840 (119904)) 119889120582
+ 119870120576
119904int
1
0
(119891119910(119904 120582) minus 119891
119910(119904)) 119889120582
+ (119876120576
119904)1015840
int
1
0
(1198911199111015840 (119904 120582) minus 119891
1199111015840 (119904)) 119889120582
+ 119876120576
119904int
1
0
(119891119911(119904 120582) minus 119891
119911(119904)) 119889120582
(89)
For any 120573 gt 0 according to Lemma 1 we obtain
E sup119905le119904le119879
11989021205731199041003816100381610038161003816120578
120576
119904
10038161003816100381610038162
+ Eint119879
119905
11989021205731199041003816100381610038161003816120577
120576
119904
10038161003816100381610038162
119889119904
le12119872
2
119860
120573int
119879
119905
1198902120573119904
E [10038161003816100381610038161003816E1015840
[119871 (119904 120576)]10038161003816100381610038161003816
2
] 119889119904
le12119872
2
119860
120573Eint
119879
119905
1198902120573119904
E1015840[|119871 (119904 120576)|
2] 119889119904
(90)
By condition (L3) we have
E [E1015840[|119871 (119904 120576)|
2]]
= E [E1015840[1003816100381610038161003816119871 (119904 120576) minus 120574
120576
119904+ 120574
120576
119904
10038161003816100381610038162
]]
le 81198622E [E
1015840[10038161003816100381610038161003816(120578
120576
119904)101584010038161003816100381610038161003816
2
+1003816100381610038161003816120578120576
119904
10038161003816100381610038162
+10038161003816100381610038161003816(120577
120576
119904)101584010038161003816100381610038161003816
2
+1003816100381610038161003816120577120576
119904
10038161003816100381610038162
]]
+ 2E [E1015840[1003816100381610038161003816120574120576
119904
10038161003816100381610038162
]]
le 161198622E [
1003816100381610038161003816120578120576
119904
10038161003816100381610038162
+1003816100381610038161003816120577120576
119904
10038161003816100381610038162
] + 2E [E1015840[1003816100381610038161003816120574120576
119904
10038161003816100381610038162
]]
(91)
Combined with (91) (90) yields
E sup119905le119904le119879
11989021205731199041003816100381610038161003816120578
120576
119904
10038161003816100381610038162
+ (1 minus192119872
2
1198601198622
120573)Eint
119879
119905
11989021205731199041003816100381610038161003816120577
120576
119904
10038161003816100381610038162
119889119904
le192119872
2
1198601198622
120573Eint
119879
119905
11989021205731199041003816100381610038161003816120578
120576
119904
10038161003816100381610038162
119889119904
+24119872
2
119860
120573Eint
119879
119905
1198902120573119904
[E1015840[1003816100381610038161003816120574120576
119904
10038161003816100381610038162
]] 119889119904
(92)
We claim that
Eint119879
119905
E1015840[1003816100381610038161003816120574120576
119904
10038161003816100381610038162
] 119889119904 997888rarr 0 as 120576 997888rarr 0 (93)
From (89)
120574120576
119904= 119868
1
119904+ 119868
2
119904+ 119868
3
119904+ 119868
4
119904 (94)
Mathematical Problems in Engineering 13
where
1198681
119904= (119870
120576
119904)1015840
int
1
0
(1198911199101015840 (119904 120582) minus 119891
1199101015840 (119904)) 119889120582
1198682
119904= 119870
120576
119904int
1
0
(119891119910(119904 120582) minus 119891
119910(119904)) 119889120582
1198683
119904= (119876
120576
119904)1015840
int
1
0
(1198911199111015840 (119904 120582) minus 119891
1199111015840 (119904)) 119889120582
1198684
119904= 119876
120576
119904int
1
0
(119891119911(119904 120582) minus 119891
119911(119904)) 119889120582
(95)
Then
Eint119879
119905
E1015840[1003816100381610038161003816120574120576
119904
10038161003816100381610038162
] 119889119904
le 4Eint119879
119905
E1015840[100381610038161003816100381610038161198681
119904
10038161003816100381610038161003816
2
+100381610038161003816100381610038161198682
119904
10038161003816100381610038161003816
2
+100381610038161003816100381610038161198683
119904
10038161003816100381610038161003816
2
+100381610038161003816100381610038161198684
119904
10038161003816100381610038161003816
2
] 119889119904
(96)
Take Eint119879119905E1015840[|1198682
119904|2
]119889119904 for example
Eint119879
119905
E1015840[100381610038161003816100381610038161198682
119904
10038161003816100381610038161003816
2
] 119889119904
= Eint119879
119905
E1015840[(119870
120576
119904int
1
0
(119891119910(119904 120582) minus 119891
119910(119904)) 119889120582)
2
]119889119904
le Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119904 120582) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582] 119889119904
le 2Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119904 120582) minus 119891
119910(119906
120576
119905)10038161003816100381610038161003816
2
119889120582] 119889119904
+ 2Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119906
120576
119905) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582] 119889119904
(97)
Note that
Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119906
120576
119905) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582] 119889119904
= Eint119903+120576
119903
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119906
120576
119905) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582] 119889119904
le sup119904isin[119905119879]
E [E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119906
120576
119905) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582]] 120576
997888rarr 0 as 120576 997888rarr 0
(98)
The inequality above holds due to the boundedness of|119870
120576
119904|2
int1
0|119891119910(119906
120576
119905) minus 119891
119910(119904)|
2
119889120582 Indeed Assumption (L3) im-plies the boundedness of 119891
119910(119906
120576
119905) minus 119891
119910(119904) Meanwhile 119870120576
119904is
the solution of mean-field BSEE (82) It can be easy to check119870120576
119904is bounded since the coefficients in (82) are boundedOn the other hand
Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119904 120582) minus 119891
119910(119906
120576
119905)10038161003816100381610038161003816
2
119889120582] 119889119904
le 11986221205822Eint
119879
119905
1003816100381610038161003816119870120576
119904
10038161003816100381610038162
times int
1
0
1198641015840[100381610038161003816100381610038161003816(119884
120576
119904minus 119884
119904)1015840100381610038161003816100381610038161003816
2
+100381610038161003816100381610038161003816(119885
120576
119904minus 119885
119904)1015840100381610038161003816100381610038161003816
2
times10038161003816100381610038161003816119884120576
119904minus 119884
119904
10038161003816100381610038161003816
2
+10038161003816100381610038161003816119885120576
119904minus 119885
119904
10038161003816100381610038161003816
2
] 119889120582 119889119904
(99)
where (119884120576
119904 119885
120576
119904) is the mild solution of the following equation
119889119884120576
119905= minus 119860119884
120576
119905119889119905
minus E1015840[119891 (119905 (119884
120576
119905)1015840
(119885120576
119905)1015840
119884120576
119905 119885
120576
119905 119906
120576
119905)] 119889119905
+ 119885120576
119905119889119882 (119905) 119905 isin [0 119879]
119884120576
119879= 120585
(100)
and (119884119904 119885
119904) is the mild solution of
119889119884119905= minus 119860119884
119905119889119905
minus E1015840[119891 (119905 (119884
119905)1015840
(119885119905)1015840
119884119905 119885
119905 119906
119905)] 119889119905
+ 119885119905119889119882 (119905) 119905 isin [0 119879]
119884119879= 120585
(101)
By the definition of 119906120576119905 according to (L2) we have
E1015840[119891 (119904 119884
1015840
(119904) 1198851015840
(119904) 119884 (119904) 119885 (119904) 119906120576
119905)]
997888rarr E1015840[119891 (119904 119884
1015840
(119904) 1198851015840
(119904) 119884 (119904) 119885 (119904) 119906119905)]
(102)
in 1198712([0 119879]119867) as 120576 rarr 0 Using the continuous dependencetheorem Corollary 5 we obtain
(119884120576
119904 119885
120576
119904) 997888rarr (119884
119904 119885
119904) as 120576 997888rarr 0 (103)
Then
Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119904 120582) minus 119891
119910(119906
120576
119905)10038161003816100381610038161003816
2
119889120582] 119889119904
le 11986221205822Eint
119879
119905
1003816100381610038161003816119870120576
119904
10038161003816100381610038162
times int
1
0
E1015840[100381610038161003816100381610038161003816(119884
120576
119904minus 119884
119904)1015840100381610038161003816100381610038161003816
2
+100381610038161003816100381610038161003816(119885
120576
119904minus 119885
119904)1015840100381610038161003816100381610038161003816
2
times10038161003816100381610038161003816119884120576
119904minus 119884
119904
10038161003816100381610038161003816
2
+10038161003816100381610038161003816119885120576
119904minus 119885
119904
10038161003816100381610038161003816
2
] 119889120582 119889119904
997888rarr 0 as 120576 997888rarr 0
(104)
14 Mathematical Problems in Engineering
Combining (98) with (104) we finally haveEint
119879
119905E1015840[|1198682
119904|2
]119889119904 rarr 0 as 120576 rarr 0The required result (93) follows by using the similar
estimations for 1198681119904 1198683
119904 and 1198684
119904
Note that 1 minus 1921198722
1198601198622120573 gt 0 if 120573 is sufficiently large
Now to prove the desired result (84) it suffices to applyGronwallrsquos lemma and estimate (93) to inequality (92)
To deal with the state constraint (75) we need to recall theEkeland variational principle
Lemma 16 (Ekelandrsquos variational principle see [16 Lemma41]) Let (119878 119889) be a complete metric space and let 119865(sdot) 119878 rarr
R be lower semicontinuous and bounded from below If for 120588 gt0 there exists 119906 isin 119878 such that
119865 (119906) le infVisin119878
119865 (V) + 120588 (105)
then there exists 119906120588 isin 119878 satisfying
(i) 119865 (119906120588) le 119865 (119906)
(ii) 119889 (119906120588 119906) le 120588
(iii) 119865 (119906120588) le 119865 (V) + 120588 sdot 119889 (119906120588 119906) forallV = 119906120588
(106)
Now fix V isin Uad and set
119878 = V (sdot) isin Uad | sup0le119905le119879
E1003816100381610038161003816V11990510038161003816100381610038162
le E100381610038161003816100381611990611990510038161003816100381610038162
+ |V|2
119889 (V (sdot) V (sdot)) = 119898 119905 isin [0 119879] | E1003816100381610038161003816V119905 minus V
119905
10038161003816100381610038162
gt 0
forallV (sdot) V (sdot) isin 119878
(107)
where119898 denotes the Lebesgue measure on RThe following result is proved as Proposition 41 in [16]
Lemma 17 (119878 119889(sdot sdot)) is a complete metric space and 119869120588 is
continuous and bounded on 119878 where
119869120588
(V (sdot)) = (119869 (V (sdot)) minus 119869 (119906 (sdot)) + 120588)2
+
10038161003816100381610038161003816100381610038161003816Eint
O
Φ(119909 (1198840(119909))
1015840
1198840(119909)) 119889119909
10038161003816100381610038161003816100381610038161003816
2
12
forallV (sdot) isin 119878(108)
and (119884 119885) is the mild solution of (72) corresponding to thecontrol V
Now we consider the following free initial state optimalcontrol problem
infV(sdot)isin119878
119869120588
(V (sdot)) (109)
It is easy to check that
0 le infV(sdot)isin119878
119869120588
(V (sdot)) le 119869120588
(119906 (sdot)) = 120588 (110)
According to Ekelandrsquos variational principle there exists a119906120588(sdot) isin 119881 such that
(i) 119869120588 (119906120588 (sdot)) le 120588
(ii) 119889 (119906120588 (sdot) 119906 (sdot)) le 120588
(iii) 119869120588 (119906120588 (sdot)) le 119869120588
(V (sdot)) + 120588119889 (119906120588 (sdot) 119906 (sdot)) forallV (sdot) isin 119878(111)
Using the spike variationmethod we can construct 119906120576120588(sdot) isin 119878as follows
119906120576120588
119905=
V119905 119905 isin [119904 119904 + 120576]
119906120588
119905 119905 isin [0 119879] [119904 119904 + 120576]
(112)
It is clear that 119889(119906120576120588(sdot) 119906120588(sdot)) le 120576 Let (119884120576120588(sdot) 119885
120576120588(sdot)) (resp
(119884120588(sdot) 119885
120588(sdot))) be the solution of (72) with respect to the
control 119906120576120588(sdot) (resp 119906120588(sdot)) Following (82) (119870120576120588
119905 119876
120576120588
119905) is the
mild solution of
119870120576120588
119905= int
119879
119905
119890119860(119904minus119905)
E1015840
times [1198911199101015840 (119904 Λ
120588
119904 119906
120588
119904) (119870
120576120588
119904)1015840
+ 119891119910(119904 Λ
120588
119904 119906
120588
119904)119870
120576120588
119904
+ 1198911199111015840 (119904 Λ
120588
119904 119906
120588
119904) (119876
120576120588
119904)1015840
+ 119891119911(119904 Λ
120588
119904 119906
120588
119904) 119876
120576120588
119904
+1
120576(119891 (119904 Λ
120588
119904 119906
120576120588
119904) minus 119891 (119904 Λ
120588
119904 119906
120588
119904))] 119889119904
minus int
119879
119905
119890119860(119904minus119905)
119876120576120588
119904119889119882 (119904)
(113)
ByTheorem 15 we know that
lim120576rarr0
E[ sup119904isin[119905119879]
100381610038161003816100381610038161003816100381610038161003816
119884120576120588
119904minus 119884
120588
119904
120576minus 119870
120576120588
119904
100381610038161003816100381610038161003816100381610038161003816
2
] = 0 forall119905 isin [0 119879]
lim120576rarr0
Eint119879
0
100381610038161003816100381610038161003816100381610038161003816
119885120576120588
119904minus 119885
120588
119904
120576minus 119876
120576120588
119904
100381610038161003816100381610038161003816100381610038161003816
2
119889119904 = 0
(114)
The proof of the following proposition is technical butbased on the arguments above and we omit it
Proposition 18 One has
1
120576E [Φ ((119884
120576120588
0)1015840
119884120576120588
0) minus Φ((119884
120588
0)1015840
119884120588
0)]
= E [Φ1199101015840 ((119884
120588
0)1015840
119884120588
0) (119870
120576120588
0)1015840
+ Φ119910((119884
120588
0)1015840
119884120588
0)119870
120576120588
0]
+ 119900 (120576)
Mathematical Problems in Engineering 15
1
120576(119869 (119906
120576120588) minus 119869 (119906
120588))
= E [1198921199101015840 ((119884
120588
0)1015840
119884120588
0) (119870
120576120588
0)1015840
+ 119892119910((119884
120588
0)1015840
119884120588
0)119870
120576120588
0]
+ Δ120576+1
120576Eint
119879
0
E1015840[ℎ (119904 Λ
120588
119904 119906
120576120588
119904)
minus ℎ (119904 Λ120588
119904 119906
120588
119904)] 119889119904 + 119900 (120576)
(115)
where
Δ120576= Eint
119879
0
E1015840[ℎ
1199101015840 (119904 Λ
120588
119904 119906
120576120588
119904) (119870
120576120588
119904)1015840
+ ℎ1199111015840 (119904 Λ
120588
119904 119906
120576120588
119904) (119876
120576120588
119904)1015840
+ ℎ119910(119904 Λ
120588
119904 119906
120576120588
119904)119870
120576120588
119904
+ ℎ119911(119904 Λ
120588
119904 119906
120576120588
119904) 119876
120576120588
119904] 119889119904
(116)
53 Variational Inequality and Adjoint Equation In thissubsection the adjoint process is introduced to deduce thevariational inequality
If we set V(sdot) = 119906120576120588(sdot) in (111) and notice that
119889(119906120576120588(sdot) 119906
120588(sdot)) le 120576 we get
minus120588 le1
120576(119869
120588(119906
120576120588
(sdot)) minus 119869120588(119906
120588
(sdot))) (117)
By Lemma 17
1
120576(119869
120588(119906
120576120588
(sdot)) minus 119869120588(119906
120588
(sdot)))
=(119869
120588(119906
120576120588(sdot)))
2
minus (119869120588(119906
120588(sdot)))
2
120576 (119869120588 (119906120576120588 (sdot)) + 119869120588 (119906120588 (sdot)))
=119869 (119906
120576120588(sdot)) + 119869 (119906
120588(sdot)) minus 2119869 (119906 (sdot)) + 2120588
119869120588 (119906120576120588 (sdot)) + 119869120588 (119906120588 (sdot))
times119869 (119906
120576120588(sdot)) minus 119869 (119906
120588(sdot))
120576
+
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] + E [Φ ((119884
120588
0)1015840
119884120588
0)]
119869120588 (119906120576120588 (sdot)) + 119869120588 (119906120588 (sdot))
times
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] minus E [Φ ((119884
120588
0)1015840
119884120588
0)]
120576
997888rarr 119897120588
1
119869 (119906120576120588(sdot)) minus 119869 (119906
120588(sdot))
120576
+ 119897120588
2
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] minus E [Φ ((119884
120588
0)1015840
119884120588
0)]
120576
(118)
where we set
119897120588
1=119869 (119906
120588(sdot)) minus 119869 (119906 (sdot)) + 120588
119869120588 (119906120588 (sdot))
119897120588
2=
E [Φ ((119884120588
0)1015840
119884120588
0)]
119869120588 (119906120588 (sdot))
(119)
and use the limit
119869 (119906120576120588
(sdot)) 997888rarr 119869 (119906120588
(sdot))
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] 997888rarr E [Φ ((119884
120588
0)1015840
119884120588
0)]
(120)
as 120576 rarr 0 according to (115)As |119897120588
1|2
+ |119897120588
2|2
= 1 for all 120588 gt 0 we know that there existsa subsequence of 119897120588
1 119897120588
2 (still denoted by 119897120588
1 119897120588
2) such that
lim120588rarr0
119897120588
1 119897120588
2 = 119897
1 1198972
1003816100381610038161003816119897110038161003816100381610038162
+1003816100381610038161003816119897210038161003816100381610038162
= 1
(121)
Combining (115) (117) with (118) we get
minus120588 le 119897120588
1
119869 (119906120576120588(sdot)) minus 119869 (119906
120588(sdot))
120576
+ 119897120588
2
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] minus E [Φ ((119884
120588
0)1015840
119884120588
0)]
120576
= 119897120588
1Δ120576+ 119897
120588
1E [119892
1199101015840 ((119884
120588
0)1015840
119884120588
0) (119870
120576120588
0)1015840
+119892119910((119884
120588
0)1015840
119884120588
0)119870
120576120588
0]
+119897120588
1
120576Eint
119879
0
E1015840[ℎ (119904 Λ
120588
119904 119906
120576120588
119904) minus ℎ (119904 Λ
120588
119904 119906
120588
119904)] 119889119904
+ 119897120588
2E [Φ
1199101015840 ((119884
120588
0)1015840
119884120588
0) (119870
120576120588
0)1015840
+Φ119910((119884
120588
0)1015840
119884120588
0)119870
120576120588
0] + (119897
120588
1+ 119897
120588
2) 119900 (120576)
(122)
Next we introduce the adjoint equation corresponding tovariational equation (113) whose solution is denoted by119875120588(119905)
119889119875120588
(119905)
= 119860lowast119875120588
(119905) 119889119905
+ E1015840[119891
1199101015840 (119905 Λ
120588
119905 119906
120588
119905) (119875
120588
(119905))1015840
+ 119891119910(119905 Λ
120588
119905 119906
120588
119905) 119875
120588
(119905) + 119897120588
1ℎ1199101015840 (119905 Λ
120588
119905 119906
120588
119905)
+ 119897120588
1ℎ119910(119905 Λ
120588
119905 119906
120588
119905)] 119889119905
16 Mathematical Problems in Engineering
+ E1015840[119891
1199111015840 (119905 Λ
120588
119905 119906
120588
119905) (119875
120588
(119905))1015840
+ 119891119911(119905 Λ
120588
119905 119906
120588
119905) 119875
120588
(119905)
+ 119897120588
1ℎ1199111015840 (119905 Λ
120588
119905 119906
120588
119905) + 119897
120588
1ℎ119911(119905 Λ
120588
119905 119906
120588
119905)] 119889119882 (119905)
119875120588
(0) = 119897120588
1E [119892
1199101015840 ((119884
120588
0(119909))
1015840
119884120588
0(119909))
+ 119892119910((119884
120588
0(119909))
1015840
119884120588
0(119909))]
+ 119897120588
2E [Φ
1199101015840 ((119884
120588
0(119909))
1015840
119884120588
0(119909))
+ Φ119910((119884
120588
0(119909))
1015840
119884120588
0(119909))]
(123)
where 119860lowast is the 1198712(O)-adjoint operator of 119860 Under assump-tions (L1)ndash(L3) this is a linear mean-field SEE with boundedcoefficients An application ofTheorem 11 implies that it has aunique adaptedmild solution such that119875120588(119905) isin S2
F ([0 119879] 119870)When 120588 rarr 0 according to Corollaries 5 and 13 119875120588(119905)
converges to 119875(119905) where
119875 (119905) isin S2
F ([0 119879] 119870) (124)
is the solution of the following equation
119875 (119905) = 1198971E [119892
1199101015840 (119884
1015840
0 119884
0) + 119892
119910(119884
1015840
0 119884
0)]
+ 1198972E [Φ
1199101015840 (119884
1015840
0 119884
0) + Φ
119910(119884
1015840
0 119884
0)]
+ int
119905
0
119890119860lowast(119905minus119904)
E1015840[119891
1199101015840 (119904) 119875
1015840
(119904) + 119891119910(119904) 119875 (119904)
+ 1198971ℎ1199101015840 (119904) + 119897
1ℎ119910(119904)] 119889119904
+ int
119905
0
119890119860lowast(119905minus119904)
E1015840[119891
1199111015840 (119904) 119875
1015840
(119904) + 119891119911(119904) 119875 (119904)
+ 1198971ℎ1199111015840 (119904) + 119897
1ℎ119911(119904)] 119889119882 (119904)
(125)
The following proposition which formally follows fromProposition 18 gives the relation between 119875120588(119905) and119870120576120588
119905
Proposition 19 Consider the following
E [119875120588
(0)119870120576120588
0]
= Eint119879
0
1
120576119875120588
(119904)E1015840[119891 (119904 Λ
120588
119904 119906
120576120588
119904) minus 119891 (119904 Λ
120588
119904 119906
120588
119904)]
minus 119897120588
1119870120576120588
119904E1015840[ℎ
1199101015840 (119904 Λ
120588
119904 119906
120588
119904) + ℎ
119910(119904 Λ
120588
119904 119906
120588
119904)]
minus 119897120588
1119876120576120588
119904E1015840[ℎ
1199111015840 (119904 Λ
120588
119904 119906
120588
119904) + ℎ
119911(119904 Λ
120588
119904 119906
120588
119904)] 119889119904
(126)
The following theorem constitutes the main contributionof this section themaximumprinciple for the BSPDE controlsystem
Theorem 20 Let assumptions (L1)ndash(L3) hold Suppose 119906(sdot) isan optimal control and (119884(sdot) 119885(sdot)) is the corresponding optimalstate trajectory for the BSPDE control systems (72) and (73)with the initial state constraint (75) Then there exists 119875(119905) isinS2
F ([0 119879] 119870) which satisfies (125) such that
H (119905 1198841015840
119905 119885
1015840
119905 119884
119905 119885
119905 V
119905 119875 (119905))
ge H (119905 1198841015840
119905 119885
1015840
119905 119884
119905 119885
119905 119906
119905 119875 (119905))
119886119890 119886119904 forallV isin U119886119889
(127)
whereH [0 119879]times119867timesL(Γ119867)times119867timesL(Γ119867)times119880times119870 rarr R
is the Hamiltonian function defined by
H (119905 119910 119910 119911 V 119901) = 1198971ℎ (119905 119910 119910 119911 V)
+ 119901119891 (119905 119910 119910 119911 V) (128)
Proof By (122) and Proposition 19 we obtain
minus120588 le1
120576Eint
119879
0
119875120588
(119904)E1015840[119891 (119904 Λ
120588
119904 119906
120576120588
119904)
minus 119891 (119904 Λ120588
119904 119906
120588
119904)] 119889119904
+119897120588
1
120576Eint
119879
0
E1015840[ℎ (119904 Λ
120588
119904 119906
120576120588
119904)
minus ℎ (119904 Λ120588
119904 119906
120588
119904)] 119889119904 + (119897
120588
1+ 119897
120588
2) 119900 (120576)
(129)
Letting 120576 rarr 0+ in (129) we derive for ae 120591 isin [0 119879]
minus120588 le 119897120588
1E1015840[ℎ (120591 Λ
120588
120591 V
120591) minus ℎ (120591 Λ
120588
120591 119906
120588
120591)]
+ 119875120588
(119904)E1015840[119891 (120591 Λ
120588
120591 V
120591) minus 119891 (120591 Λ
120588
120591 119906
120588
120591)]
(130)
for all V isin UadFinally taking 120588 rarr 0 in (130) we derive the desired
result
Remark 21 We note that if the coefficients do not dependexplicitly on the marginal law of the underlying diffusionthe result reduces to the classical case that is the SMP forBSPDEs without mean-field term
Remark 22 When we remove the initial state constraint (75)we obtain the general maximum principle for the mean-fieldBSPDEs system (ie without the constraint) with 119897
1= 1
54 Application A Backward Linear Quadratic Control Prob-lem Now we apply our maximum principle to solve an LQproblem For notational simplicity we restrict ourselves tothe free case (ie without the initial state constraint (75)) thegeneral case being handled in a similar way
Mathematical Problems in Engineering 17
Consider the following problem
119869 (119906) =1
2E [(119884 (0))
2] +
1
2Eint
119879
0
119873V2 (119905) 119889119905 997888rarr min
(131)
subject to
119889119884 (119905) = minus119860119884 (119905) 119889119905
minus 119861119884 (119905) + 119861E [119884 (119905)]
+119862V (119905) + 119863119885 (119905) + 119863E [119885 (119905)] 119889119905
+ 119885 (119905) 119889119882 (119905)
119884 (119879) = 120585 119905 isin [0 119879]
(132)
where 119860 is a partial differential operator satisfying condition(L1) and 119861 119861 119862119863119863 and119873 are bounded and deterministicconstantsWe also assume that119873 gt 0 and V isin 1198712F (0 119879 119880) It iseasy to verify that BSPDE (132) admits a uniquemild solution(119884(119905) 119885(119905))
119875(119905) the adjoint process of state equation (132) is thesolution of
119889119875 (119905) = 119860lowast119875 (119905) 119889119905 + 119861119875 (119905) + 119861E [119875 (119905)] 119889119905
+ 119863119875 (119905) + 119863E [119875 (119905)] 119889119882 (119905)
119875 (0) = 119884 (0)
(133)
Let 119906(sdot) be an optimal control and let (119884(sdot) 119885(sdot)) bethe corresponding state process By maximum principle ofTheorem 20 (note that 119897
1= 1 in this problem)
1
2119873V2 (119905) + 119875 (119905) 119862V (119905) ge
1
2119873119906
2
(119905) + 119875 (119905) 119862119906 (119905) (134)
for all V isin 119880ad since the state equation has the form (132)Thisin turn implies
119906 (119905) = minus119862
119873119875 (119905) (135)
It is clear that (131) is a positive quadratic functional of controlbecause of 119873 gt 0 Hence an optimal control exists Thecandidate optimal control (135) is indeed an optimal controlof this LQ problem for it is the only control which satisfies themaximum principle
Next we want to obtain a more explicit representationof the optimal control (135) from the state equation (132)Substituting (135) into state equation (132) yields
119889119884 (119905) = minus119860119884 (119905) 119889119905
minus 119861119884 (119905) + 119861E [119884 (119905)] minus1198622
119873119875 (119905)
+119863119885 (119905) + 119863E [119885 (119905)] 119889119905 + 119885 (119905) 119889119882 (119905)
119884 (119879) = 120585 119905 isin [0 119879]
(136)
Combining the above equation with (133) we obtain thefollowing related feedback control system
119889119884 (119905) = minus119860119884 (119905) 119889119905
minus 119861119884 (119905) + 119861E [119884 (119905)] minus1198622
119873119875 (119905)
+ 119863119885 (119905) + 119863E [119885 (119905)] 119889119905 + 119885 (119905) 119889119882 (119905)
119884 (119879) = 120585
119889119875 (119905) = 119860lowast119875 (119905) 119889119905 + 119861119875 (119905) + 119861E [119875 (119905)] 119889119905
+ 119863119875 (119905) + 119863E [119875 (119905)] 119889119882 (119905)
119875 (0) = 119884 (0)
(137)
Looking at the terminal condition of 119875(119905) in (133) andconsidering the mean-field type of (132) it is reasonable toconjecture that 119875(119905) has the following form
119875 (119905) = 120593 (119905) 119884 (119905) + 120601 (119905)E [119884 (119905)] (138)
where120593(119905)120601(119905) are deterministic differential functionswhichwill be specified below Moreover 120593(0) = 1 120601(0) = 0
Inserting this form into adjoint equation (133) and notic-ing that 119884(119905) satisfies (136) we can compare the coefficientsof 119889119905 and 119889119882(119905) to obtain the following equation
2120593 (119905) 119860119884 (119905) + 2120601 (119905) 119860E [119884 (119905)] + 119861120593 (119905) 119884 (119905)
+ 2 (119861120601 (119905) + 119861120593 (119905) + 119861120601 (119905))E [119884 (119905)]
= minus120593 (119905) 119861119884 (119905) minus1198622
119873120593 (119905) 119884 (119905) + 119863119885 (119905)
+ 119884 (119905)119889120593 (119905)
119889119905+ E [119884 (119905)]
119889120601 (119905)
119889119905
+ (1206012
(119905)1198622
119873+ 2
1198622
119873120593 (119905) 120601 (119905))E [119884 (119905)]
minus (119863120601 (119905) + 119863120601 (119905) + 119863120593 (119905))E [119885 (119905)]
120593 (119905) 119885 (119905) = 119863120593 (119905) 119884 (119905)
+ (119863120601 (119905) + 119863120593 (119905) + 119863120601 (119905))E [119884 (119905)]
(139)
Then subtracting 119885(119905) we have
2120593 (119905) 119860119884 (119905) + 2120601 (119905) 119860E [119884 (119905)] + 119861120593 (119905) 119884 (119905)
+ 2 (119861120601 (119905) + 119861120593 (119905) + 119861120601 (119905))E [119884 (119905)]
18 Mathematical Problems in Engineering
= 120593 (119905) minus119861 +1198622
119873120593 (119905) minus 119863
2119884 (119905)
+ 119884 (119905)119889120593 (119905)
119889119905+ E [119884 (119905)]
119889120601 (119905)
119889119905
+ (1206012
(119905)1198622
119873+ 2
1198622
119873120593 (119905) 120601 (119905))E [119884 (119905)]
minus1
120593 (119905)(119863120601 (119905) + 119863120593 (119905) + 119863120601 (119905))
2
E [119884 (119905)]
minus 2119863 (119863120601 (119905) + 119863120593 (119905) + 119863120601 (119905))E [119884 (119905)]
(140)
Comparing the coefficients of 119884(119905) and E[119884(119905)] respec-tively we get
119889
119889119905120593 (119905) = 2119860
lowast120593 (119905) + 2119861120593 (119905) + 119863
2120593 (119905) minus
1198622
1198731205932
(119905)
120593 (0) = 1
(141)
119889
119889119905120601 (119905) = 2119860
lowast120601 (119905) + (
(119863 + 119863)2
120593 (119905)minus1198622
119873)120601
2
(119905)
+ 2119877 (119905) 120601 (119905) + (1198632+ 2119861 + 2119863119863)120593 (119905)
120601 (0) = 0
(142)
where 119877(119905) = 119861 + 119861 + (119863 + 119863)2
minus (1198622119873)120593(119905)
We solve (141) to get
120593 (119905) = (119890minus(2119860lowast+2119861+119863
2)119905(1 minus
1198622
119873(2119860lowast + 2119861 + 1198632))
+1198622
119873(2119860lowast + 2119861 + 1198632))
minus1
(143)
Then (142) exists a unique solution from the classical Riccatiequation theory
We now conclude the above discussions in the followingresult
Theorem 23 For onersquos linear quadratic stochastic partialdifferential control problem (131)-(132) the unique optimalcontrol 119906(sdot) isin 119880ad is given by
119906 (119905) = minus119862
119873(120593 (119905) 119884 (119905) + 120601 (119905)E [119884 (119905)]) (144)
with 120593(119905) satisfying (143) and 120601(119905) solving (142)
Conflict of Interests
The authors declare that they have no conflict of interestsregarding the publication of this paper
Acknowledgment
This research is partially supported by the Natural ScienceFoundation of China under Grant no 11301310
References
[1] Y Hu and S Peng ldquoAdapted solution of a backward semilinearstochastic evolution equationrdquo Stochastic Analysis and Applica-tions vol 9 no 4 pp 445ndash459 1991
[2] N IMahmudov andMAMcKibben ldquoOnbackward stochasticevolution equations in Hilbert spaces and optimal controlrdquoNonlinear Analysis vol 67 no 4 pp 1260ndash1274 2007
[3] M Fuhrman and G Tessitore ldquoNonlinear Kolmogorov equa-tions in infinite dimensional spaces the backward stochasticdifferential equations approach and applications to optimalcontrolrdquoThe Annals of Probability vol 30 no 3 pp 1397ndash14652002
[4] M Fuhrman and G Tessitore ldquoInfinite horizon backwardstochastic differential equations and elliptic equations inHilbert spacesrdquo Annals of Probability vol 32 no 1B pp 607ndash660 2004
[5] X Mao ldquoAdapted solutions of backward stochastic differentialequations with non-Lipschitz coefficientsrdquo Stochastic Processesand their Applications vol 58 no 2 pp 281ndash292 1995
[6] J Lasry and P Lions ldquoMean field gamesrdquo Japanese Journal ofMathematics vol 2 no 1 pp 229ndash260 2007
[7] R Buckdahn J Li and S Peng ldquoMean-field backward stochas-tic differential equations and related partial differential equa-tionsrdquo Stochastic Processes and their Applications vol 119 no 10pp 3133ndash3154 2009
[8] D Andersson and B Djehiche ldquoAmaximum principle for SDEsofmean-field typerdquoAppliedMathematics andOptimization vol63 no 3 pp 341ndash356 2011
[9] R Buckdahn B Djehiche J Li and S Peng ldquoMean-fieldbackward stochastic differential equations a limit approachrdquoThe Annals of Probability vol 37 no 4 pp 1524ndash1565 2009
[10] R Xu ldquoMean-field backward doubly stochastic differentialequations and related SPDEsrdquo Boundary Value Problems vol2012 article 114 2012
[11] R Buckdahn B Djehiche and J Li ldquoA general stochasticmaximum principle for SDEs of mean-field typerdquo AppliedMathematics and Optimization vol 64 no 2 pp 197ndash216 2011
[12] J Li ldquoStochasticmaximumprinciple in themean-field controlsrdquoAutomatica vol 48 no 2 pp 366ndash373 2012
[13] G Wang C Zhang and W Zhang ldquoStochastic maximumprinciple for mean-field type optimal control under partialinformationrdquo IEEE Transactions on Automatic Control vol 59no 2 pp 522ndash528 2014
[14] M Hafayed ldquoA mean-field necessary and sufficient conditionsfor optimal singular stochastic controlrdquo Communications inMathematics and Statistics vol 1 no 4 pp 417ndash435 2013
[15] M Hafayed ldquoA mean-field maximum principle for optimalcontrol of forward-backward stochastic differential equationswith Poisson jumpprocessesrdquo International Journal ofDynamicsand Control vol 1 no 4 pp 300ndash315 2013
[16] Y Hu and S Peng ldquoMaximum principle for semilinearstochastic evolution control systemsrdquo Stochastics and StochasticsReports vol 33 no 3-4 pp 159ndash180 1990
[17] M Fuhrman Y Hu and G Tessitore ldquoStochastic maximumprinciple for optimal control of SPDEsrdquo Comptes Rendus Math-ematique vol 350 no 13-14 pp 683ndash688 2012
Submit your manuscripts athttpwwwhindawicom
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Mathematical Problems in Engineering
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Differential EquationsInternational Journal of
Volume 2014
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Abstract and Applied AnalysisHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Volume 2014 Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Stochastic AnalysisInternational Journal of
Mathematical Problems in Engineering 7
From formula (32) we know that
int
119879
119905
sup119899isinN
E [11989021205731199041003816100381610038161003816119885119899
(119904)10038161003816100381610038162
] 119889119904
le 21198621+ int
119879
119905
sup119899isinN
E[ sup119904le119903le119879
11989021205731199031003816100381610038161003816119884119899minus1 (119903)
10038161003816100381610038162
]119889119904
le 21198621+ 2119862
1int
119879
119905
exp (119879 minus 119904) 119889119904
= 21198621exp (119879 minus 119905)
(34)
This proves point (ii) of the LemmaTo prove point (iii) we note that
E1015840[10038161003816100381610038161003816119891 (119904 119884
1015840
119899(119904) 119885
1015840
119899+1(119904) 119884
119899(119904) 119885
119899+1(119904))
minus 119891 (119904 1198841015840
119899minus1(119904) 119885
1015840
119899(119904) 119884
119899minus1(119904) 119885
119899(119904))
10038161003816100381610038161003816
2
]
le E1015840[120579 (
100381610038161003816100381610038161198841015840
119899(119904) minus 119884
1015840
119899minus1(119904)10038161003816100381610038161003816
2
) + 119897100381610038161003816100381610038161198851015840
119899+1(119904) minus 119885
1015840
119899(119904)10038161003816100381610038161003816
2
]
+ 120579 (1003816100381610038161003816119884119899 (119904) minus 119884119899minus1 (119904)
10038161003816100381610038162
) + 1198971003816100381610038161003816119885119899+1
(119904) minus 119885119899(119904)10038161003816100381610038162
= E [120579 (1003816100381610038161003816119884119899 (119904) minus 119884119899minus1 (119904)
10038161003816100381610038162
) + 1198971003816100381610038161003816119885119899+1
(119904) minus 119885119899(119904)10038161003816100381610038162
]
+ 120579 (1003816100381610038161003816119884119899 (119904) minus 119884119899minus1 (119904)
10038161003816100381610038162
) + 1198971003816100381610038161003816119885119899+1
(119904) minus 119885119899(119904)10038161003816100381610038162
(35)
By Lemma 1 we have
E( sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899+1 (119904) minus 119884119899 (119904)
10038161003816100381610038162
)
+ Eint119879
119905
11989021205731199041003816100381610038161003816119885119899+1
(119904) minus 119885119899(119904)10038161003816100381610038162
119889119904
le12119872
2
119860
120573E
times int
119879
119905
1198902120573119904 10038161003816100381610038161003816
E1015840[119891 (119904 119884
1015840
119899(119904) 119885
1015840
119899+1(119904) 119884
119899(119904) 119885
119899+1(119904))
minus 119891 (119904 1198841015840
119899minus1(119904) 119885
1015840
119899(119904)
119884119899minus1
(119904) 119885119899(119904))]
10038161003816100381610038161003816
2
119889119904
le12119872
2
119860
120573E
times int
119879
119905
1198902120573119904
E1015840[10038161003816100381610038161003816119891 (119904 119884
1015840
119899(119904) 119885
1015840
119899+1(119904) 119884
119899(119904) 119885
119899+1(119904))
minus 119891 (119904 1198841015840
119899minus1(119904) 119885
1015840
119899(119904)
119884119899minus1
(119904) 119885119899(119904))
10038161003816100381610038161003816
2
] 119889119904
le24119872
2
119860
120573Eint
119879
119905
1198902120573119904
[120579 (1003816100381610038161003816119884119899 (119904) minus 119884119899minus1 (119904)
10038161003816100381610038162
)
+ 1198971003816100381610038161003816119885119899+1
(119904) minus 119885119899(119904)10038161003816100381610038162
] 119889119904
(36)
We can choose 120573 gt 0 sufficiently large such that
(1 minus24119872
2
119860119897
120573)Eint
119879
119905
11989021205731199041003817100381710038171003817119885119899+1
(119904) minus 119885119899(119904)10038171003817100381710038172
119889119904 ge 0 (37)
Then
E( sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899+1 (119904) minus 119884119899 (119904)
10038161003816100381610038162
)
le24119872
2
119860
120573Eint
119879
119905
1198902120573119904120579 (1003816100381610038161003816119884119899 (119904) minus 119884119899minus1 (119904)
10038161003816100381610038162
) 119889119904
le 1198622Eint
119879
119905
120579(E sup119904le119903le119879
11989021205731199031003816100381610038161003816119884119899 (119903) minus 119884119899minus1 (119903)
10038161003816100381610038162
)119889119904
(38)
where we set 1198622= (24119872
2
119860120573)119890
2120573119879
We divide the interval [0 119879] into subintervals 0 = 1205910lt
1205911lt sdot sdot sdot lt 120591
119898= 119879 by setting 120591
119896= 119896120575 119896 = 1 2 3 119898 with
120575 = 119879119898
Lemma 7 For all 119905 isin [120591119896minus1
120591119896] define
1198623= 119862
2120579 (2119862
1exp (119879))
1205931198961(119905) = 119862
3(120591119896minus 119905)
120593119896119899+1
(119905) = 1198622int
120591119896
119905
120579 (120593119896119899(119904)) 119889119904 119899 ge 1
(39)
Then for all 119899 ge 1 the following inequality holds for a suitable120575 gt 0
0 le 120593119896119899(119905) le 120593
119896119899minus1(119905) le sdot sdot sdot le 120593
1198961(119905) (40)
Proof Firstly it needs to be verified that for all 119905 isin [120591119896minus1
120591119896]
the following inequality
1205931198962(119905) = 119862
2int
120591119896
119905
120579 (1205931198961(119904)) 119889119904 = 119862
2int
120591119896
119905
120579 (1198623(120591119896minus 119904)) 119889119904
le 1198623(120591119896minus 119905) = 120593
1198961(119905)
(41)
holds provided 120575 gt 0 is chosen sufficiently smallActually this inequality holds provided that
1198622120579 (119862
3(120591119896minus 119905)) le 119862
3= 119862
2120579 (2119862
1exp (119879)) (42)
8 Mathematical Problems in Engineering
or
1198623(120591119896minus 119905) = 119862
2120579 (2119862
1exp (119879)) (120591
119896minus 119905) le 2119862
1exp (119879)
(43)
Since 1198621gt 1 from 120579(119906) le 119886 + 119887119906 the above inequality holds
if
1198622(119886 + 119887) (120591
119896minus 119905) le 1 (44)
Thus (41) holds for any 119905 isin [120591119896minus1
120591119896] 119896 = 1 2 119898 if 120591
119896minus
120591119896minus1
le 11198622(119886 + 119887) Therefore we can choose a sufficiently
large119898 isin N such that 120575 = 119879119898 le 11198622(119886 + 119887) Clearly such a
120575 only depends on 119886 119887 119897 119879 and119872119860
Now assume that (40) holds for some 119899 ge 2 Then wehave
120593119896119899+1
(119905) = 1198622int
120591119896
119905
120579 (120593119896119899(119904)) 119889119904 le 119862
2int
120591119896
119905
120579 (120593119896119899minus1
(119904)) 119889119904
= 120593119896119899(119905) forall119905 isin [120591
119896minus1 120591
119896]
(45)
This completes the proof
Now we can give the main result of this section
Theorem 8 Assume that (A2) and (A3) hold Then thereexists a unique mild solution (119884 119885) to (11)
Proof Consider the following
Uniqueness To show the uniqueness let both (119884 119885) and( 119885) be solutions of (11) For any 120573 gt 0 similar to the proofof (36) one can obtain
E( sup119905le119904le119879
119890212057311990410038161003816100381610038161003816
119884 (119904) minus (119904)10038161003816100381610038161003816
2
) + Eint119879
119905
119890212057311990410038161003816100381610038161003816
119885 (119904) minus 119885 (119904)10038161003816100381610038161003816
2
119889119904
le24119872
2
119860
120573E
times int
119879
119905
1198902120573119904
[120579 (10038161003816100381610038161003816119884 (119904) minus (119904)
10038161003816100381610038161003816
2
) + 11989710038161003816100381610038161003816119885 (119904) minus 119885 (119904)
10038161003816100381610038161003816
2
] 119889119904
(46)
That is if 120573 is sufficiently large
E( sup119905le119904le119879
119890212057311990410038161003816100381610038161003816
119884 (119904) minus (119904)10038161003816100381610038161003816
2
)
le24119872
2
119860
120573Eint
119879
119905
1198902120573119904
[120579 (10038161003816100381610038161003816119884 (119904) minus (119904)
10038161003816100381610038161003816
2
)] 119889119904
le 1198622Eint
119879
119905
120579(E sup119904le119903le119879
119890212057311990310038161003816100381610038161003816
119884 (119904) minus (119904)10038161003816100381610038161003816
2
)119889119904
(47)
An application of Bihari inequality yields
E( sup119905le119904le119879
119890212057311990410038161003816100381610038161003816
119884 (119904) minus (119904)10038161003816100381610038161003816
2
) = 0 (48)
So 119884(119905) = (119905) for all 119905 isin [0 119879] as It then follows from (46)that 119885(119905) = 119885(119905) for all 119905 isin [0 119879] as as well This establishesthe uniqueness
ExistenceWe claim that the sequence (119884119899 119885
119899) defined by (27)
satisfies
E sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899+1 (119904) minus 119884119899 (119904)
10038161003816100381610038162
997888rarr 0 forall0 le 119905 le 119879 (49)
as 119899 rarr infinIndeed for all 119905 isin [120591
119896minus1 120591
119896] we set 120593
119896119899(119905) =
E sup119904isin[119905120591119896]
1198902120573119904|119884119899+1
(119904) minus 119884119899(119904)|
2 By Lemmas 6 and 7
1205931198961(119905) = E sup
119904isin[119905120591119896]
119890212057311990410038161003816100381610038161198842 (119904) minus 1198841 (119904)
10038161003816100381610038162
le 1198622int
120591119896
119905
120579(E sup119904le119903le120591119896
119890212057311990310038161003816100381610038161198841 (119903) minus 1198840 (119903)
10038161003816100381610038162
)119889119904
le 1198622int
120591119896
119905
120579 (21198621exp (120591
119896minus 119905)) 119889119904
le 1198622120579 (2119862
1exp (119879)) (120591
119896minus 119905) = 119862
3(120591119896minus 119905) = 120593
1198961(119905)
(50)
Suppose that 120593119896119899(119905) le 120593
119896119899(119905) holds for some 119899 ge 1
According to Lemma 6(iii) and Lemma 7 for all 119905 isin [120591119896minus1
120591119896]
we obtain
120593119896119899+1
(119905) = E sup119904isin[119905120591119896]
11989021205731199041003816100381610038161003816119884119899+2 (119904) minus 119884119899+1 (119904)
10038161003816100381610038162
le 1198622int
120591119896
119905
120579(E sup119903isin[119904120591119896]
11989021205731199031003816100381610038161003816119884119899+1 (119903) minus 119884119899 (119903)
10038161003816100381610038162
)119889119904
= 1198622int
120591119896
119905
120579 (120593119896119899(119904)) 119889119904
le 1198622int
120591119896
119905
120579 (120593119896119899(119904)) 119889119904 = 120593
119896119899+1(119905)
(51)
This implies that for all 119899 isin N
120593119896119899(119905) le 120593
119896119899(119905) (52)
By definition 120593119896119899(sdot) is continuous on [120591
119896minus1 120591
119896] Note that
for each 119899 ge 1 120593119896119899(sdot) is decreasing on [120591
119896minus1 120591
119896] and for each
119905 119896 120593119896119899(119905) is a nonincreasing sequence Therefore we define
the function 120593119896(119905) by 120593
119896119899(119905) darr 120593
119896(119905) It is easy to verify that
120593119896(119905) is continuous and nonincreasing on [120591
119896minus1 120591
119896] By the
definitions of 120593119896119899(119905) and 120593
119896(119905) we get
120593119896(119905) = lim
119899rarrinfin
1198622int
120591119896
119905
120579 (120593119896119899(119904)) 119889119904 = 119862
2int
120591119896
119905
120579 (120593119896(119904)) 119889119904
(53)
for all 120591119896minus1
le 119905 le 120591119896 Since int
0+(119889119906120579(119906)) = infin the Bihari
inequality implies
120593119896(119905) = 0 for each 119905 isin [120591
119896minus1 120591
119896] (54)
For each 119896 isin 1 2 119898 (52) and (54) yield
lim119899rarrinfin
120593119896119899(119905) = 0 (55)
Mathematical Problems in Engineering 9
Then
E sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899+1 (119904) minus 119884119899 (119904)
10038161003816100381610038162
le max1le119896le119898
E sup119904isin[120591119896minus1120591119896]
11989021205731199041003816100381610038161003816119884119899+1 (119904) minus 119884119899 (119904)
10038161003816100381610038162
= max1le119896le119898
120593119896119899(119905) 997888rarr 0
(56)
as 119899 rarr infin and this proves the assertion (49)By (36) we obtain
E( sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899+1 (119904) minus 119884119899 (119904)
10038161003816100381610038162
) + (1 minus24119872
2
119860119897
120573)E
times int
119879
119905
11989021205731199041003816100381610038161003816119885119899+1
(119904) minus 119885119899(119904)10038161003816100381610038162
119889119904
le24119872
2
119860
120573Eint
119879
119905
1198902120573119904
[120579 (1003816100381610038161003816119884119899 (119904) minus 119884119899minus1 (119904)
10038161003816100381610038162
)] 119889119904
(57)
Applying (49) to the above formula we see that (119884119899 119885
119899) is
a Cauchy (hence convergent) sequence in S2
F ([0 119879]119867) times
H2
F ([0 119879]L(Γ119867)) denote the limit by (119884 119885) Now letting119899 rarr infin in (27) we obtain that
119884 (119905) = 119890119860(119879minus119905)
120585
+ int
119879
119905
E1015840[119890
119860(119904minus119905)119891 (119904 119884
1015840
(119904) 1198851015840
(119904) 119884 (119904) 119885 (119904))] 119889119904
minus int
119879
119905
119890119860(119904minus119905)
119885 (119904) 119889119882 (119904)
(58)
holds on the entire interval [0 119879]The theorem is nowproved
To illustrate the application of the obtained existenceand uniqueness result we consider the example of backwardstochastic partial differential equations (BSPDEs) of mean-field type
Example 9 Let O be an open bounded domain in R119899
with uniformly 1198622 boundary 120597O let 119861(119905) be a standard 119899-dimensional Brownian motion (equipped with the normalfiltration) and let 120585 O rarr R be an F
119879-measurable
random variable We also let 119871 denote the semielliptic partialdifferential operator on 1198622
(R) of the form
119871 =
119899
sum
119894119895=1
119886119894119895(119909)
1205972
120597119909119894120597119909
119895
+
119899
sum
119894=1
119887119894(119909)
120597
120597119909119894
(59)
The aim is to study the solvability of the following initialboundary value problem
119889119884 (119905 119909)
= (119871119884 (119905 119909) + E1015840
times [119892 (119905 119909 1198841015840
(119905 119909) 1198851015840
(119905 119909) 119884 (119905 119909) 119885 (119905 119909))]) 119889119905
+ 119885 (119905 119909) 119889119861 (119905) ae on (0 119879) times O
119884 (119905 119909) = 0 ae on (0 119879) times 120597O
119884 (119879 119909) = 120585 (119879 119909) ae onO(60)
where
119884 [0 119879] times O 997888rarr R
119885 [0 119879] times O 997888rarr L (R119899 119871
2
(O))
119892 [0 119879] times O timesR timesL (R119899 119871
2
(O))
timesR timesL (R119899 119871
2
(O)) 997888rarr R
(61)
The following assumptions will have to be in force
(H1) 119886119894119895 119887
119894 R119899
rarr R are uniformly continuous andbounded and satisfy the usual uniform ellipticitycondition sum119899
119894119895=1119886119894119895(119909)119908
119894119908119895ge 120582|119908|
2 for some 120582 gt 0
and all 119909 isin O 119908 isin R119899(H2) 119892 is measurable in (119905 119909 119910 119910 119911) and continuous in
( 119911) and there exists 119862 gt 0 such that1003816100381610038161003816119892 (119905 119909 1199101 1 1199101 1199111) minus 119892 (119905 119909 1199102 2 1199102 1199112)
1003816100381610038161003816
le 119862 [10038161003816100381610038161199101 minus 1199102
1003816100381610038161003816 +10038161003816100381610038161 minus 2
1003816100381610038161003816 +10038161003816100381610038161199101 minus 1199102
1003816100381610038161003816 +10038161003816100381610038161199111 minus 1199112
1003816100381610038161003816]
(62)
for all 0 le 119905 le 119879 119909 isin O 1199101 119910
2 119910
1 119910
2isin R
1
2 119911
1 119911
2isin
L(R119899 119871
2(O))
Then we are now in a position of showing existence anduniqueness of the solution of BSPDEs (60)
Theorem 10 If (H1) and (H2) are satisfied then the mean-field BSPDE (60) has a unique mild solution (119884 119885) isin
1198712(0 119879 119871
2(Ω 119871
2(O))) times 1198712F (0 119879 119871
2(R119899
1198712(Ω 119871
2(O))))
Proof Let119867 = 1198712(O) and119870 = R119899 Define the operator 119860 by
119860119884 (119905 sdot) = 119871119884 (119905 sdot) (63)
It is shown in [17] (see Example 21 in [17]) that 119860 generatesa strongly continuous semigroup on 119867 Define the maps 119891
[0 119879] times 119867 timesL(119870119867) times 119867 timesL(119870119867) rarr 119867 by
119891 (119905 1198841015840
(119905) 1198851015840
(119905) 119884 (119905) 119885 (119905)) (119909)
= 119892 (119905 119909 1198841015840
(119905 119909) 1198851015840
(119905 119909) 119884 (119905 119909) 119885 (119905 119909))
(64)
10 Mathematical Problems in Engineering
for all 0 le 119905 le 119879 119909 isin O With these identifications(60) can be written in the form of (11) By (H2) we know119891 satisfy condition (A1) Hence an application of Theorem 4concludes that (60) has a unique mild solution (119884 119885) isin
1198712(0 119879 119871
2(Ω 119871
2(O))) times 1198712F (0 119879 119871
2(R119899
1198712(Ω 119871
2(O))))
4 Mean-Field Stochastic Evolution Equations
Let 119882(119905) 119905 isin [0 119879] be a cylindrical Wiener process withvalues in a Hilbert space Γ defined on a probability space(ΩFP) We fix an interval [119905 119879] sub [0 119879] and considerthe stochastic evolution equations of mean-field type for anunknown process 119883(119904) 119904 isin [119905 119879] with values in a Hilbertspace 119870
119889119883 (119904) = 119861119883 (119904) 119889119904 + E1015840[119887 (119904 119883
1015840
(119904) 119883 (119904))] 119889119904
+ E1015840[120590 (119904 119883
1015840
(119904) 119883 (119904))] 119889119882 (119904)
119883 (119905) = 119909 isin 119870
(65)
where operator 119861 is the generator of a strongly continuoussemigroup 119890
119905119861 119905 ge 0 in the Hilbert space 119870 with 119872119861≜
sup119905isin[0119879]
|119890119905119861|
By a mild solution of (65) we mean an F119904-measurable
process 119883(119904) 119904 isin [119905 119879] with continuous paths in 119870 suchthat P-as
119883 (119904) = 119890119861(119904minus119905)
119909
+ int
119904
119905
119890119861(119904minus120591)
E1015840[119887 (120591 119883
1015840
(120591) 119883 (120591))] 119889120591
+ int
119904
119905
119890119861(119904minus120591)
E1015840[120590 (120591119883
1015840
(120591) 119883 (120591))] 119889119882 (120591)
119904 isin [119905 119879]
(66)
We suppose the following(A4) 119887 [0 119879] times 119870 times 119870 rarr 119870 is a measurable mapping
which satisfies10038161003816100381610038161003816119887 (119905 119909
1015840 119909) minus 119887 (119905 119910
1015840 119910)
10038161003816100381610038161003816
2
le 1198711(100381610038161003816100381610038161199091015840minus 119910
101584010038161003816100381610038161003816
2
+1003816100381610038161003816119909 minus 119910
10038161003816100381610038162
)
119905 isin [0 119879] 1199091015840 119909 119910
1015840 119910 isin 119870
(67)
for some constant 1198711gt 0
(A5) The mapping 120590 [0 119879] times 119870 times 119870 rarr L(Γ 119870) fulfillsthat for every V isin Γ the map 120590V [0 119879] times 119870 times 119870 rarr 119870
is measurable for every 119904 gt 0 119905 isin [0 119879] 1199091015840 1199101015840 119909 119910 isin 119870119890119904119861120590(119905 119909
1015840 119909) isin L(Γ 119870) and
10038161003816100381610038161003816119890119904119861120590 (119905 119909
1015840 119909)
10038161003816100381610038161003816L(Γ119870)le 119871
2119904minus120574(1 +
10038161003816100381610038161003816119909101584010038161003816100381610038161003816+ |119909|)
10038161003816100381610038161003816119890119904119861120590 (119905 119909
1015840 119909) minus 119890
119904119861120590 (119905 119910
1015840 119910)
10038161003816100381610038161003816L(Γ119870)
le 1198712119904minus120574(100381610038161003816100381610038161199091015840minus 119910
101584010038161003816100381610038161003816+1003816100381610038161003816119909 minus 119910
1003816100381610038161003816)
10038161003816100381610038161003816120590 (119905 119909
1015840 119909)
10038161003816100381610038161003816L(Γ119870)le 119871
2(1 +
10038161003816100381610038161003816119909101584010038161003816100381610038161003816+ |119909|)
(68)
for some constants 1198712gt 0 and 120574 isin [0 12)
Theorem 11 Under assumptions (A3) and (A4) (65) has aunique mild solution119883(sdot) isin S2
F ([119905 119879] 119870)
The proof is constructed in two steps like that ofTheorem 4 and it uses standard arguments for stochastic evo-lution equations introduced in the proof of Proposition 32 in[3] Since the proof is straightforward we prefer to omit it
Remark 12 In our paper Lipchitz condtion (A4) is givento get the well-posedness of mean-field stochastic evolutionequations In fact (A4) can be replaced by a weaker conditionsuch as (A3) We just give the condition (A4) for simplicity
From standard arguments we can also get the followingcontinuous dependence theorem
Corollary 13 Assume that for all 120572 in a metric space 119865(119887120572 120590
120572) satisfy (A4) and (A5)with119871
1and119871
2independent of 120572
Also assume that
Eint119879
0
10038161003816100381610038161003816E1015840[119887120572(119904 119883
1015840
(119904) 119883 (119904))]
minus E1015840[1198871205720(119904 119883
1015840
(119904) 119883 (119904))]10038161003816100381610038161003816
2
119889119904 997888rarr 0
Eint119879
0
10038161003816100381610038161003816E1015840[120590
120572(119904 119883
1015840
(119904) 119883 (119904))]
minusE1015840[120590
1205720(119904 119883
1015840
(119904) 119883 (119904))]10038161003816100381610038161003816
2
119889119904 997888rarr 0
(69)
as 120572 rarr 1205720for all119883 isin S2
F ([0 119879] 119870)If we denote by 119883120572
(sdot) the mild solution of mean-field SEE(65) corresponding to the functions (119887
120572 120590
120572) and to the initial
data 119909 then we have
sup119904isin[0119879]
E1003816100381610038161003816119883
120572
(119904) minus 1198831205720 (119904)
10038161003816100381610038162
997888rarr 0 119886119904 120572 997888rarr 1205720 (70)
5 Maximum Principle for BSPDEs ofMean-Field Type
51 Formulation of the Problem Let O isin R119899 be a boundedopen set with smooth boundary 120597O and let 119880 the space ofcontrols be a separable real Hilbert space We denote
U = V (sdot) isin 1198712F(0 119879 119880)
| V119905(120596
1015840 120596) [0 119879] times Ω times Ω
997888rarr 119880 is F otimesF119905-progressively measurable
(71)
Mathematical Problems in Engineering 11
An element ofU is called an admissible controlFor any V isin U we consider the following controlled
BSPDE system in the state space119867 = 1198712(O) (norm | sdot | scalar
product ⟨sdot sdot⟩)
119889119884119905(119909) = minus119860119884
119905(119909) 119889119905
minus E1015840[119891 (119905 119909 (119884
119905(119909))
1015840
(119885119905(119909))
1015840
119884119905(119909) 119885
119905(119909) V
119905)] 119889119905
+ 119885119905(119909) 119889119882 (119905) 119905 isin [0 119879]
119884119879(119909) = 120585 (119909) 119909 isin O
(72)
where 119860 is a partial differential operator 119891 [0 119879] timesO times119867 times
L(Γ119867)times119867timesL(Γ119867)times119880 rarr 119867 and 120585 isin 1198712(ΩF119879P 119867)
The cost functional is given by
119869 (V) = Eint119879
0
intO
E1015840[ℎ (119904 119909 (119884
119904(119909))
1015840
(119885119904(119909))
1015840
119884119904(119909) 119885
119904(119909) V
119904)] 119889119909 119889119904
+E1015840intO
119892 (119909 (1198840(119909))
1015840
1198840(119909)) 119889119909
(73)
where
ℎ [0 119879] times O times 119867 timesL (Γ119867) times 119867 timesL (Γ119867) times 119880 997888rarr R
119892 O times 119867 times119867 997888rarr R
(74)
Our purpose is to minimize the functional 119869(sdot) over Uadsubject to the following state constraint
EintO
Φ(119909 (1198840(119909))
1015840
1198840(119909)) 119889119909 = 0 (75)
where
Φ O times 119867 times119867 997888rarr R (76)
An admissible control 119906 isin Uad that satisfies
119869 (119906) = minVisinUad
119869 (V) (77)
is called optimalThrough what follows the following assumptions will be
in force
(L1) 119860 is a partial differential operator with appropri-ate boundary conditions We assume that 119860 is theinfinitesimal generator of a strongly continuous semi-group 119890119905119860 119905 ge 0 in119867 Moreover for every 119905 isin [0 119879]119890
119905119860119891
1198712(O) le 119872
119860119891
1198712(O) for some constant 119872
119860
independent of 119905 and 119891(L2) 119891 ℎ 119892 and Φ are continuously Gateaux differen-
tiable with respect to (1199101015840 1199111015840 119910 119911) 119891 is continuouslyGateaux differentiable with respect to V and ℎ iscontinuous with respect to V
(L3) The derivatives of 119891 ℎ 119892 and Φ are Lipschitzcontinuous and bounded by
100381610038161003816100381610038161198911199101015840
10038161003816100381610038161003816+10038161003816100381610038161198911199111015840
1003816100381610038161003816 +10038161003816100381610038161003816119891119910
10038161003816100381610038161003816+1003816100381610038161003816119891119911
1003816100381610038161003816 +1003816100381610038161003816119891V
1003816100381610038161003816 +10038161003816100381610038161003816Φ1199101015840
10038161003816100381610038161003816+10038161003816100381610038161003816Φ119910
10038161003816100381610038161003816le 119862
10038161003816100381610038161003816ℎ1199101015840
10038161003816100381610038161003816+1003816100381610038161003816ℎ1199111015840
1003816100381610038161003816 +10038161003816100381610038161003816ℎ119910
10038161003816100381610038161003816+1003816100381610038161003816ℎ119911
1003816100381610038161003816 +100381610038161003816100381610038161198921199101015840
10038161003816100381610038161003816+10038161003816100381610038161003816119892119910
10038161003816100381610038161003816
le 119862 (1 +10038161003816100381610038161003816119910101584010038161003816100381610038161003816+10038161003816100381610038161003816119911101584010038161003816100381610038161003816+10038161003816100381610038161199101003816100381610038161003816 + |119911| + |V|)
(78)
where 119862 is a positive constant
Obviously according to Theorem 4 state equation (72)has a unique mild solution under the above assumptions
Remark 14 We can define the second order differentialoperator
(119860119891) (119909) =
119899
sum
119894119895=1
119886119894119895(119909)
1205972119891
120597119909119894120597119909
119895
(119909) +
119899
sum
119894=1
119887119894(119909)
120597119891
120597119909119894
(119909) (79)
By Example 9 119860 fulfills assumption (L1) if 119886119894119895 119887
119894satisfy
condition (H1)
52 Variation of the Trajectory Let 119906 be an optimal controlwith (119884(sdot) 119885(sdot)) being the corresponding optimal state Let120576 gt 0 and [119903 119903 + 120576] sube [0 119879] For any given V isin Uad weintroduce the spike variation of the control 119906(sdot)
119906120576
119905=
V119905 119905 isin [119903 119903 + 120576]
119906119905 119905 isin [0 119879] [119904 119904 + 120576]
(80)
It is clear that 119906120576(sdot) isin UadLet (119884120576
(sdot) 119885120576(sdot)) be the trajectory corresponding to 119906120576(sdot)
We use the following short notation for brevity
119891 (119905) = 119891 (119905 119909 (119884119905(119909))
1015840
(119885119905(119909))
1015840
119884119905(119909) 119885
119905(119909) 119906
119905)
119891 (119906120576
119905) = 119891 (119905 119909 (119884
119905(119909))
1015840
(119885119905(119909))
1015840
119884119905(119909) 119885
119905(119909) 119906
120576
119905)
(81)
Consider the following equation
119889119870120576
119905(119909) = minus119860119870
120576
119905(119909) 119889119905
minus E1015840[119891
1199101015840 (119905) (119870
120576
119905(119909))
1015840
+ 119891119910(119905) 119870
120576
119905(119909)
+ 1198911199111015840 (119905) (119876
120576
119905(119909))
1015840
+ 119891119911(119905) 119876
120576
119905(119909)
+1
120576(119891 (119906
120576
119905) minus 119891 (119905))] 119889119905 + 119876
120576
119905(119909) 119889119882 (119905)
119870120576
119879(119909) = 0
(82)
Since the coefficients in (82) are bounded it is easy to checkthat there exists a unique mild solution such that
E[ sup119905isin[0119879]
1003816100381610038161003816119870120576
119905
10038161003816100381610038162
+ int
119879
0
1003816100381610038161003816119876120576
119905
10038161003816100381610038162
119889119905] lt infin (83)
We have the following estimate
12 Mathematical Problems in Engineering
Theorem 15 There holds
lim120576rarr0
E[ sup119904isin[119905119879]
1003816100381610038161003816100381610038161003816100381610038161003816
119884120576
119904minus 119884
119904
120576minus 119870
120576
119904
1003816100381610038161003816100381610038161003816100381610038161003816
2
] = 0 forall119905 isin [0 119879]
lim120576rarr0
Eint119879
0
1003816100381610038161003816100381610038161003816100381610038161003816
119885120576
119904minus 119885
119904
120576minus 119876
120576
119904
1003816100381610038161003816100381610038161003816100381610038161003816
2
119889119904 = 0
(84)
Proof We define
120578120576
119904=119884120576
119904minus 119884
119904
120576minus 119870
120576
119904 120577
120576
119904=119885120576
119904minus 119885
119904
120576minus 119876
120576
119904 119904 isin [0 119879]
(85)
For simplicity let us define
Λ120576
119904= ((119884
120576
119904)1015840
(119885120576
119904)1015840
119884120576
119904 119885
120576
119904)
119891 (119904 120582) = 119891 (119904 1198841015840
119904+ 120582(119884
120576
119904minus 119884
119904)1015840
1198851015840
119904+ 120582(119885
120576
119904minus 119885
119904)1015840
119884119904+ 120582 (119884
120576
119904minus 119884
119904)
119885119904+ 120582 (119885
120576
119904minus 119885
119904) 119906
120576
119904)
(86)
By the definition of (119884120576
119904 119885
120576
119904) (119884
119904 119885
119904) and (119870120576
119904 119876
120576
119904) (120578120576
119904 120577
120576
119904)
is the mild solution of
119889120578120576
119904= minus119860120578
120576
119904119889119904 minus E
1015840
[119871 (119904 120576)] 119889119904 + 120577120576
119904119889119882 (119904)
120578120576
119879= 0
(87)
with
119871 (119904 120576) =1
120576(119891 (119904 Λ
120576
119904 119906
120576
119904) minus 119891 (119906
120576
119904))
minus 1198911199101015840 (119904) (119870
120576
119904)1015840
minus 119891119910(119904) 119870
120576
119904minus 119891
1199111015840 (119904) (119876
120576
119904)1015840
minus 119891119911(119904) 119876
120576
119904
= ((120578120576
119904)1015840
+ (119870120576
119904)1015840
) int
1
0
1198911199101015840 (119904 120582) 119889120582 + (120578
120576
119904+ 119870
120576
119904)
times int
1
0
119891119910(119904 120582) 119889120582 minus 119891
1199101015840 (119904) (119870
120576
119904)1015840
minus 119891119910(119904) 119870
120576
119904
+ ((120577120576
119904)1015840
+ (119876120576
119904)1015840
)int
1
0
1198911199111015840 (119904 120582) 119889120582 + (120577
120576
119904+ 119876
120576
119904)
times int
1
0
119891119911(119904 120582) 119889120582 minus 119891
1199111015840 (119904) (119876
120576
119904)1015840
minus 119891119911(119904) 119876
120576
119904
= (120578120576
119904)1015840
int
1
0
1198911199101015840 (119904 120582) 119889120582 + 120578
120576
119904int
1
0
119891119910(119904 120582) 119889120582
+ (120577120576
119904)1015840
int
1
0
1198911199111015840 (119904 120582) 119889120582 + 120577
120576
119904int
1
0
119891119911(119904 120582) 119889120582 + 120574
120576
119904
(88)
where we denote
120574120576
119904= (119870
120576
119904)1015840
int
1
0
(1198911199101015840 (119904 120582) minus 119891
1199101015840 (119904)) 119889120582
+ 119870120576
119904int
1
0
(119891119910(119904 120582) minus 119891
119910(119904)) 119889120582
+ (119876120576
119904)1015840
int
1
0
(1198911199111015840 (119904 120582) minus 119891
1199111015840 (119904)) 119889120582
+ 119876120576
119904int
1
0
(119891119911(119904 120582) minus 119891
119911(119904)) 119889120582
(89)
For any 120573 gt 0 according to Lemma 1 we obtain
E sup119905le119904le119879
11989021205731199041003816100381610038161003816120578
120576
119904
10038161003816100381610038162
+ Eint119879
119905
11989021205731199041003816100381610038161003816120577
120576
119904
10038161003816100381610038162
119889119904
le12119872
2
119860
120573int
119879
119905
1198902120573119904
E [10038161003816100381610038161003816E1015840
[119871 (119904 120576)]10038161003816100381610038161003816
2
] 119889119904
le12119872
2
119860
120573Eint
119879
119905
1198902120573119904
E1015840[|119871 (119904 120576)|
2] 119889119904
(90)
By condition (L3) we have
E [E1015840[|119871 (119904 120576)|
2]]
= E [E1015840[1003816100381610038161003816119871 (119904 120576) minus 120574
120576
119904+ 120574
120576
119904
10038161003816100381610038162
]]
le 81198622E [E
1015840[10038161003816100381610038161003816(120578
120576
119904)101584010038161003816100381610038161003816
2
+1003816100381610038161003816120578120576
119904
10038161003816100381610038162
+10038161003816100381610038161003816(120577
120576
119904)101584010038161003816100381610038161003816
2
+1003816100381610038161003816120577120576
119904
10038161003816100381610038162
]]
+ 2E [E1015840[1003816100381610038161003816120574120576
119904
10038161003816100381610038162
]]
le 161198622E [
1003816100381610038161003816120578120576
119904
10038161003816100381610038162
+1003816100381610038161003816120577120576
119904
10038161003816100381610038162
] + 2E [E1015840[1003816100381610038161003816120574120576
119904
10038161003816100381610038162
]]
(91)
Combined with (91) (90) yields
E sup119905le119904le119879
11989021205731199041003816100381610038161003816120578
120576
119904
10038161003816100381610038162
+ (1 minus192119872
2
1198601198622
120573)Eint
119879
119905
11989021205731199041003816100381610038161003816120577
120576
119904
10038161003816100381610038162
119889119904
le192119872
2
1198601198622
120573Eint
119879
119905
11989021205731199041003816100381610038161003816120578
120576
119904
10038161003816100381610038162
119889119904
+24119872
2
119860
120573Eint
119879
119905
1198902120573119904
[E1015840[1003816100381610038161003816120574120576
119904
10038161003816100381610038162
]] 119889119904
(92)
We claim that
Eint119879
119905
E1015840[1003816100381610038161003816120574120576
119904
10038161003816100381610038162
] 119889119904 997888rarr 0 as 120576 997888rarr 0 (93)
From (89)
120574120576
119904= 119868
1
119904+ 119868
2
119904+ 119868
3
119904+ 119868
4
119904 (94)
Mathematical Problems in Engineering 13
where
1198681
119904= (119870
120576
119904)1015840
int
1
0
(1198911199101015840 (119904 120582) minus 119891
1199101015840 (119904)) 119889120582
1198682
119904= 119870
120576
119904int
1
0
(119891119910(119904 120582) minus 119891
119910(119904)) 119889120582
1198683
119904= (119876
120576
119904)1015840
int
1
0
(1198911199111015840 (119904 120582) minus 119891
1199111015840 (119904)) 119889120582
1198684
119904= 119876
120576
119904int
1
0
(119891119911(119904 120582) minus 119891
119911(119904)) 119889120582
(95)
Then
Eint119879
119905
E1015840[1003816100381610038161003816120574120576
119904
10038161003816100381610038162
] 119889119904
le 4Eint119879
119905
E1015840[100381610038161003816100381610038161198681
119904
10038161003816100381610038161003816
2
+100381610038161003816100381610038161198682
119904
10038161003816100381610038161003816
2
+100381610038161003816100381610038161198683
119904
10038161003816100381610038161003816
2
+100381610038161003816100381610038161198684
119904
10038161003816100381610038161003816
2
] 119889119904
(96)
Take Eint119879119905E1015840[|1198682
119904|2
]119889119904 for example
Eint119879
119905
E1015840[100381610038161003816100381610038161198682
119904
10038161003816100381610038161003816
2
] 119889119904
= Eint119879
119905
E1015840[(119870
120576
119904int
1
0
(119891119910(119904 120582) minus 119891
119910(119904)) 119889120582)
2
]119889119904
le Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119904 120582) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582] 119889119904
le 2Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119904 120582) minus 119891
119910(119906
120576
119905)10038161003816100381610038161003816
2
119889120582] 119889119904
+ 2Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119906
120576
119905) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582] 119889119904
(97)
Note that
Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119906
120576
119905) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582] 119889119904
= Eint119903+120576
119903
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119906
120576
119905) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582] 119889119904
le sup119904isin[119905119879]
E [E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119906
120576
119905) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582]] 120576
997888rarr 0 as 120576 997888rarr 0
(98)
The inequality above holds due to the boundedness of|119870
120576
119904|2
int1
0|119891119910(119906
120576
119905) minus 119891
119910(119904)|
2
119889120582 Indeed Assumption (L3) im-plies the boundedness of 119891
119910(119906
120576
119905) minus 119891
119910(119904) Meanwhile 119870120576
119904is
the solution of mean-field BSEE (82) It can be easy to check119870120576
119904is bounded since the coefficients in (82) are boundedOn the other hand
Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119904 120582) minus 119891
119910(119906
120576
119905)10038161003816100381610038161003816
2
119889120582] 119889119904
le 11986221205822Eint
119879
119905
1003816100381610038161003816119870120576
119904
10038161003816100381610038162
times int
1
0
1198641015840[100381610038161003816100381610038161003816(119884
120576
119904minus 119884
119904)1015840100381610038161003816100381610038161003816
2
+100381610038161003816100381610038161003816(119885
120576
119904minus 119885
119904)1015840100381610038161003816100381610038161003816
2
times10038161003816100381610038161003816119884120576
119904minus 119884
119904
10038161003816100381610038161003816
2
+10038161003816100381610038161003816119885120576
119904minus 119885
119904
10038161003816100381610038161003816
2
] 119889120582 119889119904
(99)
where (119884120576
119904 119885
120576
119904) is the mild solution of the following equation
119889119884120576
119905= minus 119860119884
120576
119905119889119905
minus E1015840[119891 (119905 (119884
120576
119905)1015840
(119885120576
119905)1015840
119884120576
119905 119885
120576
119905 119906
120576
119905)] 119889119905
+ 119885120576
119905119889119882 (119905) 119905 isin [0 119879]
119884120576
119879= 120585
(100)
and (119884119904 119885
119904) is the mild solution of
119889119884119905= minus 119860119884
119905119889119905
minus E1015840[119891 (119905 (119884
119905)1015840
(119885119905)1015840
119884119905 119885
119905 119906
119905)] 119889119905
+ 119885119905119889119882 (119905) 119905 isin [0 119879]
119884119879= 120585
(101)
By the definition of 119906120576119905 according to (L2) we have
E1015840[119891 (119904 119884
1015840
(119904) 1198851015840
(119904) 119884 (119904) 119885 (119904) 119906120576
119905)]
997888rarr E1015840[119891 (119904 119884
1015840
(119904) 1198851015840
(119904) 119884 (119904) 119885 (119904) 119906119905)]
(102)
in 1198712([0 119879]119867) as 120576 rarr 0 Using the continuous dependencetheorem Corollary 5 we obtain
(119884120576
119904 119885
120576
119904) 997888rarr (119884
119904 119885
119904) as 120576 997888rarr 0 (103)
Then
Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119904 120582) minus 119891
119910(119906
120576
119905)10038161003816100381610038161003816
2
119889120582] 119889119904
le 11986221205822Eint
119879
119905
1003816100381610038161003816119870120576
119904
10038161003816100381610038162
times int
1
0
E1015840[100381610038161003816100381610038161003816(119884
120576
119904minus 119884
119904)1015840100381610038161003816100381610038161003816
2
+100381610038161003816100381610038161003816(119885
120576
119904minus 119885
119904)1015840100381610038161003816100381610038161003816
2
times10038161003816100381610038161003816119884120576
119904minus 119884
119904
10038161003816100381610038161003816
2
+10038161003816100381610038161003816119885120576
119904minus 119885
119904
10038161003816100381610038161003816
2
] 119889120582 119889119904
997888rarr 0 as 120576 997888rarr 0
(104)
14 Mathematical Problems in Engineering
Combining (98) with (104) we finally haveEint
119879
119905E1015840[|1198682
119904|2
]119889119904 rarr 0 as 120576 rarr 0The required result (93) follows by using the similar
estimations for 1198681119904 1198683
119904 and 1198684
119904
Note that 1 minus 1921198722
1198601198622120573 gt 0 if 120573 is sufficiently large
Now to prove the desired result (84) it suffices to applyGronwallrsquos lemma and estimate (93) to inequality (92)
To deal with the state constraint (75) we need to recall theEkeland variational principle
Lemma 16 (Ekelandrsquos variational principle see [16 Lemma41]) Let (119878 119889) be a complete metric space and let 119865(sdot) 119878 rarr
R be lower semicontinuous and bounded from below If for 120588 gt0 there exists 119906 isin 119878 such that
119865 (119906) le infVisin119878
119865 (V) + 120588 (105)
then there exists 119906120588 isin 119878 satisfying
(i) 119865 (119906120588) le 119865 (119906)
(ii) 119889 (119906120588 119906) le 120588
(iii) 119865 (119906120588) le 119865 (V) + 120588 sdot 119889 (119906120588 119906) forallV = 119906120588
(106)
Now fix V isin Uad and set
119878 = V (sdot) isin Uad | sup0le119905le119879
E1003816100381610038161003816V11990510038161003816100381610038162
le E100381610038161003816100381611990611990510038161003816100381610038162
+ |V|2
119889 (V (sdot) V (sdot)) = 119898 119905 isin [0 119879] | E1003816100381610038161003816V119905 minus V
119905
10038161003816100381610038162
gt 0
forallV (sdot) V (sdot) isin 119878
(107)
where119898 denotes the Lebesgue measure on RThe following result is proved as Proposition 41 in [16]
Lemma 17 (119878 119889(sdot sdot)) is a complete metric space and 119869120588 is
continuous and bounded on 119878 where
119869120588
(V (sdot)) = (119869 (V (sdot)) minus 119869 (119906 (sdot)) + 120588)2
+
10038161003816100381610038161003816100381610038161003816Eint
O
Φ(119909 (1198840(119909))
1015840
1198840(119909)) 119889119909
10038161003816100381610038161003816100381610038161003816
2
12
forallV (sdot) isin 119878(108)
and (119884 119885) is the mild solution of (72) corresponding to thecontrol V
Now we consider the following free initial state optimalcontrol problem
infV(sdot)isin119878
119869120588
(V (sdot)) (109)
It is easy to check that
0 le infV(sdot)isin119878
119869120588
(V (sdot)) le 119869120588
(119906 (sdot)) = 120588 (110)
According to Ekelandrsquos variational principle there exists a119906120588(sdot) isin 119881 such that
(i) 119869120588 (119906120588 (sdot)) le 120588
(ii) 119889 (119906120588 (sdot) 119906 (sdot)) le 120588
(iii) 119869120588 (119906120588 (sdot)) le 119869120588
(V (sdot)) + 120588119889 (119906120588 (sdot) 119906 (sdot)) forallV (sdot) isin 119878(111)
Using the spike variationmethod we can construct 119906120576120588(sdot) isin 119878as follows
119906120576120588
119905=
V119905 119905 isin [119904 119904 + 120576]
119906120588
119905 119905 isin [0 119879] [119904 119904 + 120576]
(112)
It is clear that 119889(119906120576120588(sdot) 119906120588(sdot)) le 120576 Let (119884120576120588(sdot) 119885
120576120588(sdot)) (resp
(119884120588(sdot) 119885
120588(sdot))) be the solution of (72) with respect to the
control 119906120576120588(sdot) (resp 119906120588(sdot)) Following (82) (119870120576120588
119905 119876
120576120588
119905) is the
mild solution of
119870120576120588
119905= int
119879
119905
119890119860(119904minus119905)
E1015840
times [1198911199101015840 (119904 Λ
120588
119904 119906
120588
119904) (119870
120576120588
119904)1015840
+ 119891119910(119904 Λ
120588
119904 119906
120588
119904)119870
120576120588
119904
+ 1198911199111015840 (119904 Λ
120588
119904 119906
120588
119904) (119876
120576120588
119904)1015840
+ 119891119911(119904 Λ
120588
119904 119906
120588
119904) 119876
120576120588
119904
+1
120576(119891 (119904 Λ
120588
119904 119906
120576120588
119904) minus 119891 (119904 Λ
120588
119904 119906
120588
119904))] 119889119904
minus int
119879
119905
119890119860(119904minus119905)
119876120576120588
119904119889119882 (119904)
(113)
ByTheorem 15 we know that
lim120576rarr0
E[ sup119904isin[119905119879]
100381610038161003816100381610038161003816100381610038161003816
119884120576120588
119904minus 119884
120588
119904
120576minus 119870
120576120588
119904
100381610038161003816100381610038161003816100381610038161003816
2
] = 0 forall119905 isin [0 119879]
lim120576rarr0
Eint119879
0
100381610038161003816100381610038161003816100381610038161003816
119885120576120588
119904minus 119885
120588
119904
120576minus 119876
120576120588
119904
100381610038161003816100381610038161003816100381610038161003816
2
119889119904 = 0
(114)
The proof of the following proposition is technical butbased on the arguments above and we omit it
Proposition 18 One has
1
120576E [Φ ((119884
120576120588
0)1015840
119884120576120588
0) minus Φ((119884
120588
0)1015840
119884120588
0)]
= E [Φ1199101015840 ((119884
120588
0)1015840
119884120588
0) (119870
120576120588
0)1015840
+ Φ119910((119884
120588
0)1015840
119884120588
0)119870
120576120588
0]
+ 119900 (120576)
Mathematical Problems in Engineering 15
1
120576(119869 (119906
120576120588) minus 119869 (119906
120588))
= E [1198921199101015840 ((119884
120588
0)1015840
119884120588
0) (119870
120576120588
0)1015840
+ 119892119910((119884
120588
0)1015840
119884120588
0)119870
120576120588
0]
+ Δ120576+1
120576Eint
119879
0
E1015840[ℎ (119904 Λ
120588
119904 119906
120576120588
119904)
minus ℎ (119904 Λ120588
119904 119906
120588
119904)] 119889119904 + 119900 (120576)
(115)
where
Δ120576= Eint
119879
0
E1015840[ℎ
1199101015840 (119904 Λ
120588
119904 119906
120576120588
119904) (119870
120576120588
119904)1015840
+ ℎ1199111015840 (119904 Λ
120588
119904 119906
120576120588
119904) (119876
120576120588
119904)1015840
+ ℎ119910(119904 Λ
120588
119904 119906
120576120588
119904)119870
120576120588
119904
+ ℎ119911(119904 Λ
120588
119904 119906
120576120588
119904) 119876
120576120588
119904] 119889119904
(116)
53 Variational Inequality and Adjoint Equation In thissubsection the adjoint process is introduced to deduce thevariational inequality
If we set V(sdot) = 119906120576120588(sdot) in (111) and notice that
119889(119906120576120588(sdot) 119906
120588(sdot)) le 120576 we get
minus120588 le1
120576(119869
120588(119906
120576120588
(sdot)) minus 119869120588(119906
120588
(sdot))) (117)
By Lemma 17
1
120576(119869
120588(119906
120576120588
(sdot)) minus 119869120588(119906
120588
(sdot)))
=(119869
120588(119906
120576120588(sdot)))
2
minus (119869120588(119906
120588(sdot)))
2
120576 (119869120588 (119906120576120588 (sdot)) + 119869120588 (119906120588 (sdot)))
=119869 (119906
120576120588(sdot)) + 119869 (119906
120588(sdot)) minus 2119869 (119906 (sdot)) + 2120588
119869120588 (119906120576120588 (sdot)) + 119869120588 (119906120588 (sdot))
times119869 (119906
120576120588(sdot)) minus 119869 (119906
120588(sdot))
120576
+
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] + E [Φ ((119884
120588
0)1015840
119884120588
0)]
119869120588 (119906120576120588 (sdot)) + 119869120588 (119906120588 (sdot))
times
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] minus E [Φ ((119884
120588
0)1015840
119884120588
0)]
120576
997888rarr 119897120588
1
119869 (119906120576120588(sdot)) minus 119869 (119906
120588(sdot))
120576
+ 119897120588
2
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] minus E [Φ ((119884
120588
0)1015840
119884120588
0)]
120576
(118)
where we set
119897120588
1=119869 (119906
120588(sdot)) minus 119869 (119906 (sdot)) + 120588
119869120588 (119906120588 (sdot))
119897120588
2=
E [Φ ((119884120588
0)1015840
119884120588
0)]
119869120588 (119906120588 (sdot))
(119)
and use the limit
119869 (119906120576120588
(sdot)) 997888rarr 119869 (119906120588
(sdot))
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] 997888rarr E [Φ ((119884
120588
0)1015840
119884120588
0)]
(120)
as 120576 rarr 0 according to (115)As |119897120588
1|2
+ |119897120588
2|2
= 1 for all 120588 gt 0 we know that there existsa subsequence of 119897120588
1 119897120588
2 (still denoted by 119897120588
1 119897120588
2) such that
lim120588rarr0
119897120588
1 119897120588
2 = 119897
1 1198972
1003816100381610038161003816119897110038161003816100381610038162
+1003816100381610038161003816119897210038161003816100381610038162
= 1
(121)
Combining (115) (117) with (118) we get
minus120588 le 119897120588
1
119869 (119906120576120588(sdot)) minus 119869 (119906
120588(sdot))
120576
+ 119897120588
2
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] minus E [Φ ((119884
120588
0)1015840
119884120588
0)]
120576
= 119897120588
1Δ120576+ 119897
120588
1E [119892
1199101015840 ((119884
120588
0)1015840
119884120588
0) (119870
120576120588
0)1015840
+119892119910((119884
120588
0)1015840
119884120588
0)119870
120576120588
0]
+119897120588
1
120576Eint
119879
0
E1015840[ℎ (119904 Λ
120588
119904 119906
120576120588
119904) minus ℎ (119904 Λ
120588
119904 119906
120588
119904)] 119889119904
+ 119897120588
2E [Φ
1199101015840 ((119884
120588
0)1015840
119884120588
0) (119870
120576120588
0)1015840
+Φ119910((119884
120588
0)1015840
119884120588
0)119870
120576120588
0] + (119897
120588
1+ 119897
120588
2) 119900 (120576)
(122)
Next we introduce the adjoint equation corresponding tovariational equation (113) whose solution is denoted by119875120588(119905)
119889119875120588
(119905)
= 119860lowast119875120588
(119905) 119889119905
+ E1015840[119891
1199101015840 (119905 Λ
120588
119905 119906
120588
119905) (119875
120588
(119905))1015840
+ 119891119910(119905 Λ
120588
119905 119906
120588
119905) 119875
120588
(119905) + 119897120588
1ℎ1199101015840 (119905 Λ
120588
119905 119906
120588
119905)
+ 119897120588
1ℎ119910(119905 Λ
120588
119905 119906
120588
119905)] 119889119905
16 Mathematical Problems in Engineering
+ E1015840[119891
1199111015840 (119905 Λ
120588
119905 119906
120588
119905) (119875
120588
(119905))1015840
+ 119891119911(119905 Λ
120588
119905 119906
120588
119905) 119875
120588
(119905)
+ 119897120588
1ℎ1199111015840 (119905 Λ
120588
119905 119906
120588
119905) + 119897
120588
1ℎ119911(119905 Λ
120588
119905 119906
120588
119905)] 119889119882 (119905)
119875120588
(0) = 119897120588
1E [119892
1199101015840 ((119884
120588
0(119909))
1015840
119884120588
0(119909))
+ 119892119910((119884
120588
0(119909))
1015840
119884120588
0(119909))]
+ 119897120588
2E [Φ
1199101015840 ((119884
120588
0(119909))
1015840
119884120588
0(119909))
+ Φ119910((119884
120588
0(119909))
1015840
119884120588
0(119909))]
(123)
where 119860lowast is the 1198712(O)-adjoint operator of 119860 Under assump-tions (L1)ndash(L3) this is a linear mean-field SEE with boundedcoefficients An application ofTheorem 11 implies that it has aunique adaptedmild solution such that119875120588(119905) isin S2
F ([0 119879] 119870)When 120588 rarr 0 according to Corollaries 5 and 13 119875120588(119905)
converges to 119875(119905) where
119875 (119905) isin S2
F ([0 119879] 119870) (124)
is the solution of the following equation
119875 (119905) = 1198971E [119892
1199101015840 (119884
1015840
0 119884
0) + 119892
119910(119884
1015840
0 119884
0)]
+ 1198972E [Φ
1199101015840 (119884
1015840
0 119884
0) + Φ
119910(119884
1015840
0 119884
0)]
+ int
119905
0
119890119860lowast(119905minus119904)
E1015840[119891
1199101015840 (119904) 119875
1015840
(119904) + 119891119910(119904) 119875 (119904)
+ 1198971ℎ1199101015840 (119904) + 119897
1ℎ119910(119904)] 119889119904
+ int
119905
0
119890119860lowast(119905minus119904)
E1015840[119891
1199111015840 (119904) 119875
1015840
(119904) + 119891119911(119904) 119875 (119904)
+ 1198971ℎ1199111015840 (119904) + 119897
1ℎ119911(119904)] 119889119882 (119904)
(125)
The following proposition which formally follows fromProposition 18 gives the relation between 119875120588(119905) and119870120576120588
119905
Proposition 19 Consider the following
E [119875120588
(0)119870120576120588
0]
= Eint119879
0
1
120576119875120588
(119904)E1015840[119891 (119904 Λ
120588
119904 119906
120576120588
119904) minus 119891 (119904 Λ
120588
119904 119906
120588
119904)]
minus 119897120588
1119870120576120588
119904E1015840[ℎ
1199101015840 (119904 Λ
120588
119904 119906
120588
119904) + ℎ
119910(119904 Λ
120588
119904 119906
120588
119904)]
minus 119897120588
1119876120576120588
119904E1015840[ℎ
1199111015840 (119904 Λ
120588
119904 119906
120588
119904) + ℎ
119911(119904 Λ
120588
119904 119906
120588
119904)] 119889119904
(126)
The following theorem constitutes the main contributionof this section themaximumprinciple for the BSPDE controlsystem
Theorem 20 Let assumptions (L1)ndash(L3) hold Suppose 119906(sdot) isan optimal control and (119884(sdot) 119885(sdot)) is the corresponding optimalstate trajectory for the BSPDE control systems (72) and (73)with the initial state constraint (75) Then there exists 119875(119905) isinS2
F ([0 119879] 119870) which satisfies (125) such that
H (119905 1198841015840
119905 119885
1015840
119905 119884
119905 119885
119905 V
119905 119875 (119905))
ge H (119905 1198841015840
119905 119885
1015840
119905 119884
119905 119885
119905 119906
119905 119875 (119905))
119886119890 119886119904 forallV isin U119886119889
(127)
whereH [0 119879]times119867timesL(Γ119867)times119867timesL(Γ119867)times119880times119870 rarr R
is the Hamiltonian function defined by
H (119905 119910 119910 119911 V 119901) = 1198971ℎ (119905 119910 119910 119911 V)
+ 119901119891 (119905 119910 119910 119911 V) (128)
Proof By (122) and Proposition 19 we obtain
minus120588 le1
120576Eint
119879
0
119875120588
(119904)E1015840[119891 (119904 Λ
120588
119904 119906
120576120588
119904)
minus 119891 (119904 Λ120588
119904 119906
120588
119904)] 119889119904
+119897120588
1
120576Eint
119879
0
E1015840[ℎ (119904 Λ
120588
119904 119906
120576120588
119904)
minus ℎ (119904 Λ120588
119904 119906
120588
119904)] 119889119904 + (119897
120588
1+ 119897
120588
2) 119900 (120576)
(129)
Letting 120576 rarr 0+ in (129) we derive for ae 120591 isin [0 119879]
minus120588 le 119897120588
1E1015840[ℎ (120591 Λ
120588
120591 V
120591) minus ℎ (120591 Λ
120588
120591 119906
120588
120591)]
+ 119875120588
(119904)E1015840[119891 (120591 Λ
120588
120591 V
120591) minus 119891 (120591 Λ
120588
120591 119906
120588
120591)]
(130)
for all V isin UadFinally taking 120588 rarr 0 in (130) we derive the desired
result
Remark 21 We note that if the coefficients do not dependexplicitly on the marginal law of the underlying diffusionthe result reduces to the classical case that is the SMP forBSPDEs without mean-field term
Remark 22 When we remove the initial state constraint (75)we obtain the general maximum principle for the mean-fieldBSPDEs system (ie without the constraint) with 119897
1= 1
54 Application A Backward Linear Quadratic Control Prob-lem Now we apply our maximum principle to solve an LQproblem For notational simplicity we restrict ourselves tothe free case (ie without the initial state constraint (75)) thegeneral case being handled in a similar way
Mathematical Problems in Engineering 17
Consider the following problem
119869 (119906) =1
2E [(119884 (0))
2] +
1
2Eint
119879
0
119873V2 (119905) 119889119905 997888rarr min
(131)
subject to
119889119884 (119905) = minus119860119884 (119905) 119889119905
minus 119861119884 (119905) + 119861E [119884 (119905)]
+119862V (119905) + 119863119885 (119905) + 119863E [119885 (119905)] 119889119905
+ 119885 (119905) 119889119882 (119905)
119884 (119879) = 120585 119905 isin [0 119879]
(132)
where 119860 is a partial differential operator satisfying condition(L1) and 119861 119861 119862119863119863 and119873 are bounded and deterministicconstantsWe also assume that119873 gt 0 and V isin 1198712F (0 119879 119880) It iseasy to verify that BSPDE (132) admits a uniquemild solution(119884(119905) 119885(119905))
119875(119905) the adjoint process of state equation (132) is thesolution of
119889119875 (119905) = 119860lowast119875 (119905) 119889119905 + 119861119875 (119905) + 119861E [119875 (119905)] 119889119905
+ 119863119875 (119905) + 119863E [119875 (119905)] 119889119882 (119905)
119875 (0) = 119884 (0)
(133)
Let 119906(sdot) be an optimal control and let (119884(sdot) 119885(sdot)) bethe corresponding state process By maximum principle ofTheorem 20 (note that 119897
1= 1 in this problem)
1
2119873V2 (119905) + 119875 (119905) 119862V (119905) ge
1
2119873119906
2
(119905) + 119875 (119905) 119862119906 (119905) (134)
for all V isin 119880ad since the state equation has the form (132)Thisin turn implies
119906 (119905) = minus119862
119873119875 (119905) (135)
It is clear that (131) is a positive quadratic functional of controlbecause of 119873 gt 0 Hence an optimal control exists Thecandidate optimal control (135) is indeed an optimal controlof this LQ problem for it is the only control which satisfies themaximum principle
Next we want to obtain a more explicit representationof the optimal control (135) from the state equation (132)Substituting (135) into state equation (132) yields
119889119884 (119905) = minus119860119884 (119905) 119889119905
minus 119861119884 (119905) + 119861E [119884 (119905)] minus1198622
119873119875 (119905)
+119863119885 (119905) + 119863E [119885 (119905)] 119889119905 + 119885 (119905) 119889119882 (119905)
119884 (119879) = 120585 119905 isin [0 119879]
(136)
Combining the above equation with (133) we obtain thefollowing related feedback control system
119889119884 (119905) = minus119860119884 (119905) 119889119905
minus 119861119884 (119905) + 119861E [119884 (119905)] minus1198622
119873119875 (119905)
+ 119863119885 (119905) + 119863E [119885 (119905)] 119889119905 + 119885 (119905) 119889119882 (119905)
119884 (119879) = 120585
119889119875 (119905) = 119860lowast119875 (119905) 119889119905 + 119861119875 (119905) + 119861E [119875 (119905)] 119889119905
+ 119863119875 (119905) + 119863E [119875 (119905)] 119889119882 (119905)
119875 (0) = 119884 (0)
(137)
Looking at the terminal condition of 119875(119905) in (133) andconsidering the mean-field type of (132) it is reasonable toconjecture that 119875(119905) has the following form
119875 (119905) = 120593 (119905) 119884 (119905) + 120601 (119905)E [119884 (119905)] (138)
where120593(119905)120601(119905) are deterministic differential functionswhichwill be specified below Moreover 120593(0) = 1 120601(0) = 0
Inserting this form into adjoint equation (133) and notic-ing that 119884(119905) satisfies (136) we can compare the coefficientsof 119889119905 and 119889119882(119905) to obtain the following equation
2120593 (119905) 119860119884 (119905) + 2120601 (119905) 119860E [119884 (119905)] + 119861120593 (119905) 119884 (119905)
+ 2 (119861120601 (119905) + 119861120593 (119905) + 119861120601 (119905))E [119884 (119905)]
= minus120593 (119905) 119861119884 (119905) minus1198622
119873120593 (119905) 119884 (119905) + 119863119885 (119905)
+ 119884 (119905)119889120593 (119905)
119889119905+ E [119884 (119905)]
119889120601 (119905)
119889119905
+ (1206012
(119905)1198622
119873+ 2
1198622
119873120593 (119905) 120601 (119905))E [119884 (119905)]
minus (119863120601 (119905) + 119863120601 (119905) + 119863120593 (119905))E [119885 (119905)]
120593 (119905) 119885 (119905) = 119863120593 (119905) 119884 (119905)
+ (119863120601 (119905) + 119863120593 (119905) + 119863120601 (119905))E [119884 (119905)]
(139)
Then subtracting 119885(119905) we have
2120593 (119905) 119860119884 (119905) + 2120601 (119905) 119860E [119884 (119905)] + 119861120593 (119905) 119884 (119905)
+ 2 (119861120601 (119905) + 119861120593 (119905) + 119861120601 (119905))E [119884 (119905)]
18 Mathematical Problems in Engineering
= 120593 (119905) minus119861 +1198622
119873120593 (119905) minus 119863
2119884 (119905)
+ 119884 (119905)119889120593 (119905)
119889119905+ E [119884 (119905)]
119889120601 (119905)
119889119905
+ (1206012
(119905)1198622
119873+ 2
1198622
119873120593 (119905) 120601 (119905))E [119884 (119905)]
minus1
120593 (119905)(119863120601 (119905) + 119863120593 (119905) + 119863120601 (119905))
2
E [119884 (119905)]
minus 2119863 (119863120601 (119905) + 119863120593 (119905) + 119863120601 (119905))E [119884 (119905)]
(140)
Comparing the coefficients of 119884(119905) and E[119884(119905)] respec-tively we get
119889
119889119905120593 (119905) = 2119860
lowast120593 (119905) + 2119861120593 (119905) + 119863
2120593 (119905) minus
1198622
1198731205932
(119905)
120593 (0) = 1
(141)
119889
119889119905120601 (119905) = 2119860
lowast120601 (119905) + (
(119863 + 119863)2
120593 (119905)minus1198622
119873)120601
2
(119905)
+ 2119877 (119905) 120601 (119905) + (1198632+ 2119861 + 2119863119863)120593 (119905)
120601 (0) = 0
(142)
where 119877(119905) = 119861 + 119861 + (119863 + 119863)2
minus (1198622119873)120593(119905)
We solve (141) to get
120593 (119905) = (119890minus(2119860lowast+2119861+119863
2)119905(1 minus
1198622
119873(2119860lowast + 2119861 + 1198632))
+1198622
119873(2119860lowast + 2119861 + 1198632))
minus1
(143)
Then (142) exists a unique solution from the classical Riccatiequation theory
We now conclude the above discussions in the followingresult
Theorem 23 For onersquos linear quadratic stochastic partialdifferential control problem (131)-(132) the unique optimalcontrol 119906(sdot) isin 119880ad is given by
119906 (119905) = minus119862
119873(120593 (119905) 119884 (119905) + 120601 (119905)E [119884 (119905)]) (144)
with 120593(119905) satisfying (143) and 120601(119905) solving (142)
Conflict of Interests
The authors declare that they have no conflict of interestsregarding the publication of this paper
Acknowledgment
This research is partially supported by the Natural ScienceFoundation of China under Grant no 11301310
References
[1] Y Hu and S Peng ldquoAdapted solution of a backward semilinearstochastic evolution equationrdquo Stochastic Analysis and Applica-tions vol 9 no 4 pp 445ndash459 1991
[2] N IMahmudov andMAMcKibben ldquoOnbackward stochasticevolution equations in Hilbert spaces and optimal controlrdquoNonlinear Analysis vol 67 no 4 pp 1260ndash1274 2007
[3] M Fuhrman and G Tessitore ldquoNonlinear Kolmogorov equa-tions in infinite dimensional spaces the backward stochasticdifferential equations approach and applications to optimalcontrolrdquoThe Annals of Probability vol 30 no 3 pp 1397ndash14652002
[4] M Fuhrman and G Tessitore ldquoInfinite horizon backwardstochastic differential equations and elliptic equations inHilbert spacesrdquo Annals of Probability vol 32 no 1B pp 607ndash660 2004
[5] X Mao ldquoAdapted solutions of backward stochastic differentialequations with non-Lipschitz coefficientsrdquo Stochastic Processesand their Applications vol 58 no 2 pp 281ndash292 1995
[6] J Lasry and P Lions ldquoMean field gamesrdquo Japanese Journal ofMathematics vol 2 no 1 pp 229ndash260 2007
[7] R Buckdahn J Li and S Peng ldquoMean-field backward stochas-tic differential equations and related partial differential equa-tionsrdquo Stochastic Processes and their Applications vol 119 no 10pp 3133ndash3154 2009
[8] D Andersson and B Djehiche ldquoAmaximum principle for SDEsofmean-field typerdquoAppliedMathematics andOptimization vol63 no 3 pp 341ndash356 2011
[9] R Buckdahn B Djehiche J Li and S Peng ldquoMean-fieldbackward stochastic differential equations a limit approachrdquoThe Annals of Probability vol 37 no 4 pp 1524ndash1565 2009
[10] R Xu ldquoMean-field backward doubly stochastic differentialequations and related SPDEsrdquo Boundary Value Problems vol2012 article 114 2012
[11] R Buckdahn B Djehiche and J Li ldquoA general stochasticmaximum principle for SDEs of mean-field typerdquo AppliedMathematics and Optimization vol 64 no 2 pp 197ndash216 2011
[12] J Li ldquoStochasticmaximumprinciple in themean-field controlsrdquoAutomatica vol 48 no 2 pp 366ndash373 2012
[13] G Wang C Zhang and W Zhang ldquoStochastic maximumprinciple for mean-field type optimal control under partialinformationrdquo IEEE Transactions on Automatic Control vol 59no 2 pp 522ndash528 2014
[14] M Hafayed ldquoA mean-field necessary and sufficient conditionsfor optimal singular stochastic controlrdquo Communications inMathematics and Statistics vol 1 no 4 pp 417ndash435 2013
[15] M Hafayed ldquoA mean-field maximum principle for optimalcontrol of forward-backward stochastic differential equationswith Poisson jumpprocessesrdquo International Journal ofDynamicsand Control vol 1 no 4 pp 300ndash315 2013
[16] Y Hu and S Peng ldquoMaximum principle for semilinearstochastic evolution control systemsrdquo Stochastics and StochasticsReports vol 33 no 3-4 pp 159ndash180 1990
[17] M Fuhrman Y Hu and G Tessitore ldquoStochastic maximumprinciple for optimal control of SPDEsrdquo Comptes Rendus Math-ematique vol 350 no 13-14 pp 683ndash688 2012
Submit your manuscripts athttpwwwhindawicom
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Mathematical Problems in Engineering
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Differential EquationsInternational Journal of
Volume 2014
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Abstract and Applied AnalysisHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Volume 2014 Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Stochastic AnalysisInternational Journal of
8 Mathematical Problems in Engineering
or
1198623(120591119896minus 119905) = 119862
2120579 (2119862
1exp (119879)) (120591
119896minus 119905) le 2119862
1exp (119879)
(43)
Since 1198621gt 1 from 120579(119906) le 119886 + 119887119906 the above inequality holds
if
1198622(119886 + 119887) (120591
119896minus 119905) le 1 (44)
Thus (41) holds for any 119905 isin [120591119896minus1
120591119896] 119896 = 1 2 119898 if 120591
119896minus
120591119896minus1
le 11198622(119886 + 119887) Therefore we can choose a sufficiently
large119898 isin N such that 120575 = 119879119898 le 11198622(119886 + 119887) Clearly such a
120575 only depends on 119886 119887 119897 119879 and119872119860
Now assume that (40) holds for some 119899 ge 2 Then wehave
120593119896119899+1
(119905) = 1198622int
120591119896
119905
120579 (120593119896119899(119904)) 119889119904 le 119862
2int
120591119896
119905
120579 (120593119896119899minus1
(119904)) 119889119904
= 120593119896119899(119905) forall119905 isin [120591
119896minus1 120591
119896]
(45)
This completes the proof
Now we can give the main result of this section
Theorem 8 Assume that (A2) and (A3) hold Then thereexists a unique mild solution (119884 119885) to (11)
Proof Consider the following
Uniqueness To show the uniqueness let both (119884 119885) and( 119885) be solutions of (11) For any 120573 gt 0 similar to the proofof (36) one can obtain
E( sup119905le119904le119879
119890212057311990410038161003816100381610038161003816
119884 (119904) minus (119904)10038161003816100381610038161003816
2
) + Eint119879
119905
119890212057311990410038161003816100381610038161003816
119885 (119904) minus 119885 (119904)10038161003816100381610038161003816
2
119889119904
le24119872
2
119860
120573E
times int
119879
119905
1198902120573119904
[120579 (10038161003816100381610038161003816119884 (119904) minus (119904)
10038161003816100381610038161003816
2
) + 11989710038161003816100381610038161003816119885 (119904) minus 119885 (119904)
10038161003816100381610038161003816
2
] 119889119904
(46)
That is if 120573 is sufficiently large
E( sup119905le119904le119879
119890212057311990410038161003816100381610038161003816
119884 (119904) minus (119904)10038161003816100381610038161003816
2
)
le24119872
2
119860
120573Eint
119879
119905
1198902120573119904
[120579 (10038161003816100381610038161003816119884 (119904) minus (119904)
10038161003816100381610038161003816
2
)] 119889119904
le 1198622Eint
119879
119905
120579(E sup119904le119903le119879
119890212057311990310038161003816100381610038161003816
119884 (119904) minus (119904)10038161003816100381610038161003816
2
)119889119904
(47)
An application of Bihari inequality yields
E( sup119905le119904le119879
119890212057311990410038161003816100381610038161003816
119884 (119904) minus (119904)10038161003816100381610038161003816
2
) = 0 (48)
So 119884(119905) = (119905) for all 119905 isin [0 119879] as It then follows from (46)that 119885(119905) = 119885(119905) for all 119905 isin [0 119879] as as well This establishesthe uniqueness
ExistenceWe claim that the sequence (119884119899 119885
119899) defined by (27)
satisfies
E sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899+1 (119904) minus 119884119899 (119904)
10038161003816100381610038162
997888rarr 0 forall0 le 119905 le 119879 (49)
as 119899 rarr infinIndeed for all 119905 isin [120591
119896minus1 120591
119896] we set 120593
119896119899(119905) =
E sup119904isin[119905120591119896]
1198902120573119904|119884119899+1
(119904) minus 119884119899(119904)|
2 By Lemmas 6 and 7
1205931198961(119905) = E sup
119904isin[119905120591119896]
119890212057311990410038161003816100381610038161198842 (119904) minus 1198841 (119904)
10038161003816100381610038162
le 1198622int
120591119896
119905
120579(E sup119904le119903le120591119896
119890212057311990310038161003816100381610038161198841 (119903) minus 1198840 (119903)
10038161003816100381610038162
)119889119904
le 1198622int
120591119896
119905
120579 (21198621exp (120591
119896minus 119905)) 119889119904
le 1198622120579 (2119862
1exp (119879)) (120591
119896minus 119905) = 119862
3(120591119896minus 119905) = 120593
1198961(119905)
(50)
Suppose that 120593119896119899(119905) le 120593
119896119899(119905) holds for some 119899 ge 1
According to Lemma 6(iii) and Lemma 7 for all 119905 isin [120591119896minus1
120591119896]
we obtain
120593119896119899+1
(119905) = E sup119904isin[119905120591119896]
11989021205731199041003816100381610038161003816119884119899+2 (119904) minus 119884119899+1 (119904)
10038161003816100381610038162
le 1198622int
120591119896
119905
120579(E sup119903isin[119904120591119896]
11989021205731199031003816100381610038161003816119884119899+1 (119903) minus 119884119899 (119903)
10038161003816100381610038162
)119889119904
= 1198622int
120591119896
119905
120579 (120593119896119899(119904)) 119889119904
le 1198622int
120591119896
119905
120579 (120593119896119899(119904)) 119889119904 = 120593
119896119899+1(119905)
(51)
This implies that for all 119899 isin N
120593119896119899(119905) le 120593
119896119899(119905) (52)
By definition 120593119896119899(sdot) is continuous on [120591
119896minus1 120591
119896] Note that
for each 119899 ge 1 120593119896119899(sdot) is decreasing on [120591
119896minus1 120591
119896] and for each
119905 119896 120593119896119899(119905) is a nonincreasing sequence Therefore we define
the function 120593119896(119905) by 120593
119896119899(119905) darr 120593
119896(119905) It is easy to verify that
120593119896(119905) is continuous and nonincreasing on [120591
119896minus1 120591
119896] By the
definitions of 120593119896119899(119905) and 120593
119896(119905) we get
120593119896(119905) = lim
119899rarrinfin
1198622int
120591119896
119905
120579 (120593119896119899(119904)) 119889119904 = 119862
2int
120591119896
119905
120579 (120593119896(119904)) 119889119904
(53)
for all 120591119896minus1
le 119905 le 120591119896 Since int
0+(119889119906120579(119906)) = infin the Bihari
inequality implies
120593119896(119905) = 0 for each 119905 isin [120591
119896minus1 120591
119896] (54)
For each 119896 isin 1 2 119898 (52) and (54) yield
lim119899rarrinfin
120593119896119899(119905) = 0 (55)
Mathematical Problems in Engineering 9
Then
E sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899+1 (119904) minus 119884119899 (119904)
10038161003816100381610038162
le max1le119896le119898
E sup119904isin[120591119896minus1120591119896]
11989021205731199041003816100381610038161003816119884119899+1 (119904) minus 119884119899 (119904)
10038161003816100381610038162
= max1le119896le119898
120593119896119899(119905) 997888rarr 0
(56)
as 119899 rarr infin and this proves the assertion (49)By (36) we obtain
E( sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899+1 (119904) minus 119884119899 (119904)
10038161003816100381610038162
) + (1 minus24119872
2
119860119897
120573)E
times int
119879
119905
11989021205731199041003816100381610038161003816119885119899+1
(119904) minus 119885119899(119904)10038161003816100381610038162
119889119904
le24119872
2
119860
120573Eint
119879
119905
1198902120573119904
[120579 (1003816100381610038161003816119884119899 (119904) minus 119884119899minus1 (119904)
10038161003816100381610038162
)] 119889119904
(57)
Applying (49) to the above formula we see that (119884119899 119885
119899) is
a Cauchy (hence convergent) sequence in S2
F ([0 119879]119867) times
H2
F ([0 119879]L(Γ119867)) denote the limit by (119884 119885) Now letting119899 rarr infin in (27) we obtain that
119884 (119905) = 119890119860(119879minus119905)
120585
+ int
119879
119905
E1015840[119890
119860(119904minus119905)119891 (119904 119884
1015840
(119904) 1198851015840
(119904) 119884 (119904) 119885 (119904))] 119889119904
minus int
119879
119905
119890119860(119904minus119905)
119885 (119904) 119889119882 (119904)
(58)
holds on the entire interval [0 119879]The theorem is nowproved
To illustrate the application of the obtained existenceand uniqueness result we consider the example of backwardstochastic partial differential equations (BSPDEs) of mean-field type
Example 9 Let O be an open bounded domain in R119899
with uniformly 1198622 boundary 120597O let 119861(119905) be a standard 119899-dimensional Brownian motion (equipped with the normalfiltration) and let 120585 O rarr R be an F
119879-measurable
random variable We also let 119871 denote the semielliptic partialdifferential operator on 1198622
(R) of the form
119871 =
119899
sum
119894119895=1
119886119894119895(119909)
1205972
120597119909119894120597119909
119895
+
119899
sum
119894=1
119887119894(119909)
120597
120597119909119894
(59)
The aim is to study the solvability of the following initialboundary value problem
119889119884 (119905 119909)
= (119871119884 (119905 119909) + E1015840
times [119892 (119905 119909 1198841015840
(119905 119909) 1198851015840
(119905 119909) 119884 (119905 119909) 119885 (119905 119909))]) 119889119905
+ 119885 (119905 119909) 119889119861 (119905) ae on (0 119879) times O
119884 (119905 119909) = 0 ae on (0 119879) times 120597O
119884 (119879 119909) = 120585 (119879 119909) ae onO(60)
where
119884 [0 119879] times O 997888rarr R
119885 [0 119879] times O 997888rarr L (R119899 119871
2
(O))
119892 [0 119879] times O timesR timesL (R119899 119871
2
(O))
timesR timesL (R119899 119871
2
(O)) 997888rarr R
(61)
The following assumptions will have to be in force
(H1) 119886119894119895 119887
119894 R119899
rarr R are uniformly continuous andbounded and satisfy the usual uniform ellipticitycondition sum119899
119894119895=1119886119894119895(119909)119908
119894119908119895ge 120582|119908|
2 for some 120582 gt 0
and all 119909 isin O 119908 isin R119899(H2) 119892 is measurable in (119905 119909 119910 119910 119911) and continuous in
( 119911) and there exists 119862 gt 0 such that1003816100381610038161003816119892 (119905 119909 1199101 1 1199101 1199111) minus 119892 (119905 119909 1199102 2 1199102 1199112)
1003816100381610038161003816
le 119862 [10038161003816100381610038161199101 minus 1199102
1003816100381610038161003816 +10038161003816100381610038161 minus 2
1003816100381610038161003816 +10038161003816100381610038161199101 minus 1199102
1003816100381610038161003816 +10038161003816100381610038161199111 minus 1199112
1003816100381610038161003816]
(62)
for all 0 le 119905 le 119879 119909 isin O 1199101 119910
2 119910
1 119910
2isin R
1
2 119911
1 119911
2isin
L(R119899 119871
2(O))
Then we are now in a position of showing existence anduniqueness of the solution of BSPDEs (60)
Theorem 10 If (H1) and (H2) are satisfied then the mean-field BSPDE (60) has a unique mild solution (119884 119885) isin
1198712(0 119879 119871
2(Ω 119871
2(O))) times 1198712F (0 119879 119871
2(R119899
1198712(Ω 119871
2(O))))
Proof Let119867 = 1198712(O) and119870 = R119899 Define the operator 119860 by
119860119884 (119905 sdot) = 119871119884 (119905 sdot) (63)
It is shown in [17] (see Example 21 in [17]) that 119860 generatesa strongly continuous semigroup on 119867 Define the maps 119891
[0 119879] times 119867 timesL(119870119867) times 119867 timesL(119870119867) rarr 119867 by
119891 (119905 1198841015840
(119905) 1198851015840
(119905) 119884 (119905) 119885 (119905)) (119909)
= 119892 (119905 119909 1198841015840
(119905 119909) 1198851015840
(119905 119909) 119884 (119905 119909) 119885 (119905 119909))
(64)
10 Mathematical Problems in Engineering
for all 0 le 119905 le 119879 119909 isin O With these identifications(60) can be written in the form of (11) By (H2) we know119891 satisfy condition (A1) Hence an application of Theorem 4concludes that (60) has a unique mild solution (119884 119885) isin
1198712(0 119879 119871
2(Ω 119871
2(O))) times 1198712F (0 119879 119871
2(R119899
1198712(Ω 119871
2(O))))
4 Mean-Field Stochastic Evolution Equations
Let 119882(119905) 119905 isin [0 119879] be a cylindrical Wiener process withvalues in a Hilbert space Γ defined on a probability space(ΩFP) We fix an interval [119905 119879] sub [0 119879] and considerthe stochastic evolution equations of mean-field type for anunknown process 119883(119904) 119904 isin [119905 119879] with values in a Hilbertspace 119870
119889119883 (119904) = 119861119883 (119904) 119889119904 + E1015840[119887 (119904 119883
1015840
(119904) 119883 (119904))] 119889119904
+ E1015840[120590 (119904 119883
1015840
(119904) 119883 (119904))] 119889119882 (119904)
119883 (119905) = 119909 isin 119870
(65)
where operator 119861 is the generator of a strongly continuoussemigroup 119890
119905119861 119905 ge 0 in the Hilbert space 119870 with 119872119861≜
sup119905isin[0119879]
|119890119905119861|
By a mild solution of (65) we mean an F119904-measurable
process 119883(119904) 119904 isin [119905 119879] with continuous paths in 119870 suchthat P-as
119883 (119904) = 119890119861(119904minus119905)
119909
+ int
119904
119905
119890119861(119904minus120591)
E1015840[119887 (120591 119883
1015840
(120591) 119883 (120591))] 119889120591
+ int
119904
119905
119890119861(119904minus120591)
E1015840[120590 (120591119883
1015840
(120591) 119883 (120591))] 119889119882 (120591)
119904 isin [119905 119879]
(66)
We suppose the following(A4) 119887 [0 119879] times 119870 times 119870 rarr 119870 is a measurable mapping
which satisfies10038161003816100381610038161003816119887 (119905 119909
1015840 119909) minus 119887 (119905 119910
1015840 119910)
10038161003816100381610038161003816
2
le 1198711(100381610038161003816100381610038161199091015840minus 119910
101584010038161003816100381610038161003816
2
+1003816100381610038161003816119909 minus 119910
10038161003816100381610038162
)
119905 isin [0 119879] 1199091015840 119909 119910
1015840 119910 isin 119870
(67)
for some constant 1198711gt 0
(A5) The mapping 120590 [0 119879] times 119870 times 119870 rarr L(Γ 119870) fulfillsthat for every V isin Γ the map 120590V [0 119879] times 119870 times 119870 rarr 119870
is measurable for every 119904 gt 0 119905 isin [0 119879] 1199091015840 1199101015840 119909 119910 isin 119870119890119904119861120590(119905 119909
1015840 119909) isin L(Γ 119870) and
10038161003816100381610038161003816119890119904119861120590 (119905 119909
1015840 119909)
10038161003816100381610038161003816L(Γ119870)le 119871
2119904minus120574(1 +
10038161003816100381610038161003816119909101584010038161003816100381610038161003816+ |119909|)
10038161003816100381610038161003816119890119904119861120590 (119905 119909
1015840 119909) minus 119890
119904119861120590 (119905 119910
1015840 119910)
10038161003816100381610038161003816L(Γ119870)
le 1198712119904minus120574(100381610038161003816100381610038161199091015840minus 119910
101584010038161003816100381610038161003816+1003816100381610038161003816119909 minus 119910
1003816100381610038161003816)
10038161003816100381610038161003816120590 (119905 119909
1015840 119909)
10038161003816100381610038161003816L(Γ119870)le 119871
2(1 +
10038161003816100381610038161003816119909101584010038161003816100381610038161003816+ |119909|)
(68)
for some constants 1198712gt 0 and 120574 isin [0 12)
Theorem 11 Under assumptions (A3) and (A4) (65) has aunique mild solution119883(sdot) isin S2
F ([119905 119879] 119870)
The proof is constructed in two steps like that ofTheorem 4 and it uses standard arguments for stochastic evo-lution equations introduced in the proof of Proposition 32 in[3] Since the proof is straightforward we prefer to omit it
Remark 12 In our paper Lipchitz condtion (A4) is givento get the well-posedness of mean-field stochastic evolutionequations In fact (A4) can be replaced by a weaker conditionsuch as (A3) We just give the condition (A4) for simplicity
From standard arguments we can also get the followingcontinuous dependence theorem
Corollary 13 Assume that for all 120572 in a metric space 119865(119887120572 120590
120572) satisfy (A4) and (A5)with119871
1and119871
2independent of 120572
Also assume that
Eint119879
0
10038161003816100381610038161003816E1015840[119887120572(119904 119883
1015840
(119904) 119883 (119904))]
minus E1015840[1198871205720(119904 119883
1015840
(119904) 119883 (119904))]10038161003816100381610038161003816
2
119889119904 997888rarr 0
Eint119879
0
10038161003816100381610038161003816E1015840[120590
120572(119904 119883
1015840
(119904) 119883 (119904))]
minusE1015840[120590
1205720(119904 119883
1015840
(119904) 119883 (119904))]10038161003816100381610038161003816
2
119889119904 997888rarr 0
(69)
as 120572 rarr 1205720for all119883 isin S2
F ([0 119879] 119870)If we denote by 119883120572
(sdot) the mild solution of mean-field SEE(65) corresponding to the functions (119887
120572 120590
120572) and to the initial
data 119909 then we have
sup119904isin[0119879]
E1003816100381610038161003816119883
120572
(119904) minus 1198831205720 (119904)
10038161003816100381610038162
997888rarr 0 119886119904 120572 997888rarr 1205720 (70)
5 Maximum Principle for BSPDEs ofMean-Field Type
51 Formulation of the Problem Let O isin R119899 be a boundedopen set with smooth boundary 120597O and let 119880 the space ofcontrols be a separable real Hilbert space We denote
U = V (sdot) isin 1198712F(0 119879 119880)
| V119905(120596
1015840 120596) [0 119879] times Ω times Ω
997888rarr 119880 is F otimesF119905-progressively measurable
(71)
Mathematical Problems in Engineering 11
An element ofU is called an admissible controlFor any V isin U we consider the following controlled
BSPDE system in the state space119867 = 1198712(O) (norm | sdot | scalar
product ⟨sdot sdot⟩)
119889119884119905(119909) = minus119860119884
119905(119909) 119889119905
minus E1015840[119891 (119905 119909 (119884
119905(119909))
1015840
(119885119905(119909))
1015840
119884119905(119909) 119885
119905(119909) V
119905)] 119889119905
+ 119885119905(119909) 119889119882 (119905) 119905 isin [0 119879]
119884119879(119909) = 120585 (119909) 119909 isin O
(72)
where 119860 is a partial differential operator 119891 [0 119879] timesO times119867 times
L(Γ119867)times119867timesL(Γ119867)times119880 rarr 119867 and 120585 isin 1198712(ΩF119879P 119867)
The cost functional is given by
119869 (V) = Eint119879
0
intO
E1015840[ℎ (119904 119909 (119884
119904(119909))
1015840
(119885119904(119909))
1015840
119884119904(119909) 119885
119904(119909) V
119904)] 119889119909 119889119904
+E1015840intO
119892 (119909 (1198840(119909))
1015840
1198840(119909)) 119889119909
(73)
where
ℎ [0 119879] times O times 119867 timesL (Γ119867) times 119867 timesL (Γ119867) times 119880 997888rarr R
119892 O times 119867 times119867 997888rarr R
(74)
Our purpose is to minimize the functional 119869(sdot) over Uadsubject to the following state constraint
EintO
Φ(119909 (1198840(119909))
1015840
1198840(119909)) 119889119909 = 0 (75)
where
Φ O times 119867 times119867 997888rarr R (76)
An admissible control 119906 isin Uad that satisfies
119869 (119906) = minVisinUad
119869 (V) (77)
is called optimalThrough what follows the following assumptions will be
in force
(L1) 119860 is a partial differential operator with appropri-ate boundary conditions We assume that 119860 is theinfinitesimal generator of a strongly continuous semi-group 119890119905119860 119905 ge 0 in119867 Moreover for every 119905 isin [0 119879]119890
119905119860119891
1198712(O) le 119872
119860119891
1198712(O) for some constant 119872
119860
independent of 119905 and 119891(L2) 119891 ℎ 119892 and Φ are continuously Gateaux differen-
tiable with respect to (1199101015840 1199111015840 119910 119911) 119891 is continuouslyGateaux differentiable with respect to V and ℎ iscontinuous with respect to V
(L3) The derivatives of 119891 ℎ 119892 and Φ are Lipschitzcontinuous and bounded by
100381610038161003816100381610038161198911199101015840
10038161003816100381610038161003816+10038161003816100381610038161198911199111015840
1003816100381610038161003816 +10038161003816100381610038161003816119891119910
10038161003816100381610038161003816+1003816100381610038161003816119891119911
1003816100381610038161003816 +1003816100381610038161003816119891V
1003816100381610038161003816 +10038161003816100381610038161003816Φ1199101015840
10038161003816100381610038161003816+10038161003816100381610038161003816Φ119910
10038161003816100381610038161003816le 119862
10038161003816100381610038161003816ℎ1199101015840
10038161003816100381610038161003816+1003816100381610038161003816ℎ1199111015840
1003816100381610038161003816 +10038161003816100381610038161003816ℎ119910
10038161003816100381610038161003816+1003816100381610038161003816ℎ119911
1003816100381610038161003816 +100381610038161003816100381610038161198921199101015840
10038161003816100381610038161003816+10038161003816100381610038161003816119892119910
10038161003816100381610038161003816
le 119862 (1 +10038161003816100381610038161003816119910101584010038161003816100381610038161003816+10038161003816100381610038161003816119911101584010038161003816100381610038161003816+10038161003816100381610038161199101003816100381610038161003816 + |119911| + |V|)
(78)
where 119862 is a positive constant
Obviously according to Theorem 4 state equation (72)has a unique mild solution under the above assumptions
Remark 14 We can define the second order differentialoperator
(119860119891) (119909) =
119899
sum
119894119895=1
119886119894119895(119909)
1205972119891
120597119909119894120597119909
119895
(119909) +
119899
sum
119894=1
119887119894(119909)
120597119891
120597119909119894
(119909) (79)
By Example 9 119860 fulfills assumption (L1) if 119886119894119895 119887
119894satisfy
condition (H1)
52 Variation of the Trajectory Let 119906 be an optimal controlwith (119884(sdot) 119885(sdot)) being the corresponding optimal state Let120576 gt 0 and [119903 119903 + 120576] sube [0 119879] For any given V isin Uad weintroduce the spike variation of the control 119906(sdot)
119906120576
119905=
V119905 119905 isin [119903 119903 + 120576]
119906119905 119905 isin [0 119879] [119904 119904 + 120576]
(80)
It is clear that 119906120576(sdot) isin UadLet (119884120576
(sdot) 119885120576(sdot)) be the trajectory corresponding to 119906120576(sdot)
We use the following short notation for brevity
119891 (119905) = 119891 (119905 119909 (119884119905(119909))
1015840
(119885119905(119909))
1015840
119884119905(119909) 119885
119905(119909) 119906
119905)
119891 (119906120576
119905) = 119891 (119905 119909 (119884
119905(119909))
1015840
(119885119905(119909))
1015840
119884119905(119909) 119885
119905(119909) 119906
120576
119905)
(81)
Consider the following equation
119889119870120576
119905(119909) = minus119860119870
120576
119905(119909) 119889119905
minus E1015840[119891
1199101015840 (119905) (119870
120576
119905(119909))
1015840
+ 119891119910(119905) 119870
120576
119905(119909)
+ 1198911199111015840 (119905) (119876
120576
119905(119909))
1015840
+ 119891119911(119905) 119876
120576
119905(119909)
+1
120576(119891 (119906
120576
119905) minus 119891 (119905))] 119889119905 + 119876
120576
119905(119909) 119889119882 (119905)
119870120576
119879(119909) = 0
(82)
Since the coefficients in (82) are bounded it is easy to checkthat there exists a unique mild solution such that
E[ sup119905isin[0119879]
1003816100381610038161003816119870120576
119905
10038161003816100381610038162
+ int
119879
0
1003816100381610038161003816119876120576
119905
10038161003816100381610038162
119889119905] lt infin (83)
We have the following estimate
12 Mathematical Problems in Engineering
Theorem 15 There holds
lim120576rarr0
E[ sup119904isin[119905119879]
1003816100381610038161003816100381610038161003816100381610038161003816
119884120576
119904minus 119884
119904
120576minus 119870
120576
119904
1003816100381610038161003816100381610038161003816100381610038161003816
2
] = 0 forall119905 isin [0 119879]
lim120576rarr0
Eint119879
0
1003816100381610038161003816100381610038161003816100381610038161003816
119885120576
119904minus 119885
119904
120576minus 119876
120576
119904
1003816100381610038161003816100381610038161003816100381610038161003816
2
119889119904 = 0
(84)
Proof We define
120578120576
119904=119884120576
119904minus 119884
119904
120576minus 119870
120576
119904 120577
120576
119904=119885120576
119904minus 119885
119904
120576minus 119876
120576
119904 119904 isin [0 119879]
(85)
For simplicity let us define
Λ120576
119904= ((119884
120576
119904)1015840
(119885120576
119904)1015840
119884120576
119904 119885
120576
119904)
119891 (119904 120582) = 119891 (119904 1198841015840
119904+ 120582(119884
120576
119904minus 119884
119904)1015840
1198851015840
119904+ 120582(119885
120576
119904minus 119885
119904)1015840
119884119904+ 120582 (119884
120576
119904minus 119884
119904)
119885119904+ 120582 (119885
120576
119904minus 119885
119904) 119906
120576
119904)
(86)
By the definition of (119884120576
119904 119885
120576
119904) (119884
119904 119885
119904) and (119870120576
119904 119876
120576
119904) (120578120576
119904 120577
120576
119904)
is the mild solution of
119889120578120576
119904= minus119860120578
120576
119904119889119904 minus E
1015840
[119871 (119904 120576)] 119889119904 + 120577120576
119904119889119882 (119904)
120578120576
119879= 0
(87)
with
119871 (119904 120576) =1
120576(119891 (119904 Λ
120576
119904 119906
120576
119904) minus 119891 (119906
120576
119904))
minus 1198911199101015840 (119904) (119870
120576
119904)1015840
minus 119891119910(119904) 119870
120576
119904minus 119891
1199111015840 (119904) (119876
120576
119904)1015840
minus 119891119911(119904) 119876
120576
119904
= ((120578120576
119904)1015840
+ (119870120576
119904)1015840
) int
1
0
1198911199101015840 (119904 120582) 119889120582 + (120578
120576
119904+ 119870
120576
119904)
times int
1
0
119891119910(119904 120582) 119889120582 minus 119891
1199101015840 (119904) (119870
120576
119904)1015840
minus 119891119910(119904) 119870
120576
119904
+ ((120577120576
119904)1015840
+ (119876120576
119904)1015840
)int
1
0
1198911199111015840 (119904 120582) 119889120582 + (120577
120576
119904+ 119876
120576
119904)
times int
1
0
119891119911(119904 120582) 119889120582 minus 119891
1199111015840 (119904) (119876
120576
119904)1015840
minus 119891119911(119904) 119876
120576
119904
= (120578120576
119904)1015840
int
1
0
1198911199101015840 (119904 120582) 119889120582 + 120578
120576
119904int
1
0
119891119910(119904 120582) 119889120582
+ (120577120576
119904)1015840
int
1
0
1198911199111015840 (119904 120582) 119889120582 + 120577
120576
119904int
1
0
119891119911(119904 120582) 119889120582 + 120574
120576
119904
(88)
where we denote
120574120576
119904= (119870
120576
119904)1015840
int
1
0
(1198911199101015840 (119904 120582) minus 119891
1199101015840 (119904)) 119889120582
+ 119870120576
119904int
1
0
(119891119910(119904 120582) minus 119891
119910(119904)) 119889120582
+ (119876120576
119904)1015840
int
1
0
(1198911199111015840 (119904 120582) minus 119891
1199111015840 (119904)) 119889120582
+ 119876120576
119904int
1
0
(119891119911(119904 120582) minus 119891
119911(119904)) 119889120582
(89)
For any 120573 gt 0 according to Lemma 1 we obtain
E sup119905le119904le119879
11989021205731199041003816100381610038161003816120578
120576
119904
10038161003816100381610038162
+ Eint119879
119905
11989021205731199041003816100381610038161003816120577
120576
119904
10038161003816100381610038162
119889119904
le12119872
2
119860
120573int
119879
119905
1198902120573119904
E [10038161003816100381610038161003816E1015840
[119871 (119904 120576)]10038161003816100381610038161003816
2
] 119889119904
le12119872
2
119860
120573Eint
119879
119905
1198902120573119904
E1015840[|119871 (119904 120576)|
2] 119889119904
(90)
By condition (L3) we have
E [E1015840[|119871 (119904 120576)|
2]]
= E [E1015840[1003816100381610038161003816119871 (119904 120576) minus 120574
120576
119904+ 120574
120576
119904
10038161003816100381610038162
]]
le 81198622E [E
1015840[10038161003816100381610038161003816(120578
120576
119904)101584010038161003816100381610038161003816
2
+1003816100381610038161003816120578120576
119904
10038161003816100381610038162
+10038161003816100381610038161003816(120577
120576
119904)101584010038161003816100381610038161003816
2
+1003816100381610038161003816120577120576
119904
10038161003816100381610038162
]]
+ 2E [E1015840[1003816100381610038161003816120574120576
119904
10038161003816100381610038162
]]
le 161198622E [
1003816100381610038161003816120578120576
119904
10038161003816100381610038162
+1003816100381610038161003816120577120576
119904
10038161003816100381610038162
] + 2E [E1015840[1003816100381610038161003816120574120576
119904
10038161003816100381610038162
]]
(91)
Combined with (91) (90) yields
E sup119905le119904le119879
11989021205731199041003816100381610038161003816120578
120576
119904
10038161003816100381610038162
+ (1 minus192119872
2
1198601198622
120573)Eint
119879
119905
11989021205731199041003816100381610038161003816120577
120576
119904
10038161003816100381610038162
119889119904
le192119872
2
1198601198622
120573Eint
119879
119905
11989021205731199041003816100381610038161003816120578
120576
119904
10038161003816100381610038162
119889119904
+24119872
2
119860
120573Eint
119879
119905
1198902120573119904
[E1015840[1003816100381610038161003816120574120576
119904
10038161003816100381610038162
]] 119889119904
(92)
We claim that
Eint119879
119905
E1015840[1003816100381610038161003816120574120576
119904
10038161003816100381610038162
] 119889119904 997888rarr 0 as 120576 997888rarr 0 (93)
From (89)
120574120576
119904= 119868
1
119904+ 119868
2
119904+ 119868
3
119904+ 119868
4
119904 (94)
Mathematical Problems in Engineering 13
where
1198681
119904= (119870
120576
119904)1015840
int
1
0
(1198911199101015840 (119904 120582) minus 119891
1199101015840 (119904)) 119889120582
1198682
119904= 119870
120576
119904int
1
0
(119891119910(119904 120582) minus 119891
119910(119904)) 119889120582
1198683
119904= (119876
120576
119904)1015840
int
1
0
(1198911199111015840 (119904 120582) minus 119891
1199111015840 (119904)) 119889120582
1198684
119904= 119876
120576
119904int
1
0
(119891119911(119904 120582) minus 119891
119911(119904)) 119889120582
(95)
Then
Eint119879
119905
E1015840[1003816100381610038161003816120574120576
119904
10038161003816100381610038162
] 119889119904
le 4Eint119879
119905
E1015840[100381610038161003816100381610038161198681
119904
10038161003816100381610038161003816
2
+100381610038161003816100381610038161198682
119904
10038161003816100381610038161003816
2
+100381610038161003816100381610038161198683
119904
10038161003816100381610038161003816
2
+100381610038161003816100381610038161198684
119904
10038161003816100381610038161003816
2
] 119889119904
(96)
Take Eint119879119905E1015840[|1198682
119904|2
]119889119904 for example
Eint119879
119905
E1015840[100381610038161003816100381610038161198682
119904
10038161003816100381610038161003816
2
] 119889119904
= Eint119879
119905
E1015840[(119870
120576
119904int
1
0
(119891119910(119904 120582) minus 119891
119910(119904)) 119889120582)
2
]119889119904
le Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119904 120582) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582] 119889119904
le 2Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119904 120582) minus 119891
119910(119906
120576
119905)10038161003816100381610038161003816
2
119889120582] 119889119904
+ 2Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119906
120576
119905) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582] 119889119904
(97)
Note that
Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119906
120576
119905) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582] 119889119904
= Eint119903+120576
119903
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119906
120576
119905) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582] 119889119904
le sup119904isin[119905119879]
E [E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119906
120576
119905) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582]] 120576
997888rarr 0 as 120576 997888rarr 0
(98)
The inequality above holds due to the boundedness of|119870
120576
119904|2
int1
0|119891119910(119906
120576
119905) minus 119891
119910(119904)|
2
119889120582 Indeed Assumption (L3) im-plies the boundedness of 119891
119910(119906
120576
119905) minus 119891
119910(119904) Meanwhile 119870120576
119904is
the solution of mean-field BSEE (82) It can be easy to check119870120576
119904is bounded since the coefficients in (82) are boundedOn the other hand
Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119904 120582) minus 119891
119910(119906
120576
119905)10038161003816100381610038161003816
2
119889120582] 119889119904
le 11986221205822Eint
119879
119905
1003816100381610038161003816119870120576
119904
10038161003816100381610038162
times int
1
0
1198641015840[100381610038161003816100381610038161003816(119884
120576
119904minus 119884
119904)1015840100381610038161003816100381610038161003816
2
+100381610038161003816100381610038161003816(119885
120576
119904minus 119885
119904)1015840100381610038161003816100381610038161003816
2
times10038161003816100381610038161003816119884120576
119904minus 119884
119904
10038161003816100381610038161003816
2
+10038161003816100381610038161003816119885120576
119904minus 119885
119904
10038161003816100381610038161003816
2
] 119889120582 119889119904
(99)
where (119884120576
119904 119885
120576
119904) is the mild solution of the following equation
119889119884120576
119905= minus 119860119884
120576
119905119889119905
minus E1015840[119891 (119905 (119884
120576
119905)1015840
(119885120576
119905)1015840
119884120576
119905 119885
120576
119905 119906
120576
119905)] 119889119905
+ 119885120576
119905119889119882 (119905) 119905 isin [0 119879]
119884120576
119879= 120585
(100)
and (119884119904 119885
119904) is the mild solution of
119889119884119905= minus 119860119884
119905119889119905
minus E1015840[119891 (119905 (119884
119905)1015840
(119885119905)1015840
119884119905 119885
119905 119906
119905)] 119889119905
+ 119885119905119889119882 (119905) 119905 isin [0 119879]
119884119879= 120585
(101)
By the definition of 119906120576119905 according to (L2) we have
E1015840[119891 (119904 119884
1015840
(119904) 1198851015840
(119904) 119884 (119904) 119885 (119904) 119906120576
119905)]
997888rarr E1015840[119891 (119904 119884
1015840
(119904) 1198851015840
(119904) 119884 (119904) 119885 (119904) 119906119905)]
(102)
in 1198712([0 119879]119867) as 120576 rarr 0 Using the continuous dependencetheorem Corollary 5 we obtain
(119884120576
119904 119885
120576
119904) 997888rarr (119884
119904 119885
119904) as 120576 997888rarr 0 (103)
Then
Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119904 120582) minus 119891
119910(119906
120576
119905)10038161003816100381610038161003816
2
119889120582] 119889119904
le 11986221205822Eint
119879
119905
1003816100381610038161003816119870120576
119904
10038161003816100381610038162
times int
1
0
E1015840[100381610038161003816100381610038161003816(119884
120576
119904minus 119884
119904)1015840100381610038161003816100381610038161003816
2
+100381610038161003816100381610038161003816(119885
120576
119904minus 119885
119904)1015840100381610038161003816100381610038161003816
2
times10038161003816100381610038161003816119884120576
119904minus 119884
119904
10038161003816100381610038161003816
2
+10038161003816100381610038161003816119885120576
119904minus 119885
119904
10038161003816100381610038161003816
2
] 119889120582 119889119904
997888rarr 0 as 120576 997888rarr 0
(104)
14 Mathematical Problems in Engineering
Combining (98) with (104) we finally haveEint
119879
119905E1015840[|1198682
119904|2
]119889119904 rarr 0 as 120576 rarr 0The required result (93) follows by using the similar
estimations for 1198681119904 1198683
119904 and 1198684
119904
Note that 1 minus 1921198722
1198601198622120573 gt 0 if 120573 is sufficiently large
Now to prove the desired result (84) it suffices to applyGronwallrsquos lemma and estimate (93) to inequality (92)
To deal with the state constraint (75) we need to recall theEkeland variational principle
Lemma 16 (Ekelandrsquos variational principle see [16 Lemma41]) Let (119878 119889) be a complete metric space and let 119865(sdot) 119878 rarr
R be lower semicontinuous and bounded from below If for 120588 gt0 there exists 119906 isin 119878 such that
119865 (119906) le infVisin119878
119865 (V) + 120588 (105)
then there exists 119906120588 isin 119878 satisfying
(i) 119865 (119906120588) le 119865 (119906)
(ii) 119889 (119906120588 119906) le 120588
(iii) 119865 (119906120588) le 119865 (V) + 120588 sdot 119889 (119906120588 119906) forallV = 119906120588
(106)
Now fix V isin Uad and set
119878 = V (sdot) isin Uad | sup0le119905le119879
E1003816100381610038161003816V11990510038161003816100381610038162
le E100381610038161003816100381611990611990510038161003816100381610038162
+ |V|2
119889 (V (sdot) V (sdot)) = 119898 119905 isin [0 119879] | E1003816100381610038161003816V119905 minus V
119905
10038161003816100381610038162
gt 0
forallV (sdot) V (sdot) isin 119878
(107)
where119898 denotes the Lebesgue measure on RThe following result is proved as Proposition 41 in [16]
Lemma 17 (119878 119889(sdot sdot)) is a complete metric space and 119869120588 is
continuous and bounded on 119878 where
119869120588
(V (sdot)) = (119869 (V (sdot)) minus 119869 (119906 (sdot)) + 120588)2
+
10038161003816100381610038161003816100381610038161003816Eint
O
Φ(119909 (1198840(119909))
1015840
1198840(119909)) 119889119909
10038161003816100381610038161003816100381610038161003816
2
12
forallV (sdot) isin 119878(108)
and (119884 119885) is the mild solution of (72) corresponding to thecontrol V
Now we consider the following free initial state optimalcontrol problem
infV(sdot)isin119878
119869120588
(V (sdot)) (109)
It is easy to check that
0 le infV(sdot)isin119878
119869120588
(V (sdot)) le 119869120588
(119906 (sdot)) = 120588 (110)
According to Ekelandrsquos variational principle there exists a119906120588(sdot) isin 119881 such that
(i) 119869120588 (119906120588 (sdot)) le 120588
(ii) 119889 (119906120588 (sdot) 119906 (sdot)) le 120588
(iii) 119869120588 (119906120588 (sdot)) le 119869120588
(V (sdot)) + 120588119889 (119906120588 (sdot) 119906 (sdot)) forallV (sdot) isin 119878(111)
Using the spike variationmethod we can construct 119906120576120588(sdot) isin 119878as follows
119906120576120588
119905=
V119905 119905 isin [119904 119904 + 120576]
119906120588
119905 119905 isin [0 119879] [119904 119904 + 120576]
(112)
It is clear that 119889(119906120576120588(sdot) 119906120588(sdot)) le 120576 Let (119884120576120588(sdot) 119885
120576120588(sdot)) (resp
(119884120588(sdot) 119885
120588(sdot))) be the solution of (72) with respect to the
control 119906120576120588(sdot) (resp 119906120588(sdot)) Following (82) (119870120576120588
119905 119876
120576120588
119905) is the
mild solution of
119870120576120588
119905= int
119879
119905
119890119860(119904minus119905)
E1015840
times [1198911199101015840 (119904 Λ
120588
119904 119906
120588
119904) (119870
120576120588
119904)1015840
+ 119891119910(119904 Λ
120588
119904 119906
120588
119904)119870
120576120588
119904
+ 1198911199111015840 (119904 Λ
120588
119904 119906
120588
119904) (119876
120576120588
119904)1015840
+ 119891119911(119904 Λ
120588
119904 119906
120588
119904) 119876
120576120588
119904
+1
120576(119891 (119904 Λ
120588
119904 119906
120576120588
119904) minus 119891 (119904 Λ
120588
119904 119906
120588
119904))] 119889119904
minus int
119879
119905
119890119860(119904minus119905)
119876120576120588
119904119889119882 (119904)
(113)
ByTheorem 15 we know that
lim120576rarr0
E[ sup119904isin[119905119879]
100381610038161003816100381610038161003816100381610038161003816
119884120576120588
119904minus 119884
120588
119904
120576minus 119870
120576120588
119904
100381610038161003816100381610038161003816100381610038161003816
2
] = 0 forall119905 isin [0 119879]
lim120576rarr0
Eint119879
0
100381610038161003816100381610038161003816100381610038161003816
119885120576120588
119904minus 119885
120588
119904
120576minus 119876
120576120588
119904
100381610038161003816100381610038161003816100381610038161003816
2
119889119904 = 0
(114)
The proof of the following proposition is technical butbased on the arguments above and we omit it
Proposition 18 One has
1
120576E [Φ ((119884
120576120588
0)1015840
119884120576120588
0) minus Φ((119884
120588
0)1015840
119884120588
0)]
= E [Φ1199101015840 ((119884
120588
0)1015840
119884120588
0) (119870
120576120588
0)1015840
+ Φ119910((119884
120588
0)1015840
119884120588
0)119870
120576120588
0]
+ 119900 (120576)
Mathematical Problems in Engineering 15
1
120576(119869 (119906
120576120588) minus 119869 (119906
120588))
= E [1198921199101015840 ((119884
120588
0)1015840
119884120588
0) (119870
120576120588
0)1015840
+ 119892119910((119884
120588
0)1015840
119884120588
0)119870
120576120588
0]
+ Δ120576+1
120576Eint
119879
0
E1015840[ℎ (119904 Λ
120588
119904 119906
120576120588
119904)
minus ℎ (119904 Λ120588
119904 119906
120588
119904)] 119889119904 + 119900 (120576)
(115)
where
Δ120576= Eint
119879
0
E1015840[ℎ
1199101015840 (119904 Λ
120588
119904 119906
120576120588
119904) (119870
120576120588
119904)1015840
+ ℎ1199111015840 (119904 Λ
120588
119904 119906
120576120588
119904) (119876
120576120588
119904)1015840
+ ℎ119910(119904 Λ
120588
119904 119906
120576120588
119904)119870
120576120588
119904
+ ℎ119911(119904 Λ
120588
119904 119906
120576120588
119904) 119876
120576120588
119904] 119889119904
(116)
53 Variational Inequality and Adjoint Equation In thissubsection the adjoint process is introduced to deduce thevariational inequality
If we set V(sdot) = 119906120576120588(sdot) in (111) and notice that
119889(119906120576120588(sdot) 119906
120588(sdot)) le 120576 we get
minus120588 le1
120576(119869
120588(119906
120576120588
(sdot)) minus 119869120588(119906
120588
(sdot))) (117)
By Lemma 17
1
120576(119869
120588(119906
120576120588
(sdot)) minus 119869120588(119906
120588
(sdot)))
=(119869
120588(119906
120576120588(sdot)))
2
minus (119869120588(119906
120588(sdot)))
2
120576 (119869120588 (119906120576120588 (sdot)) + 119869120588 (119906120588 (sdot)))
=119869 (119906
120576120588(sdot)) + 119869 (119906
120588(sdot)) minus 2119869 (119906 (sdot)) + 2120588
119869120588 (119906120576120588 (sdot)) + 119869120588 (119906120588 (sdot))
times119869 (119906
120576120588(sdot)) minus 119869 (119906
120588(sdot))
120576
+
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] + E [Φ ((119884
120588
0)1015840
119884120588
0)]
119869120588 (119906120576120588 (sdot)) + 119869120588 (119906120588 (sdot))
times
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] minus E [Φ ((119884
120588
0)1015840
119884120588
0)]
120576
997888rarr 119897120588
1
119869 (119906120576120588(sdot)) minus 119869 (119906
120588(sdot))
120576
+ 119897120588
2
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] minus E [Φ ((119884
120588
0)1015840
119884120588
0)]
120576
(118)
where we set
119897120588
1=119869 (119906
120588(sdot)) minus 119869 (119906 (sdot)) + 120588
119869120588 (119906120588 (sdot))
119897120588
2=
E [Φ ((119884120588
0)1015840
119884120588
0)]
119869120588 (119906120588 (sdot))
(119)
and use the limit
119869 (119906120576120588
(sdot)) 997888rarr 119869 (119906120588
(sdot))
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] 997888rarr E [Φ ((119884
120588
0)1015840
119884120588
0)]
(120)
as 120576 rarr 0 according to (115)As |119897120588
1|2
+ |119897120588
2|2
= 1 for all 120588 gt 0 we know that there existsa subsequence of 119897120588
1 119897120588
2 (still denoted by 119897120588
1 119897120588
2) such that
lim120588rarr0
119897120588
1 119897120588
2 = 119897
1 1198972
1003816100381610038161003816119897110038161003816100381610038162
+1003816100381610038161003816119897210038161003816100381610038162
= 1
(121)
Combining (115) (117) with (118) we get
minus120588 le 119897120588
1
119869 (119906120576120588(sdot)) minus 119869 (119906
120588(sdot))
120576
+ 119897120588
2
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] minus E [Φ ((119884
120588
0)1015840
119884120588
0)]
120576
= 119897120588
1Δ120576+ 119897
120588
1E [119892
1199101015840 ((119884
120588
0)1015840
119884120588
0) (119870
120576120588
0)1015840
+119892119910((119884
120588
0)1015840
119884120588
0)119870
120576120588
0]
+119897120588
1
120576Eint
119879
0
E1015840[ℎ (119904 Λ
120588
119904 119906
120576120588
119904) minus ℎ (119904 Λ
120588
119904 119906
120588
119904)] 119889119904
+ 119897120588
2E [Φ
1199101015840 ((119884
120588
0)1015840
119884120588
0) (119870
120576120588
0)1015840
+Φ119910((119884
120588
0)1015840
119884120588
0)119870
120576120588
0] + (119897
120588
1+ 119897
120588
2) 119900 (120576)
(122)
Next we introduce the adjoint equation corresponding tovariational equation (113) whose solution is denoted by119875120588(119905)
119889119875120588
(119905)
= 119860lowast119875120588
(119905) 119889119905
+ E1015840[119891
1199101015840 (119905 Λ
120588
119905 119906
120588
119905) (119875
120588
(119905))1015840
+ 119891119910(119905 Λ
120588
119905 119906
120588
119905) 119875
120588
(119905) + 119897120588
1ℎ1199101015840 (119905 Λ
120588
119905 119906
120588
119905)
+ 119897120588
1ℎ119910(119905 Λ
120588
119905 119906
120588
119905)] 119889119905
16 Mathematical Problems in Engineering
+ E1015840[119891
1199111015840 (119905 Λ
120588
119905 119906
120588
119905) (119875
120588
(119905))1015840
+ 119891119911(119905 Λ
120588
119905 119906
120588
119905) 119875
120588
(119905)
+ 119897120588
1ℎ1199111015840 (119905 Λ
120588
119905 119906
120588
119905) + 119897
120588
1ℎ119911(119905 Λ
120588
119905 119906
120588
119905)] 119889119882 (119905)
119875120588
(0) = 119897120588
1E [119892
1199101015840 ((119884
120588
0(119909))
1015840
119884120588
0(119909))
+ 119892119910((119884
120588
0(119909))
1015840
119884120588
0(119909))]
+ 119897120588
2E [Φ
1199101015840 ((119884
120588
0(119909))
1015840
119884120588
0(119909))
+ Φ119910((119884
120588
0(119909))
1015840
119884120588
0(119909))]
(123)
where 119860lowast is the 1198712(O)-adjoint operator of 119860 Under assump-tions (L1)ndash(L3) this is a linear mean-field SEE with boundedcoefficients An application ofTheorem 11 implies that it has aunique adaptedmild solution such that119875120588(119905) isin S2
F ([0 119879] 119870)When 120588 rarr 0 according to Corollaries 5 and 13 119875120588(119905)
converges to 119875(119905) where
119875 (119905) isin S2
F ([0 119879] 119870) (124)
is the solution of the following equation
119875 (119905) = 1198971E [119892
1199101015840 (119884
1015840
0 119884
0) + 119892
119910(119884
1015840
0 119884
0)]
+ 1198972E [Φ
1199101015840 (119884
1015840
0 119884
0) + Φ
119910(119884
1015840
0 119884
0)]
+ int
119905
0
119890119860lowast(119905minus119904)
E1015840[119891
1199101015840 (119904) 119875
1015840
(119904) + 119891119910(119904) 119875 (119904)
+ 1198971ℎ1199101015840 (119904) + 119897
1ℎ119910(119904)] 119889119904
+ int
119905
0
119890119860lowast(119905minus119904)
E1015840[119891
1199111015840 (119904) 119875
1015840
(119904) + 119891119911(119904) 119875 (119904)
+ 1198971ℎ1199111015840 (119904) + 119897
1ℎ119911(119904)] 119889119882 (119904)
(125)
The following proposition which formally follows fromProposition 18 gives the relation between 119875120588(119905) and119870120576120588
119905
Proposition 19 Consider the following
E [119875120588
(0)119870120576120588
0]
= Eint119879
0
1
120576119875120588
(119904)E1015840[119891 (119904 Λ
120588
119904 119906
120576120588
119904) minus 119891 (119904 Λ
120588
119904 119906
120588
119904)]
minus 119897120588
1119870120576120588
119904E1015840[ℎ
1199101015840 (119904 Λ
120588
119904 119906
120588
119904) + ℎ
119910(119904 Λ
120588
119904 119906
120588
119904)]
minus 119897120588
1119876120576120588
119904E1015840[ℎ
1199111015840 (119904 Λ
120588
119904 119906
120588
119904) + ℎ
119911(119904 Λ
120588
119904 119906
120588
119904)] 119889119904
(126)
The following theorem constitutes the main contributionof this section themaximumprinciple for the BSPDE controlsystem
Theorem 20 Let assumptions (L1)ndash(L3) hold Suppose 119906(sdot) isan optimal control and (119884(sdot) 119885(sdot)) is the corresponding optimalstate trajectory for the BSPDE control systems (72) and (73)with the initial state constraint (75) Then there exists 119875(119905) isinS2
F ([0 119879] 119870) which satisfies (125) such that
H (119905 1198841015840
119905 119885
1015840
119905 119884
119905 119885
119905 V
119905 119875 (119905))
ge H (119905 1198841015840
119905 119885
1015840
119905 119884
119905 119885
119905 119906
119905 119875 (119905))
119886119890 119886119904 forallV isin U119886119889
(127)
whereH [0 119879]times119867timesL(Γ119867)times119867timesL(Γ119867)times119880times119870 rarr R
is the Hamiltonian function defined by
H (119905 119910 119910 119911 V 119901) = 1198971ℎ (119905 119910 119910 119911 V)
+ 119901119891 (119905 119910 119910 119911 V) (128)
Proof By (122) and Proposition 19 we obtain
minus120588 le1
120576Eint
119879
0
119875120588
(119904)E1015840[119891 (119904 Λ
120588
119904 119906
120576120588
119904)
minus 119891 (119904 Λ120588
119904 119906
120588
119904)] 119889119904
+119897120588
1
120576Eint
119879
0
E1015840[ℎ (119904 Λ
120588
119904 119906
120576120588
119904)
minus ℎ (119904 Λ120588
119904 119906
120588
119904)] 119889119904 + (119897
120588
1+ 119897
120588
2) 119900 (120576)
(129)
Letting 120576 rarr 0+ in (129) we derive for ae 120591 isin [0 119879]
minus120588 le 119897120588
1E1015840[ℎ (120591 Λ
120588
120591 V
120591) minus ℎ (120591 Λ
120588
120591 119906
120588
120591)]
+ 119875120588
(119904)E1015840[119891 (120591 Λ
120588
120591 V
120591) minus 119891 (120591 Λ
120588
120591 119906
120588
120591)]
(130)
for all V isin UadFinally taking 120588 rarr 0 in (130) we derive the desired
result
Remark 21 We note that if the coefficients do not dependexplicitly on the marginal law of the underlying diffusionthe result reduces to the classical case that is the SMP forBSPDEs without mean-field term
Remark 22 When we remove the initial state constraint (75)we obtain the general maximum principle for the mean-fieldBSPDEs system (ie without the constraint) with 119897
1= 1
54 Application A Backward Linear Quadratic Control Prob-lem Now we apply our maximum principle to solve an LQproblem For notational simplicity we restrict ourselves tothe free case (ie without the initial state constraint (75)) thegeneral case being handled in a similar way
Mathematical Problems in Engineering 17
Consider the following problem
119869 (119906) =1
2E [(119884 (0))
2] +
1
2Eint
119879
0
119873V2 (119905) 119889119905 997888rarr min
(131)
subject to
119889119884 (119905) = minus119860119884 (119905) 119889119905
minus 119861119884 (119905) + 119861E [119884 (119905)]
+119862V (119905) + 119863119885 (119905) + 119863E [119885 (119905)] 119889119905
+ 119885 (119905) 119889119882 (119905)
119884 (119879) = 120585 119905 isin [0 119879]
(132)
where 119860 is a partial differential operator satisfying condition(L1) and 119861 119861 119862119863119863 and119873 are bounded and deterministicconstantsWe also assume that119873 gt 0 and V isin 1198712F (0 119879 119880) It iseasy to verify that BSPDE (132) admits a uniquemild solution(119884(119905) 119885(119905))
119875(119905) the adjoint process of state equation (132) is thesolution of
119889119875 (119905) = 119860lowast119875 (119905) 119889119905 + 119861119875 (119905) + 119861E [119875 (119905)] 119889119905
+ 119863119875 (119905) + 119863E [119875 (119905)] 119889119882 (119905)
119875 (0) = 119884 (0)
(133)
Let 119906(sdot) be an optimal control and let (119884(sdot) 119885(sdot)) bethe corresponding state process By maximum principle ofTheorem 20 (note that 119897
1= 1 in this problem)
1
2119873V2 (119905) + 119875 (119905) 119862V (119905) ge
1
2119873119906
2
(119905) + 119875 (119905) 119862119906 (119905) (134)
for all V isin 119880ad since the state equation has the form (132)Thisin turn implies
119906 (119905) = minus119862
119873119875 (119905) (135)
It is clear that (131) is a positive quadratic functional of controlbecause of 119873 gt 0 Hence an optimal control exists Thecandidate optimal control (135) is indeed an optimal controlof this LQ problem for it is the only control which satisfies themaximum principle
Next we want to obtain a more explicit representationof the optimal control (135) from the state equation (132)Substituting (135) into state equation (132) yields
119889119884 (119905) = minus119860119884 (119905) 119889119905
minus 119861119884 (119905) + 119861E [119884 (119905)] minus1198622
119873119875 (119905)
+119863119885 (119905) + 119863E [119885 (119905)] 119889119905 + 119885 (119905) 119889119882 (119905)
119884 (119879) = 120585 119905 isin [0 119879]
(136)
Combining the above equation with (133) we obtain thefollowing related feedback control system
119889119884 (119905) = minus119860119884 (119905) 119889119905
minus 119861119884 (119905) + 119861E [119884 (119905)] minus1198622
119873119875 (119905)
+ 119863119885 (119905) + 119863E [119885 (119905)] 119889119905 + 119885 (119905) 119889119882 (119905)
119884 (119879) = 120585
119889119875 (119905) = 119860lowast119875 (119905) 119889119905 + 119861119875 (119905) + 119861E [119875 (119905)] 119889119905
+ 119863119875 (119905) + 119863E [119875 (119905)] 119889119882 (119905)
119875 (0) = 119884 (0)
(137)
Looking at the terminal condition of 119875(119905) in (133) andconsidering the mean-field type of (132) it is reasonable toconjecture that 119875(119905) has the following form
119875 (119905) = 120593 (119905) 119884 (119905) + 120601 (119905)E [119884 (119905)] (138)
where120593(119905)120601(119905) are deterministic differential functionswhichwill be specified below Moreover 120593(0) = 1 120601(0) = 0
Inserting this form into adjoint equation (133) and notic-ing that 119884(119905) satisfies (136) we can compare the coefficientsof 119889119905 and 119889119882(119905) to obtain the following equation
2120593 (119905) 119860119884 (119905) + 2120601 (119905) 119860E [119884 (119905)] + 119861120593 (119905) 119884 (119905)
+ 2 (119861120601 (119905) + 119861120593 (119905) + 119861120601 (119905))E [119884 (119905)]
= minus120593 (119905) 119861119884 (119905) minus1198622
119873120593 (119905) 119884 (119905) + 119863119885 (119905)
+ 119884 (119905)119889120593 (119905)
119889119905+ E [119884 (119905)]
119889120601 (119905)
119889119905
+ (1206012
(119905)1198622
119873+ 2
1198622
119873120593 (119905) 120601 (119905))E [119884 (119905)]
minus (119863120601 (119905) + 119863120601 (119905) + 119863120593 (119905))E [119885 (119905)]
120593 (119905) 119885 (119905) = 119863120593 (119905) 119884 (119905)
+ (119863120601 (119905) + 119863120593 (119905) + 119863120601 (119905))E [119884 (119905)]
(139)
Then subtracting 119885(119905) we have
2120593 (119905) 119860119884 (119905) + 2120601 (119905) 119860E [119884 (119905)] + 119861120593 (119905) 119884 (119905)
+ 2 (119861120601 (119905) + 119861120593 (119905) + 119861120601 (119905))E [119884 (119905)]
18 Mathematical Problems in Engineering
= 120593 (119905) minus119861 +1198622
119873120593 (119905) minus 119863
2119884 (119905)
+ 119884 (119905)119889120593 (119905)
119889119905+ E [119884 (119905)]
119889120601 (119905)
119889119905
+ (1206012
(119905)1198622
119873+ 2
1198622
119873120593 (119905) 120601 (119905))E [119884 (119905)]
minus1
120593 (119905)(119863120601 (119905) + 119863120593 (119905) + 119863120601 (119905))
2
E [119884 (119905)]
minus 2119863 (119863120601 (119905) + 119863120593 (119905) + 119863120601 (119905))E [119884 (119905)]
(140)
Comparing the coefficients of 119884(119905) and E[119884(119905)] respec-tively we get
119889
119889119905120593 (119905) = 2119860
lowast120593 (119905) + 2119861120593 (119905) + 119863
2120593 (119905) minus
1198622
1198731205932
(119905)
120593 (0) = 1
(141)
119889
119889119905120601 (119905) = 2119860
lowast120601 (119905) + (
(119863 + 119863)2
120593 (119905)minus1198622
119873)120601
2
(119905)
+ 2119877 (119905) 120601 (119905) + (1198632+ 2119861 + 2119863119863)120593 (119905)
120601 (0) = 0
(142)
where 119877(119905) = 119861 + 119861 + (119863 + 119863)2
minus (1198622119873)120593(119905)
We solve (141) to get
120593 (119905) = (119890minus(2119860lowast+2119861+119863
2)119905(1 minus
1198622
119873(2119860lowast + 2119861 + 1198632))
+1198622
119873(2119860lowast + 2119861 + 1198632))
minus1
(143)
Then (142) exists a unique solution from the classical Riccatiequation theory
We now conclude the above discussions in the followingresult
Theorem 23 For onersquos linear quadratic stochastic partialdifferential control problem (131)-(132) the unique optimalcontrol 119906(sdot) isin 119880ad is given by
119906 (119905) = minus119862
119873(120593 (119905) 119884 (119905) + 120601 (119905)E [119884 (119905)]) (144)
with 120593(119905) satisfying (143) and 120601(119905) solving (142)
Conflict of Interests
The authors declare that they have no conflict of interestsregarding the publication of this paper
Acknowledgment
This research is partially supported by the Natural ScienceFoundation of China under Grant no 11301310
References
[1] Y Hu and S Peng ldquoAdapted solution of a backward semilinearstochastic evolution equationrdquo Stochastic Analysis and Applica-tions vol 9 no 4 pp 445ndash459 1991
[2] N IMahmudov andMAMcKibben ldquoOnbackward stochasticevolution equations in Hilbert spaces and optimal controlrdquoNonlinear Analysis vol 67 no 4 pp 1260ndash1274 2007
[3] M Fuhrman and G Tessitore ldquoNonlinear Kolmogorov equa-tions in infinite dimensional spaces the backward stochasticdifferential equations approach and applications to optimalcontrolrdquoThe Annals of Probability vol 30 no 3 pp 1397ndash14652002
[4] M Fuhrman and G Tessitore ldquoInfinite horizon backwardstochastic differential equations and elliptic equations inHilbert spacesrdquo Annals of Probability vol 32 no 1B pp 607ndash660 2004
[5] X Mao ldquoAdapted solutions of backward stochastic differentialequations with non-Lipschitz coefficientsrdquo Stochastic Processesand their Applications vol 58 no 2 pp 281ndash292 1995
[6] J Lasry and P Lions ldquoMean field gamesrdquo Japanese Journal ofMathematics vol 2 no 1 pp 229ndash260 2007
[7] R Buckdahn J Li and S Peng ldquoMean-field backward stochas-tic differential equations and related partial differential equa-tionsrdquo Stochastic Processes and their Applications vol 119 no 10pp 3133ndash3154 2009
[8] D Andersson and B Djehiche ldquoAmaximum principle for SDEsofmean-field typerdquoAppliedMathematics andOptimization vol63 no 3 pp 341ndash356 2011
[9] R Buckdahn B Djehiche J Li and S Peng ldquoMean-fieldbackward stochastic differential equations a limit approachrdquoThe Annals of Probability vol 37 no 4 pp 1524ndash1565 2009
[10] R Xu ldquoMean-field backward doubly stochastic differentialequations and related SPDEsrdquo Boundary Value Problems vol2012 article 114 2012
[11] R Buckdahn B Djehiche and J Li ldquoA general stochasticmaximum principle for SDEs of mean-field typerdquo AppliedMathematics and Optimization vol 64 no 2 pp 197ndash216 2011
[12] J Li ldquoStochasticmaximumprinciple in themean-field controlsrdquoAutomatica vol 48 no 2 pp 366ndash373 2012
[13] G Wang C Zhang and W Zhang ldquoStochastic maximumprinciple for mean-field type optimal control under partialinformationrdquo IEEE Transactions on Automatic Control vol 59no 2 pp 522ndash528 2014
[14] M Hafayed ldquoA mean-field necessary and sufficient conditionsfor optimal singular stochastic controlrdquo Communications inMathematics and Statistics vol 1 no 4 pp 417ndash435 2013
[15] M Hafayed ldquoA mean-field maximum principle for optimalcontrol of forward-backward stochastic differential equationswith Poisson jumpprocessesrdquo International Journal ofDynamicsand Control vol 1 no 4 pp 300ndash315 2013
[16] Y Hu and S Peng ldquoMaximum principle for semilinearstochastic evolution control systemsrdquo Stochastics and StochasticsReports vol 33 no 3-4 pp 159ndash180 1990
[17] M Fuhrman Y Hu and G Tessitore ldquoStochastic maximumprinciple for optimal control of SPDEsrdquo Comptes Rendus Math-ematique vol 350 no 13-14 pp 683ndash688 2012
Submit your manuscripts athttpwwwhindawicom
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Mathematical Problems in Engineering
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Stochastic AnalysisInternational Journal of
Mathematical Problems in Engineering 9
Then
E sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899+1 (119904) minus 119884119899 (119904)
10038161003816100381610038162
le max1le119896le119898
E sup119904isin[120591119896minus1120591119896]
11989021205731199041003816100381610038161003816119884119899+1 (119904) minus 119884119899 (119904)
10038161003816100381610038162
= max1le119896le119898
120593119896119899(119905) 997888rarr 0
(56)
as 119899 rarr infin and this proves the assertion (49)By (36) we obtain
E( sup119905le119904le119879
11989021205731199041003816100381610038161003816119884119899+1 (119904) minus 119884119899 (119904)
10038161003816100381610038162
) + (1 minus24119872
2
119860119897
120573)E
times int
119879
119905
11989021205731199041003816100381610038161003816119885119899+1
(119904) minus 119885119899(119904)10038161003816100381610038162
119889119904
le24119872
2
119860
120573Eint
119879
119905
1198902120573119904
[120579 (1003816100381610038161003816119884119899 (119904) minus 119884119899minus1 (119904)
10038161003816100381610038162
)] 119889119904
(57)
Applying (49) to the above formula we see that (119884119899 119885
119899) is
a Cauchy (hence convergent) sequence in S2
F ([0 119879]119867) times
H2
F ([0 119879]L(Γ119867)) denote the limit by (119884 119885) Now letting119899 rarr infin in (27) we obtain that
119884 (119905) = 119890119860(119879minus119905)
120585
+ int
119879
119905
E1015840[119890
119860(119904minus119905)119891 (119904 119884
1015840
(119904) 1198851015840
(119904) 119884 (119904) 119885 (119904))] 119889119904
minus int
119879
119905
119890119860(119904minus119905)
119885 (119904) 119889119882 (119904)
(58)
holds on the entire interval [0 119879]The theorem is nowproved
To illustrate the application of the obtained existenceand uniqueness result we consider the example of backwardstochastic partial differential equations (BSPDEs) of mean-field type
Example 9 Let O be an open bounded domain in R119899
with uniformly 1198622 boundary 120597O let 119861(119905) be a standard 119899-dimensional Brownian motion (equipped with the normalfiltration) and let 120585 O rarr R be an F
119879-measurable
random variable We also let 119871 denote the semielliptic partialdifferential operator on 1198622
(R) of the form
119871 =
119899
sum
119894119895=1
119886119894119895(119909)
1205972
120597119909119894120597119909
119895
+
119899
sum
119894=1
119887119894(119909)
120597
120597119909119894
(59)
The aim is to study the solvability of the following initialboundary value problem
119889119884 (119905 119909)
= (119871119884 (119905 119909) + E1015840
times [119892 (119905 119909 1198841015840
(119905 119909) 1198851015840
(119905 119909) 119884 (119905 119909) 119885 (119905 119909))]) 119889119905
+ 119885 (119905 119909) 119889119861 (119905) ae on (0 119879) times O
119884 (119905 119909) = 0 ae on (0 119879) times 120597O
119884 (119879 119909) = 120585 (119879 119909) ae onO(60)
where
119884 [0 119879] times O 997888rarr R
119885 [0 119879] times O 997888rarr L (R119899 119871
2
(O))
119892 [0 119879] times O timesR timesL (R119899 119871
2
(O))
timesR timesL (R119899 119871
2
(O)) 997888rarr R
(61)
The following assumptions will have to be in force
(H1) 119886119894119895 119887
119894 R119899
rarr R are uniformly continuous andbounded and satisfy the usual uniform ellipticitycondition sum119899
119894119895=1119886119894119895(119909)119908
119894119908119895ge 120582|119908|
2 for some 120582 gt 0
and all 119909 isin O 119908 isin R119899(H2) 119892 is measurable in (119905 119909 119910 119910 119911) and continuous in
( 119911) and there exists 119862 gt 0 such that1003816100381610038161003816119892 (119905 119909 1199101 1 1199101 1199111) minus 119892 (119905 119909 1199102 2 1199102 1199112)
1003816100381610038161003816
le 119862 [10038161003816100381610038161199101 minus 1199102
1003816100381610038161003816 +10038161003816100381610038161 minus 2
1003816100381610038161003816 +10038161003816100381610038161199101 minus 1199102
1003816100381610038161003816 +10038161003816100381610038161199111 minus 1199112
1003816100381610038161003816]
(62)
for all 0 le 119905 le 119879 119909 isin O 1199101 119910
2 119910
1 119910
2isin R
1
2 119911
1 119911
2isin
L(R119899 119871
2(O))
Then we are now in a position of showing existence anduniqueness of the solution of BSPDEs (60)
Theorem 10 If (H1) and (H2) are satisfied then the mean-field BSPDE (60) has a unique mild solution (119884 119885) isin
1198712(0 119879 119871
2(Ω 119871
2(O))) times 1198712F (0 119879 119871
2(R119899
1198712(Ω 119871
2(O))))
Proof Let119867 = 1198712(O) and119870 = R119899 Define the operator 119860 by
119860119884 (119905 sdot) = 119871119884 (119905 sdot) (63)
It is shown in [17] (see Example 21 in [17]) that 119860 generatesa strongly continuous semigroup on 119867 Define the maps 119891
[0 119879] times 119867 timesL(119870119867) times 119867 timesL(119870119867) rarr 119867 by
119891 (119905 1198841015840
(119905) 1198851015840
(119905) 119884 (119905) 119885 (119905)) (119909)
= 119892 (119905 119909 1198841015840
(119905 119909) 1198851015840
(119905 119909) 119884 (119905 119909) 119885 (119905 119909))
(64)
10 Mathematical Problems in Engineering
for all 0 le 119905 le 119879 119909 isin O With these identifications(60) can be written in the form of (11) By (H2) we know119891 satisfy condition (A1) Hence an application of Theorem 4concludes that (60) has a unique mild solution (119884 119885) isin
1198712(0 119879 119871
2(Ω 119871
2(O))) times 1198712F (0 119879 119871
2(R119899
1198712(Ω 119871
2(O))))
4 Mean-Field Stochastic Evolution Equations
Let 119882(119905) 119905 isin [0 119879] be a cylindrical Wiener process withvalues in a Hilbert space Γ defined on a probability space(ΩFP) We fix an interval [119905 119879] sub [0 119879] and considerthe stochastic evolution equations of mean-field type for anunknown process 119883(119904) 119904 isin [119905 119879] with values in a Hilbertspace 119870
119889119883 (119904) = 119861119883 (119904) 119889119904 + E1015840[119887 (119904 119883
1015840
(119904) 119883 (119904))] 119889119904
+ E1015840[120590 (119904 119883
1015840
(119904) 119883 (119904))] 119889119882 (119904)
119883 (119905) = 119909 isin 119870
(65)
where operator 119861 is the generator of a strongly continuoussemigroup 119890
119905119861 119905 ge 0 in the Hilbert space 119870 with 119872119861≜
sup119905isin[0119879]
|119890119905119861|
By a mild solution of (65) we mean an F119904-measurable
process 119883(119904) 119904 isin [119905 119879] with continuous paths in 119870 suchthat P-as
119883 (119904) = 119890119861(119904minus119905)
119909
+ int
119904
119905
119890119861(119904minus120591)
E1015840[119887 (120591 119883
1015840
(120591) 119883 (120591))] 119889120591
+ int
119904
119905
119890119861(119904minus120591)
E1015840[120590 (120591119883
1015840
(120591) 119883 (120591))] 119889119882 (120591)
119904 isin [119905 119879]
(66)
We suppose the following(A4) 119887 [0 119879] times 119870 times 119870 rarr 119870 is a measurable mapping
which satisfies10038161003816100381610038161003816119887 (119905 119909
1015840 119909) minus 119887 (119905 119910
1015840 119910)
10038161003816100381610038161003816
2
le 1198711(100381610038161003816100381610038161199091015840minus 119910
101584010038161003816100381610038161003816
2
+1003816100381610038161003816119909 minus 119910
10038161003816100381610038162
)
119905 isin [0 119879] 1199091015840 119909 119910
1015840 119910 isin 119870
(67)
for some constant 1198711gt 0
(A5) The mapping 120590 [0 119879] times 119870 times 119870 rarr L(Γ 119870) fulfillsthat for every V isin Γ the map 120590V [0 119879] times 119870 times 119870 rarr 119870
is measurable for every 119904 gt 0 119905 isin [0 119879] 1199091015840 1199101015840 119909 119910 isin 119870119890119904119861120590(119905 119909
1015840 119909) isin L(Γ 119870) and
10038161003816100381610038161003816119890119904119861120590 (119905 119909
1015840 119909)
10038161003816100381610038161003816L(Γ119870)le 119871
2119904minus120574(1 +
10038161003816100381610038161003816119909101584010038161003816100381610038161003816+ |119909|)
10038161003816100381610038161003816119890119904119861120590 (119905 119909
1015840 119909) minus 119890
119904119861120590 (119905 119910
1015840 119910)
10038161003816100381610038161003816L(Γ119870)
le 1198712119904minus120574(100381610038161003816100381610038161199091015840minus 119910
101584010038161003816100381610038161003816+1003816100381610038161003816119909 minus 119910
1003816100381610038161003816)
10038161003816100381610038161003816120590 (119905 119909
1015840 119909)
10038161003816100381610038161003816L(Γ119870)le 119871
2(1 +
10038161003816100381610038161003816119909101584010038161003816100381610038161003816+ |119909|)
(68)
for some constants 1198712gt 0 and 120574 isin [0 12)
Theorem 11 Under assumptions (A3) and (A4) (65) has aunique mild solution119883(sdot) isin S2
F ([119905 119879] 119870)
The proof is constructed in two steps like that ofTheorem 4 and it uses standard arguments for stochastic evo-lution equations introduced in the proof of Proposition 32 in[3] Since the proof is straightforward we prefer to omit it
Remark 12 In our paper Lipchitz condtion (A4) is givento get the well-posedness of mean-field stochastic evolutionequations In fact (A4) can be replaced by a weaker conditionsuch as (A3) We just give the condition (A4) for simplicity
From standard arguments we can also get the followingcontinuous dependence theorem
Corollary 13 Assume that for all 120572 in a metric space 119865(119887120572 120590
120572) satisfy (A4) and (A5)with119871
1and119871
2independent of 120572
Also assume that
Eint119879
0
10038161003816100381610038161003816E1015840[119887120572(119904 119883
1015840
(119904) 119883 (119904))]
minus E1015840[1198871205720(119904 119883
1015840
(119904) 119883 (119904))]10038161003816100381610038161003816
2
119889119904 997888rarr 0
Eint119879
0
10038161003816100381610038161003816E1015840[120590
120572(119904 119883
1015840
(119904) 119883 (119904))]
minusE1015840[120590
1205720(119904 119883
1015840
(119904) 119883 (119904))]10038161003816100381610038161003816
2
119889119904 997888rarr 0
(69)
as 120572 rarr 1205720for all119883 isin S2
F ([0 119879] 119870)If we denote by 119883120572
(sdot) the mild solution of mean-field SEE(65) corresponding to the functions (119887
120572 120590
120572) and to the initial
data 119909 then we have
sup119904isin[0119879]
E1003816100381610038161003816119883
120572
(119904) minus 1198831205720 (119904)
10038161003816100381610038162
997888rarr 0 119886119904 120572 997888rarr 1205720 (70)
5 Maximum Principle for BSPDEs ofMean-Field Type
51 Formulation of the Problem Let O isin R119899 be a boundedopen set with smooth boundary 120597O and let 119880 the space ofcontrols be a separable real Hilbert space We denote
U = V (sdot) isin 1198712F(0 119879 119880)
| V119905(120596
1015840 120596) [0 119879] times Ω times Ω
997888rarr 119880 is F otimesF119905-progressively measurable
(71)
Mathematical Problems in Engineering 11
An element ofU is called an admissible controlFor any V isin U we consider the following controlled
BSPDE system in the state space119867 = 1198712(O) (norm | sdot | scalar
product ⟨sdot sdot⟩)
119889119884119905(119909) = minus119860119884
119905(119909) 119889119905
minus E1015840[119891 (119905 119909 (119884
119905(119909))
1015840
(119885119905(119909))
1015840
119884119905(119909) 119885
119905(119909) V
119905)] 119889119905
+ 119885119905(119909) 119889119882 (119905) 119905 isin [0 119879]
119884119879(119909) = 120585 (119909) 119909 isin O
(72)
where 119860 is a partial differential operator 119891 [0 119879] timesO times119867 times
L(Γ119867)times119867timesL(Γ119867)times119880 rarr 119867 and 120585 isin 1198712(ΩF119879P 119867)
The cost functional is given by
119869 (V) = Eint119879
0
intO
E1015840[ℎ (119904 119909 (119884
119904(119909))
1015840
(119885119904(119909))
1015840
119884119904(119909) 119885
119904(119909) V
119904)] 119889119909 119889119904
+E1015840intO
119892 (119909 (1198840(119909))
1015840
1198840(119909)) 119889119909
(73)
where
ℎ [0 119879] times O times 119867 timesL (Γ119867) times 119867 timesL (Γ119867) times 119880 997888rarr R
119892 O times 119867 times119867 997888rarr R
(74)
Our purpose is to minimize the functional 119869(sdot) over Uadsubject to the following state constraint
EintO
Φ(119909 (1198840(119909))
1015840
1198840(119909)) 119889119909 = 0 (75)
where
Φ O times 119867 times119867 997888rarr R (76)
An admissible control 119906 isin Uad that satisfies
119869 (119906) = minVisinUad
119869 (V) (77)
is called optimalThrough what follows the following assumptions will be
in force
(L1) 119860 is a partial differential operator with appropri-ate boundary conditions We assume that 119860 is theinfinitesimal generator of a strongly continuous semi-group 119890119905119860 119905 ge 0 in119867 Moreover for every 119905 isin [0 119879]119890
119905119860119891
1198712(O) le 119872
119860119891
1198712(O) for some constant 119872
119860
independent of 119905 and 119891(L2) 119891 ℎ 119892 and Φ are continuously Gateaux differen-
tiable with respect to (1199101015840 1199111015840 119910 119911) 119891 is continuouslyGateaux differentiable with respect to V and ℎ iscontinuous with respect to V
(L3) The derivatives of 119891 ℎ 119892 and Φ are Lipschitzcontinuous and bounded by
100381610038161003816100381610038161198911199101015840
10038161003816100381610038161003816+10038161003816100381610038161198911199111015840
1003816100381610038161003816 +10038161003816100381610038161003816119891119910
10038161003816100381610038161003816+1003816100381610038161003816119891119911
1003816100381610038161003816 +1003816100381610038161003816119891V
1003816100381610038161003816 +10038161003816100381610038161003816Φ1199101015840
10038161003816100381610038161003816+10038161003816100381610038161003816Φ119910
10038161003816100381610038161003816le 119862
10038161003816100381610038161003816ℎ1199101015840
10038161003816100381610038161003816+1003816100381610038161003816ℎ1199111015840
1003816100381610038161003816 +10038161003816100381610038161003816ℎ119910
10038161003816100381610038161003816+1003816100381610038161003816ℎ119911
1003816100381610038161003816 +100381610038161003816100381610038161198921199101015840
10038161003816100381610038161003816+10038161003816100381610038161003816119892119910
10038161003816100381610038161003816
le 119862 (1 +10038161003816100381610038161003816119910101584010038161003816100381610038161003816+10038161003816100381610038161003816119911101584010038161003816100381610038161003816+10038161003816100381610038161199101003816100381610038161003816 + |119911| + |V|)
(78)
where 119862 is a positive constant
Obviously according to Theorem 4 state equation (72)has a unique mild solution under the above assumptions
Remark 14 We can define the second order differentialoperator
(119860119891) (119909) =
119899
sum
119894119895=1
119886119894119895(119909)
1205972119891
120597119909119894120597119909
119895
(119909) +
119899
sum
119894=1
119887119894(119909)
120597119891
120597119909119894
(119909) (79)
By Example 9 119860 fulfills assumption (L1) if 119886119894119895 119887
119894satisfy
condition (H1)
52 Variation of the Trajectory Let 119906 be an optimal controlwith (119884(sdot) 119885(sdot)) being the corresponding optimal state Let120576 gt 0 and [119903 119903 + 120576] sube [0 119879] For any given V isin Uad weintroduce the spike variation of the control 119906(sdot)
119906120576
119905=
V119905 119905 isin [119903 119903 + 120576]
119906119905 119905 isin [0 119879] [119904 119904 + 120576]
(80)
It is clear that 119906120576(sdot) isin UadLet (119884120576
(sdot) 119885120576(sdot)) be the trajectory corresponding to 119906120576(sdot)
We use the following short notation for brevity
119891 (119905) = 119891 (119905 119909 (119884119905(119909))
1015840
(119885119905(119909))
1015840
119884119905(119909) 119885
119905(119909) 119906
119905)
119891 (119906120576
119905) = 119891 (119905 119909 (119884
119905(119909))
1015840
(119885119905(119909))
1015840
119884119905(119909) 119885
119905(119909) 119906
120576
119905)
(81)
Consider the following equation
119889119870120576
119905(119909) = minus119860119870
120576
119905(119909) 119889119905
minus E1015840[119891
1199101015840 (119905) (119870
120576
119905(119909))
1015840
+ 119891119910(119905) 119870
120576
119905(119909)
+ 1198911199111015840 (119905) (119876
120576
119905(119909))
1015840
+ 119891119911(119905) 119876
120576
119905(119909)
+1
120576(119891 (119906
120576
119905) minus 119891 (119905))] 119889119905 + 119876
120576
119905(119909) 119889119882 (119905)
119870120576
119879(119909) = 0
(82)
Since the coefficients in (82) are bounded it is easy to checkthat there exists a unique mild solution such that
E[ sup119905isin[0119879]
1003816100381610038161003816119870120576
119905
10038161003816100381610038162
+ int
119879
0
1003816100381610038161003816119876120576
119905
10038161003816100381610038162
119889119905] lt infin (83)
We have the following estimate
12 Mathematical Problems in Engineering
Theorem 15 There holds
lim120576rarr0
E[ sup119904isin[119905119879]
1003816100381610038161003816100381610038161003816100381610038161003816
119884120576
119904minus 119884
119904
120576minus 119870
120576
119904
1003816100381610038161003816100381610038161003816100381610038161003816
2
] = 0 forall119905 isin [0 119879]
lim120576rarr0
Eint119879
0
1003816100381610038161003816100381610038161003816100381610038161003816
119885120576
119904minus 119885
119904
120576minus 119876
120576
119904
1003816100381610038161003816100381610038161003816100381610038161003816
2
119889119904 = 0
(84)
Proof We define
120578120576
119904=119884120576
119904minus 119884
119904
120576minus 119870
120576
119904 120577
120576
119904=119885120576
119904minus 119885
119904
120576minus 119876
120576
119904 119904 isin [0 119879]
(85)
For simplicity let us define
Λ120576
119904= ((119884
120576
119904)1015840
(119885120576
119904)1015840
119884120576
119904 119885
120576
119904)
119891 (119904 120582) = 119891 (119904 1198841015840
119904+ 120582(119884
120576
119904minus 119884
119904)1015840
1198851015840
119904+ 120582(119885
120576
119904minus 119885
119904)1015840
119884119904+ 120582 (119884
120576
119904minus 119884
119904)
119885119904+ 120582 (119885
120576
119904minus 119885
119904) 119906
120576
119904)
(86)
By the definition of (119884120576
119904 119885
120576
119904) (119884
119904 119885
119904) and (119870120576
119904 119876
120576
119904) (120578120576
119904 120577
120576
119904)
is the mild solution of
119889120578120576
119904= minus119860120578
120576
119904119889119904 minus E
1015840
[119871 (119904 120576)] 119889119904 + 120577120576
119904119889119882 (119904)
120578120576
119879= 0
(87)
with
119871 (119904 120576) =1
120576(119891 (119904 Λ
120576
119904 119906
120576
119904) minus 119891 (119906
120576
119904))
minus 1198911199101015840 (119904) (119870
120576
119904)1015840
minus 119891119910(119904) 119870
120576
119904minus 119891
1199111015840 (119904) (119876
120576
119904)1015840
minus 119891119911(119904) 119876
120576
119904
= ((120578120576
119904)1015840
+ (119870120576
119904)1015840
) int
1
0
1198911199101015840 (119904 120582) 119889120582 + (120578
120576
119904+ 119870
120576
119904)
times int
1
0
119891119910(119904 120582) 119889120582 minus 119891
1199101015840 (119904) (119870
120576
119904)1015840
minus 119891119910(119904) 119870
120576
119904
+ ((120577120576
119904)1015840
+ (119876120576
119904)1015840
)int
1
0
1198911199111015840 (119904 120582) 119889120582 + (120577
120576
119904+ 119876
120576
119904)
times int
1
0
119891119911(119904 120582) 119889120582 minus 119891
1199111015840 (119904) (119876
120576
119904)1015840
minus 119891119911(119904) 119876
120576
119904
= (120578120576
119904)1015840
int
1
0
1198911199101015840 (119904 120582) 119889120582 + 120578
120576
119904int
1
0
119891119910(119904 120582) 119889120582
+ (120577120576
119904)1015840
int
1
0
1198911199111015840 (119904 120582) 119889120582 + 120577
120576
119904int
1
0
119891119911(119904 120582) 119889120582 + 120574
120576
119904
(88)
where we denote
120574120576
119904= (119870
120576
119904)1015840
int
1
0
(1198911199101015840 (119904 120582) minus 119891
1199101015840 (119904)) 119889120582
+ 119870120576
119904int
1
0
(119891119910(119904 120582) minus 119891
119910(119904)) 119889120582
+ (119876120576
119904)1015840
int
1
0
(1198911199111015840 (119904 120582) minus 119891
1199111015840 (119904)) 119889120582
+ 119876120576
119904int
1
0
(119891119911(119904 120582) minus 119891
119911(119904)) 119889120582
(89)
For any 120573 gt 0 according to Lemma 1 we obtain
E sup119905le119904le119879
11989021205731199041003816100381610038161003816120578
120576
119904
10038161003816100381610038162
+ Eint119879
119905
11989021205731199041003816100381610038161003816120577
120576
119904
10038161003816100381610038162
119889119904
le12119872
2
119860
120573int
119879
119905
1198902120573119904
E [10038161003816100381610038161003816E1015840
[119871 (119904 120576)]10038161003816100381610038161003816
2
] 119889119904
le12119872
2
119860
120573Eint
119879
119905
1198902120573119904
E1015840[|119871 (119904 120576)|
2] 119889119904
(90)
By condition (L3) we have
E [E1015840[|119871 (119904 120576)|
2]]
= E [E1015840[1003816100381610038161003816119871 (119904 120576) minus 120574
120576
119904+ 120574
120576
119904
10038161003816100381610038162
]]
le 81198622E [E
1015840[10038161003816100381610038161003816(120578
120576
119904)101584010038161003816100381610038161003816
2
+1003816100381610038161003816120578120576
119904
10038161003816100381610038162
+10038161003816100381610038161003816(120577
120576
119904)101584010038161003816100381610038161003816
2
+1003816100381610038161003816120577120576
119904
10038161003816100381610038162
]]
+ 2E [E1015840[1003816100381610038161003816120574120576
119904
10038161003816100381610038162
]]
le 161198622E [
1003816100381610038161003816120578120576
119904
10038161003816100381610038162
+1003816100381610038161003816120577120576
119904
10038161003816100381610038162
] + 2E [E1015840[1003816100381610038161003816120574120576
119904
10038161003816100381610038162
]]
(91)
Combined with (91) (90) yields
E sup119905le119904le119879
11989021205731199041003816100381610038161003816120578
120576
119904
10038161003816100381610038162
+ (1 minus192119872
2
1198601198622
120573)Eint
119879
119905
11989021205731199041003816100381610038161003816120577
120576
119904
10038161003816100381610038162
119889119904
le192119872
2
1198601198622
120573Eint
119879
119905
11989021205731199041003816100381610038161003816120578
120576
119904
10038161003816100381610038162
119889119904
+24119872
2
119860
120573Eint
119879
119905
1198902120573119904
[E1015840[1003816100381610038161003816120574120576
119904
10038161003816100381610038162
]] 119889119904
(92)
We claim that
Eint119879
119905
E1015840[1003816100381610038161003816120574120576
119904
10038161003816100381610038162
] 119889119904 997888rarr 0 as 120576 997888rarr 0 (93)
From (89)
120574120576
119904= 119868
1
119904+ 119868
2
119904+ 119868
3
119904+ 119868
4
119904 (94)
Mathematical Problems in Engineering 13
where
1198681
119904= (119870
120576
119904)1015840
int
1
0
(1198911199101015840 (119904 120582) minus 119891
1199101015840 (119904)) 119889120582
1198682
119904= 119870
120576
119904int
1
0
(119891119910(119904 120582) minus 119891
119910(119904)) 119889120582
1198683
119904= (119876
120576
119904)1015840
int
1
0
(1198911199111015840 (119904 120582) minus 119891
1199111015840 (119904)) 119889120582
1198684
119904= 119876
120576
119904int
1
0
(119891119911(119904 120582) minus 119891
119911(119904)) 119889120582
(95)
Then
Eint119879
119905
E1015840[1003816100381610038161003816120574120576
119904
10038161003816100381610038162
] 119889119904
le 4Eint119879
119905
E1015840[100381610038161003816100381610038161198681
119904
10038161003816100381610038161003816
2
+100381610038161003816100381610038161198682
119904
10038161003816100381610038161003816
2
+100381610038161003816100381610038161198683
119904
10038161003816100381610038161003816
2
+100381610038161003816100381610038161198684
119904
10038161003816100381610038161003816
2
] 119889119904
(96)
Take Eint119879119905E1015840[|1198682
119904|2
]119889119904 for example
Eint119879
119905
E1015840[100381610038161003816100381610038161198682
119904
10038161003816100381610038161003816
2
] 119889119904
= Eint119879
119905
E1015840[(119870
120576
119904int
1
0
(119891119910(119904 120582) minus 119891
119910(119904)) 119889120582)
2
]119889119904
le Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119904 120582) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582] 119889119904
le 2Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119904 120582) minus 119891
119910(119906
120576
119905)10038161003816100381610038161003816
2
119889120582] 119889119904
+ 2Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119906
120576
119905) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582] 119889119904
(97)
Note that
Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119906
120576
119905) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582] 119889119904
= Eint119903+120576
119903
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119906
120576
119905) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582] 119889119904
le sup119904isin[119905119879]
E [E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119906
120576
119905) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582]] 120576
997888rarr 0 as 120576 997888rarr 0
(98)
The inequality above holds due to the boundedness of|119870
120576
119904|2
int1
0|119891119910(119906
120576
119905) minus 119891
119910(119904)|
2
119889120582 Indeed Assumption (L3) im-plies the boundedness of 119891
119910(119906
120576
119905) minus 119891
119910(119904) Meanwhile 119870120576
119904is
the solution of mean-field BSEE (82) It can be easy to check119870120576
119904is bounded since the coefficients in (82) are boundedOn the other hand
Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119904 120582) minus 119891
119910(119906
120576
119905)10038161003816100381610038161003816
2
119889120582] 119889119904
le 11986221205822Eint
119879
119905
1003816100381610038161003816119870120576
119904
10038161003816100381610038162
times int
1
0
1198641015840[100381610038161003816100381610038161003816(119884
120576
119904minus 119884
119904)1015840100381610038161003816100381610038161003816
2
+100381610038161003816100381610038161003816(119885
120576
119904minus 119885
119904)1015840100381610038161003816100381610038161003816
2
times10038161003816100381610038161003816119884120576
119904minus 119884
119904
10038161003816100381610038161003816
2
+10038161003816100381610038161003816119885120576
119904minus 119885
119904
10038161003816100381610038161003816
2
] 119889120582 119889119904
(99)
where (119884120576
119904 119885
120576
119904) is the mild solution of the following equation
119889119884120576
119905= minus 119860119884
120576
119905119889119905
minus E1015840[119891 (119905 (119884
120576
119905)1015840
(119885120576
119905)1015840
119884120576
119905 119885
120576
119905 119906
120576
119905)] 119889119905
+ 119885120576
119905119889119882 (119905) 119905 isin [0 119879]
119884120576
119879= 120585
(100)
and (119884119904 119885
119904) is the mild solution of
119889119884119905= minus 119860119884
119905119889119905
minus E1015840[119891 (119905 (119884
119905)1015840
(119885119905)1015840
119884119905 119885
119905 119906
119905)] 119889119905
+ 119885119905119889119882 (119905) 119905 isin [0 119879]
119884119879= 120585
(101)
By the definition of 119906120576119905 according to (L2) we have
E1015840[119891 (119904 119884
1015840
(119904) 1198851015840
(119904) 119884 (119904) 119885 (119904) 119906120576
119905)]
997888rarr E1015840[119891 (119904 119884
1015840
(119904) 1198851015840
(119904) 119884 (119904) 119885 (119904) 119906119905)]
(102)
in 1198712([0 119879]119867) as 120576 rarr 0 Using the continuous dependencetheorem Corollary 5 we obtain
(119884120576
119904 119885
120576
119904) 997888rarr (119884
119904 119885
119904) as 120576 997888rarr 0 (103)
Then
Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119904 120582) minus 119891
119910(119906
120576
119905)10038161003816100381610038161003816
2
119889120582] 119889119904
le 11986221205822Eint
119879
119905
1003816100381610038161003816119870120576
119904
10038161003816100381610038162
times int
1
0
E1015840[100381610038161003816100381610038161003816(119884
120576
119904minus 119884
119904)1015840100381610038161003816100381610038161003816
2
+100381610038161003816100381610038161003816(119885
120576
119904minus 119885
119904)1015840100381610038161003816100381610038161003816
2
times10038161003816100381610038161003816119884120576
119904minus 119884
119904
10038161003816100381610038161003816
2
+10038161003816100381610038161003816119885120576
119904minus 119885
119904
10038161003816100381610038161003816
2
] 119889120582 119889119904
997888rarr 0 as 120576 997888rarr 0
(104)
14 Mathematical Problems in Engineering
Combining (98) with (104) we finally haveEint
119879
119905E1015840[|1198682
119904|2
]119889119904 rarr 0 as 120576 rarr 0The required result (93) follows by using the similar
estimations for 1198681119904 1198683
119904 and 1198684
119904
Note that 1 minus 1921198722
1198601198622120573 gt 0 if 120573 is sufficiently large
Now to prove the desired result (84) it suffices to applyGronwallrsquos lemma and estimate (93) to inequality (92)
To deal with the state constraint (75) we need to recall theEkeland variational principle
Lemma 16 (Ekelandrsquos variational principle see [16 Lemma41]) Let (119878 119889) be a complete metric space and let 119865(sdot) 119878 rarr
R be lower semicontinuous and bounded from below If for 120588 gt0 there exists 119906 isin 119878 such that
119865 (119906) le infVisin119878
119865 (V) + 120588 (105)
then there exists 119906120588 isin 119878 satisfying
(i) 119865 (119906120588) le 119865 (119906)
(ii) 119889 (119906120588 119906) le 120588
(iii) 119865 (119906120588) le 119865 (V) + 120588 sdot 119889 (119906120588 119906) forallV = 119906120588
(106)
Now fix V isin Uad and set
119878 = V (sdot) isin Uad | sup0le119905le119879
E1003816100381610038161003816V11990510038161003816100381610038162
le E100381610038161003816100381611990611990510038161003816100381610038162
+ |V|2
119889 (V (sdot) V (sdot)) = 119898 119905 isin [0 119879] | E1003816100381610038161003816V119905 minus V
119905
10038161003816100381610038162
gt 0
forallV (sdot) V (sdot) isin 119878
(107)
where119898 denotes the Lebesgue measure on RThe following result is proved as Proposition 41 in [16]
Lemma 17 (119878 119889(sdot sdot)) is a complete metric space and 119869120588 is
continuous and bounded on 119878 where
119869120588
(V (sdot)) = (119869 (V (sdot)) minus 119869 (119906 (sdot)) + 120588)2
+
10038161003816100381610038161003816100381610038161003816Eint
O
Φ(119909 (1198840(119909))
1015840
1198840(119909)) 119889119909
10038161003816100381610038161003816100381610038161003816
2
12
forallV (sdot) isin 119878(108)
and (119884 119885) is the mild solution of (72) corresponding to thecontrol V
Now we consider the following free initial state optimalcontrol problem
infV(sdot)isin119878
119869120588
(V (sdot)) (109)
It is easy to check that
0 le infV(sdot)isin119878
119869120588
(V (sdot)) le 119869120588
(119906 (sdot)) = 120588 (110)
According to Ekelandrsquos variational principle there exists a119906120588(sdot) isin 119881 such that
(i) 119869120588 (119906120588 (sdot)) le 120588
(ii) 119889 (119906120588 (sdot) 119906 (sdot)) le 120588
(iii) 119869120588 (119906120588 (sdot)) le 119869120588
(V (sdot)) + 120588119889 (119906120588 (sdot) 119906 (sdot)) forallV (sdot) isin 119878(111)
Using the spike variationmethod we can construct 119906120576120588(sdot) isin 119878as follows
119906120576120588
119905=
V119905 119905 isin [119904 119904 + 120576]
119906120588
119905 119905 isin [0 119879] [119904 119904 + 120576]
(112)
It is clear that 119889(119906120576120588(sdot) 119906120588(sdot)) le 120576 Let (119884120576120588(sdot) 119885
120576120588(sdot)) (resp
(119884120588(sdot) 119885
120588(sdot))) be the solution of (72) with respect to the
control 119906120576120588(sdot) (resp 119906120588(sdot)) Following (82) (119870120576120588
119905 119876
120576120588
119905) is the
mild solution of
119870120576120588
119905= int
119879
119905
119890119860(119904minus119905)
E1015840
times [1198911199101015840 (119904 Λ
120588
119904 119906
120588
119904) (119870
120576120588
119904)1015840
+ 119891119910(119904 Λ
120588
119904 119906
120588
119904)119870
120576120588
119904
+ 1198911199111015840 (119904 Λ
120588
119904 119906
120588
119904) (119876
120576120588
119904)1015840
+ 119891119911(119904 Λ
120588
119904 119906
120588
119904) 119876
120576120588
119904
+1
120576(119891 (119904 Λ
120588
119904 119906
120576120588
119904) minus 119891 (119904 Λ
120588
119904 119906
120588
119904))] 119889119904
minus int
119879
119905
119890119860(119904minus119905)
119876120576120588
119904119889119882 (119904)
(113)
ByTheorem 15 we know that
lim120576rarr0
E[ sup119904isin[119905119879]
100381610038161003816100381610038161003816100381610038161003816
119884120576120588
119904minus 119884
120588
119904
120576minus 119870
120576120588
119904
100381610038161003816100381610038161003816100381610038161003816
2
] = 0 forall119905 isin [0 119879]
lim120576rarr0
Eint119879
0
100381610038161003816100381610038161003816100381610038161003816
119885120576120588
119904minus 119885
120588
119904
120576minus 119876
120576120588
119904
100381610038161003816100381610038161003816100381610038161003816
2
119889119904 = 0
(114)
The proof of the following proposition is technical butbased on the arguments above and we omit it
Proposition 18 One has
1
120576E [Φ ((119884
120576120588
0)1015840
119884120576120588
0) minus Φ((119884
120588
0)1015840
119884120588
0)]
= E [Φ1199101015840 ((119884
120588
0)1015840
119884120588
0) (119870
120576120588
0)1015840
+ Φ119910((119884
120588
0)1015840
119884120588
0)119870
120576120588
0]
+ 119900 (120576)
Mathematical Problems in Engineering 15
1
120576(119869 (119906
120576120588) minus 119869 (119906
120588))
= E [1198921199101015840 ((119884
120588
0)1015840
119884120588
0) (119870
120576120588
0)1015840
+ 119892119910((119884
120588
0)1015840
119884120588
0)119870
120576120588
0]
+ Δ120576+1
120576Eint
119879
0
E1015840[ℎ (119904 Λ
120588
119904 119906
120576120588
119904)
minus ℎ (119904 Λ120588
119904 119906
120588
119904)] 119889119904 + 119900 (120576)
(115)
where
Δ120576= Eint
119879
0
E1015840[ℎ
1199101015840 (119904 Λ
120588
119904 119906
120576120588
119904) (119870
120576120588
119904)1015840
+ ℎ1199111015840 (119904 Λ
120588
119904 119906
120576120588
119904) (119876
120576120588
119904)1015840
+ ℎ119910(119904 Λ
120588
119904 119906
120576120588
119904)119870
120576120588
119904
+ ℎ119911(119904 Λ
120588
119904 119906
120576120588
119904) 119876
120576120588
119904] 119889119904
(116)
53 Variational Inequality and Adjoint Equation In thissubsection the adjoint process is introduced to deduce thevariational inequality
If we set V(sdot) = 119906120576120588(sdot) in (111) and notice that
119889(119906120576120588(sdot) 119906
120588(sdot)) le 120576 we get
minus120588 le1
120576(119869
120588(119906
120576120588
(sdot)) minus 119869120588(119906
120588
(sdot))) (117)
By Lemma 17
1
120576(119869
120588(119906
120576120588
(sdot)) minus 119869120588(119906
120588
(sdot)))
=(119869
120588(119906
120576120588(sdot)))
2
minus (119869120588(119906
120588(sdot)))
2
120576 (119869120588 (119906120576120588 (sdot)) + 119869120588 (119906120588 (sdot)))
=119869 (119906
120576120588(sdot)) + 119869 (119906
120588(sdot)) minus 2119869 (119906 (sdot)) + 2120588
119869120588 (119906120576120588 (sdot)) + 119869120588 (119906120588 (sdot))
times119869 (119906
120576120588(sdot)) minus 119869 (119906
120588(sdot))
120576
+
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] + E [Φ ((119884
120588
0)1015840
119884120588
0)]
119869120588 (119906120576120588 (sdot)) + 119869120588 (119906120588 (sdot))
times
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] minus E [Φ ((119884
120588
0)1015840
119884120588
0)]
120576
997888rarr 119897120588
1
119869 (119906120576120588(sdot)) minus 119869 (119906
120588(sdot))
120576
+ 119897120588
2
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] minus E [Φ ((119884
120588
0)1015840
119884120588
0)]
120576
(118)
where we set
119897120588
1=119869 (119906
120588(sdot)) minus 119869 (119906 (sdot)) + 120588
119869120588 (119906120588 (sdot))
119897120588
2=
E [Φ ((119884120588
0)1015840
119884120588
0)]
119869120588 (119906120588 (sdot))
(119)
and use the limit
119869 (119906120576120588
(sdot)) 997888rarr 119869 (119906120588
(sdot))
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] 997888rarr E [Φ ((119884
120588
0)1015840
119884120588
0)]
(120)
as 120576 rarr 0 according to (115)As |119897120588
1|2
+ |119897120588
2|2
= 1 for all 120588 gt 0 we know that there existsa subsequence of 119897120588
1 119897120588
2 (still denoted by 119897120588
1 119897120588
2) such that
lim120588rarr0
119897120588
1 119897120588
2 = 119897
1 1198972
1003816100381610038161003816119897110038161003816100381610038162
+1003816100381610038161003816119897210038161003816100381610038162
= 1
(121)
Combining (115) (117) with (118) we get
minus120588 le 119897120588
1
119869 (119906120576120588(sdot)) minus 119869 (119906
120588(sdot))
120576
+ 119897120588
2
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] minus E [Φ ((119884
120588
0)1015840
119884120588
0)]
120576
= 119897120588
1Δ120576+ 119897
120588
1E [119892
1199101015840 ((119884
120588
0)1015840
119884120588
0) (119870
120576120588
0)1015840
+119892119910((119884
120588
0)1015840
119884120588
0)119870
120576120588
0]
+119897120588
1
120576Eint
119879
0
E1015840[ℎ (119904 Λ
120588
119904 119906
120576120588
119904) minus ℎ (119904 Λ
120588
119904 119906
120588
119904)] 119889119904
+ 119897120588
2E [Φ
1199101015840 ((119884
120588
0)1015840
119884120588
0) (119870
120576120588
0)1015840
+Φ119910((119884
120588
0)1015840
119884120588
0)119870
120576120588
0] + (119897
120588
1+ 119897
120588
2) 119900 (120576)
(122)
Next we introduce the adjoint equation corresponding tovariational equation (113) whose solution is denoted by119875120588(119905)
119889119875120588
(119905)
= 119860lowast119875120588
(119905) 119889119905
+ E1015840[119891
1199101015840 (119905 Λ
120588
119905 119906
120588
119905) (119875
120588
(119905))1015840
+ 119891119910(119905 Λ
120588
119905 119906
120588
119905) 119875
120588
(119905) + 119897120588
1ℎ1199101015840 (119905 Λ
120588
119905 119906
120588
119905)
+ 119897120588
1ℎ119910(119905 Λ
120588
119905 119906
120588
119905)] 119889119905
16 Mathematical Problems in Engineering
+ E1015840[119891
1199111015840 (119905 Λ
120588
119905 119906
120588
119905) (119875
120588
(119905))1015840
+ 119891119911(119905 Λ
120588
119905 119906
120588
119905) 119875
120588
(119905)
+ 119897120588
1ℎ1199111015840 (119905 Λ
120588
119905 119906
120588
119905) + 119897
120588
1ℎ119911(119905 Λ
120588
119905 119906
120588
119905)] 119889119882 (119905)
119875120588
(0) = 119897120588
1E [119892
1199101015840 ((119884
120588
0(119909))
1015840
119884120588
0(119909))
+ 119892119910((119884
120588
0(119909))
1015840
119884120588
0(119909))]
+ 119897120588
2E [Φ
1199101015840 ((119884
120588
0(119909))
1015840
119884120588
0(119909))
+ Φ119910((119884
120588
0(119909))
1015840
119884120588
0(119909))]
(123)
where 119860lowast is the 1198712(O)-adjoint operator of 119860 Under assump-tions (L1)ndash(L3) this is a linear mean-field SEE with boundedcoefficients An application ofTheorem 11 implies that it has aunique adaptedmild solution such that119875120588(119905) isin S2
F ([0 119879] 119870)When 120588 rarr 0 according to Corollaries 5 and 13 119875120588(119905)
converges to 119875(119905) where
119875 (119905) isin S2
F ([0 119879] 119870) (124)
is the solution of the following equation
119875 (119905) = 1198971E [119892
1199101015840 (119884
1015840
0 119884
0) + 119892
119910(119884
1015840
0 119884
0)]
+ 1198972E [Φ
1199101015840 (119884
1015840
0 119884
0) + Φ
119910(119884
1015840
0 119884
0)]
+ int
119905
0
119890119860lowast(119905minus119904)
E1015840[119891
1199101015840 (119904) 119875
1015840
(119904) + 119891119910(119904) 119875 (119904)
+ 1198971ℎ1199101015840 (119904) + 119897
1ℎ119910(119904)] 119889119904
+ int
119905
0
119890119860lowast(119905minus119904)
E1015840[119891
1199111015840 (119904) 119875
1015840
(119904) + 119891119911(119904) 119875 (119904)
+ 1198971ℎ1199111015840 (119904) + 119897
1ℎ119911(119904)] 119889119882 (119904)
(125)
The following proposition which formally follows fromProposition 18 gives the relation between 119875120588(119905) and119870120576120588
119905
Proposition 19 Consider the following
E [119875120588
(0)119870120576120588
0]
= Eint119879
0
1
120576119875120588
(119904)E1015840[119891 (119904 Λ
120588
119904 119906
120576120588
119904) minus 119891 (119904 Λ
120588
119904 119906
120588
119904)]
minus 119897120588
1119870120576120588
119904E1015840[ℎ
1199101015840 (119904 Λ
120588
119904 119906
120588
119904) + ℎ
119910(119904 Λ
120588
119904 119906
120588
119904)]
minus 119897120588
1119876120576120588
119904E1015840[ℎ
1199111015840 (119904 Λ
120588
119904 119906
120588
119904) + ℎ
119911(119904 Λ
120588
119904 119906
120588
119904)] 119889119904
(126)
The following theorem constitutes the main contributionof this section themaximumprinciple for the BSPDE controlsystem
Theorem 20 Let assumptions (L1)ndash(L3) hold Suppose 119906(sdot) isan optimal control and (119884(sdot) 119885(sdot)) is the corresponding optimalstate trajectory for the BSPDE control systems (72) and (73)with the initial state constraint (75) Then there exists 119875(119905) isinS2
F ([0 119879] 119870) which satisfies (125) such that
H (119905 1198841015840
119905 119885
1015840
119905 119884
119905 119885
119905 V
119905 119875 (119905))
ge H (119905 1198841015840
119905 119885
1015840
119905 119884
119905 119885
119905 119906
119905 119875 (119905))
119886119890 119886119904 forallV isin U119886119889
(127)
whereH [0 119879]times119867timesL(Γ119867)times119867timesL(Γ119867)times119880times119870 rarr R
is the Hamiltonian function defined by
H (119905 119910 119910 119911 V 119901) = 1198971ℎ (119905 119910 119910 119911 V)
+ 119901119891 (119905 119910 119910 119911 V) (128)
Proof By (122) and Proposition 19 we obtain
minus120588 le1
120576Eint
119879
0
119875120588
(119904)E1015840[119891 (119904 Λ
120588
119904 119906
120576120588
119904)
minus 119891 (119904 Λ120588
119904 119906
120588
119904)] 119889119904
+119897120588
1
120576Eint
119879
0
E1015840[ℎ (119904 Λ
120588
119904 119906
120576120588
119904)
minus ℎ (119904 Λ120588
119904 119906
120588
119904)] 119889119904 + (119897
120588
1+ 119897
120588
2) 119900 (120576)
(129)
Letting 120576 rarr 0+ in (129) we derive for ae 120591 isin [0 119879]
minus120588 le 119897120588
1E1015840[ℎ (120591 Λ
120588
120591 V
120591) minus ℎ (120591 Λ
120588
120591 119906
120588
120591)]
+ 119875120588
(119904)E1015840[119891 (120591 Λ
120588
120591 V
120591) minus 119891 (120591 Λ
120588
120591 119906
120588
120591)]
(130)
for all V isin UadFinally taking 120588 rarr 0 in (130) we derive the desired
result
Remark 21 We note that if the coefficients do not dependexplicitly on the marginal law of the underlying diffusionthe result reduces to the classical case that is the SMP forBSPDEs without mean-field term
Remark 22 When we remove the initial state constraint (75)we obtain the general maximum principle for the mean-fieldBSPDEs system (ie without the constraint) with 119897
1= 1
54 Application A Backward Linear Quadratic Control Prob-lem Now we apply our maximum principle to solve an LQproblem For notational simplicity we restrict ourselves tothe free case (ie without the initial state constraint (75)) thegeneral case being handled in a similar way
Mathematical Problems in Engineering 17
Consider the following problem
119869 (119906) =1
2E [(119884 (0))
2] +
1
2Eint
119879
0
119873V2 (119905) 119889119905 997888rarr min
(131)
subject to
119889119884 (119905) = minus119860119884 (119905) 119889119905
minus 119861119884 (119905) + 119861E [119884 (119905)]
+119862V (119905) + 119863119885 (119905) + 119863E [119885 (119905)] 119889119905
+ 119885 (119905) 119889119882 (119905)
119884 (119879) = 120585 119905 isin [0 119879]
(132)
where 119860 is a partial differential operator satisfying condition(L1) and 119861 119861 119862119863119863 and119873 are bounded and deterministicconstantsWe also assume that119873 gt 0 and V isin 1198712F (0 119879 119880) It iseasy to verify that BSPDE (132) admits a uniquemild solution(119884(119905) 119885(119905))
119875(119905) the adjoint process of state equation (132) is thesolution of
119889119875 (119905) = 119860lowast119875 (119905) 119889119905 + 119861119875 (119905) + 119861E [119875 (119905)] 119889119905
+ 119863119875 (119905) + 119863E [119875 (119905)] 119889119882 (119905)
119875 (0) = 119884 (0)
(133)
Let 119906(sdot) be an optimal control and let (119884(sdot) 119885(sdot)) bethe corresponding state process By maximum principle ofTheorem 20 (note that 119897
1= 1 in this problem)
1
2119873V2 (119905) + 119875 (119905) 119862V (119905) ge
1
2119873119906
2
(119905) + 119875 (119905) 119862119906 (119905) (134)
for all V isin 119880ad since the state equation has the form (132)Thisin turn implies
119906 (119905) = minus119862
119873119875 (119905) (135)
It is clear that (131) is a positive quadratic functional of controlbecause of 119873 gt 0 Hence an optimal control exists Thecandidate optimal control (135) is indeed an optimal controlof this LQ problem for it is the only control which satisfies themaximum principle
Next we want to obtain a more explicit representationof the optimal control (135) from the state equation (132)Substituting (135) into state equation (132) yields
119889119884 (119905) = minus119860119884 (119905) 119889119905
minus 119861119884 (119905) + 119861E [119884 (119905)] minus1198622
119873119875 (119905)
+119863119885 (119905) + 119863E [119885 (119905)] 119889119905 + 119885 (119905) 119889119882 (119905)
119884 (119879) = 120585 119905 isin [0 119879]
(136)
Combining the above equation with (133) we obtain thefollowing related feedback control system
119889119884 (119905) = minus119860119884 (119905) 119889119905
minus 119861119884 (119905) + 119861E [119884 (119905)] minus1198622
119873119875 (119905)
+ 119863119885 (119905) + 119863E [119885 (119905)] 119889119905 + 119885 (119905) 119889119882 (119905)
119884 (119879) = 120585
119889119875 (119905) = 119860lowast119875 (119905) 119889119905 + 119861119875 (119905) + 119861E [119875 (119905)] 119889119905
+ 119863119875 (119905) + 119863E [119875 (119905)] 119889119882 (119905)
119875 (0) = 119884 (0)
(137)
Looking at the terminal condition of 119875(119905) in (133) andconsidering the mean-field type of (132) it is reasonable toconjecture that 119875(119905) has the following form
119875 (119905) = 120593 (119905) 119884 (119905) + 120601 (119905)E [119884 (119905)] (138)
where120593(119905)120601(119905) are deterministic differential functionswhichwill be specified below Moreover 120593(0) = 1 120601(0) = 0
Inserting this form into adjoint equation (133) and notic-ing that 119884(119905) satisfies (136) we can compare the coefficientsof 119889119905 and 119889119882(119905) to obtain the following equation
2120593 (119905) 119860119884 (119905) + 2120601 (119905) 119860E [119884 (119905)] + 119861120593 (119905) 119884 (119905)
+ 2 (119861120601 (119905) + 119861120593 (119905) + 119861120601 (119905))E [119884 (119905)]
= minus120593 (119905) 119861119884 (119905) minus1198622
119873120593 (119905) 119884 (119905) + 119863119885 (119905)
+ 119884 (119905)119889120593 (119905)
119889119905+ E [119884 (119905)]
119889120601 (119905)
119889119905
+ (1206012
(119905)1198622
119873+ 2
1198622
119873120593 (119905) 120601 (119905))E [119884 (119905)]
minus (119863120601 (119905) + 119863120601 (119905) + 119863120593 (119905))E [119885 (119905)]
120593 (119905) 119885 (119905) = 119863120593 (119905) 119884 (119905)
+ (119863120601 (119905) + 119863120593 (119905) + 119863120601 (119905))E [119884 (119905)]
(139)
Then subtracting 119885(119905) we have
2120593 (119905) 119860119884 (119905) + 2120601 (119905) 119860E [119884 (119905)] + 119861120593 (119905) 119884 (119905)
+ 2 (119861120601 (119905) + 119861120593 (119905) + 119861120601 (119905))E [119884 (119905)]
18 Mathematical Problems in Engineering
= 120593 (119905) minus119861 +1198622
119873120593 (119905) minus 119863
2119884 (119905)
+ 119884 (119905)119889120593 (119905)
119889119905+ E [119884 (119905)]
119889120601 (119905)
119889119905
+ (1206012
(119905)1198622
119873+ 2
1198622
119873120593 (119905) 120601 (119905))E [119884 (119905)]
minus1
120593 (119905)(119863120601 (119905) + 119863120593 (119905) + 119863120601 (119905))
2
E [119884 (119905)]
minus 2119863 (119863120601 (119905) + 119863120593 (119905) + 119863120601 (119905))E [119884 (119905)]
(140)
Comparing the coefficients of 119884(119905) and E[119884(119905)] respec-tively we get
119889
119889119905120593 (119905) = 2119860
lowast120593 (119905) + 2119861120593 (119905) + 119863
2120593 (119905) minus
1198622
1198731205932
(119905)
120593 (0) = 1
(141)
119889
119889119905120601 (119905) = 2119860
lowast120601 (119905) + (
(119863 + 119863)2
120593 (119905)minus1198622
119873)120601
2
(119905)
+ 2119877 (119905) 120601 (119905) + (1198632+ 2119861 + 2119863119863)120593 (119905)
120601 (0) = 0
(142)
where 119877(119905) = 119861 + 119861 + (119863 + 119863)2
minus (1198622119873)120593(119905)
We solve (141) to get
120593 (119905) = (119890minus(2119860lowast+2119861+119863
2)119905(1 minus
1198622
119873(2119860lowast + 2119861 + 1198632))
+1198622
119873(2119860lowast + 2119861 + 1198632))
minus1
(143)
Then (142) exists a unique solution from the classical Riccatiequation theory
We now conclude the above discussions in the followingresult
Theorem 23 For onersquos linear quadratic stochastic partialdifferential control problem (131)-(132) the unique optimalcontrol 119906(sdot) isin 119880ad is given by
119906 (119905) = minus119862
119873(120593 (119905) 119884 (119905) + 120601 (119905)E [119884 (119905)]) (144)
with 120593(119905) satisfying (143) and 120601(119905) solving (142)
Conflict of Interests
The authors declare that they have no conflict of interestsregarding the publication of this paper
Acknowledgment
This research is partially supported by the Natural ScienceFoundation of China under Grant no 11301310
References
[1] Y Hu and S Peng ldquoAdapted solution of a backward semilinearstochastic evolution equationrdquo Stochastic Analysis and Applica-tions vol 9 no 4 pp 445ndash459 1991
[2] N IMahmudov andMAMcKibben ldquoOnbackward stochasticevolution equations in Hilbert spaces and optimal controlrdquoNonlinear Analysis vol 67 no 4 pp 1260ndash1274 2007
[3] M Fuhrman and G Tessitore ldquoNonlinear Kolmogorov equa-tions in infinite dimensional spaces the backward stochasticdifferential equations approach and applications to optimalcontrolrdquoThe Annals of Probability vol 30 no 3 pp 1397ndash14652002
[4] M Fuhrman and G Tessitore ldquoInfinite horizon backwardstochastic differential equations and elliptic equations inHilbert spacesrdquo Annals of Probability vol 32 no 1B pp 607ndash660 2004
[5] X Mao ldquoAdapted solutions of backward stochastic differentialequations with non-Lipschitz coefficientsrdquo Stochastic Processesand their Applications vol 58 no 2 pp 281ndash292 1995
[6] J Lasry and P Lions ldquoMean field gamesrdquo Japanese Journal ofMathematics vol 2 no 1 pp 229ndash260 2007
[7] R Buckdahn J Li and S Peng ldquoMean-field backward stochas-tic differential equations and related partial differential equa-tionsrdquo Stochastic Processes and their Applications vol 119 no 10pp 3133ndash3154 2009
[8] D Andersson and B Djehiche ldquoAmaximum principle for SDEsofmean-field typerdquoAppliedMathematics andOptimization vol63 no 3 pp 341ndash356 2011
[9] R Buckdahn B Djehiche J Li and S Peng ldquoMean-fieldbackward stochastic differential equations a limit approachrdquoThe Annals of Probability vol 37 no 4 pp 1524ndash1565 2009
[10] R Xu ldquoMean-field backward doubly stochastic differentialequations and related SPDEsrdquo Boundary Value Problems vol2012 article 114 2012
[11] R Buckdahn B Djehiche and J Li ldquoA general stochasticmaximum principle for SDEs of mean-field typerdquo AppliedMathematics and Optimization vol 64 no 2 pp 197ndash216 2011
[12] J Li ldquoStochasticmaximumprinciple in themean-field controlsrdquoAutomatica vol 48 no 2 pp 366ndash373 2012
[13] G Wang C Zhang and W Zhang ldquoStochastic maximumprinciple for mean-field type optimal control under partialinformationrdquo IEEE Transactions on Automatic Control vol 59no 2 pp 522ndash528 2014
[14] M Hafayed ldquoA mean-field necessary and sufficient conditionsfor optimal singular stochastic controlrdquo Communications inMathematics and Statistics vol 1 no 4 pp 417ndash435 2013
[15] M Hafayed ldquoA mean-field maximum principle for optimalcontrol of forward-backward stochastic differential equationswith Poisson jumpprocessesrdquo International Journal ofDynamicsand Control vol 1 no 4 pp 300ndash315 2013
[16] Y Hu and S Peng ldquoMaximum principle for semilinearstochastic evolution control systemsrdquo Stochastics and StochasticsReports vol 33 no 3-4 pp 159ndash180 1990
[17] M Fuhrman Y Hu and G Tessitore ldquoStochastic maximumprinciple for optimal control of SPDEsrdquo Comptes Rendus Math-ematique vol 350 no 13-14 pp 683ndash688 2012
Submit your manuscripts athttpwwwhindawicom
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Mathematical Problems in Engineering
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Differential EquationsInternational Journal of
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Function Spaces
Abstract and Applied AnalysisHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Discrete Dynamics in Nature and Society
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Stochastic AnalysisInternational Journal of
10 Mathematical Problems in Engineering
for all 0 le 119905 le 119879 119909 isin O With these identifications(60) can be written in the form of (11) By (H2) we know119891 satisfy condition (A1) Hence an application of Theorem 4concludes that (60) has a unique mild solution (119884 119885) isin
1198712(0 119879 119871
2(Ω 119871
2(O))) times 1198712F (0 119879 119871
2(R119899
1198712(Ω 119871
2(O))))
4 Mean-Field Stochastic Evolution Equations
Let 119882(119905) 119905 isin [0 119879] be a cylindrical Wiener process withvalues in a Hilbert space Γ defined on a probability space(ΩFP) We fix an interval [119905 119879] sub [0 119879] and considerthe stochastic evolution equations of mean-field type for anunknown process 119883(119904) 119904 isin [119905 119879] with values in a Hilbertspace 119870
119889119883 (119904) = 119861119883 (119904) 119889119904 + E1015840[119887 (119904 119883
1015840
(119904) 119883 (119904))] 119889119904
+ E1015840[120590 (119904 119883
1015840
(119904) 119883 (119904))] 119889119882 (119904)
119883 (119905) = 119909 isin 119870
(65)
where operator 119861 is the generator of a strongly continuoussemigroup 119890
119905119861 119905 ge 0 in the Hilbert space 119870 with 119872119861≜
sup119905isin[0119879]
|119890119905119861|
By a mild solution of (65) we mean an F119904-measurable
process 119883(119904) 119904 isin [119905 119879] with continuous paths in 119870 suchthat P-as
119883 (119904) = 119890119861(119904minus119905)
119909
+ int
119904
119905
119890119861(119904minus120591)
E1015840[119887 (120591 119883
1015840
(120591) 119883 (120591))] 119889120591
+ int
119904
119905
119890119861(119904minus120591)
E1015840[120590 (120591119883
1015840
(120591) 119883 (120591))] 119889119882 (120591)
119904 isin [119905 119879]
(66)
We suppose the following(A4) 119887 [0 119879] times 119870 times 119870 rarr 119870 is a measurable mapping
which satisfies10038161003816100381610038161003816119887 (119905 119909
1015840 119909) minus 119887 (119905 119910
1015840 119910)
10038161003816100381610038161003816
2
le 1198711(100381610038161003816100381610038161199091015840minus 119910
101584010038161003816100381610038161003816
2
+1003816100381610038161003816119909 minus 119910
10038161003816100381610038162
)
119905 isin [0 119879] 1199091015840 119909 119910
1015840 119910 isin 119870
(67)
for some constant 1198711gt 0
(A5) The mapping 120590 [0 119879] times 119870 times 119870 rarr L(Γ 119870) fulfillsthat for every V isin Γ the map 120590V [0 119879] times 119870 times 119870 rarr 119870
is measurable for every 119904 gt 0 119905 isin [0 119879] 1199091015840 1199101015840 119909 119910 isin 119870119890119904119861120590(119905 119909
1015840 119909) isin L(Γ 119870) and
10038161003816100381610038161003816119890119904119861120590 (119905 119909
1015840 119909)
10038161003816100381610038161003816L(Γ119870)le 119871
2119904minus120574(1 +
10038161003816100381610038161003816119909101584010038161003816100381610038161003816+ |119909|)
10038161003816100381610038161003816119890119904119861120590 (119905 119909
1015840 119909) minus 119890
119904119861120590 (119905 119910
1015840 119910)
10038161003816100381610038161003816L(Γ119870)
le 1198712119904minus120574(100381610038161003816100381610038161199091015840minus 119910
101584010038161003816100381610038161003816+1003816100381610038161003816119909 minus 119910
1003816100381610038161003816)
10038161003816100381610038161003816120590 (119905 119909
1015840 119909)
10038161003816100381610038161003816L(Γ119870)le 119871
2(1 +
10038161003816100381610038161003816119909101584010038161003816100381610038161003816+ |119909|)
(68)
for some constants 1198712gt 0 and 120574 isin [0 12)
Theorem 11 Under assumptions (A3) and (A4) (65) has aunique mild solution119883(sdot) isin S2
F ([119905 119879] 119870)
The proof is constructed in two steps like that ofTheorem 4 and it uses standard arguments for stochastic evo-lution equations introduced in the proof of Proposition 32 in[3] Since the proof is straightforward we prefer to omit it
Remark 12 In our paper Lipchitz condtion (A4) is givento get the well-posedness of mean-field stochastic evolutionequations In fact (A4) can be replaced by a weaker conditionsuch as (A3) We just give the condition (A4) for simplicity
From standard arguments we can also get the followingcontinuous dependence theorem
Corollary 13 Assume that for all 120572 in a metric space 119865(119887120572 120590
120572) satisfy (A4) and (A5)with119871
1and119871
2independent of 120572
Also assume that
Eint119879
0
10038161003816100381610038161003816E1015840[119887120572(119904 119883
1015840
(119904) 119883 (119904))]
minus E1015840[1198871205720(119904 119883
1015840
(119904) 119883 (119904))]10038161003816100381610038161003816
2
119889119904 997888rarr 0
Eint119879
0
10038161003816100381610038161003816E1015840[120590
120572(119904 119883
1015840
(119904) 119883 (119904))]
minusE1015840[120590
1205720(119904 119883
1015840
(119904) 119883 (119904))]10038161003816100381610038161003816
2
119889119904 997888rarr 0
(69)
as 120572 rarr 1205720for all119883 isin S2
F ([0 119879] 119870)If we denote by 119883120572
(sdot) the mild solution of mean-field SEE(65) corresponding to the functions (119887
120572 120590
120572) and to the initial
data 119909 then we have
sup119904isin[0119879]
E1003816100381610038161003816119883
120572
(119904) minus 1198831205720 (119904)
10038161003816100381610038162
997888rarr 0 119886119904 120572 997888rarr 1205720 (70)
5 Maximum Principle for BSPDEs ofMean-Field Type
51 Formulation of the Problem Let O isin R119899 be a boundedopen set with smooth boundary 120597O and let 119880 the space ofcontrols be a separable real Hilbert space We denote
U = V (sdot) isin 1198712F(0 119879 119880)
| V119905(120596
1015840 120596) [0 119879] times Ω times Ω
997888rarr 119880 is F otimesF119905-progressively measurable
(71)
Mathematical Problems in Engineering 11
An element ofU is called an admissible controlFor any V isin U we consider the following controlled
BSPDE system in the state space119867 = 1198712(O) (norm | sdot | scalar
product ⟨sdot sdot⟩)
119889119884119905(119909) = minus119860119884
119905(119909) 119889119905
minus E1015840[119891 (119905 119909 (119884
119905(119909))
1015840
(119885119905(119909))
1015840
119884119905(119909) 119885
119905(119909) V
119905)] 119889119905
+ 119885119905(119909) 119889119882 (119905) 119905 isin [0 119879]
119884119879(119909) = 120585 (119909) 119909 isin O
(72)
where 119860 is a partial differential operator 119891 [0 119879] timesO times119867 times
L(Γ119867)times119867timesL(Γ119867)times119880 rarr 119867 and 120585 isin 1198712(ΩF119879P 119867)
The cost functional is given by
119869 (V) = Eint119879
0
intO
E1015840[ℎ (119904 119909 (119884
119904(119909))
1015840
(119885119904(119909))
1015840
119884119904(119909) 119885
119904(119909) V
119904)] 119889119909 119889119904
+E1015840intO
119892 (119909 (1198840(119909))
1015840
1198840(119909)) 119889119909
(73)
where
ℎ [0 119879] times O times 119867 timesL (Γ119867) times 119867 timesL (Γ119867) times 119880 997888rarr R
119892 O times 119867 times119867 997888rarr R
(74)
Our purpose is to minimize the functional 119869(sdot) over Uadsubject to the following state constraint
EintO
Φ(119909 (1198840(119909))
1015840
1198840(119909)) 119889119909 = 0 (75)
where
Φ O times 119867 times119867 997888rarr R (76)
An admissible control 119906 isin Uad that satisfies
119869 (119906) = minVisinUad
119869 (V) (77)
is called optimalThrough what follows the following assumptions will be
in force
(L1) 119860 is a partial differential operator with appropri-ate boundary conditions We assume that 119860 is theinfinitesimal generator of a strongly continuous semi-group 119890119905119860 119905 ge 0 in119867 Moreover for every 119905 isin [0 119879]119890
119905119860119891
1198712(O) le 119872
119860119891
1198712(O) for some constant 119872
119860
independent of 119905 and 119891(L2) 119891 ℎ 119892 and Φ are continuously Gateaux differen-
tiable with respect to (1199101015840 1199111015840 119910 119911) 119891 is continuouslyGateaux differentiable with respect to V and ℎ iscontinuous with respect to V
(L3) The derivatives of 119891 ℎ 119892 and Φ are Lipschitzcontinuous and bounded by
100381610038161003816100381610038161198911199101015840
10038161003816100381610038161003816+10038161003816100381610038161198911199111015840
1003816100381610038161003816 +10038161003816100381610038161003816119891119910
10038161003816100381610038161003816+1003816100381610038161003816119891119911
1003816100381610038161003816 +1003816100381610038161003816119891V
1003816100381610038161003816 +10038161003816100381610038161003816Φ1199101015840
10038161003816100381610038161003816+10038161003816100381610038161003816Φ119910
10038161003816100381610038161003816le 119862
10038161003816100381610038161003816ℎ1199101015840
10038161003816100381610038161003816+1003816100381610038161003816ℎ1199111015840
1003816100381610038161003816 +10038161003816100381610038161003816ℎ119910
10038161003816100381610038161003816+1003816100381610038161003816ℎ119911
1003816100381610038161003816 +100381610038161003816100381610038161198921199101015840
10038161003816100381610038161003816+10038161003816100381610038161003816119892119910
10038161003816100381610038161003816
le 119862 (1 +10038161003816100381610038161003816119910101584010038161003816100381610038161003816+10038161003816100381610038161003816119911101584010038161003816100381610038161003816+10038161003816100381610038161199101003816100381610038161003816 + |119911| + |V|)
(78)
where 119862 is a positive constant
Obviously according to Theorem 4 state equation (72)has a unique mild solution under the above assumptions
Remark 14 We can define the second order differentialoperator
(119860119891) (119909) =
119899
sum
119894119895=1
119886119894119895(119909)
1205972119891
120597119909119894120597119909
119895
(119909) +
119899
sum
119894=1
119887119894(119909)
120597119891
120597119909119894
(119909) (79)
By Example 9 119860 fulfills assumption (L1) if 119886119894119895 119887
119894satisfy
condition (H1)
52 Variation of the Trajectory Let 119906 be an optimal controlwith (119884(sdot) 119885(sdot)) being the corresponding optimal state Let120576 gt 0 and [119903 119903 + 120576] sube [0 119879] For any given V isin Uad weintroduce the spike variation of the control 119906(sdot)
119906120576
119905=
V119905 119905 isin [119903 119903 + 120576]
119906119905 119905 isin [0 119879] [119904 119904 + 120576]
(80)
It is clear that 119906120576(sdot) isin UadLet (119884120576
(sdot) 119885120576(sdot)) be the trajectory corresponding to 119906120576(sdot)
We use the following short notation for brevity
119891 (119905) = 119891 (119905 119909 (119884119905(119909))
1015840
(119885119905(119909))
1015840
119884119905(119909) 119885
119905(119909) 119906
119905)
119891 (119906120576
119905) = 119891 (119905 119909 (119884
119905(119909))
1015840
(119885119905(119909))
1015840
119884119905(119909) 119885
119905(119909) 119906
120576
119905)
(81)
Consider the following equation
119889119870120576
119905(119909) = minus119860119870
120576
119905(119909) 119889119905
minus E1015840[119891
1199101015840 (119905) (119870
120576
119905(119909))
1015840
+ 119891119910(119905) 119870
120576
119905(119909)
+ 1198911199111015840 (119905) (119876
120576
119905(119909))
1015840
+ 119891119911(119905) 119876
120576
119905(119909)
+1
120576(119891 (119906
120576
119905) minus 119891 (119905))] 119889119905 + 119876
120576
119905(119909) 119889119882 (119905)
119870120576
119879(119909) = 0
(82)
Since the coefficients in (82) are bounded it is easy to checkthat there exists a unique mild solution such that
E[ sup119905isin[0119879]
1003816100381610038161003816119870120576
119905
10038161003816100381610038162
+ int
119879
0
1003816100381610038161003816119876120576
119905
10038161003816100381610038162
119889119905] lt infin (83)
We have the following estimate
12 Mathematical Problems in Engineering
Theorem 15 There holds
lim120576rarr0
E[ sup119904isin[119905119879]
1003816100381610038161003816100381610038161003816100381610038161003816
119884120576
119904minus 119884
119904
120576minus 119870
120576
119904
1003816100381610038161003816100381610038161003816100381610038161003816
2
] = 0 forall119905 isin [0 119879]
lim120576rarr0
Eint119879
0
1003816100381610038161003816100381610038161003816100381610038161003816
119885120576
119904minus 119885
119904
120576minus 119876
120576
119904
1003816100381610038161003816100381610038161003816100381610038161003816
2
119889119904 = 0
(84)
Proof We define
120578120576
119904=119884120576
119904minus 119884
119904
120576minus 119870
120576
119904 120577
120576
119904=119885120576
119904minus 119885
119904
120576minus 119876
120576
119904 119904 isin [0 119879]
(85)
For simplicity let us define
Λ120576
119904= ((119884
120576
119904)1015840
(119885120576
119904)1015840
119884120576
119904 119885
120576
119904)
119891 (119904 120582) = 119891 (119904 1198841015840
119904+ 120582(119884
120576
119904minus 119884
119904)1015840
1198851015840
119904+ 120582(119885
120576
119904minus 119885
119904)1015840
119884119904+ 120582 (119884
120576
119904minus 119884
119904)
119885119904+ 120582 (119885
120576
119904minus 119885
119904) 119906
120576
119904)
(86)
By the definition of (119884120576
119904 119885
120576
119904) (119884
119904 119885
119904) and (119870120576
119904 119876
120576
119904) (120578120576
119904 120577
120576
119904)
is the mild solution of
119889120578120576
119904= minus119860120578
120576
119904119889119904 minus E
1015840
[119871 (119904 120576)] 119889119904 + 120577120576
119904119889119882 (119904)
120578120576
119879= 0
(87)
with
119871 (119904 120576) =1
120576(119891 (119904 Λ
120576
119904 119906
120576
119904) minus 119891 (119906
120576
119904))
minus 1198911199101015840 (119904) (119870
120576
119904)1015840
minus 119891119910(119904) 119870
120576
119904minus 119891
1199111015840 (119904) (119876
120576
119904)1015840
minus 119891119911(119904) 119876
120576
119904
= ((120578120576
119904)1015840
+ (119870120576
119904)1015840
) int
1
0
1198911199101015840 (119904 120582) 119889120582 + (120578
120576
119904+ 119870
120576
119904)
times int
1
0
119891119910(119904 120582) 119889120582 minus 119891
1199101015840 (119904) (119870
120576
119904)1015840
minus 119891119910(119904) 119870
120576
119904
+ ((120577120576
119904)1015840
+ (119876120576
119904)1015840
)int
1
0
1198911199111015840 (119904 120582) 119889120582 + (120577
120576
119904+ 119876
120576
119904)
times int
1
0
119891119911(119904 120582) 119889120582 minus 119891
1199111015840 (119904) (119876
120576
119904)1015840
minus 119891119911(119904) 119876
120576
119904
= (120578120576
119904)1015840
int
1
0
1198911199101015840 (119904 120582) 119889120582 + 120578
120576
119904int
1
0
119891119910(119904 120582) 119889120582
+ (120577120576
119904)1015840
int
1
0
1198911199111015840 (119904 120582) 119889120582 + 120577
120576
119904int
1
0
119891119911(119904 120582) 119889120582 + 120574
120576
119904
(88)
where we denote
120574120576
119904= (119870
120576
119904)1015840
int
1
0
(1198911199101015840 (119904 120582) minus 119891
1199101015840 (119904)) 119889120582
+ 119870120576
119904int
1
0
(119891119910(119904 120582) minus 119891
119910(119904)) 119889120582
+ (119876120576
119904)1015840
int
1
0
(1198911199111015840 (119904 120582) minus 119891
1199111015840 (119904)) 119889120582
+ 119876120576
119904int
1
0
(119891119911(119904 120582) minus 119891
119911(119904)) 119889120582
(89)
For any 120573 gt 0 according to Lemma 1 we obtain
E sup119905le119904le119879
11989021205731199041003816100381610038161003816120578
120576
119904
10038161003816100381610038162
+ Eint119879
119905
11989021205731199041003816100381610038161003816120577
120576
119904
10038161003816100381610038162
119889119904
le12119872
2
119860
120573int
119879
119905
1198902120573119904
E [10038161003816100381610038161003816E1015840
[119871 (119904 120576)]10038161003816100381610038161003816
2
] 119889119904
le12119872
2
119860
120573Eint
119879
119905
1198902120573119904
E1015840[|119871 (119904 120576)|
2] 119889119904
(90)
By condition (L3) we have
E [E1015840[|119871 (119904 120576)|
2]]
= E [E1015840[1003816100381610038161003816119871 (119904 120576) minus 120574
120576
119904+ 120574
120576
119904
10038161003816100381610038162
]]
le 81198622E [E
1015840[10038161003816100381610038161003816(120578
120576
119904)101584010038161003816100381610038161003816
2
+1003816100381610038161003816120578120576
119904
10038161003816100381610038162
+10038161003816100381610038161003816(120577
120576
119904)101584010038161003816100381610038161003816
2
+1003816100381610038161003816120577120576
119904
10038161003816100381610038162
]]
+ 2E [E1015840[1003816100381610038161003816120574120576
119904
10038161003816100381610038162
]]
le 161198622E [
1003816100381610038161003816120578120576
119904
10038161003816100381610038162
+1003816100381610038161003816120577120576
119904
10038161003816100381610038162
] + 2E [E1015840[1003816100381610038161003816120574120576
119904
10038161003816100381610038162
]]
(91)
Combined with (91) (90) yields
E sup119905le119904le119879
11989021205731199041003816100381610038161003816120578
120576
119904
10038161003816100381610038162
+ (1 minus192119872
2
1198601198622
120573)Eint
119879
119905
11989021205731199041003816100381610038161003816120577
120576
119904
10038161003816100381610038162
119889119904
le192119872
2
1198601198622
120573Eint
119879
119905
11989021205731199041003816100381610038161003816120578
120576
119904
10038161003816100381610038162
119889119904
+24119872
2
119860
120573Eint
119879
119905
1198902120573119904
[E1015840[1003816100381610038161003816120574120576
119904
10038161003816100381610038162
]] 119889119904
(92)
We claim that
Eint119879
119905
E1015840[1003816100381610038161003816120574120576
119904
10038161003816100381610038162
] 119889119904 997888rarr 0 as 120576 997888rarr 0 (93)
From (89)
120574120576
119904= 119868
1
119904+ 119868
2
119904+ 119868
3
119904+ 119868
4
119904 (94)
Mathematical Problems in Engineering 13
where
1198681
119904= (119870
120576
119904)1015840
int
1
0
(1198911199101015840 (119904 120582) minus 119891
1199101015840 (119904)) 119889120582
1198682
119904= 119870
120576
119904int
1
0
(119891119910(119904 120582) minus 119891
119910(119904)) 119889120582
1198683
119904= (119876
120576
119904)1015840
int
1
0
(1198911199111015840 (119904 120582) minus 119891
1199111015840 (119904)) 119889120582
1198684
119904= 119876
120576
119904int
1
0
(119891119911(119904 120582) minus 119891
119911(119904)) 119889120582
(95)
Then
Eint119879
119905
E1015840[1003816100381610038161003816120574120576
119904
10038161003816100381610038162
] 119889119904
le 4Eint119879
119905
E1015840[100381610038161003816100381610038161198681
119904
10038161003816100381610038161003816
2
+100381610038161003816100381610038161198682
119904
10038161003816100381610038161003816
2
+100381610038161003816100381610038161198683
119904
10038161003816100381610038161003816
2
+100381610038161003816100381610038161198684
119904
10038161003816100381610038161003816
2
] 119889119904
(96)
Take Eint119879119905E1015840[|1198682
119904|2
]119889119904 for example
Eint119879
119905
E1015840[100381610038161003816100381610038161198682
119904
10038161003816100381610038161003816
2
] 119889119904
= Eint119879
119905
E1015840[(119870
120576
119904int
1
0
(119891119910(119904 120582) minus 119891
119910(119904)) 119889120582)
2
]119889119904
le Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119904 120582) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582] 119889119904
le 2Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119904 120582) minus 119891
119910(119906
120576
119905)10038161003816100381610038161003816
2
119889120582] 119889119904
+ 2Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119906
120576
119905) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582] 119889119904
(97)
Note that
Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119906
120576
119905) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582] 119889119904
= Eint119903+120576
119903
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119906
120576
119905) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582] 119889119904
le sup119904isin[119905119879]
E [E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119906
120576
119905) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582]] 120576
997888rarr 0 as 120576 997888rarr 0
(98)
The inequality above holds due to the boundedness of|119870
120576
119904|2
int1
0|119891119910(119906
120576
119905) minus 119891
119910(119904)|
2
119889120582 Indeed Assumption (L3) im-plies the boundedness of 119891
119910(119906
120576
119905) minus 119891
119910(119904) Meanwhile 119870120576
119904is
the solution of mean-field BSEE (82) It can be easy to check119870120576
119904is bounded since the coefficients in (82) are boundedOn the other hand
Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119904 120582) minus 119891
119910(119906
120576
119905)10038161003816100381610038161003816
2
119889120582] 119889119904
le 11986221205822Eint
119879
119905
1003816100381610038161003816119870120576
119904
10038161003816100381610038162
times int
1
0
1198641015840[100381610038161003816100381610038161003816(119884
120576
119904minus 119884
119904)1015840100381610038161003816100381610038161003816
2
+100381610038161003816100381610038161003816(119885
120576
119904minus 119885
119904)1015840100381610038161003816100381610038161003816
2
times10038161003816100381610038161003816119884120576
119904minus 119884
119904
10038161003816100381610038161003816
2
+10038161003816100381610038161003816119885120576
119904minus 119885
119904
10038161003816100381610038161003816
2
] 119889120582 119889119904
(99)
where (119884120576
119904 119885
120576
119904) is the mild solution of the following equation
119889119884120576
119905= minus 119860119884
120576
119905119889119905
minus E1015840[119891 (119905 (119884
120576
119905)1015840
(119885120576
119905)1015840
119884120576
119905 119885
120576
119905 119906
120576
119905)] 119889119905
+ 119885120576
119905119889119882 (119905) 119905 isin [0 119879]
119884120576
119879= 120585
(100)
and (119884119904 119885
119904) is the mild solution of
119889119884119905= minus 119860119884
119905119889119905
minus E1015840[119891 (119905 (119884
119905)1015840
(119885119905)1015840
119884119905 119885
119905 119906
119905)] 119889119905
+ 119885119905119889119882 (119905) 119905 isin [0 119879]
119884119879= 120585
(101)
By the definition of 119906120576119905 according to (L2) we have
E1015840[119891 (119904 119884
1015840
(119904) 1198851015840
(119904) 119884 (119904) 119885 (119904) 119906120576
119905)]
997888rarr E1015840[119891 (119904 119884
1015840
(119904) 1198851015840
(119904) 119884 (119904) 119885 (119904) 119906119905)]
(102)
in 1198712([0 119879]119867) as 120576 rarr 0 Using the continuous dependencetheorem Corollary 5 we obtain
(119884120576
119904 119885
120576
119904) 997888rarr (119884
119904 119885
119904) as 120576 997888rarr 0 (103)
Then
Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119904 120582) minus 119891
119910(119906
120576
119905)10038161003816100381610038161003816
2
119889120582] 119889119904
le 11986221205822Eint
119879
119905
1003816100381610038161003816119870120576
119904
10038161003816100381610038162
times int
1
0
E1015840[100381610038161003816100381610038161003816(119884
120576
119904minus 119884
119904)1015840100381610038161003816100381610038161003816
2
+100381610038161003816100381610038161003816(119885
120576
119904minus 119885
119904)1015840100381610038161003816100381610038161003816
2
times10038161003816100381610038161003816119884120576
119904minus 119884
119904
10038161003816100381610038161003816
2
+10038161003816100381610038161003816119885120576
119904minus 119885
119904
10038161003816100381610038161003816
2
] 119889120582 119889119904
997888rarr 0 as 120576 997888rarr 0
(104)
14 Mathematical Problems in Engineering
Combining (98) with (104) we finally haveEint
119879
119905E1015840[|1198682
119904|2
]119889119904 rarr 0 as 120576 rarr 0The required result (93) follows by using the similar
estimations for 1198681119904 1198683
119904 and 1198684
119904
Note that 1 minus 1921198722
1198601198622120573 gt 0 if 120573 is sufficiently large
Now to prove the desired result (84) it suffices to applyGronwallrsquos lemma and estimate (93) to inequality (92)
To deal with the state constraint (75) we need to recall theEkeland variational principle
Lemma 16 (Ekelandrsquos variational principle see [16 Lemma41]) Let (119878 119889) be a complete metric space and let 119865(sdot) 119878 rarr
R be lower semicontinuous and bounded from below If for 120588 gt0 there exists 119906 isin 119878 such that
119865 (119906) le infVisin119878
119865 (V) + 120588 (105)
then there exists 119906120588 isin 119878 satisfying
(i) 119865 (119906120588) le 119865 (119906)
(ii) 119889 (119906120588 119906) le 120588
(iii) 119865 (119906120588) le 119865 (V) + 120588 sdot 119889 (119906120588 119906) forallV = 119906120588
(106)
Now fix V isin Uad and set
119878 = V (sdot) isin Uad | sup0le119905le119879
E1003816100381610038161003816V11990510038161003816100381610038162
le E100381610038161003816100381611990611990510038161003816100381610038162
+ |V|2
119889 (V (sdot) V (sdot)) = 119898 119905 isin [0 119879] | E1003816100381610038161003816V119905 minus V
119905
10038161003816100381610038162
gt 0
forallV (sdot) V (sdot) isin 119878
(107)
where119898 denotes the Lebesgue measure on RThe following result is proved as Proposition 41 in [16]
Lemma 17 (119878 119889(sdot sdot)) is a complete metric space and 119869120588 is
continuous and bounded on 119878 where
119869120588
(V (sdot)) = (119869 (V (sdot)) minus 119869 (119906 (sdot)) + 120588)2
+
10038161003816100381610038161003816100381610038161003816Eint
O
Φ(119909 (1198840(119909))
1015840
1198840(119909)) 119889119909
10038161003816100381610038161003816100381610038161003816
2
12
forallV (sdot) isin 119878(108)
and (119884 119885) is the mild solution of (72) corresponding to thecontrol V
Now we consider the following free initial state optimalcontrol problem
infV(sdot)isin119878
119869120588
(V (sdot)) (109)
It is easy to check that
0 le infV(sdot)isin119878
119869120588
(V (sdot)) le 119869120588
(119906 (sdot)) = 120588 (110)
According to Ekelandrsquos variational principle there exists a119906120588(sdot) isin 119881 such that
(i) 119869120588 (119906120588 (sdot)) le 120588
(ii) 119889 (119906120588 (sdot) 119906 (sdot)) le 120588
(iii) 119869120588 (119906120588 (sdot)) le 119869120588
(V (sdot)) + 120588119889 (119906120588 (sdot) 119906 (sdot)) forallV (sdot) isin 119878(111)
Using the spike variationmethod we can construct 119906120576120588(sdot) isin 119878as follows
119906120576120588
119905=
V119905 119905 isin [119904 119904 + 120576]
119906120588
119905 119905 isin [0 119879] [119904 119904 + 120576]
(112)
It is clear that 119889(119906120576120588(sdot) 119906120588(sdot)) le 120576 Let (119884120576120588(sdot) 119885
120576120588(sdot)) (resp
(119884120588(sdot) 119885
120588(sdot))) be the solution of (72) with respect to the
control 119906120576120588(sdot) (resp 119906120588(sdot)) Following (82) (119870120576120588
119905 119876
120576120588
119905) is the
mild solution of
119870120576120588
119905= int
119879
119905
119890119860(119904minus119905)
E1015840
times [1198911199101015840 (119904 Λ
120588
119904 119906
120588
119904) (119870
120576120588
119904)1015840
+ 119891119910(119904 Λ
120588
119904 119906
120588
119904)119870
120576120588
119904
+ 1198911199111015840 (119904 Λ
120588
119904 119906
120588
119904) (119876
120576120588
119904)1015840
+ 119891119911(119904 Λ
120588
119904 119906
120588
119904) 119876
120576120588
119904
+1
120576(119891 (119904 Λ
120588
119904 119906
120576120588
119904) minus 119891 (119904 Λ
120588
119904 119906
120588
119904))] 119889119904
minus int
119879
119905
119890119860(119904minus119905)
119876120576120588
119904119889119882 (119904)
(113)
ByTheorem 15 we know that
lim120576rarr0
E[ sup119904isin[119905119879]
100381610038161003816100381610038161003816100381610038161003816
119884120576120588
119904minus 119884
120588
119904
120576minus 119870
120576120588
119904
100381610038161003816100381610038161003816100381610038161003816
2
] = 0 forall119905 isin [0 119879]
lim120576rarr0
Eint119879
0
100381610038161003816100381610038161003816100381610038161003816
119885120576120588
119904minus 119885
120588
119904
120576minus 119876
120576120588
119904
100381610038161003816100381610038161003816100381610038161003816
2
119889119904 = 0
(114)
The proof of the following proposition is technical butbased on the arguments above and we omit it
Proposition 18 One has
1
120576E [Φ ((119884
120576120588
0)1015840
119884120576120588
0) minus Φ((119884
120588
0)1015840
119884120588
0)]
= E [Φ1199101015840 ((119884
120588
0)1015840
119884120588
0) (119870
120576120588
0)1015840
+ Φ119910((119884
120588
0)1015840
119884120588
0)119870
120576120588
0]
+ 119900 (120576)
Mathematical Problems in Engineering 15
1
120576(119869 (119906
120576120588) minus 119869 (119906
120588))
= E [1198921199101015840 ((119884
120588
0)1015840
119884120588
0) (119870
120576120588
0)1015840
+ 119892119910((119884
120588
0)1015840
119884120588
0)119870
120576120588
0]
+ Δ120576+1
120576Eint
119879
0
E1015840[ℎ (119904 Λ
120588
119904 119906
120576120588
119904)
minus ℎ (119904 Λ120588
119904 119906
120588
119904)] 119889119904 + 119900 (120576)
(115)
where
Δ120576= Eint
119879
0
E1015840[ℎ
1199101015840 (119904 Λ
120588
119904 119906
120576120588
119904) (119870
120576120588
119904)1015840
+ ℎ1199111015840 (119904 Λ
120588
119904 119906
120576120588
119904) (119876
120576120588
119904)1015840
+ ℎ119910(119904 Λ
120588
119904 119906
120576120588
119904)119870
120576120588
119904
+ ℎ119911(119904 Λ
120588
119904 119906
120576120588
119904) 119876
120576120588
119904] 119889119904
(116)
53 Variational Inequality and Adjoint Equation In thissubsection the adjoint process is introduced to deduce thevariational inequality
If we set V(sdot) = 119906120576120588(sdot) in (111) and notice that
119889(119906120576120588(sdot) 119906
120588(sdot)) le 120576 we get
minus120588 le1
120576(119869
120588(119906
120576120588
(sdot)) minus 119869120588(119906
120588
(sdot))) (117)
By Lemma 17
1
120576(119869
120588(119906
120576120588
(sdot)) minus 119869120588(119906
120588
(sdot)))
=(119869
120588(119906
120576120588(sdot)))
2
minus (119869120588(119906
120588(sdot)))
2
120576 (119869120588 (119906120576120588 (sdot)) + 119869120588 (119906120588 (sdot)))
=119869 (119906
120576120588(sdot)) + 119869 (119906
120588(sdot)) minus 2119869 (119906 (sdot)) + 2120588
119869120588 (119906120576120588 (sdot)) + 119869120588 (119906120588 (sdot))
times119869 (119906
120576120588(sdot)) minus 119869 (119906
120588(sdot))
120576
+
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] + E [Φ ((119884
120588
0)1015840
119884120588
0)]
119869120588 (119906120576120588 (sdot)) + 119869120588 (119906120588 (sdot))
times
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] minus E [Φ ((119884
120588
0)1015840
119884120588
0)]
120576
997888rarr 119897120588
1
119869 (119906120576120588(sdot)) minus 119869 (119906
120588(sdot))
120576
+ 119897120588
2
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] minus E [Φ ((119884
120588
0)1015840
119884120588
0)]
120576
(118)
where we set
119897120588
1=119869 (119906
120588(sdot)) minus 119869 (119906 (sdot)) + 120588
119869120588 (119906120588 (sdot))
119897120588
2=
E [Φ ((119884120588
0)1015840
119884120588
0)]
119869120588 (119906120588 (sdot))
(119)
and use the limit
119869 (119906120576120588
(sdot)) 997888rarr 119869 (119906120588
(sdot))
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] 997888rarr E [Φ ((119884
120588
0)1015840
119884120588
0)]
(120)
as 120576 rarr 0 according to (115)As |119897120588
1|2
+ |119897120588
2|2
= 1 for all 120588 gt 0 we know that there existsa subsequence of 119897120588
1 119897120588
2 (still denoted by 119897120588
1 119897120588
2) such that
lim120588rarr0
119897120588
1 119897120588
2 = 119897
1 1198972
1003816100381610038161003816119897110038161003816100381610038162
+1003816100381610038161003816119897210038161003816100381610038162
= 1
(121)
Combining (115) (117) with (118) we get
minus120588 le 119897120588
1
119869 (119906120576120588(sdot)) minus 119869 (119906
120588(sdot))
120576
+ 119897120588
2
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] minus E [Φ ((119884
120588
0)1015840
119884120588
0)]
120576
= 119897120588
1Δ120576+ 119897
120588
1E [119892
1199101015840 ((119884
120588
0)1015840
119884120588
0) (119870
120576120588
0)1015840
+119892119910((119884
120588
0)1015840
119884120588
0)119870
120576120588
0]
+119897120588
1
120576Eint
119879
0
E1015840[ℎ (119904 Λ
120588
119904 119906
120576120588
119904) minus ℎ (119904 Λ
120588
119904 119906
120588
119904)] 119889119904
+ 119897120588
2E [Φ
1199101015840 ((119884
120588
0)1015840
119884120588
0) (119870
120576120588
0)1015840
+Φ119910((119884
120588
0)1015840
119884120588
0)119870
120576120588
0] + (119897
120588
1+ 119897
120588
2) 119900 (120576)
(122)
Next we introduce the adjoint equation corresponding tovariational equation (113) whose solution is denoted by119875120588(119905)
119889119875120588
(119905)
= 119860lowast119875120588
(119905) 119889119905
+ E1015840[119891
1199101015840 (119905 Λ
120588
119905 119906
120588
119905) (119875
120588
(119905))1015840
+ 119891119910(119905 Λ
120588
119905 119906
120588
119905) 119875
120588
(119905) + 119897120588
1ℎ1199101015840 (119905 Λ
120588
119905 119906
120588
119905)
+ 119897120588
1ℎ119910(119905 Λ
120588
119905 119906
120588
119905)] 119889119905
16 Mathematical Problems in Engineering
+ E1015840[119891
1199111015840 (119905 Λ
120588
119905 119906
120588
119905) (119875
120588
(119905))1015840
+ 119891119911(119905 Λ
120588
119905 119906
120588
119905) 119875
120588
(119905)
+ 119897120588
1ℎ1199111015840 (119905 Λ
120588
119905 119906
120588
119905) + 119897
120588
1ℎ119911(119905 Λ
120588
119905 119906
120588
119905)] 119889119882 (119905)
119875120588
(0) = 119897120588
1E [119892
1199101015840 ((119884
120588
0(119909))
1015840
119884120588
0(119909))
+ 119892119910((119884
120588
0(119909))
1015840
119884120588
0(119909))]
+ 119897120588
2E [Φ
1199101015840 ((119884
120588
0(119909))
1015840
119884120588
0(119909))
+ Φ119910((119884
120588
0(119909))
1015840
119884120588
0(119909))]
(123)
where 119860lowast is the 1198712(O)-adjoint operator of 119860 Under assump-tions (L1)ndash(L3) this is a linear mean-field SEE with boundedcoefficients An application ofTheorem 11 implies that it has aunique adaptedmild solution such that119875120588(119905) isin S2
F ([0 119879] 119870)When 120588 rarr 0 according to Corollaries 5 and 13 119875120588(119905)
converges to 119875(119905) where
119875 (119905) isin S2
F ([0 119879] 119870) (124)
is the solution of the following equation
119875 (119905) = 1198971E [119892
1199101015840 (119884
1015840
0 119884
0) + 119892
119910(119884
1015840
0 119884
0)]
+ 1198972E [Φ
1199101015840 (119884
1015840
0 119884
0) + Φ
119910(119884
1015840
0 119884
0)]
+ int
119905
0
119890119860lowast(119905minus119904)
E1015840[119891
1199101015840 (119904) 119875
1015840
(119904) + 119891119910(119904) 119875 (119904)
+ 1198971ℎ1199101015840 (119904) + 119897
1ℎ119910(119904)] 119889119904
+ int
119905
0
119890119860lowast(119905minus119904)
E1015840[119891
1199111015840 (119904) 119875
1015840
(119904) + 119891119911(119904) 119875 (119904)
+ 1198971ℎ1199111015840 (119904) + 119897
1ℎ119911(119904)] 119889119882 (119904)
(125)
The following proposition which formally follows fromProposition 18 gives the relation between 119875120588(119905) and119870120576120588
119905
Proposition 19 Consider the following
E [119875120588
(0)119870120576120588
0]
= Eint119879
0
1
120576119875120588
(119904)E1015840[119891 (119904 Λ
120588
119904 119906
120576120588
119904) minus 119891 (119904 Λ
120588
119904 119906
120588
119904)]
minus 119897120588
1119870120576120588
119904E1015840[ℎ
1199101015840 (119904 Λ
120588
119904 119906
120588
119904) + ℎ
119910(119904 Λ
120588
119904 119906
120588
119904)]
minus 119897120588
1119876120576120588
119904E1015840[ℎ
1199111015840 (119904 Λ
120588
119904 119906
120588
119904) + ℎ
119911(119904 Λ
120588
119904 119906
120588
119904)] 119889119904
(126)
The following theorem constitutes the main contributionof this section themaximumprinciple for the BSPDE controlsystem
Theorem 20 Let assumptions (L1)ndash(L3) hold Suppose 119906(sdot) isan optimal control and (119884(sdot) 119885(sdot)) is the corresponding optimalstate trajectory for the BSPDE control systems (72) and (73)with the initial state constraint (75) Then there exists 119875(119905) isinS2
F ([0 119879] 119870) which satisfies (125) such that
H (119905 1198841015840
119905 119885
1015840
119905 119884
119905 119885
119905 V
119905 119875 (119905))
ge H (119905 1198841015840
119905 119885
1015840
119905 119884
119905 119885
119905 119906
119905 119875 (119905))
119886119890 119886119904 forallV isin U119886119889
(127)
whereH [0 119879]times119867timesL(Γ119867)times119867timesL(Γ119867)times119880times119870 rarr R
is the Hamiltonian function defined by
H (119905 119910 119910 119911 V 119901) = 1198971ℎ (119905 119910 119910 119911 V)
+ 119901119891 (119905 119910 119910 119911 V) (128)
Proof By (122) and Proposition 19 we obtain
minus120588 le1
120576Eint
119879
0
119875120588
(119904)E1015840[119891 (119904 Λ
120588
119904 119906
120576120588
119904)
minus 119891 (119904 Λ120588
119904 119906
120588
119904)] 119889119904
+119897120588
1
120576Eint
119879
0
E1015840[ℎ (119904 Λ
120588
119904 119906
120576120588
119904)
minus ℎ (119904 Λ120588
119904 119906
120588
119904)] 119889119904 + (119897
120588
1+ 119897
120588
2) 119900 (120576)
(129)
Letting 120576 rarr 0+ in (129) we derive for ae 120591 isin [0 119879]
minus120588 le 119897120588
1E1015840[ℎ (120591 Λ
120588
120591 V
120591) minus ℎ (120591 Λ
120588
120591 119906
120588
120591)]
+ 119875120588
(119904)E1015840[119891 (120591 Λ
120588
120591 V
120591) minus 119891 (120591 Λ
120588
120591 119906
120588
120591)]
(130)
for all V isin UadFinally taking 120588 rarr 0 in (130) we derive the desired
result
Remark 21 We note that if the coefficients do not dependexplicitly on the marginal law of the underlying diffusionthe result reduces to the classical case that is the SMP forBSPDEs without mean-field term
Remark 22 When we remove the initial state constraint (75)we obtain the general maximum principle for the mean-fieldBSPDEs system (ie without the constraint) with 119897
1= 1
54 Application A Backward Linear Quadratic Control Prob-lem Now we apply our maximum principle to solve an LQproblem For notational simplicity we restrict ourselves tothe free case (ie without the initial state constraint (75)) thegeneral case being handled in a similar way
Mathematical Problems in Engineering 17
Consider the following problem
119869 (119906) =1
2E [(119884 (0))
2] +
1
2Eint
119879
0
119873V2 (119905) 119889119905 997888rarr min
(131)
subject to
119889119884 (119905) = minus119860119884 (119905) 119889119905
minus 119861119884 (119905) + 119861E [119884 (119905)]
+119862V (119905) + 119863119885 (119905) + 119863E [119885 (119905)] 119889119905
+ 119885 (119905) 119889119882 (119905)
119884 (119879) = 120585 119905 isin [0 119879]
(132)
where 119860 is a partial differential operator satisfying condition(L1) and 119861 119861 119862119863119863 and119873 are bounded and deterministicconstantsWe also assume that119873 gt 0 and V isin 1198712F (0 119879 119880) It iseasy to verify that BSPDE (132) admits a uniquemild solution(119884(119905) 119885(119905))
119875(119905) the adjoint process of state equation (132) is thesolution of
119889119875 (119905) = 119860lowast119875 (119905) 119889119905 + 119861119875 (119905) + 119861E [119875 (119905)] 119889119905
+ 119863119875 (119905) + 119863E [119875 (119905)] 119889119882 (119905)
119875 (0) = 119884 (0)
(133)
Let 119906(sdot) be an optimal control and let (119884(sdot) 119885(sdot)) bethe corresponding state process By maximum principle ofTheorem 20 (note that 119897
1= 1 in this problem)
1
2119873V2 (119905) + 119875 (119905) 119862V (119905) ge
1
2119873119906
2
(119905) + 119875 (119905) 119862119906 (119905) (134)
for all V isin 119880ad since the state equation has the form (132)Thisin turn implies
119906 (119905) = minus119862
119873119875 (119905) (135)
It is clear that (131) is a positive quadratic functional of controlbecause of 119873 gt 0 Hence an optimal control exists Thecandidate optimal control (135) is indeed an optimal controlof this LQ problem for it is the only control which satisfies themaximum principle
Next we want to obtain a more explicit representationof the optimal control (135) from the state equation (132)Substituting (135) into state equation (132) yields
119889119884 (119905) = minus119860119884 (119905) 119889119905
minus 119861119884 (119905) + 119861E [119884 (119905)] minus1198622
119873119875 (119905)
+119863119885 (119905) + 119863E [119885 (119905)] 119889119905 + 119885 (119905) 119889119882 (119905)
119884 (119879) = 120585 119905 isin [0 119879]
(136)
Combining the above equation with (133) we obtain thefollowing related feedback control system
119889119884 (119905) = minus119860119884 (119905) 119889119905
minus 119861119884 (119905) + 119861E [119884 (119905)] minus1198622
119873119875 (119905)
+ 119863119885 (119905) + 119863E [119885 (119905)] 119889119905 + 119885 (119905) 119889119882 (119905)
119884 (119879) = 120585
119889119875 (119905) = 119860lowast119875 (119905) 119889119905 + 119861119875 (119905) + 119861E [119875 (119905)] 119889119905
+ 119863119875 (119905) + 119863E [119875 (119905)] 119889119882 (119905)
119875 (0) = 119884 (0)
(137)
Looking at the terminal condition of 119875(119905) in (133) andconsidering the mean-field type of (132) it is reasonable toconjecture that 119875(119905) has the following form
119875 (119905) = 120593 (119905) 119884 (119905) + 120601 (119905)E [119884 (119905)] (138)
where120593(119905)120601(119905) are deterministic differential functionswhichwill be specified below Moreover 120593(0) = 1 120601(0) = 0
Inserting this form into adjoint equation (133) and notic-ing that 119884(119905) satisfies (136) we can compare the coefficientsof 119889119905 and 119889119882(119905) to obtain the following equation
2120593 (119905) 119860119884 (119905) + 2120601 (119905) 119860E [119884 (119905)] + 119861120593 (119905) 119884 (119905)
+ 2 (119861120601 (119905) + 119861120593 (119905) + 119861120601 (119905))E [119884 (119905)]
= minus120593 (119905) 119861119884 (119905) minus1198622
119873120593 (119905) 119884 (119905) + 119863119885 (119905)
+ 119884 (119905)119889120593 (119905)
119889119905+ E [119884 (119905)]
119889120601 (119905)
119889119905
+ (1206012
(119905)1198622
119873+ 2
1198622
119873120593 (119905) 120601 (119905))E [119884 (119905)]
minus (119863120601 (119905) + 119863120601 (119905) + 119863120593 (119905))E [119885 (119905)]
120593 (119905) 119885 (119905) = 119863120593 (119905) 119884 (119905)
+ (119863120601 (119905) + 119863120593 (119905) + 119863120601 (119905))E [119884 (119905)]
(139)
Then subtracting 119885(119905) we have
2120593 (119905) 119860119884 (119905) + 2120601 (119905) 119860E [119884 (119905)] + 119861120593 (119905) 119884 (119905)
+ 2 (119861120601 (119905) + 119861120593 (119905) + 119861120601 (119905))E [119884 (119905)]
18 Mathematical Problems in Engineering
= 120593 (119905) minus119861 +1198622
119873120593 (119905) minus 119863
2119884 (119905)
+ 119884 (119905)119889120593 (119905)
119889119905+ E [119884 (119905)]
119889120601 (119905)
119889119905
+ (1206012
(119905)1198622
119873+ 2
1198622
119873120593 (119905) 120601 (119905))E [119884 (119905)]
minus1
120593 (119905)(119863120601 (119905) + 119863120593 (119905) + 119863120601 (119905))
2
E [119884 (119905)]
minus 2119863 (119863120601 (119905) + 119863120593 (119905) + 119863120601 (119905))E [119884 (119905)]
(140)
Comparing the coefficients of 119884(119905) and E[119884(119905)] respec-tively we get
119889
119889119905120593 (119905) = 2119860
lowast120593 (119905) + 2119861120593 (119905) + 119863
2120593 (119905) minus
1198622
1198731205932
(119905)
120593 (0) = 1
(141)
119889
119889119905120601 (119905) = 2119860
lowast120601 (119905) + (
(119863 + 119863)2
120593 (119905)minus1198622
119873)120601
2
(119905)
+ 2119877 (119905) 120601 (119905) + (1198632+ 2119861 + 2119863119863)120593 (119905)
120601 (0) = 0
(142)
where 119877(119905) = 119861 + 119861 + (119863 + 119863)2
minus (1198622119873)120593(119905)
We solve (141) to get
120593 (119905) = (119890minus(2119860lowast+2119861+119863
2)119905(1 minus
1198622
119873(2119860lowast + 2119861 + 1198632))
+1198622
119873(2119860lowast + 2119861 + 1198632))
minus1
(143)
Then (142) exists a unique solution from the classical Riccatiequation theory
We now conclude the above discussions in the followingresult
Theorem 23 For onersquos linear quadratic stochastic partialdifferential control problem (131)-(132) the unique optimalcontrol 119906(sdot) isin 119880ad is given by
119906 (119905) = minus119862
119873(120593 (119905) 119884 (119905) + 120601 (119905)E [119884 (119905)]) (144)
with 120593(119905) satisfying (143) and 120601(119905) solving (142)
Conflict of Interests
The authors declare that they have no conflict of interestsregarding the publication of this paper
Acknowledgment
This research is partially supported by the Natural ScienceFoundation of China under Grant no 11301310
References
[1] Y Hu and S Peng ldquoAdapted solution of a backward semilinearstochastic evolution equationrdquo Stochastic Analysis and Applica-tions vol 9 no 4 pp 445ndash459 1991
[2] N IMahmudov andMAMcKibben ldquoOnbackward stochasticevolution equations in Hilbert spaces and optimal controlrdquoNonlinear Analysis vol 67 no 4 pp 1260ndash1274 2007
[3] M Fuhrman and G Tessitore ldquoNonlinear Kolmogorov equa-tions in infinite dimensional spaces the backward stochasticdifferential equations approach and applications to optimalcontrolrdquoThe Annals of Probability vol 30 no 3 pp 1397ndash14652002
[4] M Fuhrman and G Tessitore ldquoInfinite horizon backwardstochastic differential equations and elliptic equations inHilbert spacesrdquo Annals of Probability vol 32 no 1B pp 607ndash660 2004
[5] X Mao ldquoAdapted solutions of backward stochastic differentialequations with non-Lipschitz coefficientsrdquo Stochastic Processesand their Applications vol 58 no 2 pp 281ndash292 1995
[6] J Lasry and P Lions ldquoMean field gamesrdquo Japanese Journal ofMathematics vol 2 no 1 pp 229ndash260 2007
[7] R Buckdahn J Li and S Peng ldquoMean-field backward stochas-tic differential equations and related partial differential equa-tionsrdquo Stochastic Processes and their Applications vol 119 no 10pp 3133ndash3154 2009
[8] D Andersson and B Djehiche ldquoAmaximum principle for SDEsofmean-field typerdquoAppliedMathematics andOptimization vol63 no 3 pp 341ndash356 2011
[9] R Buckdahn B Djehiche J Li and S Peng ldquoMean-fieldbackward stochastic differential equations a limit approachrdquoThe Annals of Probability vol 37 no 4 pp 1524ndash1565 2009
[10] R Xu ldquoMean-field backward doubly stochastic differentialequations and related SPDEsrdquo Boundary Value Problems vol2012 article 114 2012
[11] R Buckdahn B Djehiche and J Li ldquoA general stochasticmaximum principle for SDEs of mean-field typerdquo AppliedMathematics and Optimization vol 64 no 2 pp 197ndash216 2011
[12] J Li ldquoStochasticmaximumprinciple in themean-field controlsrdquoAutomatica vol 48 no 2 pp 366ndash373 2012
[13] G Wang C Zhang and W Zhang ldquoStochastic maximumprinciple for mean-field type optimal control under partialinformationrdquo IEEE Transactions on Automatic Control vol 59no 2 pp 522ndash528 2014
[14] M Hafayed ldquoA mean-field necessary and sufficient conditionsfor optimal singular stochastic controlrdquo Communications inMathematics and Statistics vol 1 no 4 pp 417ndash435 2013
[15] M Hafayed ldquoA mean-field maximum principle for optimalcontrol of forward-backward stochastic differential equationswith Poisson jumpprocessesrdquo International Journal ofDynamicsand Control vol 1 no 4 pp 300ndash315 2013
[16] Y Hu and S Peng ldquoMaximum principle for semilinearstochastic evolution control systemsrdquo Stochastics and StochasticsReports vol 33 no 3-4 pp 159ndash180 1990
[17] M Fuhrman Y Hu and G Tessitore ldquoStochastic maximumprinciple for optimal control of SPDEsrdquo Comptes Rendus Math-ematique vol 350 no 13-14 pp 683ndash688 2012
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Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Mathematical Problems in Engineering
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Stochastic AnalysisInternational Journal of
Mathematical Problems in Engineering 11
An element ofU is called an admissible controlFor any V isin U we consider the following controlled
BSPDE system in the state space119867 = 1198712(O) (norm | sdot | scalar
product ⟨sdot sdot⟩)
119889119884119905(119909) = minus119860119884
119905(119909) 119889119905
minus E1015840[119891 (119905 119909 (119884
119905(119909))
1015840
(119885119905(119909))
1015840
119884119905(119909) 119885
119905(119909) V
119905)] 119889119905
+ 119885119905(119909) 119889119882 (119905) 119905 isin [0 119879]
119884119879(119909) = 120585 (119909) 119909 isin O
(72)
where 119860 is a partial differential operator 119891 [0 119879] timesO times119867 times
L(Γ119867)times119867timesL(Γ119867)times119880 rarr 119867 and 120585 isin 1198712(ΩF119879P 119867)
The cost functional is given by
119869 (V) = Eint119879
0
intO
E1015840[ℎ (119904 119909 (119884
119904(119909))
1015840
(119885119904(119909))
1015840
119884119904(119909) 119885
119904(119909) V
119904)] 119889119909 119889119904
+E1015840intO
119892 (119909 (1198840(119909))
1015840
1198840(119909)) 119889119909
(73)
where
ℎ [0 119879] times O times 119867 timesL (Γ119867) times 119867 timesL (Γ119867) times 119880 997888rarr R
119892 O times 119867 times119867 997888rarr R
(74)
Our purpose is to minimize the functional 119869(sdot) over Uadsubject to the following state constraint
EintO
Φ(119909 (1198840(119909))
1015840
1198840(119909)) 119889119909 = 0 (75)
where
Φ O times 119867 times119867 997888rarr R (76)
An admissible control 119906 isin Uad that satisfies
119869 (119906) = minVisinUad
119869 (V) (77)
is called optimalThrough what follows the following assumptions will be
in force
(L1) 119860 is a partial differential operator with appropri-ate boundary conditions We assume that 119860 is theinfinitesimal generator of a strongly continuous semi-group 119890119905119860 119905 ge 0 in119867 Moreover for every 119905 isin [0 119879]119890
119905119860119891
1198712(O) le 119872
119860119891
1198712(O) for some constant 119872
119860
independent of 119905 and 119891(L2) 119891 ℎ 119892 and Φ are continuously Gateaux differen-
tiable with respect to (1199101015840 1199111015840 119910 119911) 119891 is continuouslyGateaux differentiable with respect to V and ℎ iscontinuous with respect to V
(L3) The derivatives of 119891 ℎ 119892 and Φ are Lipschitzcontinuous and bounded by
100381610038161003816100381610038161198911199101015840
10038161003816100381610038161003816+10038161003816100381610038161198911199111015840
1003816100381610038161003816 +10038161003816100381610038161003816119891119910
10038161003816100381610038161003816+1003816100381610038161003816119891119911
1003816100381610038161003816 +1003816100381610038161003816119891V
1003816100381610038161003816 +10038161003816100381610038161003816Φ1199101015840
10038161003816100381610038161003816+10038161003816100381610038161003816Φ119910
10038161003816100381610038161003816le 119862
10038161003816100381610038161003816ℎ1199101015840
10038161003816100381610038161003816+1003816100381610038161003816ℎ1199111015840
1003816100381610038161003816 +10038161003816100381610038161003816ℎ119910
10038161003816100381610038161003816+1003816100381610038161003816ℎ119911
1003816100381610038161003816 +100381610038161003816100381610038161198921199101015840
10038161003816100381610038161003816+10038161003816100381610038161003816119892119910
10038161003816100381610038161003816
le 119862 (1 +10038161003816100381610038161003816119910101584010038161003816100381610038161003816+10038161003816100381610038161003816119911101584010038161003816100381610038161003816+10038161003816100381610038161199101003816100381610038161003816 + |119911| + |V|)
(78)
where 119862 is a positive constant
Obviously according to Theorem 4 state equation (72)has a unique mild solution under the above assumptions
Remark 14 We can define the second order differentialoperator
(119860119891) (119909) =
119899
sum
119894119895=1
119886119894119895(119909)
1205972119891
120597119909119894120597119909
119895
(119909) +
119899
sum
119894=1
119887119894(119909)
120597119891
120597119909119894
(119909) (79)
By Example 9 119860 fulfills assumption (L1) if 119886119894119895 119887
119894satisfy
condition (H1)
52 Variation of the Trajectory Let 119906 be an optimal controlwith (119884(sdot) 119885(sdot)) being the corresponding optimal state Let120576 gt 0 and [119903 119903 + 120576] sube [0 119879] For any given V isin Uad weintroduce the spike variation of the control 119906(sdot)
119906120576
119905=
V119905 119905 isin [119903 119903 + 120576]
119906119905 119905 isin [0 119879] [119904 119904 + 120576]
(80)
It is clear that 119906120576(sdot) isin UadLet (119884120576
(sdot) 119885120576(sdot)) be the trajectory corresponding to 119906120576(sdot)
We use the following short notation for brevity
119891 (119905) = 119891 (119905 119909 (119884119905(119909))
1015840
(119885119905(119909))
1015840
119884119905(119909) 119885
119905(119909) 119906
119905)
119891 (119906120576
119905) = 119891 (119905 119909 (119884
119905(119909))
1015840
(119885119905(119909))
1015840
119884119905(119909) 119885
119905(119909) 119906
120576
119905)
(81)
Consider the following equation
119889119870120576
119905(119909) = minus119860119870
120576
119905(119909) 119889119905
minus E1015840[119891
1199101015840 (119905) (119870
120576
119905(119909))
1015840
+ 119891119910(119905) 119870
120576
119905(119909)
+ 1198911199111015840 (119905) (119876
120576
119905(119909))
1015840
+ 119891119911(119905) 119876
120576
119905(119909)
+1
120576(119891 (119906
120576
119905) minus 119891 (119905))] 119889119905 + 119876
120576
119905(119909) 119889119882 (119905)
119870120576
119879(119909) = 0
(82)
Since the coefficients in (82) are bounded it is easy to checkthat there exists a unique mild solution such that
E[ sup119905isin[0119879]
1003816100381610038161003816119870120576
119905
10038161003816100381610038162
+ int
119879
0
1003816100381610038161003816119876120576
119905
10038161003816100381610038162
119889119905] lt infin (83)
We have the following estimate
12 Mathematical Problems in Engineering
Theorem 15 There holds
lim120576rarr0
E[ sup119904isin[119905119879]
1003816100381610038161003816100381610038161003816100381610038161003816
119884120576
119904minus 119884
119904
120576minus 119870
120576
119904
1003816100381610038161003816100381610038161003816100381610038161003816
2
] = 0 forall119905 isin [0 119879]
lim120576rarr0
Eint119879
0
1003816100381610038161003816100381610038161003816100381610038161003816
119885120576
119904minus 119885
119904
120576minus 119876
120576
119904
1003816100381610038161003816100381610038161003816100381610038161003816
2
119889119904 = 0
(84)
Proof We define
120578120576
119904=119884120576
119904minus 119884
119904
120576minus 119870
120576
119904 120577
120576
119904=119885120576
119904minus 119885
119904
120576minus 119876
120576
119904 119904 isin [0 119879]
(85)
For simplicity let us define
Λ120576
119904= ((119884
120576
119904)1015840
(119885120576
119904)1015840
119884120576
119904 119885
120576
119904)
119891 (119904 120582) = 119891 (119904 1198841015840
119904+ 120582(119884
120576
119904minus 119884
119904)1015840
1198851015840
119904+ 120582(119885
120576
119904minus 119885
119904)1015840
119884119904+ 120582 (119884
120576
119904minus 119884
119904)
119885119904+ 120582 (119885
120576
119904minus 119885
119904) 119906
120576
119904)
(86)
By the definition of (119884120576
119904 119885
120576
119904) (119884
119904 119885
119904) and (119870120576
119904 119876
120576
119904) (120578120576
119904 120577
120576
119904)
is the mild solution of
119889120578120576
119904= minus119860120578
120576
119904119889119904 minus E
1015840
[119871 (119904 120576)] 119889119904 + 120577120576
119904119889119882 (119904)
120578120576
119879= 0
(87)
with
119871 (119904 120576) =1
120576(119891 (119904 Λ
120576
119904 119906
120576
119904) minus 119891 (119906
120576
119904))
minus 1198911199101015840 (119904) (119870
120576
119904)1015840
minus 119891119910(119904) 119870
120576
119904minus 119891
1199111015840 (119904) (119876
120576
119904)1015840
minus 119891119911(119904) 119876
120576
119904
= ((120578120576
119904)1015840
+ (119870120576
119904)1015840
) int
1
0
1198911199101015840 (119904 120582) 119889120582 + (120578
120576
119904+ 119870
120576
119904)
times int
1
0
119891119910(119904 120582) 119889120582 minus 119891
1199101015840 (119904) (119870
120576
119904)1015840
minus 119891119910(119904) 119870
120576
119904
+ ((120577120576
119904)1015840
+ (119876120576
119904)1015840
)int
1
0
1198911199111015840 (119904 120582) 119889120582 + (120577
120576
119904+ 119876
120576
119904)
times int
1
0
119891119911(119904 120582) 119889120582 minus 119891
1199111015840 (119904) (119876
120576
119904)1015840
minus 119891119911(119904) 119876
120576
119904
= (120578120576
119904)1015840
int
1
0
1198911199101015840 (119904 120582) 119889120582 + 120578
120576
119904int
1
0
119891119910(119904 120582) 119889120582
+ (120577120576
119904)1015840
int
1
0
1198911199111015840 (119904 120582) 119889120582 + 120577
120576
119904int
1
0
119891119911(119904 120582) 119889120582 + 120574
120576
119904
(88)
where we denote
120574120576
119904= (119870
120576
119904)1015840
int
1
0
(1198911199101015840 (119904 120582) minus 119891
1199101015840 (119904)) 119889120582
+ 119870120576
119904int
1
0
(119891119910(119904 120582) minus 119891
119910(119904)) 119889120582
+ (119876120576
119904)1015840
int
1
0
(1198911199111015840 (119904 120582) minus 119891
1199111015840 (119904)) 119889120582
+ 119876120576
119904int
1
0
(119891119911(119904 120582) minus 119891
119911(119904)) 119889120582
(89)
For any 120573 gt 0 according to Lemma 1 we obtain
E sup119905le119904le119879
11989021205731199041003816100381610038161003816120578
120576
119904
10038161003816100381610038162
+ Eint119879
119905
11989021205731199041003816100381610038161003816120577
120576
119904
10038161003816100381610038162
119889119904
le12119872
2
119860
120573int
119879
119905
1198902120573119904
E [10038161003816100381610038161003816E1015840
[119871 (119904 120576)]10038161003816100381610038161003816
2
] 119889119904
le12119872
2
119860
120573Eint
119879
119905
1198902120573119904
E1015840[|119871 (119904 120576)|
2] 119889119904
(90)
By condition (L3) we have
E [E1015840[|119871 (119904 120576)|
2]]
= E [E1015840[1003816100381610038161003816119871 (119904 120576) minus 120574
120576
119904+ 120574
120576
119904
10038161003816100381610038162
]]
le 81198622E [E
1015840[10038161003816100381610038161003816(120578
120576
119904)101584010038161003816100381610038161003816
2
+1003816100381610038161003816120578120576
119904
10038161003816100381610038162
+10038161003816100381610038161003816(120577
120576
119904)101584010038161003816100381610038161003816
2
+1003816100381610038161003816120577120576
119904
10038161003816100381610038162
]]
+ 2E [E1015840[1003816100381610038161003816120574120576
119904
10038161003816100381610038162
]]
le 161198622E [
1003816100381610038161003816120578120576
119904
10038161003816100381610038162
+1003816100381610038161003816120577120576
119904
10038161003816100381610038162
] + 2E [E1015840[1003816100381610038161003816120574120576
119904
10038161003816100381610038162
]]
(91)
Combined with (91) (90) yields
E sup119905le119904le119879
11989021205731199041003816100381610038161003816120578
120576
119904
10038161003816100381610038162
+ (1 minus192119872
2
1198601198622
120573)Eint
119879
119905
11989021205731199041003816100381610038161003816120577
120576
119904
10038161003816100381610038162
119889119904
le192119872
2
1198601198622
120573Eint
119879
119905
11989021205731199041003816100381610038161003816120578
120576
119904
10038161003816100381610038162
119889119904
+24119872
2
119860
120573Eint
119879
119905
1198902120573119904
[E1015840[1003816100381610038161003816120574120576
119904
10038161003816100381610038162
]] 119889119904
(92)
We claim that
Eint119879
119905
E1015840[1003816100381610038161003816120574120576
119904
10038161003816100381610038162
] 119889119904 997888rarr 0 as 120576 997888rarr 0 (93)
From (89)
120574120576
119904= 119868
1
119904+ 119868
2
119904+ 119868
3
119904+ 119868
4
119904 (94)
Mathematical Problems in Engineering 13
where
1198681
119904= (119870
120576
119904)1015840
int
1
0
(1198911199101015840 (119904 120582) minus 119891
1199101015840 (119904)) 119889120582
1198682
119904= 119870
120576
119904int
1
0
(119891119910(119904 120582) minus 119891
119910(119904)) 119889120582
1198683
119904= (119876
120576
119904)1015840
int
1
0
(1198911199111015840 (119904 120582) minus 119891
1199111015840 (119904)) 119889120582
1198684
119904= 119876
120576
119904int
1
0
(119891119911(119904 120582) minus 119891
119911(119904)) 119889120582
(95)
Then
Eint119879
119905
E1015840[1003816100381610038161003816120574120576
119904
10038161003816100381610038162
] 119889119904
le 4Eint119879
119905
E1015840[100381610038161003816100381610038161198681
119904
10038161003816100381610038161003816
2
+100381610038161003816100381610038161198682
119904
10038161003816100381610038161003816
2
+100381610038161003816100381610038161198683
119904
10038161003816100381610038161003816
2
+100381610038161003816100381610038161198684
119904
10038161003816100381610038161003816
2
] 119889119904
(96)
Take Eint119879119905E1015840[|1198682
119904|2
]119889119904 for example
Eint119879
119905
E1015840[100381610038161003816100381610038161198682
119904
10038161003816100381610038161003816
2
] 119889119904
= Eint119879
119905
E1015840[(119870
120576
119904int
1
0
(119891119910(119904 120582) minus 119891
119910(119904)) 119889120582)
2
]119889119904
le Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119904 120582) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582] 119889119904
le 2Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119904 120582) minus 119891
119910(119906
120576
119905)10038161003816100381610038161003816
2
119889120582] 119889119904
+ 2Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119906
120576
119905) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582] 119889119904
(97)
Note that
Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119906
120576
119905) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582] 119889119904
= Eint119903+120576
119903
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119906
120576
119905) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582] 119889119904
le sup119904isin[119905119879]
E [E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119906
120576
119905) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582]] 120576
997888rarr 0 as 120576 997888rarr 0
(98)
The inequality above holds due to the boundedness of|119870
120576
119904|2
int1
0|119891119910(119906
120576
119905) minus 119891
119910(119904)|
2
119889120582 Indeed Assumption (L3) im-plies the boundedness of 119891
119910(119906
120576
119905) minus 119891
119910(119904) Meanwhile 119870120576
119904is
the solution of mean-field BSEE (82) It can be easy to check119870120576
119904is bounded since the coefficients in (82) are boundedOn the other hand
Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119904 120582) minus 119891
119910(119906
120576
119905)10038161003816100381610038161003816
2
119889120582] 119889119904
le 11986221205822Eint
119879
119905
1003816100381610038161003816119870120576
119904
10038161003816100381610038162
times int
1
0
1198641015840[100381610038161003816100381610038161003816(119884
120576
119904minus 119884
119904)1015840100381610038161003816100381610038161003816
2
+100381610038161003816100381610038161003816(119885
120576
119904minus 119885
119904)1015840100381610038161003816100381610038161003816
2
times10038161003816100381610038161003816119884120576
119904minus 119884
119904
10038161003816100381610038161003816
2
+10038161003816100381610038161003816119885120576
119904minus 119885
119904
10038161003816100381610038161003816
2
] 119889120582 119889119904
(99)
where (119884120576
119904 119885
120576
119904) is the mild solution of the following equation
119889119884120576
119905= minus 119860119884
120576
119905119889119905
minus E1015840[119891 (119905 (119884
120576
119905)1015840
(119885120576
119905)1015840
119884120576
119905 119885
120576
119905 119906
120576
119905)] 119889119905
+ 119885120576
119905119889119882 (119905) 119905 isin [0 119879]
119884120576
119879= 120585
(100)
and (119884119904 119885
119904) is the mild solution of
119889119884119905= minus 119860119884
119905119889119905
minus E1015840[119891 (119905 (119884
119905)1015840
(119885119905)1015840
119884119905 119885
119905 119906
119905)] 119889119905
+ 119885119905119889119882 (119905) 119905 isin [0 119879]
119884119879= 120585
(101)
By the definition of 119906120576119905 according to (L2) we have
E1015840[119891 (119904 119884
1015840
(119904) 1198851015840
(119904) 119884 (119904) 119885 (119904) 119906120576
119905)]
997888rarr E1015840[119891 (119904 119884
1015840
(119904) 1198851015840
(119904) 119884 (119904) 119885 (119904) 119906119905)]
(102)
in 1198712([0 119879]119867) as 120576 rarr 0 Using the continuous dependencetheorem Corollary 5 we obtain
(119884120576
119904 119885
120576
119904) 997888rarr (119884
119904 119885
119904) as 120576 997888rarr 0 (103)
Then
Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119904 120582) minus 119891
119910(119906
120576
119905)10038161003816100381610038161003816
2
119889120582] 119889119904
le 11986221205822Eint
119879
119905
1003816100381610038161003816119870120576
119904
10038161003816100381610038162
times int
1
0
E1015840[100381610038161003816100381610038161003816(119884
120576
119904minus 119884
119904)1015840100381610038161003816100381610038161003816
2
+100381610038161003816100381610038161003816(119885
120576
119904minus 119885
119904)1015840100381610038161003816100381610038161003816
2
times10038161003816100381610038161003816119884120576
119904minus 119884
119904
10038161003816100381610038161003816
2
+10038161003816100381610038161003816119885120576
119904minus 119885
119904
10038161003816100381610038161003816
2
] 119889120582 119889119904
997888rarr 0 as 120576 997888rarr 0
(104)
14 Mathematical Problems in Engineering
Combining (98) with (104) we finally haveEint
119879
119905E1015840[|1198682
119904|2
]119889119904 rarr 0 as 120576 rarr 0The required result (93) follows by using the similar
estimations for 1198681119904 1198683
119904 and 1198684
119904
Note that 1 minus 1921198722
1198601198622120573 gt 0 if 120573 is sufficiently large
Now to prove the desired result (84) it suffices to applyGronwallrsquos lemma and estimate (93) to inequality (92)
To deal with the state constraint (75) we need to recall theEkeland variational principle
Lemma 16 (Ekelandrsquos variational principle see [16 Lemma41]) Let (119878 119889) be a complete metric space and let 119865(sdot) 119878 rarr
R be lower semicontinuous and bounded from below If for 120588 gt0 there exists 119906 isin 119878 such that
119865 (119906) le infVisin119878
119865 (V) + 120588 (105)
then there exists 119906120588 isin 119878 satisfying
(i) 119865 (119906120588) le 119865 (119906)
(ii) 119889 (119906120588 119906) le 120588
(iii) 119865 (119906120588) le 119865 (V) + 120588 sdot 119889 (119906120588 119906) forallV = 119906120588
(106)
Now fix V isin Uad and set
119878 = V (sdot) isin Uad | sup0le119905le119879
E1003816100381610038161003816V11990510038161003816100381610038162
le E100381610038161003816100381611990611990510038161003816100381610038162
+ |V|2
119889 (V (sdot) V (sdot)) = 119898 119905 isin [0 119879] | E1003816100381610038161003816V119905 minus V
119905
10038161003816100381610038162
gt 0
forallV (sdot) V (sdot) isin 119878
(107)
where119898 denotes the Lebesgue measure on RThe following result is proved as Proposition 41 in [16]
Lemma 17 (119878 119889(sdot sdot)) is a complete metric space and 119869120588 is
continuous and bounded on 119878 where
119869120588
(V (sdot)) = (119869 (V (sdot)) minus 119869 (119906 (sdot)) + 120588)2
+
10038161003816100381610038161003816100381610038161003816Eint
O
Φ(119909 (1198840(119909))
1015840
1198840(119909)) 119889119909
10038161003816100381610038161003816100381610038161003816
2
12
forallV (sdot) isin 119878(108)
and (119884 119885) is the mild solution of (72) corresponding to thecontrol V
Now we consider the following free initial state optimalcontrol problem
infV(sdot)isin119878
119869120588
(V (sdot)) (109)
It is easy to check that
0 le infV(sdot)isin119878
119869120588
(V (sdot)) le 119869120588
(119906 (sdot)) = 120588 (110)
According to Ekelandrsquos variational principle there exists a119906120588(sdot) isin 119881 such that
(i) 119869120588 (119906120588 (sdot)) le 120588
(ii) 119889 (119906120588 (sdot) 119906 (sdot)) le 120588
(iii) 119869120588 (119906120588 (sdot)) le 119869120588
(V (sdot)) + 120588119889 (119906120588 (sdot) 119906 (sdot)) forallV (sdot) isin 119878(111)
Using the spike variationmethod we can construct 119906120576120588(sdot) isin 119878as follows
119906120576120588
119905=
V119905 119905 isin [119904 119904 + 120576]
119906120588
119905 119905 isin [0 119879] [119904 119904 + 120576]
(112)
It is clear that 119889(119906120576120588(sdot) 119906120588(sdot)) le 120576 Let (119884120576120588(sdot) 119885
120576120588(sdot)) (resp
(119884120588(sdot) 119885
120588(sdot))) be the solution of (72) with respect to the
control 119906120576120588(sdot) (resp 119906120588(sdot)) Following (82) (119870120576120588
119905 119876
120576120588
119905) is the
mild solution of
119870120576120588
119905= int
119879
119905
119890119860(119904minus119905)
E1015840
times [1198911199101015840 (119904 Λ
120588
119904 119906
120588
119904) (119870
120576120588
119904)1015840
+ 119891119910(119904 Λ
120588
119904 119906
120588
119904)119870
120576120588
119904
+ 1198911199111015840 (119904 Λ
120588
119904 119906
120588
119904) (119876
120576120588
119904)1015840
+ 119891119911(119904 Λ
120588
119904 119906
120588
119904) 119876
120576120588
119904
+1
120576(119891 (119904 Λ
120588
119904 119906
120576120588
119904) minus 119891 (119904 Λ
120588
119904 119906
120588
119904))] 119889119904
minus int
119879
119905
119890119860(119904minus119905)
119876120576120588
119904119889119882 (119904)
(113)
ByTheorem 15 we know that
lim120576rarr0
E[ sup119904isin[119905119879]
100381610038161003816100381610038161003816100381610038161003816
119884120576120588
119904minus 119884
120588
119904
120576minus 119870
120576120588
119904
100381610038161003816100381610038161003816100381610038161003816
2
] = 0 forall119905 isin [0 119879]
lim120576rarr0
Eint119879
0
100381610038161003816100381610038161003816100381610038161003816
119885120576120588
119904minus 119885
120588
119904
120576minus 119876
120576120588
119904
100381610038161003816100381610038161003816100381610038161003816
2
119889119904 = 0
(114)
The proof of the following proposition is technical butbased on the arguments above and we omit it
Proposition 18 One has
1
120576E [Φ ((119884
120576120588
0)1015840
119884120576120588
0) minus Φ((119884
120588
0)1015840
119884120588
0)]
= E [Φ1199101015840 ((119884
120588
0)1015840
119884120588
0) (119870
120576120588
0)1015840
+ Φ119910((119884
120588
0)1015840
119884120588
0)119870
120576120588
0]
+ 119900 (120576)
Mathematical Problems in Engineering 15
1
120576(119869 (119906
120576120588) minus 119869 (119906
120588))
= E [1198921199101015840 ((119884
120588
0)1015840
119884120588
0) (119870
120576120588
0)1015840
+ 119892119910((119884
120588
0)1015840
119884120588
0)119870
120576120588
0]
+ Δ120576+1
120576Eint
119879
0
E1015840[ℎ (119904 Λ
120588
119904 119906
120576120588
119904)
minus ℎ (119904 Λ120588
119904 119906
120588
119904)] 119889119904 + 119900 (120576)
(115)
where
Δ120576= Eint
119879
0
E1015840[ℎ
1199101015840 (119904 Λ
120588
119904 119906
120576120588
119904) (119870
120576120588
119904)1015840
+ ℎ1199111015840 (119904 Λ
120588
119904 119906
120576120588
119904) (119876
120576120588
119904)1015840
+ ℎ119910(119904 Λ
120588
119904 119906
120576120588
119904)119870
120576120588
119904
+ ℎ119911(119904 Λ
120588
119904 119906
120576120588
119904) 119876
120576120588
119904] 119889119904
(116)
53 Variational Inequality and Adjoint Equation In thissubsection the adjoint process is introduced to deduce thevariational inequality
If we set V(sdot) = 119906120576120588(sdot) in (111) and notice that
119889(119906120576120588(sdot) 119906
120588(sdot)) le 120576 we get
minus120588 le1
120576(119869
120588(119906
120576120588
(sdot)) minus 119869120588(119906
120588
(sdot))) (117)
By Lemma 17
1
120576(119869
120588(119906
120576120588
(sdot)) minus 119869120588(119906
120588
(sdot)))
=(119869
120588(119906
120576120588(sdot)))
2
minus (119869120588(119906
120588(sdot)))
2
120576 (119869120588 (119906120576120588 (sdot)) + 119869120588 (119906120588 (sdot)))
=119869 (119906
120576120588(sdot)) + 119869 (119906
120588(sdot)) minus 2119869 (119906 (sdot)) + 2120588
119869120588 (119906120576120588 (sdot)) + 119869120588 (119906120588 (sdot))
times119869 (119906
120576120588(sdot)) minus 119869 (119906
120588(sdot))
120576
+
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] + E [Φ ((119884
120588
0)1015840
119884120588
0)]
119869120588 (119906120576120588 (sdot)) + 119869120588 (119906120588 (sdot))
times
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] minus E [Φ ((119884
120588
0)1015840
119884120588
0)]
120576
997888rarr 119897120588
1
119869 (119906120576120588(sdot)) minus 119869 (119906
120588(sdot))
120576
+ 119897120588
2
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] minus E [Φ ((119884
120588
0)1015840
119884120588
0)]
120576
(118)
where we set
119897120588
1=119869 (119906
120588(sdot)) minus 119869 (119906 (sdot)) + 120588
119869120588 (119906120588 (sdot))
119897120588
2=
E [Φ ((119884120588
0)1015840
119884120588
0)]
119869120588 (119906120588 (sdot))
(119)
and use the limit
119869 (119906120576120588
(sdot)) 997888rarr 119869 (119906120588
(sdot))
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] 997888rarr E [Φ ((119884
120588
0)1015840
119884120588
0)]
(120)
as 120576 rarr 0 according to (115)As |119897120588
1|2
+ |119897120588
2|2
= 1 for all 120588 gt 0 we know that there existsa subsequence of 119897120588
1 119897120588
2 (still denoted by 119897120588
1 119897120588
2) such that
lim120588rarr0
119897120588
1 119897120588
2 = 119897
1 1198972
1003816100381610038161003816119897110038161003816100381610038162
+1003816100381610038161003816119897210038161003816100381610038162
= 1
(121)
Combining (115) (117) with (118) we get
minus120588 le 119897120588
1
119869 (119906120576120588(sdot)) minus 119869 (119906
120588(sdot))
120576
+ 119897120588
2
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] minus E [Φ ((119884
120588
0)1015840
119884120588
0)]
120576
= 119897120588
1Δ120576+ 119897
120588
1E [119892
1199101015840 ((119884
120588
0)1015840
119884120588
0) (119870
120576120588
0)1015840
+119892119910((119884
120588
0)1015840
119884120588
0)119870
120576120588
0]
+119897120588
1
120576Eint
119879
0
E1015840[ℎ (119904 Λ
120588
119904 119906
120576120588
119904) minus ℎ (119904 Λ
120588
119904 119906
120588
119904)] 119889119904
+ 119897120588
2E [Φ
1199101015840 ((119884
120588
0)1015840
119884120588
0) (119870
120576120588
0)1015840
+Φ119910((119884
120588
0)1015840
119884120588
0)119870
120576120588
0] + (119897
120588
1+ 119897
120588
2) 119900 (120576)
(122)
Next we introduce the adjoint equation corresponding tovariational equation (113) whose solution is denoted by119875120588(119905)
119889119875120588
(119905)
= 119860lowast119875120588
(119905) 119889119905
+ E1015840[119891
1199101015840 (119905 Λ
120588
119905 119906
120588
119905) (119875
120588
(119905))1015840
+ 119891119910(119905 Λ
120588
119905 119906
120588
119905) 119875
120588
(119905) + 119897120588
1ℎ1199101015840 (119905 Λ
120588
119905 119906
120588
119905)
+ 119897120588
1ℎ119910(119905 Λ
120588
119905 119906
120588
119905)] 119889119905
16 Mathematical Problems in Engineering
+ E1015840[119891
1199111015840 (119905 Λ
120588
119905 119906
120588
119905) (119875
120588
(119905))1015840
+ 119891119911(119905 Λ
120588
119905 119906
120588
119905) 119875
120588
(119905)
+ 119897120588
1ℎ1199111015840 (119905 Λ
120588
119905 119906
120588
119905) + 119897
120588
1ℎ119911(119905 Λ
120588
119905 119906
120588
119905)] 119889119882 (119905)
119875120588
(0) = 119897120588
1E [119892
1199101015840 ((119884
120588
0(119909))
1015840
119884120588
0(119909))
+ 119892119910((119884
120588
0(119909))
1015840
119884120588
0(119909))]
+ 119897120588
2E [Φ
1199101015840 ((119884
120588
0(119909))
1015840
119884120588
0(119909))
+ Φ119910((119884
120588
0(119909))
1015840
119884120588
0(119909))]
(123)
where 119860lowast is the 1198712(O)-adjoint operator of 119860 Under assump-tions (L1)ndash(L3) this is a linear mean-field SEE with boundedcoefficients An application ofTheorem 11 implies that it has aunique adaptedmild solution such that119875120588(119905) isin S2
F ([0 119879] 119870)When 120588 rarr 0 according to Corollaries 5 and 13 119875120588(119905)
converges to 119875(119905) where
119875 (119905) isin S2
F ([0 119879] 119870) (124)
is the solution of the following equation
119875 (119905) = 1198971E [119892
1199101015840 (119884
1015840
0 119884
0) + 119892
119910(119884
1015840
0 119884
0)]
+ 1198972E [Φ
1199101015840 (119884
1015840
0 119884
0) + Φ
119910(119884
1015840
0 119884
0)]
+ int
119905
0
119890119860lowast(119905minus119904)
E1015840[119891
1199101015840 (119904) 119875
1015840
(119904) + 119891119910(119904) 119875 (119904)
+ 1198971ℎ1199101015840 (119904) + 119897
1ℎ119910(119904)] 119889119904
+ int
119905
0
119890119860lowast(119905minus119904)
E1015840[119891
1199111015840 (119904) 119875
1015840
(119904) + 119891119911(119904) 119875 (119904)
+ 1198971ℎ1199111015840 (119904) + 119897
1ℎ119911(119904)] 119889119882 (119904)
(125)
The following proposition which formally follows fromProposition 18 gives the relation between 119875120588(119905) and119870120576120588
119905
Proposition 19 Consider the following
E [119875120588
(0)119870120576120588
0]
= Eint119879
0
1
120576119875120588
(119904)E1015840[119891 (119904 Λ
120588
119904 119906
120576120588
119904) minus 119891 (119904 Λ
120588
119904 119906
120588
119904)]
minus 119897120588
1119870120576120588
119904E1015840[ℎ
1199101015840 (119904 Λ
120588
119904 119906
120588
119904) + ℎ
119910(119904 Λ
120588
119904 119906
120588
119904)]
minus 119897120588
1119876120576120588
119904E1015840[ℎ
1199111015840 (119904 Λ
120588
119904 119906
120588
119904) + ℎ
119911(119904 Λ
120588
119904 119906
120588
119904)] 119889119904
(126)
The following theorem constitutes the main contributionof this section themaximumprinciple for the BSPDE controlsystem
Theorem 20 Let assumptions (L1)ndash(L3) hold Suppose 119906(sdot) isan optimal control and (119884(sdot) 119885(sdot)) is the corresponding optimalstate trajectory for the BSPDE control systems (72) and (73)with the initial state constraint (75) Then there exists 119875(119905) isinS2
F ([0 119879] 119870) which satisfies (125) such that
H (119905 1198841015840
119905 119885
1015840
119905 119884
119905 119885
119905 V
119905 119875 (119905))
ge H (119905 1198841015840
119905 119885
1015840
119905 119884
119905 119885
119905 119906
119905 119875 (119905))
119886119890 119886119904 forallV isin U119886119889
(127)
whereH [0 119879]times119867timesL(Γ119867)times119867timesL(Γ119867)times119880times119870 rarr R
is the Hamiltonian function defined by
H (119905 119910 119910 119911 V 119901) = 1198971ℎ (119905 119910 119910 119911 V)
+ 119901119891 (119905 119910 119910 119911 V) (128)
Proof By (122) and Proposition 19 we obtain
minus120588 le1
120576Eint
119879
0
119875120588
(119904)E1015840[119891 (119904 Λ
120588
119904 119906
120576120588
119904)
minus 119891 (119904 Λ120588
119904 119906
120588
119904)] 119889119904
+119897120588
1
120576Eint
119879
0
E1015840[ℎ (119904 Λ
120588
119904 119906
120576120588
119904)
minus ℎ (119904 Λ120588
119904 119906
120588
119904)] 119889119904 + (119897
120588
1+ 119897
120588
2) 119900 (120576)
(129)
Letting 120576 rarr 0+ in (129) we derive for ae 120591 isin [0 119879]
minus120588 le 119897120588
1E1015840[ℎ (120591 Λ
120588
120591 V
120591) minus ℎ (120591 Λ
120588
120591 119906
120588
120591)]
+ 119875120588
(119904)E1015840[119891 (120591 Λ
120588
120591 V
120591) minus 119891 (120591 Λ
120588
120591 119906
120588
120591)]
(130)
for all V isin UadFinally taking 120588 rarr 0 in (130) we derive the desired
result
Remark 21 We note that if the coefficients do not dependexplicitly on the marginal law of the underlying diffusionthe result reduces to the classical case that is the SMP forBSPDEs without mean-field term
Remark 22 When we remove the initial state constraint (75)we obtain the general maximum principle for the mean-fieldBSPDEs system (ie without the constraint) with 119897
1= 1
54 Application A Backward Linear Quadratic Control Prob-lem Now we apply our maximum principle to solve an LQproblem For notational simplicity we restrict ourselves tothe free case (ie without the initial state constraint (75)) thegeneral case being handled in a similar way
Mathematical Problems in Engineering 17
Consider the following problem
119869 (119906) =1
2E [(119884 (0))
2] +
1
2Eint
119879
0
119873V2 (119905) 119889119905 997888rarr min
(131)
subject to
119889119884 (119905) = minus119860119884 (119905) 119889119905
minus 119861119884 (119905) + 119861E [119884 (119905)]
+119862V (119905) + 119863119885 (119905) + 119863E [119885 (119905)] 119889119905
+ 119885 (119905) 119889119882 (119905)
119884 (119879) = 120585 119905 isin [0 119879]
(132)
where 119860 is a partial differential operator satisfying condition(L1) and 119861 119861 119862119863119863 and119873 are bounded and deterministicconstantsWe also assume that119873 gt 0 and V isin 1198712F (0 119879 119880) It iseasy to verify that BSPDE (132) admits a uniquemild solution(119884(119905) 119885(119905))
119875(119905) the adjoint process of state equation (132) is thesolution of
119889119875 (119905) = 119860lowast119875 (119905) 119889119905 + 119861119875 (119905) + 119861E [119875 (119905)] 119889119905
+ 119863119875 (119905) + 119863E [119875 (119905)] 119889119882 (119905)
119875 (0) = 119884 (0)
(133)
Let 119906(sdot) be an optimal control and let (119884(sdot) 119885(sdot)) bethe corresponding state process By maximum principle ofTheorem 20 (note that 119897
1= 1 in this problem)
1
2119873V2 (119905) + 119875 (119905) 119862V (119905) ge
1
2119873119906
2
(119905) + 119875 (119905) 119862119906 (119905) (134)
for all V isin 119880ad since the state equation has the form (132)Thisin turn implies
119906 (119905) = minus119862
119873119875 (119905) (135)
It is clear that (131) is a positive quadratic functional of controlbecause of 119873 gt 0 Hence an optimal control exists Thecandidate optimal control (135) is indeed an optimal controlof this LQ problem for it is the only control which satisfies themaximum principle
Next we want to obtain a more explicit representationof the optimal control (135) from the state equation (132)Substituting (135) into state equation (132) yields
119889119884 (119905) = minus119860119884 (119905) 119889119905
minus 119861119884 (119905) + 119861E [119884 (119905)] minus1198622
119873119875 (119905)
+119863119885 (119905) + 119863E [119885 (119905)] 119889119905 + 119885 (119905) 119889119882 (119905)
119884 (119879) = 120585 119905 isin [0 119879]
(136)
Combining the above equation with (133) we obtain thefollowing related feedback control system
119889119884 (119905) = minus119860119884 (119905) 119889119905
minus 119861119884 (119905) + 119861E [119884 (119905)] minus1198622
119873119875 (119905)
+ 119863119885 (119905) + 119863E [119885 (119905)] 119889119905 + 119885 (119905) 119889119882 (119905)
119884 (119879) = 120585
119889119875 (119905) = 119860lowast119875 (119905) 119889119905 + 119861119875 (119905) + 119861E [119875 (119905)] 119889119905
+ 119863119875 (119905) + 119863E [119875 (119905)] 119889119882 (119905)
119875 (0) = 119884 (0)
(137)
Looking at the terminal condition of 119875(119905) in (133) andconsidering the mean-field type of (132) it is reasonable toconjecture that 119875(119905) has the following form
119875 (119905) = 120593 (119905) 119884 (119905) + 120601 (119905)E [119884 (119905)] (138)
where120593(119905)120601(119905) are deterministic differential functionswhichwill be specified below Moreover 120593(0) = 1 120601(0) = 0
Inserting this form into adjoint equation (133) and notic-ing that 119884(119905) satisfies (136) we can compare the coefficientsof 119889119905 and 119889119882(119905) to obtain the following equation
2120593 (119905) 119860119884 (119905) + 2120601 (119905) 119860E [119884 (119905)] + 119861120593 (119905) 119884 (119905)
+ 2 (119861120601 (119905) + 119861120593 (119905) + 119861120601 (119905))E [119884 (119905)]
= minus120593 (119905) 119861119884 (119905) minus1198622
119873120593 (119905) 119884 (119905) + 119863119885 (119905)
+ 119884 (119905)119889120593 (119905)
119889119905+ E [119884 (119905)]
119889120601 (119905)
119889119905
+ (1206012
(119905)1198622
119873+ 2
1198622
119873120593 (119905) 120601 (119905))E [119884 (119905)]
minus (119863120601 (119905) + 119863120601 (119905) + 119863120593 (119905))E [119885 (119905)]
120593 (119905) 119885 (119905) = 119863120593 (119905) 119884 (119905)
+ (119863120601 (119905) + 119863120593 (119905) + 119863120601 (119905))E [119884 (119905)]
(139)
Then subtracting 119885(119905) we have
2120593 (119905) 119860119884 (119905) + 2120601 (119905) 119860E [119884 (119905)] + 119861120593 (119905) 119884 (119905)
+ 2 (119861120601 (119905) + 119861120593 (119905) + 119861120601 (119905))E [119884 (119905)]
18 Mathematical Problems in Engineering
= 120593 (119905) minus119861 +1198622
119873120593 (119905) minus 119863
2119884 (119905)
+ 119884 (119905)119889120593 (119905)
119889119905+ E [119884 (119905)]
119889120601 (119905)
119889119905
+ (1206012
(119905)1198622
119873+ 2
1198622
119873120593 (119905) 120601 (119905))E [119884 (119905)]
minus1
120593 (119905)(119863120601 (119905) + 119863120593 (119905) + 119863120601 (119905))
2
E [119884 (119905)]
minus 2119863 (119863120601 (119905) + 119863120593 (119905) + 119863120601 (119905))E [119884 (119905)]
(140)
Comparing the coefficients of 119884(119905) and E[119884(119905)] respec-tively we get
119889
119889119905120593 (119905) = 2119860
lowast120593 (119905) + 2119861120593 (119905) + 119863
2120593 (119905) minus
1198622
1198731205932
(119905)
120593 (0) = 1
(141)
119889
119889119905120601 (119905) = 2119860
lowast120601 (119905) + (
(119863 + 119863)2
120593 (119905)minus1198622
119873)120601
2
(119905)
+ 2119877 (119905) 120601 (119905) + (1198632+ 2119861 + 2119863119863)120593 (119905)
120601 (0) = 0
(142)
where 119877(119905) = 119861 + 119861 + (119863 + 119863)2
minus (1198622119873)120593(119905)
We solve (141) to get
120593 (119905) = (119890minus(2119860lowast+2119861+119863
2)119905(1 minus
1198622
119873(2119860lowast + 2119861 + 1198632))
+1198622
119873(2119860lowast + 2119861 + 1198632))
minus1
(143)
Then (142) exists a unique solution from the classical Riccatiequation theory
We now conclude the above discussions in the followingresult
Theorem 23 For onersquos linear quadratic stochastic partialdifferential control problem (131)-(132) the unique optimalcontrol 119906(sdot) isin 119880ad is given by
119906 (119905) = minus119862
119873(120593 (119905) 119884 (119905) + 120601 (119905)E [119884 (119905)]) (144)
with 120593(119905) satisfying (143) and 120601(119905) solving (142)
Conflict of Interests
The authors declare that they have no conflict of interestsregarding the publication of this paper
Acknowledgment
This research is partially supported by the Natural ScienceFoundation of China under Grant no 11301310
References
[1] Y Hu and S Peng ldquoAdapted solution of a backward semilinearstochastic evolution equationrdquo Stochastic Analysis and Applica-tions vol 9 no 4 pp 445ndash459 1991
[2] N IMahmudov andMAMcKibben ldquoOnbackward stochasticevolution equations in Hilbert spaces and optimal controlrdquoNonlinear Analysis vol 67 no 4 pp 1260ndash1274 2007
[3] M Fuhrman and G Tessitore ldquoNonlinear Kolmogorov equa-tions in infinite dimensional spaces the backward stochasticdifferential equations approach and applications to optimalcontrolrdquoThe Annals of Probability vol 30 no 3 pp 1397ndash14652002
[4] M Fuhrman and G Tessitore ldquoInfinite horizon backwardstochastic differential equations and elliptic equations inHilbert spacesrdquo Annals of Probability vol 32 no 1B pp 607ndash660 2004
[5] X Mao ldquoAdapted solutions of backward stochastic differentialequations with non-Lipschitz coefficientsrdquo Stochastic Processesand their Applications vol 58 no 2 pp 281ndash292 1995
[6] J Lasry and P Lions ldquoMean field gamesrdquo Japanese Journal ofMathematics vol 2 no 1 pp 229ndash260 2007
[7] R Buckdahn J Li and S Peng ldquoMean-field backward stochas-tic differential equations and related partial differential equa-tionsrdquo Stochastic Processes and their Applications vol 119 no 10pp 3133ndash3154 2009
[8] D Andersson and B Djehiche ldquoAmaximum principle for SDEsofmean-field typerdquoAppliedMathematics andOptimization vol63 no 3 pp 341ndash356 2011
[9] R Buckdahn B Djehiche J Li and S Peng ldquoMean-fieldbackward stochastic differential equations a limit approachrdquoThe Annals of Probability vol 37 no 4 pp 1524ndash1565 2009
[10] R Xu ldquoMean-field backward doubly stochastic differentialequations and related SPDEsrdquo Boundary Value Problems vol2012 article 114 2012
[11] R Buckdahn B Djehiche and J Li ldquoA general stochasticmaximum principle for SDEs of mean-field typerdquo AppliedMathematics and Optimization vol 64 no 2 pp 197ndash216 2011
[12] J Li ldquoStochasticmaximumprinciple in themean-field controlsrdquoAutomatica vol 48 no 2 pp 366ndash373 2012
[13] G Wang C Zhang and W Zhang ldquoStochastic maximumprinciple for mean-field type optimal control under partialinformationrdquo IEEE Transactions on Automatic Control vol 59no 2 pp 522ndash528 2014
[14] M Hafayed ldquoA mean-field necessary and sufficient conditionsfor optimal singular stochastic controlrdquo Communications inMathematics and Statistics vol 1 no 4 pp 417ndash435 2013
[15] M Hafayed ldquoA mean-field maximum principle for optimalcontrol of forward-backward stochastic differential equationswith Poisson jumpprocessesrdquo International Journal ofDynamicsand Control vol 1 no 4 pp 300ndash315 2013
[16] Y Hu and S Peng ldquoMaximum principle for semilinearstochastic evolution control systemsrdquo Stochastics and StochasticsReports vol 33 no 3-4 pp 159ndash180 1990
[17] M Fuhrman Y Hu and G Tessitore ldquoStochastic maximumprinciple for optimal control of SPDEsrdquo Comptes Rendus Math-ematique vol 350 no 13-14 pp 683ndash688 2012
Submit your manuscripts athttpwwwhindawicom
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
MathematicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Mathematical Problems in Engineering
Hindawi Publishing Corporationhttpwwwhindawicom
Differential EquationsInternational Journal of
Volume 2014
Applied MathematicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Probability and StatisticsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
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Mathematical PhysicsAdvances in
Complex AnalysisJournal of
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OptimizationJournal of
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CombinatoricsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of
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Operations ResearchAdvances in
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Function Spaces
Abstract and Applied AnalysisHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of Mathematics and Mathematical Sciences
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The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
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Algebra
Discrete Dynamics in Nature and Society
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Decision SciencesAdvances in
Discrete MathematicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom
Volume 2014 Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Stochastic AnalysisInternational Journal of
12 Mathematical Problems in Engineering
Theorem 15 There holds
lim120576rarr0
E[ sup119904isin[119905119879]
1003816100381610038161003816100381610038161003816100381610038161003816
119884120576
119904minus 119884
119904
120576minus 119870
120576
119904
1003816100381610038161003816100381610038161003816100381610038161003816
2
] = 0 forall119905 isin [0 119879]
lim120576rarr0
Eint119879
0
1003816100381610038161003816100381610038161003816100381610038161003816
119885120576
119904minus 119885
119904
120576minus 119876
120576
119904
1003816100381610038161003816100381610038161003816100381610038161003816
2
119889119904 = 0
(84)
Proof We define
120578120576
119904=119884120576
119904minus 119884
119904
120576minus 119870
120576
119904 120577
120576
119904=119885120576
119904minus 119885
119904
120576minus 119876
120576
119904 119904 isin [0 119879]
(85)
For simplicity let us define
Λ120576
119904= ((119884
120576
119904)1015840
(119885120576
119904)1015840
119884120576
119904 119885
120576
119904)
119891 (119904 120582) = 119891 (119904 1198841015840
119904+ 120582(119884
120576
119904minus 119884
119904)1015840
1198851015840
119904+ 120582(119885
120576
119904minus 119885
119904)1015840
119884119904+ 120582 (119884
120576
119904minus 119884
119904)
119885119904+ 120582 (119885
120576
119904minus 119885
119904) 119906
120576
119904)
(86)
By the definition of (119884120576
119904 119885
120576
119904) (119884
119904 119885
119904) and (119870120576
119904 119876
120576
119904) (120578120576
119904 120577
120576
119904)
is the mild solution of
119889120578120576
119904= minus119860120578
120576
119904119889119904 minus E
1015840
[119871 (119904 120576)] 119889119904 + 120577120576
119904119889119882 (119904)
120578120576
119879= 0
(87)
with
119871 (119904 120576) =1
120576(119891 (119904 Λ
120576
119904 119906
120576
119904) minus 119891 (119906
120576
119904))
minus 1198911199101015840 (119904) (119870
120576
119904)1015840
minus 119891119910(119904) 119870
120576
119904minus 119891
1199111015840 (119904) (119876
120576
119904)1015840
minus 119891119911(119904) 119876
120576
119904
= ((120578120576
119904)1015840
+ (119870120576
119904)1015840
) int
1
0
1198911199101015840 (119904 120582) 119889120582 + (120578
120576
119904+ 119870
120576
119904)
times int
1
0
119891119910(119904 120582) 119889120582 minus 119891
1199101015840 (119904) (119870
120576
119904)1015840
minus 119891119910(119904) 119870
120576
119904
+ ((120577120576
119904)1015840
+ (119876120576
119904)1015840
)int
1
0
1198911199111015840 (119904 120582) 119889120582 + (120577
120576
119904+ 119876
120576
119904)
times int
1
0
119891119911(119904 120582) 119889120582 minus 119891
1199111015840 (119904) (119876
120576
119904)1015840
minus 119891119911(119904) 119876
120576
119904
= (120578120576
119904)1015840
int
1
0
1198911199101015840 (119904 120582) 119889120582 + 120578
120576
119904int
1
0
119891119910(119904 120582) 119889120582
+ (120577120576
119904)1015840
int
1
0
1198911199111015840 (119904 120582) 119889120582 + 120577
120576
119904int
1
0
119891119911(119904 120582) 119889120582 + 120574
120576
119904
(88)
where we denote
120574120576
119904= (119870
120576
119904)1015840
int
1
0
(1198911199101015840 (119904 120582) minus 119891
1199101015840 (119904)) 119889120582
+ 119870120576
119904int
1
0
(119891119910(119904 120582) minus 119891
119910(119904)) 119889120582
+ (119876120576
119904)1015840
int
1
0
(1198911199111015840 (119904 120582) minus 119891
1199111015840 (119904)) 119889120582
+ 119876120576
119904int
1
0
(119891119911(119904 120582) minus 119891
119911(119904)) 119889120582
(89)
For any 120573 gt 0 according to Lemma 1 we obtain
E sup119905le119904le119879
11989021205731199041003816100381610038161003816120578
120576
119904
10038161003816100381610038162
+ Eint119879
119905
11989021205731199041003816100381610038161003816120577
120576
119904
10038161003816100381610038162
119889119904
le12119872
2
119860
120573int
119879
119905
1198902120573119904
E [10038161003816100381610038161003816E1015840
[119871 (119904 120576)]10038161003816100381610038161003816
2
] 119889119904
le12119872
2
119860
120573Eint
119879
119905
1198902120573119904
E1015840[|119871 (119904 120576)|
2] 119889119904
(90)
By condition (L3) we have
E [E1015840[|119871 (119904 120576)|
2]]
= E [E1015840[1003816100381610038161003816119871 (119904 120576) minus 120574
120576
119904+ 120574
120576
119904
10038161003816100381610038162
]]
le 81198622E [E
1015840[10038161003816100381610038161003816(120578
120576
119904)101584010038161003816100381610038161003816
2
+1003816100381610038161003816120578120576
119904
10038161003816100381610038162
+10038161003816100381610038161003816(120577
120576
119904)101584010038161003816100381610038161003816
2
+1003816100381610038161003816120577120576
119904
10038161003816100381610038162
]]
+ 2E [E1015840[1003816100381610038161003816120574120576
119904
10038161003816100381610038162
]]
le 161198622E [
1003816100381610038161003816120578120576
119904
10038161003816100381610038162
+1003816100381610038161003816120577120576
119904
10038161003816100381610038162
] + 2E [E1015840[1003816100381610038161003816120574120576
119904
10038161003816100381610038162
]]
(91)
Combined with (91) (90) yields
E sup119905le119904le119879
11989021205731199041003816100381610038161003816120578
120576
119904
10038161003816100381610038162
+ (1 minus192119872
2
1198601198622
120573)Eint
119879
119905
11989021205731199041003816100381610038161003816120577
120576
119904
10038161003816100381610038162
119889119904
le192119872
2
1198601198622
120573Eint
119879
119905
11989021205731199041003816100381610038161003816120578
120576
119904
10038161003816100381610038162
119889119904
+24119872
2
119860
120573Eint
119879
119905
1198902120573119904
[E1015840[1003816100381610038161003816120574120576
119904
10038161003816100381610038162
]] 119889119904
(92)
We claim that
Eint119879
119905
E1015840[1003816100381610038161003816120574120576
119904
10038161003816100381610038162
] 119889119904 997888rarr 0 as 120576 997888rarr 0 (93)
From (89)
120574120576
119904= 119868
1
119904+ 119868
2
119904+ 119868
3
119904+ 119868
4
119904 (94)
Mathematical Problems in Engineering 13
where
1198681
119904= (119870
120576
119904)1015840
int
1
0
(1198911199101015840 (119904 120582) minus 119891
1199101015840 (119904)) 119889120582
1198682
119904= 119870
120576
119904int
1
0
(119891119910(119904 120582) minus 119891
119910(119904)) 119889120582
1198683
119904= (119876
120576
119904)1015840
int
1
0
(1198911199111015840 (119904 120582) minus 119891
1199111015840 (119904)) 119889120582
1198684
119904= 119876
120576
119904int
1
0
(119891119911(119904 120582) minus 119891
119911(119904)) 119889120582
(95)
Then
Eint119879
119905
E1015840[1003816100381610038161003816120574120576
119904
10038161003816100381610038162
] 119889119904
le 4Eint119879
119905
E1015840[100381610038161003816100381610038161198681
119904
10038161003816100381610038161003816
2
+100381610038161003816100381610038161198682
119904
10038161003816100381610038161003816
2
+100381610038161003816100381610038161198683
119904
10038161003816100381610038161003816
2
+100381610038161003816100381610038161198684
119904
10038161003816100381610038161003816
2
] 119889119904
(96)
Take Eint119879119905E1015840[|1198682
119904|2
]119889119904 for example
Eint119879
119905
E1015840[100381610038161003816100381610038161198682
119904
10038161003816100381610038161003816
2
] 119889119904
= Eint119879
119905
E1015840[(119870
120576
119904int
1
0
(119891119910(119904 120582) minus 119891
119910(119904)) 119889120582)
2
]119889119904
le Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119904 120582) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582] 119889119904
le 2Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119904 120582) minus 119891
119910(119906
120576
119905)10038161003816100381610038161003816
2
119889120582] 119889119904
+ 2Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119906
120576
119905) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582] 119889119904
(97)
Note that
Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119906
120576
119905) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582] 119889119904
= Eint119903+120576
119903
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119906
120576
119905) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582] 119889119904
le sup119904isin[119905119879]
E [E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119906
120576
119905) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582]] 120576
997888rarr 0 as 120576 997888rarr 0
(98)
The inequality above holds due to the boundedness of|119870
120576
119904|2
int1
0|119891119910(119906
120576
119905) minus 119891
119910(119904)|
2
119889120582 Indeed Assumption (L3) im-plies the boundedness of 119891
119910(119906
120576
119905) minus 119891
119910(119904) Meanwhile 119870120576
119904is
the solution of mean-field BSEE (82) It can be easy to check119870120576
119904is bounded since the coefficients in (82) are boundedOn the other hand
Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119904 120582) minus 119891
119910(119906
120576
119905)10038161003816100381610038161003816
2
119889120582] 119889119904
le 11986221205822Eint
119879
119905
1003816100381610038161003816119870120576
119904
10038161003816100381610038162
times int
1
0
1198641015840[100381610038161003816100381610038161003816(119884
120576
119904minus 119884
119904)1015840100381610038161003816100381610038161003816
2
+100381610038161003816100381610038161003816(119885
120576
119904minus 119885
119904)1015840100381610038161003816100381610038161003816
2
times10038161003816100381610038161003816119884120576
119904minus 119884
119904
10038161003816100381610038161003816
2
+10038161003816100381610038161003816119885120576
119904minus 119885
119904
10038161003816100381610038161003816
2
] 119889120582 119889119904
(99)
where (119884120576
119904 119885
120576
119904) is the mild solution of the following equation
119889119884120576
119905= minus 119860119884
120576
119905119889119905
minus E1015840[119891 (119905 (119884
120576
119905)1015840
(119885120576
119905)1015840
119884120576
119905 119885
120576
119905 119906
120576
119905)] 119889119905
+ 119885120576
119905119889119882 (119905) 119905 isin [0 119879]
119884120576
119879= 120585
(100)
and (119884119904 119885
119904) is the mild solution of
119889119884119905= minus 119860119884
119905119889119905
minus E1015840[119891 (119905 (119884
119905)1015840
(119885119905)1015840
119884119905 119885
119905 119906
119905)] 119889119905
+ 119885119905119889119882 (119905) 119905 isin [0 119879]
119884119879= 120585
(101)
By the definition of 119906120576119905 according to (L2) we have
E1015840[119891 (119904 119884
1015840
(119904) 1198851015840
(119904) 119884 (119904) 119885 (119904) 119906120576
119905)]
997888rarr E1015840[119891 (119904 119884
1015840
(119904) 1198851015840
(119904) 119884 (119904) 119885 (119904) 119906119905)]
(102)
in 1198712([0 119879]119867) as 120576 rarr 0 Using the continuous dependencetheorem Corollary 5 we obtain
(119884120576
119904 119885
120576
119904) 997888rarr (119884
119904 119885
119904) as 120576 997888rarr 0 (103)
Then
Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119904 120582) minus 119891
119910(119906
120576
119905)10038161003816100381610038161003816
2
119889120582] 119889119904
le 11986221205822Eint
119879
119905
1003816100381610038161003816119870120576
119904
10038161003816100381610038162
times int
1
0
E1015840[100381610038161003816100381610038161003816(119884
120576
119904minus 119884
119904)1015840100381610038161003816100381610038161003816
2
+100381610038161003816100381610038161003816(119885
120576
119904minus 119885
119904)1015840100381610038161003816100381610038161003816
2
times10038161003816100381610038161003816119884120576
119904minus 119884
119904
10038161003816100381610038161003816
2
+10038161003816100381610038161003816119885120576
119904minus 119885
119904
10038161003816100381610038161003816
2
] 119889120582 119889119904
997888rarr 0 as 120576 997888rarr 0
(104)
14 Mathematical Problems in Engineering
Combining (98) with (104) we finally haveEint
119879
119905E1015840[|1198682
119904|2
]119889119904 rarr 0 as 120576 rarr 0The required result (93) follows by using the similar
estimations for 1198681119904 1198683
119904 and 1198684
119904
Note that 1 minus 1921198722
1198601198622120573 gt 0 if 120573 is sufficiently large
Now to prove the desired result (84) it suffices to applyGronwallrsquos lemma and estimate (93) to inequality (92)
To deal with the state constraint (75) we need to recall theEkeland variational principle
Lemma 16 (Ekelandrsquos variational principle see [16 Lemma41]) Let (119878 119889) be a complete metric space and let 119865(sdot) 119878 rarr
R be lower semicontinuous and bounded from below If for 120588 gt0 there exists 119906 isin 119878 such that
119865 (119906) le infVisin119878
119865 (V) + 120588 (105)
then there exists 119906120588 isin 119878 satisfying
(i) 119865 (119906120588) le 119865 (119906)
(ii) 119889 (119906120588 119906) le 120588
(iii) 119865 (119906120588) le 119865 (V) + 120588 sdot 119889 (119906120588 119906) forallV = 119906120588
(106)
Now fix V isin Uad and set
119878 = V (sdot) isin Uad | sup0le119905le119879
E1003816100381610038161003816V11990510038161003816100381610038162
le E100381610038161003816100381611990611990510038161003816100381610038162
+ |V|2
119889 (V (sdot) V (sdot)) = 119898 119905 isin [0 119879] | E1003816100381610038161003816V119905 minus V
119905
10038161003816100381610038162
gt 0
forallV (sdot) V (sdot) isin 119878
(107)
where119898 denotes the Lebesgue measure on RThe following result is proved as Proposition 41 in [16]
Lemma 17 (119878 119889(sdot sdot)) is a complete metric space and 119869120588 is
continuous and bounded on 119878 where
119869120588
(V (sdot)) = (119869 (V (sdot)) minus 119869 (119906 (sdot)) + 120588)2
+
10038161003816100381610038161003816100381610038161003816Eint
O
Φ(119909 (1198840(119909))
1015840
1198840(119909)) 119889119909
10038161003816100381610038161003816100381610038161003816
2
12
forallV (sdot) isin 119878(108)
and (119884 119885) is the mild solution of (72) corresponding to thecontrol V
Now we consider the following free initial state optimalcontrol problem
infV(sdot)isin119878
119869120588
(V (sdot)) (109)
It is easy to check that
0 le infV(sdot)isin119878
119869120588
(V (sdot)) le 119869120588
(119906 (sdot)) = 120588 (110)
According to Ekelandrsquos variational principle there exists a119906120588(sdot) isin 119881 such that
(i) 119869120588 (119906120588 (sdot)) le 120588
(ii) 119889 (119906120588 (sdot) 119906 (sdot)) le 120588
(iii) 119869120588 (119906120588 (sdot)) le 119869120588
(V (sdot)) + 120588119889 (119906120588 (sdot) 119906 (sdot)) forallV (sdot) isin 119878(111)
Using the spike variationmethod we can construct 119906120576120588(sdot) isin 119878as follows
119906120576120588
119905=
V119905 119905 isin [119904 119904 + 120576]
119906120588
119905 119905 isin [0 119879] [119904 119904 + 120576]
(112)
It is clear that 119889(119906120576120588(sdot) 119906120588(sdot)) le 120576 Let (119884120576120588(sdot) 119885
120576120588(sdot)) (resp
(119884120588(sdot) 119885
120588(sdot))) be the solution of (72) with respect to the
control 119906120576120588(sdot) (resp 119906120588(sdot)) Following (82) (119870120576120588
119905 119876
120576120588
119905) is the
mild solution of
119870120576120588
119905= int
119879
119905
119890119860(119904minus119905)
E1015840
times [1198911199101015840 (119904 Λ
120588
119904 119906
120588
119904) (119870
120576120588
119904)1015840
+ 119891119910(119904 Λ
120588
119904 119906
120588
119904)119870
120576120588
119904
+ 1198911199111015840 (119904 Λ
120588
119904 119906
120588
119904) (119876
120576120588
119904)1015840
+ 119891119911(119904 Λ
120588
119904 119906
120588
119904) 119876
120576120588
119904
+1
120576(119891 (119904 Λ
120588
119904 119906
120576120588
119904) minus 119891 (119904 Λ
120588
119904 119906
120588
119904))] 119889119904
minus int
119879
119905
119890119860(119904minus119905)
119876120576120588
119904119889119882 (119904)
(113)
ByTheorem 15 we know that
lim120576rarr0
E[ sup119904isin[119905119879]
100381610038161003816100381610038161003816100381610038161003816
119884120576120588
119904minus 119884
120588
119904
120576minus 119870
120576120588
119904
100381610038161003816100381610038161003816100381610038161003816
2
] = 0 forall119905 isin [0 119879]
lim120576rarr0
Eint119879
0
100381610038161003816100381610038161003816100381610038161003816
119885120576120588
119904minus 119885
120588
119904
120576minus 119876
120576120588
119904
100381610038161003816100381610038161003816100381610038161003816
2
119889119904 = 0
(114)
The proof of the following proposition is technical butbased on the arguments above and we omit it
Proposition 18 One has
1
120576E [Φ ((119884
120576120588
0)1015840
119884120576120588
0) minus Φ((119884
120588
0)1015840
119884120588
0)]
= E [Φ1199101015840 ((119884
120588
0)1015840
119884120588
0) (119870
120576120588
0)1015840
+ Φ119910((119884
120588
0)1015840
119884120588
0)119870
120576120588
0]
+ 119900 (120576)
Mathematical Problems in Engineering 15
1
120576(119869 (119906
120576120588) minus 119869 (119906
120588))
= E [1198921199101015840 ((119884
120588
0)1015840
119884120588
0) (119870
120576120588
0)1015840
+ 119892119910((119884
120588
0)1015840
119884120588
0)119870
120576120588
0]
+ Δ120576+1
120576Eint
119879
0
E1015840[ℎ (119904 Λ
120588
119904 119906
120576120588
119904)
minus ℎ (119904 Λ120588
119904 119906
120588
119904)] 119889119904 + 119900 (120576)
(115)
where
Δ120576= Eint
119879
0
E1015840[ℎ
1199101015840 (119904 Λ
120588
119904 119906
120576120588
119904) (119870
120576120588
119904)1015840
+ ℎ1199111015840 (119904 Λ
120588
119904 119906
120576120588
119904) (119876
120576120588
119904)1015840
+ ℎ119910(119904 Λ
120588
119904 119906
120576120588
119904)119870
120576120588
119904
+ ℎ119911(119904 Λ
120588
119904 119906
120576120588
119904) 119876
120576120588
119904] 119889119904
(116)
53 Variational Inequality and Adjoint Equation In thissubsection the adjoint process is introduced to deduce thevariational inequality
If we set V(sdot) = 119906120576120588(sdot) in (111) and notice that
119889(119906120576120588(sdot) 119906
120588(sdot)) le 120576 we get
minus120588 le1
120576(119869
120588(119906
120576120588
(sdot)) minus 119869120588(119906
120588
(sdot))) (117)
By Lemma 17
1
120576(119869
120588(119906
120576120588
(sdot)) minus 119869120588(119906
120588
(sdot)))
=(119869
120588(119906
120576120588(sdot)))
2
minus (119869120588(119906
120588(sdot)))
2
120576 (119869120588 (119906120576120588 (sdot)) + 119869120588 (119906120588 (sdot)))
=119869 (119906
120576120588(sdot)) + 119869 (119906
120588(sdot)) minus 2119869 (119906 (sdot)) + 2120588
119869120588 (119906120576120588 (sdot)) + 119869120588 (119906120588 (sdot))
times119869 (119906
120576120588(sdot)) minus 119869 (119906
120588(sdot))
120576
+
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] + E [Φ ((119884
120588
0)1015840
119884120588
0)]
119869120588 (119906120576120588 (sdot)) + 119869120588 (119906120588 (sdot))
times
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] minus E [Φ ((119884
120588
0)1015840
119884120588
0)]
120576
997888rarr 119897120588
1
119869 (119906120576120588(sdot)) minus 119869 (119906
120588(sdot))
120576
+ 119897120588
2
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] minus E [Φ ((119884
120588
0)1015840
119884120588
0)]
120576
(118)
where we set
119897120588
1=119869 (119906
120588(sdot)) minus 119869 (119906 (sdot)) + 120588
119869120588 (119906120588 (sdot))
119897120588
2=
E [Φ ((119884120588
0)1015840
119884120588
0)]
119869120588 (119906120588 (sdot))
(119)
and use the limit
119869 (119906120576120588
(sdot)) 997888rarr 119869 (119906120588
(sdot))
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] 997888rarr E [Φ ((119884
120588
0)1015840
119884120588
0)]
(120)
as 120576 rarr 0 according to (115)As |119897120588
1|2
+ |119897120588
2|2
= 1 for all 120588 gt 0 we know that there existsa subsequence of 119897120588
1 119897120588
2 (still denoted by 119897120588
1 119897120588
2) such that
lim120588rarr0
119897120588
1 119897120588
2 = 119897
1 1198972
1003816100381610038161003816119897110038161003816100381610038162
+1003816100381610038161003816119897210038161003816100381610038162
= 1
(121)
Combining (115) (117) with (118) we get
minus120588 le 119897120588
1
119869 (119906120576120588(sdot)) minus 119869 (119906
120588(sdot))
120576
+ 119897120588
2
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] minus E [Φ ((119884
120588
0)1015840
119884120588
0)]
120576
= 119897120588
1Δ120576+ 119897
120588
1E [119892
1199101015840 ((119884
120588
0)1015840
119884120588
0) (119870
120576120588
0)1015840
+119892119910((119884
120588
0)1015840
119884120588
0)119870
120576120588
0]
+119897120588
1
120576Eint
119879
0
E1015840[ℎ (119904 Λ
120588
119904 119906
120576120588
119904) minus ℎ (119904 Λ
120588
119904 119906
120588
119904)] 119889119904
+ 119897120588
2E [Φ
1199101015840 ((119884
120588
0)1015840
119884120588
0) (119870
120576120588
0)1015840
+Φ119910((119884
120588
0)1015840
119884120588
0)119870
120576120588
0] + (119897
120588
1+ 119897
120588
2) 119900 (120576)
(122)
Next we introduce the adjoint equation corresponding tovariational equation (113) whose solution is denoted by119875120588(119905)
119889119875120588
(119905)
= 119860lowast119875120588
(119905) 119889119905
+ E1015840[119891
1199101015840 (119905 Λ
120588
119905 119906
120588
119905) (119875
120588
(119905))1015840
+ 119891119910(119905 Λ
120588
119905 119906
120588
119905) 119875
120588
(119905) + 119897120588
1ℎ1199101015840 (119905 Λ
120588
119905 119906
120588
119905)
+ 119897120588
1ℎ119910(119905 Λ
120588
119905 119906
120588
119905)] 119889119905
16 Mathematical Problems in Engineering
+ E1015840[119891
1199111015840 (119905 Λ
120588
119905 119906
120588
119905) (119875
120588
(119905))1015840
+ 119891119911(119905 Λ
120588
119905 119906
120588
119905) 119875
120588
(119905)
+ 119897120588
1ℎ1199111015840 (119905 Λ
120588
119905 119906
120588
119905) + 119897
120588
1ℎ119911(119905 Λ
120588
119905 119906
120588
119905)] 119889119882 (119905)
119875120588
(0) = 119897120588
1E [119892
1199101015840 ((119884
120588
0(119909))
1015840
119884120588
0(119909))
+ 119892119910((119884
120588
0(119909))
1015840
119884120588
0(119909))]
+ 119897120588
2E [Φ
1199101015840 ((119884
120588
0(119909))
1015840
119884120588
0(119909))
+ Φ119910((119884
120588
0(119909))
1015840
119884120588
0(119909))]
(123)
where 119860lowast is the 1198712(O)-adjoint operator of 119860 Under assump-tions (L1)ndash(L3) this is a linear mean-field SEE with boundedcoefficients An application ofTheorem 11 implies that it has aunique adaptedmild solution such that119875120588(119905) isin S2
F ([0 119879] 119870)When 120588 rarr 0 according to Corollaries 5 and 13 119875120588(119905)
converges to 119875(119905) where
119875 (119905) isin S2
F ([0 119879] 119870) (124)
is the solution of the following equation
119875 (119905) = 1198971E [119892
1199101015840 (119884
1015840
0 119884
0) + 119892
119910(119884
1015840
0 119884
0)]
+ 1198972E [Φ
1199101015840 (119884
1015840
0 119884
0) + Φ
119910(119884
1015840
0 119884
0)]
+ int
119905
0
119890119860lowast(119905minus119904)
E1015840[119891
1199101015840 (119904) 119875
1015840
(119904) + 119891119910(119904) 119875 (119904)
+ 1198971ℎ1199101015840 (119904) + 119897
1ℎ119910(119904)] 119889119904
+ int
119905
0
119890119860lowast(119905minus119904)
E1015840[119891
1199111015840 (119904) 119875
1015840
(119904) + 119891119911(119904) 119875 (119904)
+ 1198971ℎ1199111015840 (119904) + 119897
1ℎ119911(119904)] 119889119882 (119904)
(125)
The following proposition which formally follows fromProposition 18 gives the relation between 119875120588(119905) and119870120576120588
119905
Proposition 19 Consider the following
E [119875120588
(0)119870120576120588
0]
= Eint119879
0
1
120576119875120588
(119904)E1015840[119891 (119904 Λ
120588
119904 119906
120576120588
119904) minus 119891 (119904 Λ
120588
119904 119906
120588
119904)]
minus 119897120588
1119870120576120588
119904E1015840[ℎ
1199101015840 (119904 Λ
120588
119904 119906
120588
119904) + ℎ
119910(119904 Λ
120588
119904 119906
120588
119904)]
minus 119897120588
1119876120576120588
119904E1015840[ℎ
1199111015840 (119904 Λ
120588
119904 119906
120588
119904) + ℎ
119911(119904 Λ
120588
119904 119906
120588
119904)] 119889119904
(126)
The following theorem constitutes the main contributionof this section themaximumprinciple for the BSPDE controlsystem
Theorem 20 Let assumptions (L1)ndash(L3) hold Suppose 119906(sdot) isan optimal control and (119884(sdot) 119885(sdot)) is the corresponding optimalstate trajectory for the BSPDE control systems (72) and (73)with the initial state constraint (75) Then there exists 119875(119905) isinS2
F ([0 119879] 119870) which satisfies (125) such that
H (119905 1198841015840
119905 119885
1015840
119905 119884
119905 119885
119905 V
119905 119875 (119905))
ge H (119905 1198841015840
119905 119885
1015840
119905 119884
119905 119885
119905 119906
119905 119875 (119905))
119886119890 119886119904 forallV isin U119886119889
(127)
whereH [0 119879]times119867timesL(Γ119867)times119867timesL(Γ119867)times119880times119870 rarr R
is the Hamiltonian function defined by
H (119905 119910 119910 119911 V 119901) = 1198971ℎ (119905 119910 119910 119911 V)
+ 119901119891 (119905 119910 119910 119911 V) (128)
Proof By (122) and Proposition 19 we obtain
minus120588 le1
120576Eint
119879
0
119875120588
(119904)E1015840[119891 (119904 Λ
120588
119904 119906
120576120588
119904)
minus 119891 (119904 Λ120588
119904 119906
120588
119904)] 119889119904
+119897120588
1
120576Eint
119879
0
E1015840[ℎ (119904 Λ
120588
119904 119906
120576120588
119904)
minus ℎ (119904 Λ120588
119904 119906
120588
119904)] 119889119904 + (119897
120588
1+ 119897
120588
2) 119900 (120576)
(129)
Letting 120576 rarr 0+ in (129) we derive for ae 120591 isin [0 119879]
minus120588 le 119897120588
1E1015840[ℎ (120591 Λ
120588
120591 V
120591) minus ℎ (120591 Λ
120588
120591 119906
120588
120591)]
+ 119875120588
(119904)E1015840[119891 (120591 Λ
120588
120591 V
120591) minus 119891 (120591 Λ
120588
120591 119906
120588
120591)]
(130)
for all V isin UadFinally taking 120588 rarr 0 in (130) we derive the desired
result
Remark 21 We note that if the coefficients do not dependexplicitly on the marginal law of the underlying diffusionthe result reduces to the classical case that is the SMP forBSPDEs without mean-field term
Remark 22 When we remove the initial state constraint (75)we obtain the general maximum principle for the mean-fieldBSPDEs system (ie without the constraint) with 119897
1= 1
54 Application A Backward Linear Quadratic Control Prob-lem Now we apply our maximum principle to solve an LQproblem For notational simplicity we restrict ourselves tothe free case (ie without the initial state constraint (75)) thegeneral case being handled in a similar way
Mathematical Problems in Engineering 17
Consider the following problem
119869 (119906) =1
2E [(119884 (0))
2] +
1
2Eint
119879
0
119873V2 (119905) 119889119905 997888rarr min
(131)
subject to
119889119884 (119905) = minus119860119884 (119905) 119889119905
minus 119861119884 (119905) + 119861E [119884 (119905)]
+119862V (119905) + 119863119885 (119905) + 119863E [119885 (119905)] 119889119905
+ 119885 (119905) 119889119882 (119905)
119884 (119879) = 120585 119905 isin [0 119879]
(132)
where 119860 is a partial differential operator satisfying condition(L1) and 119861 119861 119862119863119863 and119873 are bounded and deterministicconstantsWe also assume that119873 gt 0 and V isin 1198712F (0 119879 119880) It iseasy to verify that BSPDE (132) admits a uniquemild solution(119884(119905) 119885(119905))
119875(119905) the adjoint process of state equation (132) is thesolution of
119889119875 (119905) = 119860lowast119875 (119905) 119889119905 + 119861119875 (119905) + 119861E [119875 (119905)] 119889119905
+ 119863119875 (119905) + 119863E [119875 (119905)] 119889119882 (119905)
119875 (0) = 119884 (0)
(133)
Let 119906(sdot) be an optimal control and let (119884(sdot) 119885(sdot)) bethe corresponding state process By maximum principle ofTheorem 20 (note that 119897
1= 1 in this problem)
1
2119873V2 (119905) + 119875 (119905) 119862V (119905) ge
1
2119873119906
2
(119905) + 119875 (119905) 119862119906 (119905) (134)
for all V isin 119880ad since the state equation has the form (132)Thisin turn implies
119906 (119905) = minus119862
119873119875 (119905) (135)
It is clear that (131) is a positive quadratic functional of controlbecause of 119873 gt 0 Hence an optimal control exists Thecandidate optimal control (135) is indeed an optimal controlof this LQ problem for it is the only control which satisfies themaximum principle
Next we want to obtain a more explicit representationof the optimal control (135) from the state equation (132)Substituting (135) into state equation (132) yields
119889119884 (119905) = minus119860119884 (119905) 119889119905
minus 119861119884 (119905) + 119861E [119884 (119905)] minus1198622
119873119875 (119905)
+119863119885 (119905) + 119863E [119885 (119905)] 119889119905 + 119885 (119905) 119889119882 (119905)
119884 (119879) = 120585 119905 isin [0 119879]
(136)
Combining the above equation with (133) we obtain thefollowing related feedback control system
119889119884 (119905) = minus119860119884 (119905) 119889119905
minus 119861119884 (119905) + 119861E [119884 (119905)] minus1198622
119873119875 (119905)
+ 119863119885 (119905) + 119863E [119885 (119905)] 119889119905 + 119885 (119905) 119889119882 (119905)
119884 (119879) = 120585
119889119875 (119905) = 119860lowast119875 (119905) 119889119905 + 119861119875 (119905) + 119861E [119875 (119905)] 119889119905
+ 119863119875 (119905) + 119863E [119875 (119905)] 119889119882 (119905)
119875 (0) = 119884 (0)
(137)
Looking at the terminal condition of 119875(119905) in (133) andconsidering the mean-field type of (132) it is reasonable toconjecture that 119875(119905) has the following form
119875 (119905) = 120593 (119905) 119884 (119905) + 120601 (119905)E [119884 (119905)] (138)
where120593(119905)120601(119905) are deterministic differential functionswhichwill be specified below Moreover 120593(0) = 1 120601(0) = 0
Inserting this form into adjoint equation (133) and notic-ing that 119884(119905) satisfies (136) we can compare the coefficientsof 119889119905 and 119889119882(119905) to obtain the following equation
2120593 (119905) 119860119884 (119905) + 2120601 (119905) 119860E [119884 (119905)] + 119861120593 (119905) 119884 (119905)
+ 2 (119861120601 (119905) + 119861120593 (119905) + 119861120601 (119905))E [119884 (119905)]
= minus120593 (119905) 119861119884 (119905) minus1198622
119873120593 (119905) 119884 (119905) + 119863119885 (119905)
+ 119884 (119905)119889120593 (119905)
119889119905+ E [119884 (119905)]
119889120601 (119905)
119889119905
+ (1206012
(119905)1198622
119873+ 2
1198622
119873120593 (119905) 120601 (119905))E [119884 (119905)]
minus (119863120601 (119905) + 119863120601 (119905) + 119863120593 (119905))E [119885 (119905)]
120593 (119905) 119885 (119905) = 119863120593 (119905) 119884 (119905)
+ (119863120601 (119905) + 119863120593 (119905) + 119863120601 (119905))E [119884 (119905)]
(139)
Then subtracting 119885(119905) we have
2120593 (119905) 119860119884 (119905) + 2120601 (119905) 119860E [119884 (119905)] + 119861120593 (119905) 119884 (119905)
+ 2 (119861120601 (119905) + 119861120593 (119905) + 119861120601 (119905))E [119884 (119905)]
18 Mathematical Problems in Engineering
= 120593 (119905) minus119861 +1198622
119873120593 (119905) minus 119863
2119884 (119905)
+ 119884 (119905)119889120593 (119905)
119889119905+ E [119884 (119905)]
119889120601 (119905)
119889119905
+ (1206012
(119905)1198622
119873+ 2
1198622
119873120593 (119905) 120601 (119905))E [119884 (119905)]
minus1
120593 (119905)(119863120601 (119905) + 119863120593 (119905) + 119863120601 (119905))
2
E [119884 (119905)]
minus 2119863 (119863120601 (119905) + 119863120593 (119905) + 119863120601 (119905))E [119884 (119905)]
(140)
Comparing the coefficients of 119884(119905) and E[119884(119905)] respec-tively we get
119889
119889119905120593 (119905) = 2119860
lowast120593 (119905) + 2119861120593 (119905) + 119863
2120593 (119905) minus
1198622
1198731205932
(119905)
120593 (0) = 1
(141)
119889
119889119905120601 (119905) = 2119860
lowast120601 (119905) + (
(119863 + 119863)2
120593 (119905)minus1198622
119873)120601
2
(119905)
+ 2119877 (119905) 120601 (119905) + (1198632+ 2119861 + 2119863119863)120593 (119905)
120601 (0) = 0
(142)
where 119877(119905) = 119861 + 119861 + (119863 + 119863)2
minus (1198622119873)120593(119905)
We solve (141) to get
120593 (119905) = (119890minus(2119860lowast+2119861+119863
2)119905(1 minus
1198622
119873(2119860lowast + 2119861 + 1198632))
+1198622
119873(2119860lowast + 2119861 + 1198632))
minus1
(143)
Then (142) exists a unique solution from the classical Riccatiequation theory
We now conclude the above discussions in the followingresult
Theorem 23 For onersquos linear quadratic stochastic partialdifferential control problem (131)-(132) the unique optimalcontrol 119906(sdot) isin 119880ad is given by
119906 (119905) = minus119862
119873(120593 (119905) 119884 (119905) + 120601 (119905)E [119884 (119905)]) (144)
with 120593(119905) satisfying (143) and 120601(119905) solving (142)
Conflict of Interests
The authors declare that they have no conflict of interestsregarding the publication of this paper
Acknowledgment
This research is partially supported by the Natural ScienceFoundation of China under Grant no 11301310
References
[1] Y Hu and S Peng ldquoAdapted solution of a backward semilinearstochastic evolution equationrdquo Stochastic Analysis and Applica-tions vol 9 no 4 pp 445ndash459 1991
[2] N IMahmudov andMAMcKibben ldquoOnbackward stochasticevolution equations in Hilbert spaces and optimal controlrdquoNonlinear Analysis vol 67 no 4 pp 1260ndash1274 2007
[3] M Fuhrman and G Tessitore ldquoNonlinear Kolmogorov equa-tions in infinite dimensional spaces the backward stochasticdifferential equations approach and applications to optimalcontrolrdquoThe Annals of Probability vol 30 no 3 pp 1397ndash14652002
[4] M Fuhrman and G Tessitore ldquoInfinite horizon backwardstochastic differential equations and elliptic equations inHilbert spacesrdquo Annals of Probability vol 32 no 1B pp 607ndash660 2004
[5] X Mao ldquoAdapted solutions of backward stochastic differentialequations with non-Lipschitz coefficientsrdquo Stochastic Processesand their Applications vol 58 no 2 pp 281ndash292 1995
[6] J Lasry and P Lions ldquoMean field gamesrdquo Japanese Journal ofMathematics vol 2 no 1 pp 229ndash260 2007
[7] R Buckdahn J Li and S Peng ldquoMean-field backward stochas-tic differential equations and related partial differential equa-tionsrdquo Stochastic Processes and their Applications vol 119 no 10pp 3133ndash3154 2009
[8] D Andersson and B Djehiche ldquoAmaximum principle for SDEsofmean-field typerdquoAppliedMathematics andOptimization vol63 no 3 pp 341ndash356 2011
[9] R Buckdahn B Djehiche J Li and S Peng ldquoMean-fieldbackward stochastic differential equations a limit approachrdquoThe Annals of Probability vol 37 no 4 pp 1524ndash1565 2009
[10] R Xu ldquoMean-field backward doubly stochastic differentialequations and related SPDEsrdquo Boundary Value Problems vol2012 article 114 2012
[11] R Buckdahn B Djehiche and J Li ldquoA general stochasticmaximum principle for SDEs of mean-field typerdquo AppliedMathematics and Optimization vol 64 no 2 pp 197ndash216 2011
[12] J Li ldquoStochasticmaximumprinciple in themean-field controlsrdquoAutomatica vol 48 no 2 pp 366ndash373 2012
[13] G Wang C Zhang and W Zhang ldquoStochastic maximumprinciple for mean-field type optimal control under partialinformationrdquo IEEE Transactions on Automatic Control vol 59no 2 pp 522ndash528 2014
[14] M Hafayed ldquoA mean-field necessary and sufficient conditionsfor optimal singular stochastic controlrdquo Communications inMathematics and Statistics vol 1 no 4 pp 417ndash435 2013
[15] M Hafayed ldquoA mean-field maximum principle for optimalcontrol of forward-backward stochastic differential equationswith Poisson jumpprocessesrdquo International Journal ofDynamicsand Control vol 1 no 4 pp 300ndash315 2013
[16] Y Hu and S Peng ldquoMaximum principle for semilinearstochastic evolution control systemsrdquo Stochastics and StochasticsReports vol 33 no 3-4 pp 159ndash180 1990
[17] M Fuhrman Y Hu and G Tessitore ldquoStochastic maximumprinciple for optimal control of SPDEsrdquo Comptes Rendus Math-ematique vol 350 no 13-14 pp 683ndash688 2012
Submit your manuscripts athttpwwwhindawicom
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
MathematicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Mathematical Problems in Engineering
Hindawi Publishing Corporationhttpwwwhindawicom
Differential EquationsInternational Journal of
Volume 2014
Applied MathematicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Probability and StatisticsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
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Mathematical PhysicsAdvances in
Complex AnalysisJournal of
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OptimizationJournal of
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CombinatoricsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of
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Operations ResearchAdvances in
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Function Spaces
Abstract and Applied AnalysisHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of Mathematics and Mathematical Sciences
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The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
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Algebra
Discrete Dynamics in Nature and Society
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Decision SciencesAdvances in
Discrete MathematicsJournal of
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Volume 2014 Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Stochastic AnalysisInternational Journal of
Mathematical Problems in Engineering 13
where
1198681
119904= (119870
120576
119904)1015840
int
1
0
(1198911199101015840 (119904 120582) minus 119891
1199101015840 (119904)) 119889120582
1198682
119904= 119870
120576
119904int
1
0
(119891119910(119904 120582) minus 119891
119910(119904)) 119889120582
1198683
119904= (119876
120576
119904)1015840
int
1
0
(1198911199111015840 (119904 120582) minus 119891
1199111015840 (119904)) 119889120582
1198684
119904= 119876
120576
119904int
1
0
(119891119911(119904 120582) minus 119891
119911(119904)) 119889120582
(95)
Then
Eint119879
119905
E1015840[1003816100381610038161003816120574120576
119904
10038161003816100381610038162
] 119889119904
le 4Eint119879
119905
E1015840[100381610038161003816100381610038161198681
119904
10038161003816100381610038161003816
2
+100381610038161003816100381610038161198682
119904
10038161003816100381610038161003816
2
+100381610038161003816100381610038161198683
119904
10038161003816100381610038161003816
2
+100381610038161003816100381610038161198684
119904
10038161003816100381610038161003816
2
] 119889119904
(96)
Take Eint119879119905E1015840[|1198682
119904|2
]119889119904 for example
Eint119879
119905
E1015840[100381610038161003816100381610038161198682
119904
10038161003816100381610038161003816
2
] 119889119904
= Eint119879
119905
E1015840[(119870
120576
119904int
1
0
(119891119910(119904 120582) minus 119891
119910(119904)) 119889120582)
2
]119889119904
le Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119904 120582) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582] 119889119904
le 2Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119904 120582) minus 119891
119910(119906
120576
119905)10038161003816100381610038161003816
2
119889120582] 119889119904
+ 2Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119906
120576
119905) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582] 119889119904
(97)
Note that
Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119906
120576
119905) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582] 119889119904
= Eint119903+120576
119903
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119906
120576
119905) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582] 119889119904
le sup119904isin[119905119879]
E [E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119906
120576
119905) minus 119891
119910(119904)10038161003816100381610038161003816
2
119889120582]] 120576
997888rarr 0 as 120576 997888rarr 0
(98)
The inequality above holds due to the boundedness of|119870
120576
119904|2
int1
0|119891119910(119906
120576
119905) minus 119891
119910(119904)|
2
119889120582 Indeed Assumption (L3) im-plies the boundedness of 119891
119910(119906
120576
119905) minus 119891
119910(119904) Meanwhile 119870120576
119904is
the solution of mean-field BSEE (82) It can be easy to check119870120576
119904is bounded since the coefficients in (82) are boundedOn the other hand
Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119904 120582) minus 119891
119910(119906
120576
119905)10038161003816100381610038161003816
2
119889120582] 119889119904
le 11986221205822Eint
119879
119905
1003816100381610038161003816119870120576
119904
10038161003816100381610038162
times int
1
0
1198641015840[100381610038161003816100381610038161003816(119884
120576
119904minus 119884
119904)1015840100381610038161003816100381610038161003816
2
+100381610038161003816100381610038161003816(119885
120576
119904minus 119885
119904)1015840100381610038161003816100381610038161003816
2
times10038161003816100381610038161003816119884120576
119904minus 119884
119904
10038161003816100381610038161003816
2
+10038161003816100381610038161003816119885120576
119904minus 119885
119904
10038161003816100381610038161003816
2
] 119889120582 119889119904
(99)
where (119884120576
119904 119885
120576
119904) is the mild solution of the following equation
119889119884120576
119905= minus 119860119884
120576
119905119889119905
minus E1015840[119891 (119905 (119884
120576
119905)1015840
(119885120576
119905)1015840
119884120576
119905 119885
120576
119905 119906
120576
119905)] 119889119905
+ 119885120576
119905119889119882 (119905) 119905 isin [0 119879]
119884120576
119879= 120585
(100)
and (119884119904 119885
119904) is the mild solution of
119889119884119905= minus 119860119884
119905119889119905
minus E1015840[119891 (119905 (119884
119905)1015840
(119885119905)1015840
119884119905 119885
119905 119906
119905)] 119889119905
+ 119885119905119889119882 (119905) 119905 isin [0 119879]
119884119879= 120585
(101)
By the definition of 119906120576119905 according to (L2) we have
E1015840[119891 (119904 119884
1015840
(119904) 1198851015840
(119904) 119884 (119904) 119885 (119904) 119906120576
119905)]
997888rarr E1015840[119891 (119904 119884
1015840
(119904) 1198851015840
(119904) 119884 (119904) 119885 (119904) 119906119905)]
(102)
in 1198712([0 119879]119867) as 120576 rarr 0 Using the continuous dependencetheorem Corollary 5 we obtain
(119884120576
119904 119885
120576
119904) 997888rarr (119884
119904 119885
119904) as 120576 997888rarr 0 (103)
Then
Eint119879
119905
E1015840[1003816100381610038161003816119870
120576
119904
10038161003816100381610038162
int
1
0
10038161003816100381610038161003816119891119910(119904 120582) minus 119891
119910(119906
120576
119905)10038161003816100381610038161003816
2
119889120582] 119889119904
le 11986221205822Eint
119879
119905
1003816100381610038161003816119870120576
119904
10038161003816100381610038162
times int
1
0
E1015840[100381610038161003816100381610038161003816(119884
120576
119904minus 119884
119904)1015840100381610038161003816100381610038161003816
2
+100381610038161003816100381610038161003816(119885
120576
119904minus 119885
119904)1015840100381610038161003816100381610038161003816
2
times10038161003816100381610038161003816119884120576
119904minus 119884
119904
10038161003816100381610038161003816
2
+10038161003816100381610038161003816119885120576
119904minus 119885
119904
10038161003816100381610038161003816
2
] 119889120582 119889119904
997888rarr 0 as 120576 997888rarr 0
(104)
14 Mathematical Problems in Engineering
Combining (98) with (104) we finally haveEint
119879
119905E1015840[|1198682
119904|2
]119889119904 rarr 0 as 120576 rarr 0The required result (93) follows by using the similar
estimations for 1198681119904 1198683
119904 and 1198684
119904
Note that 1 minus 1921198722
1198601198622120573 gt 0 if 120573 is sufficiently large
Now to prove the desired result (84) it suffices to applyGronwallrsquos lemma and estimate (93) to inequality (92)
To deal with the state constraint (75) we need to recall theEkeland variational principle
Lemma 16 (Ekelandrsquos variational principle see [16 Lemma41]) Let (119878 119889) be a complete metric space and let 119865(sdot) 119878 rarr
R be lower semicontinuous and bounded from below If for 120588 gt0 there exists 119906 isin 119878 such that
119865 (119906) le infVisin119878
119865 (V) + 120588 (105)
then there exists 119906120588 isin 119878 satisfying
(i) 119865 (119906120588) le 119865 (119906)
(ii) 119889 (119906120588 119906) le 120588
(iii) 119865 (119906120588) le 119865 (V) + 120588 sdot 119889 (119906120588 119906) forallV = 119906120588
(106)
Now fix V isin Uad and set
119878 = V (sdot) isin Uad | sup0le119905le119879
E1003816100381610038161003816V11990510038161003816100381610038162
le E100381610038161003816100381611990611990510038161003816100381610038162
+ |V|2
119889 (V (sdot) V (sdot)) = 119898 119905 isin [0 119879] | E1003816100381610038161003816V119905 minus V
119905
10038161003816100381610038162
gt 0
forallV (sdot) V (sdot) isin 119878
(107)
where119898 denotes the Lebesgue measure on RThe following result is proved as Proposition 41 in [16]
Lemma 17 (119878 119889(sdot sdot)) is a complete metric space and 119869120588 is
continuous and bounded on 119878 where
119869120588
(V (sdot)) = (119869 (V (sdot)) minus 119869 (119906 (sdot)) + 120588)2
+
10038161003816100381610038161003816100381610038161003816Eint
O
Φ(119909 (1198840(119909))
1015840
1198840(119909)) 119889119909
10038161003816100381610038161003816100381610038161003816
2
12
forallV (sdot) isin 119878(108)
and (119884 119885) is the mild solution of (72) corresponding to thecontrol V
Now we consider the following free initial state optimalcontrol problem
infV(sdot)isin119878
119869120588
(V (sdot)) (109)
It is easy to check that
0 le infV(sdot)isin119878
119869120588
(V (sdot)) le 119869120588
(119906 (sdot)) = 120588 (110)
According to Ekelandrsquos variational principle there exists a119906120588(sdot) isin 119881 such that
(i) 119869120588 (119906120588 (sdot)) le 120588
(ii) 119889 (119906120588 (sdot) 119906 (sdot)) le 120588
(iii) 119869120588 (119906120588 (sdot)) le 119869120588
(V (sdot)) + 120588119889 (119906120588 (sdot) 119906 (sdot)) forallV (sdot) isin 119878(111)
Using the spike variationmethod we can construct 119906120576120588(sdot) isin 119878as follows
119906120576120588
119905=
V119905 119905 isin [119904 119904 + 120576]
119906120588
119905 119905 isin [0 119879] [119904 119904 + 120576]
(112)
It is clear that 119889(119906120576120588(sdot) 119906120588(sdot)) le 120576 Let (119884120576120588(sdot) 119885
120576120588(sdot)) (resp
(119884120588(sdot) 119885
120588(sdot))) be the solution of (72) with respect to the
control 119906120576120588(sdot) (resp 119906120588(sdot)) Following (82) (119870120576120588
119905 119876
120576120588
119905) is the
mild solution of
119870120576120588
119905= int
119879
119905
119890119860(119904minus119905)
E1015840
times [1198911199101015840 (119904 Λ
120588
119904 119906
120588
119904) (119870
120576120588
119904)1015840
+ 119891119910(119904 Λ
120588
119904 119906
120588
119904)119870
120576120588
119904
+ 1198911199111015840 (119904 Λ
120588
119904 119906
120588
119904) (119876
120576120588
119904)1015840
+ 119891119911(119904 Λ
120588
119904 119906
120588
119904) 119876
120576120588
119904
+1
120576(119891 (119904 Λ
120588
119904 119906
120576120588
119904) minus 119891 (119904 Λ
120588
119904 119906
120588
119904))] 119889119904
minus int
119879
119905
119890119860(119904minus119905)
119876120576120588
119904119889119882 (119904)
(113)
ByTheorem 15 we know that
lim120576rarr0
E[ sup119904isin[119905119879]
100381610038161003816100381610038161003816100381610038161003816
119884120576120588
119904minus 119884
120588
119904
120576minus 119870
120576120588
119904
100381610038161003816100381610038161003816100381610038161003816
2
] = 0 forall119905 isin [0 119879]
lim120576rarr0
Eint119879
0
100381610038161003816100381610038161003816100381610038161003816
119885120576120588
119904minus 119885
120588
119904
120576minus 119876
120576120588
119904
100381610038161003816100381610038161003816100381610038161003816
2
119889119904 = 0
(114)
The proof of the following proposition is technical butbased on the arguments above and we omit it
Proposition 18 One has
1
120576E [Φ ((119884
120576120588
0)1015840
119884120576120588
0) minus Φ((119884
120588
0)1015840
119884120588
0)]
= E [Φ1199101015840 ((119884
120588
0)1015840
119884120588
0) (119870
120576120588
0)1015840
+ Φ119910((119884
120588
0)1015840
119884120588
0)119870
120576120588
0]
+ 119900 (120576)
Mathematical Problems in Engineering 15
1
120576(119869 (119906
120576120588) minus 119869 (119906
120588))
= E [1198921199101015840 ((119884
120588
0)1015840
119884120588
0) (119870
120576120588
0)1015840
+ 119892119910((119884
120588
0)1015840
119884120588
0)119870
120576120588
0]
+ Δ120576+1
120576Eint
119879
0
E1015840[ℎ (119904 Λ
120588
119904 119906
120576120588
119904)
minus ℎ (119904 Λ120588
119904 119906
120588
119904)] 119889119904 + 119900 (120576)
(115)
where
Δ120576= Eint
119879
0
E1015840[ℎ
1199101015840 (119904 Λ
120588
119904 119906
120576120588
119904) (119870
120576120588
119904)1015840
+ ℎ1199111015840 (119904 Λ
120588
119904 119906
120576120588
119904) (119876
120576120588
119904)1015840
+ ℎ119910(119904 Λ
120588
119904 119906
120576120588
119904)119870
120576120588
119904
+ ℎ119911(119904 Λ
120588
119904 119906
120576120588
119904) 119876
120576120588
119904] 119889119904
(116)
53 Variational Inequality and Adjoint Equation In thissubsection the adjoint process is introduced to deduce thevariational inequality
If we set V(sdot) = 119906120576120588(sdot) in (111) and notice that
119889(119906120576120588(sdot) 119906
120588(sdot)) le 120576 we get
minus120588 le1
120576(119869
120588(119906
120576120588
(sdot)) minus 119869120588(119906
120588
(sdot))) (117)
By Lemma 17
1
120576(119869
120588(119906
120576120588
(sdot)) minus 119869120588(119906
120588
(sdot)))
=(119869
120588(119906
120576120588(sdot)))
2
minus (119869120588(119906
120588(sdot)))
2
120576 (119869120588 (119906120576120588 (sdot)) + 119869120588 (119906120588 (sdot)))
=119869 (119906
120576120588(sdot)) + 119869 (119906
120588(sdot)) minus 2119869 (119906 (sdot)) + 2120588
119869120588 (119906120576120588 (sdot)) + 119869120588 (119906120588 (sdot))
times119869 (119906
120576120588(sdot)) minus 119869 (119906
120588(sdot))
120576
+
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] + E [Φ ((119884
120588
0)1015840
119884120588
0)]
119869120588 (119906120576120588 (sdot)) + 119869120588 (119906120588 (sdot))
times
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] minus E [Φ ((119884
120588
0)1015840
119884120588
0)]
120576
997888rarr 119897120588
1
119869 (119906120576120588(sdot)) minus 119869 (119906
120588(sdot))
120576
+ 119897120588
2
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] minus E [Φ ((119884
120588
0)1015840
119884120588
0)]
120576
(118)
where we set
119897120588
1=119869 (119906
120588(sdot)) minus 119869 (119906 (sdot)) + 120588
119869120588 (119906120588 (sdot))
119897120588
2=
E [Φ ((119884120588
0)1015840
119884120588
0)]
119869120588 (119906120588 (sdot))
(119)
and use the limit
119869 (119906120576120588
(sdot)) 997888rarr 119869 (119906120588
(sdot))
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] 997888rarr E [Φ ((119884
120588
0)1015840
119884120588
0)]
(120)
as 120576 rarr 0 according to (115)As |119897120588
1|2
+ |119897120588
2|2
= 1 for all 120588 gt 0 we know that there existsa subsequence of 119897120588
1 119897120588
2 (still denoted by 119897120588
1 119897120588
2) such that
lim120588rarr0
119897120588
1 119897120588
2 = 119897
1 1198972
1003816100381610038161003816119897110038161003816100381610038162
+1003816100381610038161003816119897210038161003816100381610038162
= 1
(121)
Combining (115) (117) with (118) we get
minus120588 le 119897120588
1
119869 (119906120576120588(sdot)) minus 119869 (119906
120588(sdot))
120576
+ 119897120588
2
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] minus E [Φ ((119884
120588
0)1015840
119884120588
0)]
120576
= 119897120588
1Δ120576+ 119897
120588
1E [119892
1199101015840 ((119884
120588
0)1015840
119884120588
0) (119870
120576120588
0)1015840
+119892119910((119884
120588
0)1015840
119884120588
0)119870
120576120588
0]
+119897120588
1
120576Eint
119879
0
E1015840[ℎ (119904 Λ
120588
119904 119906
120576120588
119904) minus ℎ (119904 Λ
120588
119904 119906
120588
119904)] 119889119904
+ 119897120588
2E [Φ
1199101015840 ((119884
120588
0)1015840
119884120588
0) (119870
120576120588
0)1015840
+Φ119910((119884
120588
0)1015840
119884120588
0)119870
120576120588
0] + (119897
120588
1+ 119897
120588
2) 119900 (120576)
(122)
Next we introduce the adjoint equation corresponding tovariational equation (113) whose solution is denoted by119875120588(119905)
119889119875120588
(119905)
= 119860lowast119875120588
(119905) 119889119905
+ E1015840[119891
1199101015840 (119905 Λ
120588
119905 119906
120588
119905) (119875
120588
(119905))1015840
+ 119891119910(119905 Λ
120588
119905 119906
120588
119905) 119875
120588
(119905) + 119897120588
1ℎ1199101015840 (119905 Λ
120588
119905 119906
120588
119905)
+ 119897120588
1ℎ119910(119905 Λ
120588
119905 119906
120588
119905)] 119889119905
16 Mathematical Problems in Engineering
+ E1015840[119891
1199111015840 (119905 Λ
120588
119905 119906
120588
119905) (119875
120588
(119905))1015840
+ 119891119911(119905 Λ
120588
119905 119906
120588
119905) 119875
120588
(119905)
+ 119897120588
1ℎ1199111015840 (119905 Λ
120588
119905 119906
120588
119905) + 119897
120588
1ℎ119911(119905 Λ
120588
119905 119906
120588
119905)] 119889119882 (119905)
119875120588
(0) = 119897120588
1E [119892
1199101015840 ((119884
120588
0(119909))
1015840
119884120588
0(119909))
+ 119892119910((119884
120588
0(119909))
1015840
119884120588
0(119909))]
+ 119897120588
2E [Φ
1199101015840 ((119884
120588
0(119909))
1015840
119884120588
0(119909))
+ Φ119910((119884
120588
0(119909))
1015840
119884120588
0(119909))]
(123)
where 119860lowast is the 1198712(O)-adjoint operator of 119860 Under assump-tions (L1)ndash(L3) this is a linear mean-field SEE with boundedcoefficients An application ofTheorem 11 implies that it has aunique adaptedmild solution such that119875120588(119905) isin S2
F ([0 119879] 119870)When 120588 rarr 0 according to Corollaries 5 and 13 119875120588(119905)
converges to 119875(119905) where
119875 (119905) isin S2
F ([0 119879] 119870) (124)
is the solution of the following equation
119875 (119905) = 1198971E [119892
1199101015840 (119884
1015840
0 119884
0) + 119892
119910(119884
1015840
0 119884
0)]
+ 1198972E [Φ
1199101015840 (119884
1015840
0 119884
0) + Φ
119910(119884
1015840
0 119884
0)]
+ int
119905
0
119890119860lowast(119905minus119904)
E1015840[119891
1199101015840 (119904) 119875
1015840
(119904) + 119891119910(119904) 119875 (119904)
+ 1198971ℎ1199101015840 (119904) + 119897
1ℎ119910(119904)] 119889119904
+ int
119905
0
119890119860lowast(119905minus119904)
E1015840[119891
1199111015840 (119904) 119875
1015840
(119904) + 119891119911(119904) 119875 (119904)
+ 1198971ℎ1199111015840 (119904) + 119897
1ℎ119911(119904)] 119889119882 (119904)
(125)
The following proposition which formally follows fromProposition 18 gives the relation between 119875120588(119905) and119870120576120588
119905
Proposition 19 Consider the following
E [119875120588
(0)119870120576120588
0]
= Eint119879
0
1
120576119875120588
(119904)E1015840[119891 (119904 Λ
120588
119904 119906
120576120588
119904) minus 119891 (119904 Λ
120588
119904 119906
120588
119904)]
minus 119897120588
1119870120576120588
119904E1015840[ℎ
1199101015840 (119904 Λ
120588
119904 119906
120588
119904) + ℎ
119910(119904 Λ
120588
119904 119906
120588
119904)]
minus 119897120588
1119876120576120588
119904E1015840[ℎ
1199111015840 (119904 Λ
120588
119904 119906
120588
119904) + ℎ
119911(119904 Λ
120588
119904 119906
120588
119904)] 119889119904
(126)
The following theorem constitutes the main contributionof this section themaximumprinciple for the BSPDE controlsystem
Theorem 20 Let assumptions (L1)ndash(L3) hold Suppose 119906(sdot) isan optimal control and (119884(sdot) 119885(sdot)) is the corresponding optimalstate trajectory for the BSPDE control systems (72) and (73)with the initial state constraint (75) Then there exists 119875(119905) isinS2
F ([0 119879] 119870) which satisfies (125) such that
H (119905 1198841015840
119905 119885
1015840
119905 119884
119905 119885
119905 V
119905 119875 (119905))
ge H (119905 1198841015840
119905 119885
1015840
119905 119884
119905 119885
119905 119906
119905 119875 (119905))
119886119890 119886119904 forallV isin U119886119889
(127)
whereH [0 119879]times119867timesL(Γ119867)times119867timesL(Γ119867)times119880times119870 rarr R
is the Hamiltonian function defined by
H (119905 119910 119910 119911 V 119901) = 1198971ℎ (119905 119910 119910 119911 V)
+ 119901119891 (119905 119910 119910 119911 V) (128)
Proof By (122) and Proposition 19 we obtain
minus120588 le1
120576Eint
119879
0
119875120588
(119904)E1015840[119891 (119904 Λ
120588
119904 119906
120576120588
119904)
minus 119891 (119904 Λ120588
119904 119906
120588
119904)] 119889119904
+119897120588
1
120576Eint
119879
0
E1015840[ℎ (119904 Λ
120588
119904 119906
120576120588
119904)
minus ℎ (119904 Λ120588
119904 119906
120588
119904)] 119889119904 + (119897
120588
1+ 119897
120588
2) 119900 (120576)
(129)
Letting 120576 rarr 0+ in (129) we derive for ae 120591 isin [0 119879]
minus120588 le 119897120588
1E1015840[ℎ (120591 Λ
120588
120591 V
120591) minus ℎ (120591 Λ
120588
120591 119906
120588
120591)]
+ 119875120588
(119904)E1015840[119891 (120591 Λ
120588
120591 V
120591) minus 119891 (120591 Λ
120588
120591 119906
120588
120591)]
(130)
for all V isin UadFinally taking 120588 rarr 0 in (130) we derive the desired
result
Remark 21 We note that if the coefficients do not dependexplicitly on the marginal law of the underlying diffusionthe result reduces to the classical case that is the SMP forBSPDEs without mean-field term
Remark 22 When we remove the initial state constraint (75)we obtain the general maximum principle for the mean-fieldBSPDEs system (ie without the constraint) with 119897
1= 1
54 Application A Backward Linear Quadratic Control Prob-lem Now we apply our maximum principle to solve an LQproblem For notational simplicity we restrict ourselves tothe free case (ie without the initial state constraint (75)) thegeneral case being handled in a similar way
Mathematical Problems in Engineering 17
Consider the following problem
119869 (119906) =1
2E [(119884 (0))
2] +
1
2Eint
119879
0
119873V2 (119905) 119889119905 997888rarr min
(131)
subject to
119889119884 (119905) = minus119860119884 (119905) 119889119905
minus 119861119884 (119905) + 119861E [119884 (119905)]
+119862V (119905) + 119863119885 (119905) + 119863E [119885 (119905)] 119889119905
+ 119885 (119905) 119889119882 (119905)
119884 (119879) = 120585 119905 isin [0 119879]
(132)
where 119860 is a partial differential operator satisfying condition(L1) and 119861 119861 119862119863119863 and119873 are bounded and deterministicconstantsWe also assume that119873 gt 0 and V isin 1198712F (0 119879 119880) It iseasy to verify that BSPDE (132) admits a uniquemild solution(119884(119905) 119885(119905))
119875(119905) the adjoint process of state equation (132) is thesolution of
119889119875 (119905) = 119860lowast119875 (119905) 119889119905 + 119861119875 (119905) + 119861E [119875 (119905)] 119889119905
+ 119863119875 (119905) + 119863E [119875 (119905)] 119889119882 (119905)
119875 (0) = 119884 (0)
(133)
Let 119906(sdot) be an optimal control and let (119884(sdot) 119885(sdot)) bethe corresponding state process By maximum principle ofTheorem 20 (note that 119897
1= 1 in this problem)
1
2119873V2 (119905) + 119875 (119905) 119862V (119905) ge
1
2119873119906
2
(119905) + 119875 (119905) 119862119906 (119905) (134)
for all V isin 119880ad since the state equation has the form (132)Thisin turn implies
119906 (119905) = minus119862
119873119875 (119905) (135)
It is clear that (131) is a positive quadratic functional of controlbecause of 119873 gt 0 Hence an optimal control exists Thecandidate optimal control (135) is indeed an optimal controlof this LQ problem for it is the only control which satisfies themaximum principle
Next we want to obtain a more explicit representationof the optimal control (135) from the state equation (132)Substituting (135) into state equation (132) yields
119889119884 (119905) = minus119860119884 (119905) 119889119905
minus 119861119884 (119905) + 119861E [119884 (119905)] minus1198622
119873119875 (119905)
+119863119885 (119905) + 119863E [119885 (119905)] 119889119905 + 119885 (119905) 119889119882 (119905)
119884 (119879) = 120585 119905 isin [0 119879]
(136)
Combining the above equation with (133) we obtain thefollowing related feedback control system
119889119884 (119905) = minus119860119884 (119905) 119889119905
minus 119861119884 (119905) + 119861E [119884 (119905)] minus1198622
119873119875 (119905)
+ 119863119885 (119905) + 119863E [119885 (119905)] 119889119905 + 119885 (119905) 119889119882 (119905)
119884 (119879) = 120585
119889119875 (119905) = 119860lowast119875 (119905) 119889119905 + 119861119875 (119905) + 119861E [119875 (119905)] 119889119905
+ 119863119875 (119905) + 119863E [119875 (119905)] 119889119882 (119905)
119875 (0) = 119884 (0)
(137)
Looking at the terminal condition of 119875(119905) in (133) andconsidering the mean-field type of (132) it is reasonable toconjecture that 119875(119905) has the following form
119875 (119905) = 120593 (119905) 119884 (119905) + 120601 (119905)E [119884 (119905)] (138)
where120593(119905)120601(119905) are deterministic differential functionswhichwill be specified below Moreover 120593(0) = 1 120601(0) = 0
Inserting this form into adjoint equation (133) and notic-ing that 119884(119905) satisfies (136) we can compare the coefficientsof 119889119905 and 119889119882(119905) to obtain the following equation
2120593 (119905) 119860119884 (119905) + 2120601 (119905) 119860E [119884 (119905)] + 119861120593 (119905) 119884 (119905)
+ 2 (119861120601 (119905) + 119861120593 (119905) + 119861120601 (119905))E [119884 (119905)]
= minus120593 (119905) 119861119884 (119905) minus1198622
119873120593 (119905) 119884 (119905) + 119863119885 (119905)
+ 119884 (119905)119889120593 (119905)
119889119905+ E [119884 (119905)]
119889120601 (119905)
119889119905
+ (1206012
(119905)1198622
119873+ 2
1198622
119873120593 (119905) 120601 (119905))E [119884 (119905)]
minus (119863120601 (119905) + 119863120601 (119905) + 119863120593 (119905))E [119885 (119905)]
120593 (119905) 119885 (119905) = 119863120593 (119905) 119884 (119905)
+ (119863120601 (119905) + 119863120593 (119905) + 119863120601 (119905))E [119884 (119905)]
(139)
Then subtracting 119885(119905) we have
2120593 (119905) 119860119884 (119905) + 2120601 (119905) 119860E [119884 (119905)] + 119861120593 (119905) 119884 (119905)
+ 2 (119861120601 (119905) + 119861120593 (119905) + 119861120601 (119905))E [119884 (119905)]
18 Mathematical Problems in Engineering
= 120593 (119905) minus119861 +1198622
119873120593 (119905) minus 119863
2119884 (119905)
+ 119884 (119905)119889120593 (119905)
119889119905+ E [119884 (119905)]
119889120601 (119905)
119889119905
+ (1206012
(119905)1198622
119873+ 2
1198622
119873120593 (119905) 120601 (119905))E [119884 (119905)]
minus1
120593 (119905)(119863120601 (119905) + 119863120593 (119905) + 119863120601 (119905))
2
E [119884 (119905)]
minus 2119863 (119863120601 (119905) + 119863120593 (119905) + 119863120601 (119905))E [119884 (119905)]
(140)
Comparing the coefficients of 119884(119905) and E[119884(119905)] respec-tively we get
119889
119889119905120593 (119905) = 2119860
lowast120593 (119905) + 2119861120593 (119905) + 119863
2120593 (119905) minus
1198622
1198731205932
(119905)
120593 (0) = 1
(141)
119889
119889119905120601 (119905) = 2119860
lowast120601 (119905) + (
(119863 + 119863)2
120593 (119905)minus1198622
119873)120601
2
(119905)
+ 2119877 (119905) 120601 (119905) + (1198632+ 2119861 + 2119863119863)120593 (119905)
120601 (0) = 0
(142)
where 119877(119905) = 119861 + 119861 + (119863 + 119863)2
minus (1198622119873)120593(119905)
We solve (141) to get
120593 (119905) = (119890minus(2119860lowast+2119861+119863
2)119905(1 minus
1198622
119873(2119860lowast + 2119861 + 1198632))
+1198622
119873(2119860lowast + 2119861 + 1198632))
minus1
(143)
Then (142) exists a unique solution from the classical Riccatiequation theory
We now conclude the above discussions in the followingresult
Theorem 23 For onersquos linear quadratic stochastic partialdifferential control problem (131)-(132) the unique optimalcontrol 119906(sdot) isin 119880ad is given by
119906 (119905) = minus119862
119873(120593 (119905) 119884 (119905) + 120601 (119905)E [119884 (119905)]) (144)
with 120593(119905) satisfying (143) and 120601(119905) solving (142)
Conflict of Interests
The authors declare that they have no conflict of interestsregarding the publication of this paper
Acknowledgment
This research is partially supported by the Natural ScienceFoundation of China under Grant no 11301310
References
[1] Y Hu and S Peng ldquoAdapted solution of a backward semilinearstochastic evolution equationrdquo Stochastic Analysis and Applica-tions vol 9 no 4 pp 445ndash459 1991
[2] N IMahmudov andMAMcKibben ldquoOnbackward stochasticevolution equations in Hilbert spaces and optimal controlrdquoNonlinear Analysis vol 67 no 4 pp 1260ndash1274 2007
[3] M Fuhrman and G Tessitore ldquoNonlinear Kolmogorov equa-tions in infinite dimensional spaces the backward stochasticdifferential equations approach and applications to optimalcontrolrdquoThe Annals of Probability vol 30 no 3 pp 1397ndash14652002
[4] M Fuhrman and G Tessitore ldquoInfinite horizon backwardstochastic differential equations and elliptic equations inHilbert spacesrdquo Annals of Probability vol 32 no 1B pp 607ndash660 2004
[5] X Mao ldquoAdapted solutions of backward stochastic differentialequations with non-Lipschitz coefficientsrdquo Stochastic Processesand their Applications vol 58 no 2 pp 281ndash292 1995
[6] J Lasry and P Lions ldquoMean field gamesrdquo Japanese Journal ofMathematics vol 2 no 1 pp 229ndash260 2007
[7] R Buckdahn J Li and S Peng ldquoMean-field backward stochas-tic differential equations and related partial differential equa-tionsrdquo Stochastic Processes and their Applications vol 119 no 10pp 3133ndash3154 2009
[8] D Andersson and B Djehiche ldquoAmaximum principle for SDEsofmean-field typerdquoAppliedMathematics andOptimization vol63 no 3 pp 341ndash356 2011
[9] R Buckdahn B Djehiche J Li and S Peng ldquoMean-fieldbackward stochastic differential equations a limit approachrdquoThe Annals of Probability vol 37 no 4 pp 1524ndash1565 2009
[10] R Xu ldquoMean-field backward doubly stochastic differentialequations and related SPDEsrdquo Boundary Value Problems vol2012 article 114 2012
[11] R Buckdahn B Djehiche and J Li ldquoA general stochasticmaximum principle for SDEs of mean-field typerdquo AppliedMathematics and Optimization vol 64 no 2 pp 197ndash216 2011
[12] J Li ldquoStochasticmaximumprinciple in themean-field controlsrdquoAutomatica vol 48 no 2 pp 366ndash373 2012
[13] G Wang C Zhang and W Zhang ldquoStochastic maximumprinciple for mean-field type optimal control under partialinformationrdquo IEEE Transactions on Automatic Control vol 59no 2 pp 522ndash528 2014
[14] M Hafayed ldquoA mean-field necessary and sufficient conditionsfor optimal singular stochastic controlrdquo Communications inMathematics and Statistics vol 1 no 4 pp 417ndash435 2013
[15] M Hafayed ldquoA mean-field maximum principle for optimalcontrol of forward-backward stochastic differential equationswith Poisson jumpprocessesrdquo International Journal ofDynamicsand Control vol 1 no 4 pp 300ndash315 2013
[16] Y Hu and S Peng ldquoMaximum principle for semilinearstochastic evolution control systemsrdquo Stochastics and StochasticsReports vol 33 no 3-4 pp 159ndash180 1990
[17] M Fuhrman Y Hu and G Tessitore ldquoStochastic maximumprinciple for optimal control of SPDEsrdquo Comptes Rendus Math-ematique vol 350 no 13-14 pp 683ndash688 2012
Submit your manuscripts athttpwwwhindawicom
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
MathematicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Mathematical Problems in Engineering
Hindawi Publishing Corporationhttpwwwhindawicom
Differential EquationsInternational Journal of
Volume 2014
Applied MathematicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Probability and StatisticsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Mathematical PhysicsAdvances in
Complex AnalysisJournal of
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OptimizationJournal of
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CombinatoricsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of
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Operations ResearchAdvances in
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Function Spaces
Abstract and Applied AnalysisHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of Mathematics and Mathematical Sciences
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Algebra
Discrete Dynamics in Nature and Society
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Decision SciencesAdvances in
Discrete MathematicsJournal of
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Volume 2014 Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Stochastic AnalysisInternational Journal of
14 Mathematical Problems in Engineering
Combining (98) with (104) we finally haveEint
119879
119905E1015840[|1198682
119904|2
]119889119904 rarr 0 as 120576 rarr 0The required result (93) follows by using the similar
estimations for 1198681119904 1198683
119904 and 1198684
119904
Note that 1 minus 1921198722
1198601198622120573 gt 0 if 120573 is sufficiently large
Now to prove the desired result (84) it suffices to applyGronwallrsquos lemma and estimate (93) to inequality (92)
To deal with the state constraint (75) we need to recall theEkeland variational principle
Lemma 16 (Ekelandrsquos variational principle see [16 Lemma41]) Let (119878 119889) be a complete metric space and let 119865(sdot) 119878 rarr
R be lower semicontinuous and bounded from below If for 120588 gt0 there exists 119906 isin 119878 such that
119865 (119906) le infVisin119878
119865 (V) + 120588 (105)
then there exists 119906120588 isin 119878 satisfying
(i) 119865 (119906120588) le 119865 (119906)
(ii) 119889 (119906120588 119906) le 120588
(iii) 119865 (119906120588) le 119865 (V) + 120588 sdot 119889 (119906120588 119906) forallV = 119906120588
(106)
Now fix V isin Uad and set
119878 = V (sdot) isin Uad | sup0le119905le119879
E1003816100381610038161003816V11990510038161003816100381610038162
le E100381610038161003816100381611990611990510038161003816100381610038162
+ |V|2
119889 (V (sdot) V (sdot)) = 119898 119905 isin [0 119879] | E1003816100381610038161003816V119905 minus V
119905
10038161003816100381610038162
gt 0
forallV (sdot) V (sdot) isin 119878
(107)
where119898 denotes the Lebesgue measure on RThe following result is proved as Proposition 41 in [16]
Lemma 17 (119878 119889(sdot sdot)) is a complete metric space and 119869120588 is
continuous and bounded on 119878 where
119869120588
(V (sdot)) = (119869 (V (sdot)) minus 119869 (119906 (sdot)) + 120588)2
+
10038161003816100381610038161003816100381610038161003816Eint
O
Φ(119909 (1198840(119909))
1015840
1198840(119909)) 119889119909
10038161003816100381610038161003816100381610038161003816
2
12
forallV (sdot) isin 119878(108)
and (119884 119885) is the mild solution of (72) corresponding to thecontrol V
Now we consider the following free initial state optimalcontrol problem
infV(sdot)isin119878
119869120588
(V (sdot)) (109)
It is easy to check that
0 le infV(sdot)isin119878
119869120588
(V (sdot)) le 119869120588
(119906 (sdot)) = 120588 (110)
According to Ekelandrsquos variational principle there exists a119906120588(sdot) isin 119881 such that
(i) 119869120588 (119906120588 (sdot)) le 120588
(ii) 119889 (119906120588 (sdot) 119906 (sdot)) le 120588
(iii) 119869120588 (119906120588 (sdot)) le 119869120588
(V (sdot)) + 120588119889 (119906120588 (sdot) 119906 (sdot)) forallV (sdot) isin 119878(111)
Using the spike variationmethod we can construct 119906120576120588(sdot) isin 119878as follows
119906120576120588
119905=
V119905 119905 isin [119904 119904 + 120576]
119906120588
119905 119905 isin [0 119879] [119904 119904 + 120576]
(112)
It is clear that 119889(119906120576120588(sdot) 119906120588(sdot)) le 120576 Let (119884120576120588(sdot) 119885
120576120588(sdot)) (resp
(119884120588(sdot) 119885
120588(sdot))) be the solution of (72) with respect to the
control 119906120576120588(sdot) (resp 119906120588(sdot)) Following (82) (119870120576120588
119905 119876
120576120588
119905) is the
mild solution of
119870120576120588
119905= int
119879
119905
119890119860(119904minus119905)
E1015840
times [1198911199101015840 (119904 Λ
120588
119904 119906
120588
119904) (119870
120576120588
119904)1015840
+ 119891119910(119904 Λ
120588
119904 119906
120588
119904)119870
120576120588
119904
+ 1198911199111015840 (119904 Λ
120588
119904 119906
120588
119904) (119876
120576120588
119904)1015840
+ 119891119911(119904 Λ
120588
119904 119906
120588
119904) 119876
120576120588
119904
+1
120576(119891 (119904 Λ
120588
119904 119906
120576120588
119904) minus 119891 (119904 Λ
120588
119904 119906
120588
119904))] 119889119904
minus int
119879
119905
119890119860(119904minus119905)
119876120576120588
119904119889119882 (119904)
(113)
ByTheorem 15 we know that
lim120576rarr0
E[ sup119904isin[119905119879]
100381610038161003816100381610038161003816100381610038161003816
119884120576120588
119904minus 119884
120588
119904
120576minus 119870
120576120588
119904
100381610038161003816100381610038161003816100381610038161003816
2
] = 0 forall119905 isin [0 119879]
lim120576rarr0
Eint119879
0
100381610038161003816100381610038161003816100381610038161003816
119885120576120588
119904minus 119885
120588
119904
120576minus 119876
120576120588
119904
100381610038161003816100381610038161003816100381610038161003816
2
119889119904 = 0
(114)
The proof of the following proposition is technical butbased on the arguments above and we omit it
Proposition 18 One has
1
120576E [Φ ((119884
120576120588
0)1015840
119884120576120588
0) minus Φ((119884
120588
0)1015840
119884120588
0)]
= E [Φ1199101015840 ((119884
120588
0)1015840
119884120588
0) (119870
120576120588
0)1015840
+ Φ119910((119884
120588
0)1015840
119884120588
0)119870
120576120588
0]
+ 119900 (120576)
Mathematical Problems in Engineering 15
1
120576(119869 (119906
120576120588) minus 119869 (119906
120588))
= E [1198921199101015840 ((119884
120588
0)1015840
119884120588
0) (119870
120576120588
0)1015840
+ 119892119910((119884
120588
0)1015840
119884120588
0)119870
120576120588
0]
+ Δ120576+1
120576Eint
119879
0
E1015840[ℎ (119904 Λ
120588
119904 119906
120576120588
119904)
minus ℎ (119904 Λ120588
119904 119906
120588
119904)] 119889119904 + 119900 (120576)
(115)
where
Δ120576= Eint
119879
0
E1015840[ℎ
1199101015840 (119904 Λ
120588
119904 119906
120576120588
119904) (119870
120576120588
119904)1015840
+ ℎ1199111015840 (119904 Λ
120588
119904 119906
120576120588
119904) (119876
120576120588
119904)1015840
+ ℎ119910(119904 Λ
120588
119904 119906
120576120588
119904)119870
120576120588
119904
+ ℎ119911(119904 Λ
120588
119904 119906
120576120588
119904) 119876
120576120588
119904] 119889119904
(116)
53 Variational Inequality and Adjoint Equation In thissubsection the adjoint process is introduced to deduce thevariational inequality
If we set V(sdot) = 119906120576120588(sdot) in (111) and notice that
119889(119906120576120588(sdot) 119906
120588(sdot)) le 120576 we get
minus120588 le1
120576(119869
120588(119906
120576120588
(sdot)) minus 119869120588(119906
120588
(sdot))) (117)
By Lemma 17
1
120576(119869
120588(119906
120576120588
(sdot)) minus 119869120588(119906
120588
(sdot)))
=(119869
120588(119906
120576120588(sdot)))
2
minus (119869120588(119906
120588(sdot)))
2
120576 (119869120588 (119906120576120588 (sdot)) + 119869120588 (119906120588 (sdot)))
=119869 (119906
120576120588(sdot)) + 119869 (119906
120588(sdot)) minus 2119869 (119906 (sdot)) + 2120588
119869120588 (119906120576120588 (sdot)) + 119869120588 (119906120588 (sdot))
times119869 (119906
120576120588(sdot)) minus 119869 (119906
120588(sdot))
120576
+
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] + E [Φ ((119884
120588
0)1015840
119884120588
0)]
119869120588 (119906120576120588 (sdot)) + 119869120588 (119906120588 (sdot))
times
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] minus E [Φ ((119884
120588
0)1015840
119884120588
0)]
120576
997888rarr 119897120588
1
119869 (119906120576120588(sdot)) minus 119869 (119906
120588(sdot))
120576
+ 119897120588
2
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] minus E [Φ ((119884
120588
0)1015840
119884120588
0)]
120576
(118)
where we set
119897120588
1=119869 (119906
120588(sdot)) minus 119869 (119906 (sdot)) + 120588
119869120588 (119906120588 (sdot))
119897120588
2=
E [Φ ((119884120588
0)1015840
119884120588
0)]
119869120588 (119906120588 (sdot))
(119)
and use the limit
119869 (119906120576120588
(sdot)) 997888rarr 119869 (119906120588
(sdot))
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] 997888rarr E [Φ ((119884
120588
0)1015840
119884120588
0)]
(120)
as 120576 rarr 0 according to (115)As |119897120588
1|2
+ |119897120588
2|2
= 1 for all 120588 gt 0 we know that there existsa subsequence of 119897120588
1 119897120588
2 (still denoted by 119897120588
1 119897120588
2) such that
lim120588rarr0
119897120588
1 119897120588
2 = 119897
1 1198972
1003816100381610038161003816119897110038161003816100381610038162
+1003816100381610038161003816119897210038161003816100381610038162
= 1
(121)
Combining (115) (117) with (118) we get
minus120588 le 119897120588
1
119869 (119906120576120588(sdot)) minus 119869 (119906
120588(sdot))
120576
+ 119897120588
2
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] minus E [Φ ((119884
120588
0)1015840
119884120588
0)]
120576
= 119897120588
1Δ120576+ 119897
120588
1E [119892
1199101015840 ((119884
120588
0)1015840
119884120588
0) (119870
120576120588
0)1015840
+119892119910((119884
120588
0)1015840
119884120588
0)119870
120576120588
0]
+119897120588
1
120576Eint
119879
0
E1015840[ℎ (119904 Λ
120588
119904 119906
120576120588
119904) minus ℎ (119904 Λ
120588
119904 119906
120588
119904)] 119889119904
+ 119897120588
2E [Φ
1199101015840 ((119884
120588
0)1015840
119884120588
0) (119870
120576120588
0)1015840
+Φ119910((119884
120588
0)1015840
119884120588
0)119870
120576120588
0] + (119897
120588
1+ 119897
120588
2) 119900 (120576)
(122)
Next we introduce the adjoint equation corresponding tovariational equation (113) whose solution is denoted by119875120588(119905)
119889119875120588
(119905)
= 119860lowast119875120588
(119905) 119889119905
+ E1015840[119891
1199101015840 (119905 Λ
120588
119905 119906
120588
119905) (119875
120588
(119905))1015840
+ 119891119910(119905 Λ
120588
119905 119906
120588
119905) 119875
120588
(119905) + 119897120588
1ℎ1199101015840 (119905 Λ
120588
119905 119906
120588
119905)
+ 119897120588
1ℎ119910(119905 Λ
120588
119905 119906
120588
119905)] 119889119905
16 Mathematical Problems in Engineering
+ E1015840[119891
1199111015840 (119905 Λ
120588
119905 119906
120588
119905) (119875
120588
(119905))1015840
+ 119891119911(119905 Λ
120588
119905 119906
120588
119905) 119875
120588
(119905)
+ 119897120588
1ℎ1199111015840 (119905 Λ
120588
119905 119906
120588
119905) + 119897
120588
1ℎ119911(119905 Λ
120588
119905 119906
120588
119905)] 119889119882 (119905)
119875120588
(0) = 119897120588
1E [119892
1199101015840 ((119884
120588
0(119909))
1015840
119884120588
0(119909))
+ 119892119910((119884
120588
0(119909))
1015840
119884120588
0(119909))]
+ 119897120588
2E [Φ
1199101015840 ((119884
120588
0(119909))
1015840
119884120588
0(119909))
+ Φ119910((119884
120588
0(119909))
1015840
119884120588
0(119909))]
(123)
where 119860lowast is the 1198712(O)-adjoint operator of 119860 Under assump-tions (L1)ndash(L3) this is a linear mean-field SEE with boundedcoefficients An application ofTheorem 11 implies that it has aunique adaptedmild solution such that119875120588(119905) isin S2
F ([0 119879] 119870)When 120588 rarr 0 according to Corollaries 5 and 13 119875120588(119905)
converges to 119875(119905) where
119875 (119905) isin S2
F ([0 119879] 119870) (124)
is the solution of the following equation
119875 (119905) = 1198971E [119892
1199101015840 (119884
1015840
0 119884
0) + 119892
119910(119884
1015840
0 119884
0)]
+ 1198972E [Φ
1199101015840 (119884
1015840
0 119884
0) + Φ
119910(119884
1015840
0 119884
0)]
+ int
119905
0
119890119860lowast(119905minus119904)
E1015840[119891
1199101015840 (119904) 119875
1015840
(119904) + 119891119910(119904) 119875 (119904)
+ 1198971ℎ1199101015840 (119904) + 119897
1ℎ119910(119904)] 119889119904
+ int
119905
0
119890119860lowast(119905minus119904)
E1015840[119891
1199111015840 (119904) 119875
1015840
(119904) + 119891119911(119904) 119875 (119904)
+ 1198971ℎ1199111015840 (119904) + 119897
1ℎ119911(119904)] 119889119882 (119904)
(125)
The following proposition which formally follows fromProposition 18 gives the relation between 119875120588(119905) and119870120576120588
119905
Proposition 19 Consider the following
E [119875120588
(0)119870120576120588
0]
= Eint119879
0
1
120576119875120588
(119904)E1015840[119891 (119904 Λ
120588
119904 119906
120576120588
119904) minus 119891 (119904 Λ
120588
119904 119906
120588
119904)]
minus 119897120588
1119870120576120588
119904E1015840[ℎ
1199101015840 (119904 Λ
120588
119904 119906
120588
119904) + ℎ
119910(119904 Λ
120588
119904 119906
120588
119904)]
minus 119897120588
1119876120576120588
119904E1015840[ℎ
1199111015840 (119904 Λ
120588
119904 119906
120588
119904) + ℎ
119911(119904 Λ
120588
119904 119906
120588
119904)] 119889119904
(126)
The following theorem constitutes the main contributionof this section themaximumprinciple for the BSPDE controlsystem
Theorem 20 Let assumptions (L1)ndash(L3) hold Suppose 119906(sdot) isan optimal control and (119884(sdot) 119885(sdot)) is the corresponding optimalstate trajectory for the BSPDE control systems (72) and (73)with the initial state constraint (75) Then there exists 119875(119905) isinS2
F ([0 119879] 119870) which satisfies (125) such that
H (119905 1198841015840
119905 119885
1015840
119905 119884
119905 119885
119905 V
119905 119875 (119905))
ge H (119905 1198841015840
119905 119885
1015840
119905 119884
119905 119885
119905 119906
119905 119875 (119905))
119886119890 119886119904 forallV isin U119886119889
(127)
whereH [0 119879]times119867timesL(Γ119867)times119867timesL(Γ119867)times119880times119870 rarr R
is the Hamiltonian function defined by
H (119905 119910 119910 119911 V 119901) = 1198971ℎ (119905 119910 119910 119911 V)
+ 119901119891 (119905 119910 119910 119911 V) (128)
Proof By (122) and Proposition 19 we obtain
minus120588 le1
120576Eint
119879
0
119875120588
(119904)E1015840[119891 (119904 Λ
120588
119904 119906
120576120588
119904)
minus 119891 (119904 Λ120588
119904 119906
120588
119904)] 119889119904
+119897120588
1
120576Eint
119879
0
E1015840[ℎ (119904 Λ
120588
119904 119906
120576120588
119904)
minus ℎ (119904 Λ120588
119904 119906
120588
119904)] 119889119904 + (119897
120588
1+ 119897
120588
2) 119900 (120576)
(129)
Letting 120576 rarr 0+ in (129) we derive for ae 120591 isin [0 119879]
minus120588 le 119897120588
1E1015840[ℎ (120591 Λ
120588
120591 V
120591) minus ℎ (120591 Λ
120588
120591 119906
120588
120591)]
+ 119875120588
(119904)E1015840[119891 (120591 Λ
120588
120591 V
120591) minus 119891 (120591 Λ
120588
120591 119906
120588
120591)]
(130)
for all V isin UadFinally taking 120588 rarr 0 in (130) we derive the desired
result
Remark 21 We note that if the coefficients do not dependexplicitly on the marginal law of the underlying diffusionthe result reduces to the classical case that is the SMP forBSPDEs without mean-field term
Remark 22 When we remove the initial state constraint (75)we obtain the general maximum principle for the mean-fieldBSPDEs system (ie without the constraint) with 119897
1= 1
54 Application A Backward Linear Quadratic Control Prob-lem Now we apply our maximum principle to solve an LQproblem For notational simplicity we restrict ourselves tothe free case (ie without the initial state constraint (75)) thegeneral case being handled in a similar way
Mathematical Problems in Engineering 17
Consider the following problem
119869 (119906) =1
2E [(119884 (0))
2] +
1
2Eint
119879
0
119873V2 (119905) 119889119905 997888rarr min
(131)
subject to
119889119884 (119905) = minus119860119884 (119905) 119889119905
minus 119861119884 (119905) + 119861E [119884 (119905)]
+119862V (119905) + 119863119885 (119905) + 119863E [119885 (119905)] 119889119905
+ 119885 (119905) 119889119882 (119905)
119884 (119879) = 120585 119905 isin [0 119879]
(132)
where 119860 is a partial differential operator satisfying condition(L1) and 119861 119861 119862119863119863 and119873 are bounded and deterministicconstantsWe also assume that119873 gt 0 and V isin 1198712F (0 119879 119880) It iseasy to verify that BSPDE (132) admits a uniquemild solution(119884(119905) 119885(119905))
119875(119905) the adjoint process of state equation (132) is thesolution of
119889119875 (119905) = 119860lowast119875 (119905) 119889119905 + 119861119875 (119905) + 119861E [119875 (119905)] 119889119905
+ 119863119875 (119905) + 119863E [119875 (119905)] 119889119882 (119905)
119875 (0) = 119884 (0)
(133)
Let 119906(sdot) be an optimal control and let (119884(sdot) 119885(sdot)) bethe corresponding state process By maximum principle ofTheorem 20 (note that 119897
1= 1 in this problem)
1
2119873V2 (119905) + 119875 (119905) 119862V (119905) ge
1
2119873119906
2
(119905) + 119875 (119905) 119862119906 (119905) (134)
for all V isin 119880ad since the state equation has the form (132)Thisin turn implies
119906 (119905) = minus119862
119873119875 (119905) (135)
It is clear that (131) is a positive quadratic functional of controlbecause of 119873 gt 0 Hence an optimal control exists Thecandidate optimal control (135) is indeed an optimal controlof this LQ problem for it is the only control which satisfies themaximum principle
Next we want to obtain a more explicit representationof the optimal control (135) from the state equation (132)Substituting (135) into state equation (132) yields
119889119884 (119905) = minus119860119884 (119905) 119889119905
minus 119861119884 (119905) + 119861E [119884 (119905)] minus1198622
119873119875 (119905)
+119863119885 (119905) + 119863E [119885 (119905)] 119889119905 + 119885 (119905) 119889119882 (119905)
119884 (119879) = 120585 119905 isin [0 119879]
(136)
Combining the above equation with (133) we obtain thefollowing related feedback control system
119889119884 (119905) = minus119860119884 (119905) 119889119905
minus 119861119884 (119905) + 119861E [119884 (119905)] minus1198622
119873119875 (119905)
+ 119863119885 (119905) + 119863E [119885 (119905)] 119889119905 + 119885 (119905) 119889119882 (119905)
119884 (119879) = 120585
119889119875 (119905) = 119860lowast119875 (119905) 119889119905 + 119861119875 (119905) + 119861E [119875 (119905)] 119889119905
+ 119863119875 (119905) + 119863E [119875 (119905)] 119889119882 (119905)
119875 (0) = 119884 (0)
(137)
Looking at the terminal condition of 119875(119905) in (133) andconsidering the mean-field type of (132) it is reasonable toconjecture that 119875(119905) has the following form
119875 (119905) = 120593 (119905) 119884 (119905) + 120601 (119905)E [119884 (119905)] (138)
where120593(119905)120601(119905) are deterministic differential functionswhichwill be specified below Moreover 120593(0) = 1 120601(0) = 0
Inserting this form into adjoint equation (133) and notic-ing that 119884(119905) satisfies (136) we can compare the coefficientsof 119889119905 and 119889119882(119905) to obtain the following equation
2120593 (119905) 119860119884 (119905) + 2120601 (119905) 119860E [119884 (119905)] + 119861120593 (119905) 119884 (119905)
+ 2 (119861120601 (119905) + 119861120593 (119905) + 119861120601 (119905))E [119884 (119905)]
= minus120593 (119905) 119861119884 (119905) minus1198622
119873120593 (119905) 119884 (119905) + 119863119885 (119905)
+ 119884 (119905)119889120593 (119905)
119889119905+ E [119884 (119905)]
119889120601 (119905)
119889119905
+ (1206012
(119905)1198622
119873+ 2
1198622
119873120593 (119905) 120601 (119905))E [119884 (119905)]
minus (119863120601 (119905) + 119863120601 (119905) + 119863120593 (119905))E [119885 (119905)]
120593 (119905) 119885 (119905) = 119863120593 (119905) 119884 (119905)
+ (119863120601 (119905) + 119863120593 (119905) + 119863120601 (119905))E [119884 (119905)]
(139)
Then subtracting 119885(119905) we have
2120593 (119905) 119860119884 (119905) + 2120601 (119905) 119860E [119884 (119905)] + 119861120593 (119905) 119884 (119905)
+ 2 (119861120601 (119905) + 119861120593 (119905) + 119861120601 (119905))E [119884 (119905)]
18 Mathematical Problems in Engineering
= 120593 (119905) minus119861 +1198622
119873120593 (119905) minus 119863
2119884 (119905)
+ 119884 (119905)119889120593 (119905)
119889119905+ E [119884 (119905)]
119889120601 (119905)
119889119905
+ (1206012
(119905)1198622
119873+ 2
1198622
119873120593 (119905) 120601 (119905))E [119884 (119905)]
minus1
120593 (119905)(119863120601 (119905) + 119863120593 (119905) + 119863120601 (119905))
2
E [119884 (119905)]
minus 2119863 (119863120601 (119905) + 119863120593 (119905) + 119863120601 (119905))E [119884 (119905)]
(140)
Comparing the coefficients of 119884(119905) and E[119884(119905)] respec-tively we get
119889
119889119905120593 (119905) = 2119860
lowast120593 (119905) + 2119861120593 (119905) + 119863
2120593 (119905) minus
1198622
1198731205932
(119905)
120593 (0) = 1
(141)
119889
119889119905120601 (119905) = 2119860
lowast120601 (119905) + (
(119863 + 119863)2
120593 (119905)minus1198622
119873)120601
2
(119905)
+ 2119877 (119905) 120601 (119905) + (1198632+ 2119861 + 2119863119863)120593 (119905)
120601 (0) = 0
(142)
where 119877(119905) = 119861 + 119861 + (119863 + 119863)2
minus (1198622119873)120593(119905)
We solve (141) to get
120593 (119905) = (119890minus(2119860lowast+2119861+119863
2)119905(1 minus
1198622
119873(2119860lowast + 2119861 + 1198632))
+1198622
119873(2119860lowast + 2119861 + 1198632))
minus1
(143)
Then (142) exists a unique solution from the classical Riccatiequation theory
We now conclude the above discussions in the followingresult
Theorem 23 For onersquos linear quadratic stochastic partialdifferential control problem (131)-(132) the unique optimalcontrol 119906(sdot) isin 119880ad is given by
119906 (119905) = minus119862
119873(120593 (119905) 119884 (119905) + 120601 (119905)E [119884 (119905)]) (144)
with 120593(119905) satisfying (143) and 120601(119905) solving (142)
Conflict of Interests
The authors declare that they have no conflict of interestsregarding the publication of this paper
Acknowledgment
This research is partially supported by the Natural ScienceFoundation of China under Grant no 11301310
References
[1] Y Hu and S Peng ldquoAdapted solution of a backward semilinearstochastic evolution equationrdquo Stochastic Analysis and Applica-tions vol 9 no 4 pp 445ndash459 1991
[2] N IMahmudov andMAMcKibben ldquoOnbackward stochasticevolution equations in Hilbert spaces and optimal controlrdquoNonlinear Analysis vol 67 no 4 pp 1260ndash1274 2007
[3] M Fuhrman and G Tessitore ldquoNonlinear Kolmogorov equa-tions in infinite dimensional spaces the backward stochasticdifferential equations approach and applications to optimalcontrolrdquoThe Annals of Probability vol 30 no 3 pp 1397ndash14652002
[4] M Fuhrman and G Tessitore ldquoInfinite horizon backwardstochastic differential equations and elliptic equations inHilbert spacesrdquo Annals of Probability vol 32 no 1B pp 607ndash660 2004
[5] X Mao ldquoAdapted solutions of backward stochastic differentialequations with non-Lipschitz coefficientsrdquo Stochastic Processesand their Applications vol 58 no 2 pp 281ndash292 1995
[6] J Lasry and P Lions ldquoMean field gamesrdquo Japanese Journal ofMathematics vol 2 no 1 pp 229ndash260 2007
[7] R Buckdahn J Li and S Peng ldquoMean-field backward stochas-tic differential equations and related partial differential equa-tionsrdquo Stochastic Processes and their Applications vol 119 no 10pp 3133ndash3154 2009
[8] D Andersson and B Djehiche ldquoAmaximum principle for SDEsofmean-field typerdquoAppliedMathematics andOptimization vol63 no 3 pp 341ndash356 2011
[9] R Buckdahn B Djehiche J Li and S Peng ldquoMean-fieldbackward stochastic differential equations a limit approachrdquoThe Annals of Probability vol 37 no 4 pp 1524ndash1565 2009
[10] R Xu ldquoMean-field backward doubly stochastic differentialequations and related SPDEsrdquo Boundary Value Problems vol2012 article 114 2012
[11] R Buckdahn B Djehiche and J Li ldquoA general stochasticmaximum principle for SDEs of mean-field typerdquo AppliedMathematics and Optimization vol 64 no 2 pp 197ndash216 2011
[12] J Li ldquoStochasticmaximumprinciple in themean-field controlsrdquoAutomatica vol 48 no 2 pp 366ndash373 2012
[13] G Wang C Zhang and W Zhang ldquoStochastic maximumprinciple for mean-field type optimal control under partialinformationrdquo IEEE Transactions on Automatic Control vol 59no 2 pp 522ndash528 2014
[14] M Hafayed ldquoA mean-field necessary and sufficient conditionsfor optimal singular stochastic controlrdquo Communications inMathematics and Statistics vol 1 no 4 pp 417ndash435 2013
[15] M Hafayed ldquoA mean-field maximum principle for optimalcontrol of forward-backward stochastic differential equationswith Poisson jumpprocessesrdquo International Journal ofDynamicsand Control vol 1 no 4 pp 300ndash315 2013
[16] Y Hu and S Peng ldquoMaximum principle for semilinearstochastic evolution control systemsrdquo Stochastics and StochasticsReports vol 33 no 3-4 pp 159ndash180 1990
[17] M Fuhrman Y Hu and G Tessitore ldquoStochastic maximumprinciple for optimal control of SPDEsrdquo Comptes Rendus Math-ematique vol 350 no 13-14 pp 683ndash688 2012
Submit your manuscripts athttpwwwhindawicom
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Mathematical Problems in Engineering
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Differential EquationsInternational Journal of
Volume 2014
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Mathematical PhysicsAdvances in
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OptimizationJournal of
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Function Spaces
Abstract and Applied AnalysisHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Algebra
Discrete Dynamics in Nature and Society
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Volume 2014 Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Stochastic AnalysisInternational Journal of
Mathematical Problems in Engineering 15
1
120576(119869 (119906
120576120588) minus 119869 (119906
120588))
= E [1198921199101015840 ((119884
120588
0)1015840
119884120588
0) (119870
120576120588
0)1015840
+ 119892119910((119884
120588
0)1015840
119884120588
0)119870
120576120588
0]
+ Δ120576+1
120576Eint
119879
0
E1015840[ℎ (119904 Λ
120588
119904 119906
120576120588
119904)
minus ℎ (119904 Λ120588
119904 119906
120588
119904)] 119889119904 + 119900 (120576)
(115)
where
Δ120576= Eint
119879
0
E1015840[ℎ
1199101015840 (119904 Λ
120588
119904 119906
120576120588
119904) (119870
120576120588
119904)1015840
+ ℎ1199111015840 (119904 Λ
120588
119904 119906
120576120588
119904) (119876
120576120588
119904)1015840
+ ℎ119910(119904 Λ
120588
119904 119906
120576120588
119904)119870
120576120588
119904
+ ℎ119911(119904 Λ
120588
119904 119906
120576120588
119904) 119876
120576120588
119904] 119889119904
(116)
53 Variational Inequality and Adjoint Equation In thissubsection the adjoint process is introduced to deduce thevariational inequality
If we set V(sdot) = 119906120576120588(sdot) in (111) and notice that
119889(119906120576120588(sdot) 119906
120588(sdot)) le 120576 we get
minus120588 le1
120576(119869
120588(119906
120576120588
(sdot)) minus 119869120588(119906
120588
(sdot))) (117)
By Lemma 17
1
120576(119869
120588(119906
120576120588
(sdot)) minus 119869120588(119906
120588
(sdot)))
=(119869
120588(119906
120576120588(sdot)))
2
minus (119869120588(119906
120588(sdot)))
2
120576 (119869120588 (119906120576120588 (sdot)) + 119869120588 (119906120588 (sdot)))
=119869 (119906
120576120588(sdot)) + 119869 (119906
120588(sdot)) minus 2119869 (119906 (sdot)) + 2120588
119869120588 (119906120576120588 (sdot)) + 119869120588 (119906120588 (sdot))
times119869 (119906
120576120588(sdot)) minus 119869 (119906
120588(sdot))
120576
+
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] + E [Φ ((119884
120588
0)1015840
119884120588
0)]
119869120588 (119906120576120588 (sdot)) + 119869120588 (119906120588 (sdot))
times
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] minus E [Φ ((119884
120588
0)1015840
119884120588
0)]
120576
997888rarr 119897120588
1
119869 (119906120576120588(sdot)) minus 119869 (119906
120588(sdot))
120576
+ 119897120588
2
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] minus E [Φ ((119884
120588
0)1015840
119884120588
0)]
120576
(118)
where we set
119897120588
1=119869 (119906
120588(sdot)) minus 119869 (119906 (sdot)) + 120588
119869120588 (119906120588 (sdot))
119897120588
2=
E [Φ ((119884120588
0)1015840
119884120588
0)]
119869120588 (119906120588 (sdot))
(119)
and use the limit
119869 (119906120576120588
(sdot)) 997888rarr 119869 (119906120588
(sdot))
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] 997888rarr E [Φ ((119884
120588
0)1015840
119884120588
0)]
(120)
as 120576 rarr 0 according to (115)As |119897120588
1|2
+ |119897120588
2|2
= 1 for all 120588 gt 0 we know that there existsa subsequence of 119897120588
1 119897120588
2 (still denoted by 119897120588
1 119897120588
2) such that
lim120588rarr0
119897120588
1 119897120588
2 = 119897
1 1198972
1003816100381610038161003816119897110038161003816100381610038162
+1003816100381610038161003816119897210038161003816100381610038162
= 1
(121)
Combining (115) (117) with (118) we get
minus120588 le 119897120588
1
119869 (119906120576120588(sdot)) minus 119869 (119906
120588(sdot))
120576
+ 119897120588
2
E [Φ ((119884120576120588
0)1015840
119884120576120588
0)] minus E [Φ ((119884
120588
0)1015840
119884120588
0)]
120576
= 119897120588
1Δ120576+ 119897
120588
1E [119892
1199101015840 ((119884
120588
0)1015840
119884120588
0) (119870
120576120588
0)1015840
+119892119910((119884
120588
0)1015840
119884120588
0)119870
120576120588
0]
+119897120588
1
120576Eint
119879
0
E1015840[ℎ (119904 Λ
120588
119904 119906
120576120588
119904) minus ℎ (119904 Λ
120588
119904 119906
120588
119904)] 119889119904
+ 119897120588
2E [Φ
1199101015840 ((119884
120588
0)1015840
119884120588
0) (119870
120576120588
0)1015840
+Φ119910((119884
120588
0)1015840
119884120588
0)119870
120576120588
0] + (119897
120588
1+ 119897
120588
2) 119900 (120576)
(122)
Next we introduce the adjoint equation corresponding tovariational equation (113) whose solution is denoted by119875120588(119905)
119889119875120588
(119905)
= 119860lowast119875120588
(119905) 119889119905
+ E1015840[119891
1199101015840 (119905 Λ
120588
119905 119906
120588
119905) (119875
120588
(119905))1015840
+ 119891119910(119905 Λ
120588
119905 119906
120588
119905) 119875
120588
(119905) + 119897120588
1ℎ1199101015840 (119905 Λ
120588
119905 119906
120588
119905)
+ 119897120588
1ℎ119910(119905 Λ
120588
119905 119906
120588
119905)] 119889119905
16 Mathematical Problems in Engineering
+ E1015840[119891
1199111015840 (119905 Λ
120588
119905 119906
120588
119905) (119875
120588
(119905))1015840
+ 119891119911(119905 Λ
120588
119905 119906
120588
119905) 119875
120588
(119905)
+ 119897120588
1ℎ1199111015840 (119905 Λ
120588
119905 119906
120588
119905) + 119897
120588
1ℎ119911(119905 Λ
120588
119905 119906
120588
119905)] 119889119882 (119905)
119875120588
(0) = 119897120588
1E [119892
1199101015840 ((119884
120588
0(119909))
1015840
119884120588
0(119909))
+ 119892119910((119884
120588
0(119909))
1015840
119884120588
0(119909))]
+ 119897120588
2E [Φ
1199101015840 ((119884
120588
0(119909))
1015840
119884120588
0(119909))
+ Φ119910((119884
120588
0(119909))
1015840
119884120588
0(119909))]
(123)
where 119860lowast is the 1198712(O)-adjoint operator of 119860 Under assump-tions (L1)ndash(L3) this is a linear mean-field SEE with boundedcoefficients An application ofTheorem 11 implies that it has aunique adaptedmild solution such that119875120588(119905) isin S2
F ([0 119879] 119870)When 120588 rarr 0 according to Corollaries 5 and 13 119875120588(119905)
converges to 119875(119905) where
119875 (119905) isin S2
F ([0 119879] 119870) (124)
is the solution of the following equation
119875 (119905) = 1198971E [119892
1199101015840 (119884
1015840
0 119884
0) + 119892
119910(119884
1015840
0 119884
0)]
+ 1198972E [Φ
1199101015840 (119884
1015840
0 119884
0) + Φ
119910(119884
1015840
0 119884
0)]
+ int
119905
0
119890119860lowast(119905minus119904)
E1015840[119891
1199101015840 (119904) 119875
1015840
(119904) + 119891119910(119904) 119875 (119904)
+ 1198971ℎ1199101015840 (119904) + 119897
1ℎ119910(119904)] 119889119904
+ int
119905
0
119890119860lowast(119905minus119904)
E1015840[119891
1199111015840 (119904) 119875
1015840
(119904) + 119891119911(119904) 119875 (119904)
+ 1198971ℎ1199111015840 (119904) + 119897
1ℎ119911(119904)] 119889119882 (119904)
(125)
The following proposition which formally follows fromProposition 18 gives the relation between 119875120588(119905) and119870120576120588
119905
Proposition 19 Consider the following
E [119875120588
(0)119870120576120588
0]
= Eint119879
0
1
120576119875120588
(119904)E1015840[119891 (119904 Λ
120588
119904 119906
120576120588
119904) minus 119891 (119904 Λ
120588
119904 119906
120588
119904)]
minus 119897120588
1119870120576120588
119904E1015840[ℎ
1199101015840 (119904 Λ
120588
119904 119906
120588
119904) + ℎ
119910(119904 Λ
120588
119904 119906
120588
119904)]
minus 119897120588
1119876120576120588
119904E1015840[ℎ
1199111015840 (119904 Λ
120588
119904 119906
120588
119904) + ℎ
119911(119904 Λ
120588
119904 119906
120588
119904)] 119889119904
(126)
The following theorem constitutes the main contributionof this section themaximumprinciple for the BSPDE controlsystem
Theorem 20 Let assumptions (L1)ndash(L3) hold Suppose 119906(sdot) isan optimal control and (119884(sdot) 119885(sdot)) is the corresponding optimalstate trajectory for the BSPDE control systems (72) and (73)with the initial state constraint (75) Then there exists 119875(119905) isinS2
F ([0 119879] 119870) which satisfies (125) such that
H (119905 1198841015840
119905 119885
1015840
119905 119884
119905 119885
119905 V
119905 119875 (119905))
ge H (119905 1198841015840
119905 119885
1015840
119905 119884
119905 119885
119905 119906
119905 119875 (119905))
119886119890 119886119904 forallV isin U119886119889
(127)
whereH [0 119879]times119867timesL(Γ119867)times119867timesL(Γ119867)times119880times119870 rarr R
is the Hamiltonian function defined by
H (119905 119910 119910 119911 V 119901) = 1198971ℎ (119905 119910 119910 119911 V)
+ 119901119891 (119905 119910 119910 119911 V) (128)
Proof By (122) and Proposition 19 we obtain
minus120588 le1
120576Eint
119879
0
119875120588
(119904)E1015840[119891 (119904 Λ
120588
119904 119906
120576120588
119904)
minus 119891 (119904 Λ120588
119904 119906
120588
119904)] 119889119904
+119897120588
1
120576Eint
119879
0
E1015840[ℎ (119904 Λ
120588
119904 119906
120576120588
119904)
minus ℎ (119904 Λ120588
119904 119906
120588
119904)] 119889119904 + (119897
120588
1+ 119897
120588
2) 119900 (120576)
(129)
Letting 120576 rarr 0+ in (129) we derive for ae 120591 isin [0 119879]
minus120588 le 119897120588
1E1015840[ℎ (120591 Λ
120588
120591 V
120591) minus ℎ (120591 Λ
120588
120591 119906
120588
120591)]
+ 119875120588
(119904)E1015840[119891 (120591 Λ
120588
120591 V
120591) minus 119891 (120591 Λ
120588
120591 119906
120588
120591)]
(130)
for all V isin UadFinally taking 120588 rarr 0 in (130) we derive the desired
result
Remark 21 We note that if the coefficients do not dependexplicitly on the marginal law of the underlying diffusionthe result reduces to the classical case that is the SMP forBSPDEs without mean-field term
Remark 22 When we remove the initial state constraint (75)we obtain the general maximum principle for the mean-fieldBSPDEs system (ie without the constraint) with 119897
1= 1
54 Application A Backward Linear Quadratic Control Prob-lem Now we apply our maximum principle to solve an LQproblem For notational simplicity we restrict ourselves tothe free case (ie without the initial state constraint (75)) thegeneral case being handled in a similar way
Mathematical Problems in Engineering 17
Consider the following problem
119869 (119906) =1
2E [(119884 (0))
2] +
1
2Eint
119879
0
119873V2 (119905) 119889119905 997888rarr min
(131)
subject to
119889119884 (119905) = minus119860119884 (119905) 119889119905
minus 119861119884 (119905) + 119861E [119884 (119905)]
+119862V (119905) + 119863119885 (119905) + 119863E [119885 (119905)] 119889119905
+ 119885 (119905) 119889119882 (119905)
119884 (119879) = 120585 119905 isin [0 119879]
(132)
where 119860 is a partial differential operator satisfying condition(L1) and 119861 119861 119862119863119863 and119873 are bounded and deterministicconstantsWe also assume that119873 gt 0 and V isin 1198712F (0 119879 119880) It iseasy to verify that BSPDE (132) admits a uniquemild solution(119884(119905) 119885(119905))
119875(119905) the adjoint process of state equation (132) is thesolution of
119889119875 (119905) = 119860lowast119875 (119905) 119889119905 + 119861119875 (119905) + 119861E [119875 (119905)] 119889119905
+ 119863119875 (119905) + 119863E [119875 (119905)] 119889119882 (119905)
119875 (0) = 119884 (0)
(133)
Let 119906(sdot) be an optimal control and let (119884(sdot) 119885(sdot)) bethe corresponding state process By maximum principle ofTheorem 20 (note that 119897
1= 1 in this problem)
1
2119873V2 (119905) + 119875 (119905) 119862V (119905) ge
1
2119873119906
2
(119905) + 119875 (119905) 119862119906 (119905) (134)
for all V isin 119880ad since the state equation has the form (132)Thisin turn implies
119906 (119905) = minus119862
119873119875 (119905) (135)
It is clear that (131) is a positive quadratic functional of controlbecause of 119873 gt 0 Hence an optimal control exists Thecandidate optimal control (135) is indeed an optimal controlof this LQ problem for it is the only control which satisfies themaximum principle
Next we want to obtain a more explicit representationof the optimal control (135) from the state equation (132)Substituting (135) into state equation (132) yields
119889119884 (119905) = minus119860119884 (119905) 119889119905
minus 119861119884 (119905) + 119861E [119884 (119905)] minus1198622
119873119875 (119905)
+119863119885 (119905) + 119863E [119885 (119905)] 119889119905 + 119885 (119905) 119889119882 (119905)
119884 (119879) = 120585 119905 isin [0 119879]
(136)
Combining the above equation with (133) we obtain thefollowing related feedback control system
119889119884 (119905) = minus119860119884 (119905) 119889119905
minus 119861119884 (119905) + 119861E [119884 (119905)] minus1198622
119873119875 (119905)
+ 119863119885 (119905) + 119863E [119885 (119905)] 119889119905 + 119885 (119905) 119889119882 (119905)
119884 (119879) = 120585
119889119875 (119905) = 119860lowast119875 (119905) 119889119905 + 119861119875 (119905) + 119861E [119875 (119905)] 119889119905
+ 119863119875 (119905) + 119863E [119875 (119905)] 119889119882 (119905)
119875 (0) = 119884 (0)
(137)
Looking at the terminal condition of 119875(119905) in (133) andconsidering the mean-field type of (132) it is reasonable toconjecture that 119875(119905) has the following form
119875 (119905) = 120593 (119905) 119884 (119905) + 120601 (119905)E [119884 (119905)] (138)
where120593(119905)120601(119905) are deterministic differential functionswhichwill be specified below Moreover 120593(0) = 1 120601(0) = 0
Inserting this form into adjoint equation (133) and notic-ing that 119884(119905) satisfies (136) we can compare the coefficientsof 119889119905 and 119889119882(119905) to obtain the following equation
2120593 (119905) 119860119884 (119905) + 2120601 (119905) 119860E [119884 (119905)] + 119861120593 (119905) 119884 (119905)
+ 2 (119861120601 (119905) + 119861120593 (119905) + 119861120601 (119905))E [119884 (119905)]
= minus120593 (119905) 119861119884 (119905) minus1198622
119873120593 (119905) 119884 (119905) + 119863119885 (119905)
+ 119884 (119905)119889120593 (119905)
119889119905+ E [119884 (119905)]
119889120601 (119905)
119889119905
+ (1206012
(119905)1198622
119873+ 2
1198622
119873120593 (119905) 120601 (119905))E [119884 (119905)]
minus (119863120601 (119905) + 119863120601 (119905) + 119863120593 (119905))E [119885 (119905)]
120593 (119905) 119885 (119905) = 119863120593 (119905) 119884 (119905)
+ (119863120601 (119905) + 119863120593 (119905) + 119863120601 (119905))E [119884 (119905)]
(139)
Then subtracting 119885(119905) we have
2120593 (119905) 119860119884 (119905) + 2120601 (119905) 119860E [119884 (119905)] + 119861120593 (119905) 119884 (119905)
+ 2 (119861120601 (119905) + 119861120593 (119905) + 119861120601 (119905))E [119884 (119905)]
18 Mathematical Problems in Engineering
= 120593 (119905) minus119861 +1198622
119873120593 (119905) minus 119863
2119884 (119905)
+ 119884 (119905)119889120593 (119905)
119889119905+ E [119884 (119905)]
119889120601 (119905)
119889119905
+ (1206012
(119905)1198622
119873+ 2
1198622
119873120593 (119905) 120601 (119905))E [119884 (119905)]
minus1
120593 (119905)(119863120601 (119905) + 119863120593 (119905) + 119863120601 (119905))
2
E [119884 (119905)]
minus 2119863 (119863120601 (119905) + 119863120593 (119905) + 119863120601 (119905))E [119884 (119905)]
(140)
Comparing the coefficients of 119884(119905) and E[119884(119905)] respec-tively we get
119889
119889119905120593 (119905) = 2119860
lowast120593 (119905) + 2119861120593 (119905) + 119863
2120593 (119905) minus
1198622
1198731205932
(119905)
120593 (0) = 1
(141)
119889
119889119905120601 (119905) = 2119860
lowast120601 (119905) + (
(119863 + 119863)2
120593 (119905)minus1198622
119873)120601
2
(119905)
+ 2119877 (119905) 120601 (119905) + (1198632+ 2119861 + 2119863119863)120593 (119905)
120601 (0) = 0
(142)
where 119877(119905) = 119861 + 119861 + (119863 + 119863)2
minus (1198622119873)120593(119905)
We solve (141) to get
120593 (119905) = (119890minus(2119860lowast+2119861+119863
2)119905(1 minus
1198622
119873(2119860lowast + 2119861 + 1198632))
+1198622
119873(2119860lowast + 2119861 + 1198632))
minus1
(143)
Then (142) exists a unique solution from the classical Riccatiequation theory
We now conclude the above discussions in the followingresult
Theorem 23 For onersquos linear quadratic stochastic partialdifferential control problem (131)-(132) the unique optimalcontrol 119906(sdot) isin 119880ad is given by
119906 (119905) = minus119862
119873(120593 (119905) 119884 (119905) + 120601 (119905)E [119884 (119905)]) (144)
with 120593(119905) satisfying (143) and 120601(119905) solving (142)
Conflict of Interests
The authors declare that they have no conflict of interestsregarding the publication of this paper
Acknowledgment
This research is partially supported by the Natural ScienceFoundation of China under Grant no 11301310
References
[1] Y Hu and S Peng ldquoAdapted solution of a backward semilinearstochastic evolution equationrdquo Stochastic Analysis and Applica-tions vol 9 no 4 pp 445ndash459 1991
[2] N IMahmudov andMAMcKibben ldquoOnbackward stochasticevolution equations in Hilbert spaces and optimal controlrdquoNonlinear Analysis vol 67 no 4 pp 1260ndash1274 2007
[3] M Fuhrman and G Tessitore ldquoNonlinear Kolmogorov equa-tions in infinite dimensional spaces the backward stochasticdifferential equations approach and applications to optimalcontrolrdquoThe Annals of Probability vol 30 no 3 pp 1397ndash14652002
[4] M Fuhrman and G Tessitore ldquoInfinite horizon backwardstochastic differential equations and elliptic equations inHilbert spacesrdquo Annals of Probability vol 32 no 1B pp 607ndash660 2004
[5] X Mao ldquoAdapted solutions of backward stochastic differentialequations with non-Lipschitz coefficientsrdquo Stochastic Processesand their Applications vol 58 no 2 pp 281ndash292 1995
[6] J Lasry and P Lions ldquoMean field gamesrdquo Japanese Journal ofMathematics vol 2 no 1 pp 229ndash260 2007
[7] R Buckdahn J Li and S Peng ldquoMean-field backward stochas-tic differential equations and related partial differential equa-tionsrdquo Stochastic Processes and their Applications vol 119 no 10pp 3133ndash3154 2009
[8] D Andersson and B Djehiche ldquoAmaximum principle for SDEsofmean-field typerdquoAppliedMathematics andOptimization vol63 no 3 pp 341ndash356 2011
[9] R Buckdahn B Djehiche J Li and S Peng ldquoMean-fieldbackward stochastic differential equations a limit approachrdquoThe Annals of Probability vol 37 no 4 pp 1524ndash1565 2009
[10] R Xu ldquoMean-field backward doubly stochastic differentialequations and related SPDEsrdquo Boundary Value Problems vol2012 article 114 2012
[11] R Buckdahn B Djehiche and J Li ldquoA general stochasticmaximum principle for SDEs of mean-field typerdquo AppliedMathematics and Optimization vol 64 no 2 pp 197ndash216 2011
[12] J Li ldquoStochasticmaximumprinciple in themean-field controlsrdquoAutomatica vol 48 no 2 pp 366ndash373 2012
[13] G Wang C Zhang and W Zhang ldquoStochastic maximumprinciple for mean-field type optimal control under partialinformationrdquo IEEE Transactions on Automatic Control vol 59no 2 pp 522ndash528 2014
[14] M Hafayed ldquoA mean-field necessary and sufficient conditionsfor optimal singular stochastic controlrdquo Communications inMathematics and Statistics vol 1 no 4 pp 417ndash435 2013
[15] M Hafayed ldquoA mean-field maximum principle for optimalcontrol of forward-backward stochastic differential equationswith Poisson jumpprocessesrdquo International Journal ofDynamicsand Control vol 1 no 4 pp 300ndash315 2013
[16] Y Hu and S Peng ldquoMaximum principle for semilinearstochastic evolution control systemsrdquo Stochastics and StochasticsReports vol 33 no 3-4 pp 159ndash180 1990
[17] M Fuhrman Y Hu and G Tessitore ldquoStochastic maximumprinciple for optimal control of SPDEsrdquo Comptes Rendus Math-ematique vol 350 no 13-14 pp 683ndash688 2012
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Stochastic AnalysisInternational Journal of
16 Mathematical Problems in Engineering
+ E1015840[119891
1199111015840 (119905 Λ
120588
119905 119906
120588
119905) (119875
120588
(119905))1015840
+ 119891119911(119905 Λ
120588
119905 119906
120588
119905) 119875
120588
(119905)
+ 119897120588
1ℎ1199111015840 (119905 Λ
120588
119905 119906
120588
119905) + 119897
120588
1ℎ119911(119905 Λ
120588
119905 119906
120588
119905)] 119889119882 (119905)
119875120588
(0) = 119897120588
1E [119892
1199101015840 ((119884
120588
0(119909))
1015840
119884120588
0(119909))
+ 119892119910((119884
120588
0(119909))
1015840
119884120588
0(119909))]
+ 119897120588
2E [Φ
1199101015840 ((119884
120588
0(119909))
1015840
119884120588
0(119909))
+ Φ119910((119884
120588
0(119909))
1015840
119884120588
0(119909))]
(123)
where 119860lowast is the 1198712(O)-adjoint operator of 119860 Under assump-tions (L1)ndash(L3) this is a linear mean-field SEE with boundedcoefficients An application ofTheorem 11 implies that it has aunique adaptedmild solution such that119875120588(119905) isin S2
F ([0 119879] 119870)When 120588 rarr 0 according to Corollaries 5 and 13 119875120588(119905)
converges to 119875(119905) where
119875 (119905) isin S2
F ([0 119879] 119870) (124)
is the solution of the following equation
119875 (119905) = 1198971E [119892
1199101015840 (119884
1015840
0 119884
0) + 119892
119910(119884
1015840
0 119884
0)]
+ 1198972E [Φ
1199101015840 (119884
1015840
0 119884
0) + Φ
119910(119884
1015840
0 119884
0)]
+ int
119905
0
119890119860lowast(119905minus119904)
E1015840[119891
1199101015840 (119904) 119875
1015840
(119904) + 119891119910(119904) 119875 (119904)
+ 1198971ℎ1199101015840 (119904) + 119897
1ℎ119910(119904)] 119889119904
+ int
119905
0
119890119860lowast(119905minus119904)
E1015840[119891
1199111015840 (119904) 119875
1015840
(119904) + 119891119911(119904) 119875 (119904)
+ 1198971ℎ1199111015840 (119904) + 119897
1ℎ119911(119904)] 119889119882 (119904)
(125)
The following proposition which formally follows fromProposition 18 gives the relation between 119875120588(119905) and119870120576120588
119905
Proposition 19 Consider the following
E [119875120588
(0)119870120576120588
0]
= Eint119879
0
1
120576119875120588
(119904)E1015840[119891 (119904 Λ
120588
119904 119906
120576120588
119904) minus 119891 (119904 Λ
120588
119904 119906
120588
119904)]
minus 119897120588
1119870120576120588
119904E1015840[ℎ
1199101015840 (119904 Λ
120588
119904 119906
120588
119904) + ℎ
119910(119904 Λ
120588
119904 119906
120588
119904)]
minus 119897120588
1119876120576120588
119904E1015840[ℎ
1199111015840 (119904 Λ
120588
119904 119906
120588
119904) + ℎ
119911(119904 Λ
120588
119904 119906
120588
119904)] 119889119904
(126)
The following theorem constitutes the main contributionof this section themaximumprinciple for the BSPDE controlsystem
Theorem 20 Let assumptions (L1)ndash(L3) hold Suppose 119906(sdot) isan optimal control and (119884(sdot) 119885(sdot)) is the corresponding optimalstate trajectory for the BSPDE control systems (72) and (73)with the initial state constraint (75) Then there exists 119875(119905) isinS2
F ([0 119879] 119870) which satisfies (125) such that
H (119905 1198841015840
119905 119885
1015840
119905 119884
119905 119885
119905 V
119905 119875 (119905))
ge H (119905 1198841015840
119905 119885
1015840
119905 119884
119905 119885
119905 119906
119905 119875 (119905))
119886119890 119886119904 forallV isin U119886119889
(127)
whereH [0 119879]times119867timesL(Γ119867)times119867timesL(Γ119867)times119880times119870 rarr R
is the Hamiltonian function defined by
H (119905 119910 119910 119911 V 119901) = 1198971ℎ (119905 119910 119910 119911 V)
+ 119901119891 (119905 119910 119910 119911 V) (128)
Proof By (122) and Proposition 19 we obtain
minus120588 le1
120576Eint
119879
0
119875120588
(119904)E1015840[119891 (119904 Λ
120588
119904 119906
120576120588
119904)
minus 119891 (119904 Λ120588
119904 119906
120588
119904)] 119889119904
+119897120588
1
120576Eint
119879
0
E1015840[ℎ (119904 Λ
120588
119904 119906
120576120588
119904)
minus ℎ (119904 Λ120588
119904 119906
120588
119904)] 119889119904 + (119897
120588
1+ 119897
120588
2) 119900 (120576)
(129)
Letting 120576 rarr 0+ in (129) we derive for ae 120591 isin [0 119879]
minus120588 le 119897120588
1E1015840[ℎ (120591 Λ
120588
120591 V
120591) minus ℎ (120591 Λ
120588
120591 119906
120588
120591)]
+ 119875120588
(119904)E1015840[119891 (120591 Λ
120588
120591 V
120591) minus 119891 (120591 Λ
120588
120591 119906
120588
120591)]
(130)
for all V isin UadFinally taking 120588 rarr 0 in (130) we derive the desired
result
Remark 21 We note that if the coefficients do not dependexplicitly on the marginal law of the underlying diffusionthe result reduces to the classical case that is the SMP forBSPDEs without mean-field term
Remark 22 When we remove the initial state constraint (75)we obtain the general maximum principle for the mean-fieldBSPDEs system (ie without the constraint) with 119897
1= 1
54 Application A Backward Linear Quadratic Control Prob-lem Now we apply our maximum principle to solve an LQproblem For notational simplicity we restrict ourselves tothe free case (ie without the initial state constraint (75)) thegeneral case being handled in a similar way
Mathematical Problems in Engineering 17
Consider the following problem
119869 (119906) =1
2E [(119884 (0))
2] +
1
2Eint
119879
0
119873V2 (119905) 119889119905 997888rarr min
(131)
subject to
119889119884 (119905) = minus119860119884 (119905) 119889119905
minus 119861119884 (119905) + 119861E [119884 (119905)]
+119862V (119905) + 119863119885 (119905) + 119863E [119885 (119905)] 119889119905
+ 119885 (119905) 119889119882 (119905)
119884 (119879) = 120585 119905 isin [0 119879]
(132)
where 119860 is a partial differential operator satisfying condition(L1) and 119861 119861 119862119863119863 and119873 are bounded and deterministicconstantsWe also assume that119873 gt 0 and V isin 1198712F (0 119879 119880) It iseasy to verify that BSPDE (132) admits a uniquemild solution(119884(119905) 119885(119905))
119875(119905) the adjoint process of state equation (132) is thesolution of
119889119875 (119905) = 119860lowast119875 (119905) 119889119905 + 119861119875 (119905) + 119861E [119875 (119905)] 119889119905
+ 119863119875 (119905) + 119863E [119875 (119905)] 119889119882 (119905)
119875 (0) = 119884 (0)
(133)
Let 119906(sdot) be an optimal control and let (119884(sdot) 119885(sdot)) bethe corresponding state process By maximum principle ofTheorem 20 (note that 119897
1= 1 in this problem)
1
2119873V2 (119905) + 119875 (119905) 119862V (119905) ge
1
2119873119906
2
(119905) + 119875 (119905) 119862119906 (119905) (134)
for all V isin 119880ad since the state equation has the form (132)Thisin turn implies
119906 (119905) = minus119862
119873119875 (119905) (135)
It is clear that (131) is a positive quadratic functional of controlbecause of 119873 gt 0 Hence an optimal control exists Thecandidate optimal control (135) is indeed an optimal controlof this LQ problem for it is the only control which satisfies themaximum principle
Next we want to obtain a more explicit representationof the optimal control (135) from the state equation (132)Substituting (135) into state equation (132) yields
119889119884 (119905) = minus119860119884 (119905) 119889119905
minus 119861119884 (119905) + 119861E [119884 (119905)] minus1198622
119873119875 (119905)
+119863119885 (119905) + 119863E [119885 (119905)] 119889119905 + 119885 (119905) 119889119882 (119905)
119884 (119879) = 120585 119905 isin [0 119879]
(136)
Combining the above equation with (133) we obtain thefollowing related feedback control system
119889119884 (119905) = minus119860119884 (119905) 119889119905
minus 119861119884 (119905) + 119861E [119884 (119905)] minus1198622
119873119875 (119905)
+ 119863119885 (119905) + 119863E [119885 (119905)] 119889119905 + 119885 (119905) 119889119882 (119905)
119884 (119879) = 120585
119889119875 (119905) = 119860lowast119875 (119905) 119889119905 + 119861119875 (119905) + 119861E [119875 (119905)] 119889119905
+ 119863119875 (119905) + 119863E [119875 (119905)] 119889119882 (119905)
119875 (0) = 119884 (0)
(137)
Looking at the terminal condition of 119875(119905) in (133) andconsidering the mean-field type of (132) it is reasonable toconjecture that 119875(119905) has the following form
119875 (119905) = 120593 (119905) 119884 (119905) + 120601 (119905)E [119884 (119905)] (138)
where120593(119905)120601(119905) are deterministic differential functionswhichwill be specified below Moreover 120593(0) = 1 120601(0) = 0
Inserting this form into adjoint equation (133) and notic-ing that 119884(119905) satisfies (136) we can compare the coefficientsof 119889119905 and 119889119882(119905) to obtain the following equation
2120593 (119905) 119860119884 (119905) + 2120601 (119905) 119860E [119884 (119905)] + 119861120593 (119905) 119884 (119905)
+ 2 (119861120601 (119905) + 119861120593 (119905) + 119861120601 (119905))E [119884 (119905)]
= minus120593 (119905) 119861119884 (119905) minus1198622
119873120593 (119905) 119884 (119905) + 119863119885 (119905)
+ 119884 (119905)119889120593 (119905)
119889119905+ E [119884 (119905)]
119889120601 (119905)
119889119905
+ (1206012
(119905)1198622
119873+ 2
1198622
119873120593 (119905) 120601 (119905))E [119884 (119905)]
minus (119863120601 (119905) + 119863120601 (119905) + 119863120593 (119905))E [119885 (119905)]
120593 (119905) 119885 (119905) = 119863120593 (119905) 119884 (119905)
+ (119863120601 (119905) + 119863120593 (119905) + 119863120601 (119905))E [119884 (119905)]
(139)
Then subtracting 119885(119905) we have
2120593 (119905) 119860119884 (119905) + 2120601 (119905) 119860E [119884 (119905)] + 119861120593 (119905) 119884 (119905)
+ 2 (119861120601 (119905) + 119861120593 (119905) + 119861120601 (119905))E [119884 (119905)]
18 Mathematical Problems in Engineering
= 120593 (119905) minus119861 +1198622
119873120593 (119905) minus 119863
2119884 (119905)
+ 119884 (119905)119889120593 (119905)
119889119905+ E [119884 (119905)]
119889120601 (119905)
119889119905
+ (1206012
(119905)1198622
119873+ 2
1198622
119873120593 (119905) 120601 (119905))E [119884 (119905)]
minus1
120593 (119905)(119863120601 (119905) + 119863120593 (119905) + 119863120601 (119905))
2
E [119884 (119905)]
minus 2119863 (119863120601 (119905) + 119863120593 (119905) + 119863120601 (119905))E [119884 (119905)]
(140)
Comparing the coefficients of 119884(119905) and E[119884(119905)] respec-tively we get
119889
119889119905120593 (119905) = 2119860
lowast120593 (119905) + 2119861120593 (119905) + 119863
2120593 (119905) minus
1198622
1198731205932
(119905)
120593 (0) = 1
(141)
119889
119889119905120601 (119905) = 2119860
lowast120601 (119905) + (
(119863 + 119863)2
120593 (119905)minus1198622
119873)120601
2
(119905)
+ 2119877 (119905) 120601 (119905) + (1198632+ 2119861 + 2119863119863)120593 (119905)
120601 (0) = 0
(142)
where 119877(119905) = 119861 + 119861 + (119863 + 119863)2
minus (1198622119873)120593(119905)
We solve (141) to get
120593 (119905) = (119890minus(2119860lowast+2119861+119863
2)119905(1 minus
1198622
119873(2119860lowast + 2119861 + 1198632))
+1198622
119873(2119860lowast + 2119861 + 1198632))
minus1
(143)
Then (142) exists a unique solution from the classical Riccatiequation theory
We now conclude the above discussions in the followingresult
Theorem 23 For onersquos linear quadratic stochastic partialdifferential control problem (131)-(132) the unique optimalcontrol 119906(sdot) isin 119880ad is given by
119906 (119905) = minus119862
119873(120593 (119905) 119884 (119905) + 120601 (119905)E [119884 (119905)]) (144)
with 120593(119905) satisfying (143) and 120601(119905) solving (142)
Conflict of Interests
The authors declare that they have no conflict of interestsregarding the publication of this paper
Acknowledgment
This research is partially supported by the Natural ScienceFoundation of China under Grant no 11301310
References
[1] Y Hu and S Peng ldquoAdapted solution of a backward semilinearstochastic evolution equationrdquo Stochastic Analysis and Applica-tions vol 9 no 4 pp 445ndash459 1991
[2] N IMahmudov andMAMcKibben ldquoOnbackward stochasticevolution equations in Hilbert spaces and optimal controlrdquoNonlinear Analysis vol 67 no 4 pp 1260ndash1274 2007
[3] M Fuhrman and G Tessitore ldquoNonlinear Kolmogorov equa-tions in infinite dimensional spaces the backward stochasticdifferential equations approach and applications to optimalcontrolrdquoThe Annals of Probability vol 30 no 3 pp 1397ndash14652002
[4] M Fuhrman and G Tessitore ldquoInfinite horizon backwardstochastic differential equations and elliptic equations inHilbert spacesrdquo Annals of Probability vol 32 no 1B pp 607ndash660 2004
[5] X Mao ldquoAdapted solutions of backward stochastic differentialequations with non-Lipschitz coefficientsrdquo Stochastic Processesand their Applications vol 58 no 2 pp 281ndash292 1995
[6] J Lasry and P Lions ldquoMean field gamesrdquo Japanese Journal ofMathematics vol 2 no 1 pp 229ndash260 2007
[7] R Buckdahn J Li and S Peng ldquoMean-field backward stochas-tic differential equations and related partial differential equa-tionsrdquo Stochastic Processes and their Applications vol 119 no 10pp 3133ndash3154 2009
[8] D Andersson and B Djehiche ldquoAmaximum principle for SDEsofmean-field typerdquoAppliedMathematics andOptimization vol63 no 3 pp 341ndash356 2011
[9] R Buckdahn B Djehiche J Li and S Peng ldquoMean-fieldbackward stochastic differential equations a limit approachrdquoThe Annals of Probability vol 37 no 4 pp 1524ndash1565 2009
[10] R Xu ldquoMean-field backward doubly stochastic differentialequations and related SPDEsrdquo Boundary Value Problems vol2012 article 114 2012
[11] R Buckdahn B Djehiche and J Li ldquoA general stochasticmaximum principle for SDEs of mean-field typerdquo AppliedMathematics and Optimization vol 64 no 2 pp 197ndash216 2011
[12] J Li ldquoStochasticmaximumprinciple in themean-field controlsrdquoAutomatica vol 48 no 2 pp 366ndash373 2012
[13] G Wang C Zhang and W Zhang ldquoStochastic maximumprinciple for mean-field type optimal control under partialinformationrdquo IEEE Transactions on Automatic Control vol 59no 2 pp 522ndash528 2014
[14] M Hafayed ldquoA mean-field necessary and sufficient conditionsfor optimal singular stochastic controlrdquo Communications inMathematics and Statistics vol 1 no 4 pp 417ndash435 2013
[15] M Hafayed ldquoA mean-field maximum principle for optimalcontrol of forward-backward stochastic differential equationswith Poisson jumpprocessesrdquo International Journal ofDynamicsand Control vol 1 no 4 pp 300ndash315 2013
[16] Y Hu and S Peng ldquoMaximum principle for semilinearstochastic evolution control systemsrdquo Stochastics and StochasticsReports vol 33 no 3-4 pp 159ndash180 1990
[17] M Fuhrman Y Hu and G Tessitore ldquoStochastic maximumprinciple for optimal control of SPDEsrdquo Comptes Rendus Math-ematique vol 350 no 13-14 pp 683ndash688 2012
Submit your manuscripts athttpwwwhindawicom
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Mathematical Problems in Engineering
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Volume 2014 Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Stochastic AnalysisInternational Journal of
Mathematical Problems in Engineering 17
Consider the following problem
119869 (119906) =1
2E [(119884 (0))
2] +
1
2Eint
119879
0
119873V2 (119905) 119889119905 997888rarr min
(131)
subject to
119889119884 (119905) = minus119860119884 (119905) 119889119905
minus 119861119884 (119905) + 119861E [119884 (119905)]
+119862V (119905) + 119863119885 (119905) + 119863E [119885 (119905)] 119889119905
+ 119885 (119905) 119889119882 (119905)
119884 (119879) = 120585 119905 isin [0 119879]
(132)
where 119860 is a partial differential operator satisfying condition(L1) and 119861 119861 119862119863119863 and119873 are bounded and deterministicconstantsWe also assume that119873 gt 0 and V isin 1198712F (0 119879 119880) It iseasy to verify that BSPDE (132) admits a uniquemild solution(119884(119905) 119885(119905))
119875(119905) the adjoint process of state equation (132) is thesolution of
119889119875 (119905) = 119860lowast119875 (119905) 119889119905 + 119861119875 (119905) + 119861E [119875 (119905)] 119889119905
+ 119863119875 (119905) + 119863E [119875 (119905)] 119889119882 (119905)
119875 (0) = 119884 (0)
(133)
Let 119906(sdot) be an optimal control and let (119884(sdot) 119885(sdot)) bethe corresponding state process By maximum principle ofTheorem 20 (note that 119897
1= 1 in this problem)
1
2119873V2 (119905) + 119875 (119905) 119862V (119905) ge
1
2119873119906
2
(119905) + 119875 (119905) 119862119906 (119905) (134)
for all V isin 119880ad since the state equation has the form (132)Thisin turn implies
119906 (119905) = minus119862
119873119875 (119905) (135)
It is clear that (131) is a positive quadratic functional of controlbecause of 119873 gt 0 Hence an optimal control exists Thecandidate optimal control (135) is indeed an optimal controlof this LQ problem for it is the only control which satisfies themaximum principle
Next we want to obtain a more explicit representationof the optimal control (135) from the state equation (132)Substituting (135) into state equation (132) yields
119889119884 (119905) = minus119860119884 (119905) 119889119905
minus 119861119884 (119905) + 119861E [119884 (119905)] minus1198622
119873119875 (119905)
+119863119885 (119905) + 119863E [119885 (119905)] 119889119905 + 119885 (119905) 119889119882 (119905)
119884 (119879) = 120585 119905 isin [0 119879]
(136)
Combining the above equation with (133) we obtain thefollowing related feedback control system
119889119884 (119905) = minus119860119884 (119905) 119889119905
minus 119861119884 (119905) + 119861E [119884 (119905)] minus1198622
119873119875 (119905)
+ 119863119885 (119905) + 119863E [119885 (119905)] 119889119905 + 119885 (119905) 119889119882 (119905)
119884 (119879) = 120585
119889119875 (119905) = 119860lowast119875 (119905) 119889119905 + 119861119875 (119905) + 119861E [119875 (119905)] 119889119905
+ 119863119875 (119905) + 119863E [119875 (119905)] 119889119882 (119905)
119875 (0) = 119884 (0)
(137)
Looking at the terminal condition of 119875(119905) in (133) andconsidering the mean-field type of (132) it is reasonable toconjecture that 119875(119905) has the following form
119875 (119905) = 120593 (119905) 119884 (119905) + 120601 (119905)E [119884 (119905)] (138)
where120593(119905)120601(119905) are deterministic differential functionswhichwill be specified below Moreover 120593(0) = 1 120601(0) = 0
Inserting this form into adjoint equation (133) and notic-ing that 119884(119905) satisfies (136) we can compare the coefficientsof 119889119905 and 119889119882(119905) to obtain the following equation
2120593 (119905) 119860119884 (119905) + 2120601 (119905) 119860E [119884 (119905)] + 119861120593 (119905) 119884 (119905)
+ 2 (119861120601 (119905) + 119861120593 (119905) + 119861120601 (119905))E [119884 (119905)]
= minus120593 (119905) 119861119884 (119905) minus1198622
119873120593 (119905) 119884 (119905) + 119863119885 (119905)
+ 119884 (119905)119889120593 (119905)
119889119905+ E [119884 (119905)]
119889120601 (119905)
119889119905
+ (1206012
(119905)1198622
119873+ 2
1198622
119873120593 (119905) 120601 (119905))E [119884 (119905)]
minus (119863120601 (119905) + 119863120601 (119905) + 119863120593 (119905))E [119885 (119905)]
120593 (119905) 119885 (119905) = 119863120593 (119905) 119884 (119905)
+ (119863120601 (119905) + 119863120593 (119905) + 119863120601 (119905))E [119884 (119905)]
(139)
Then subtracting 119885(119905) we have
2120593 (119905) 119860119884 (119905) + 2120601 (119905) 119860E [119884 (119905)] + 119861120593 (119905) 119884 (119905)
+ 2 (119861120601 (119905) + 119861120593 (119905) + 119861120601 (119905))E [119884 (119905)]
18 Mathematical Problems in Engineering
= 120593 (119905) minus119861 +1198622
119873120593 (119905) minus 119863
2119884 (119905)
+ 119884 (119905)119889120593 (119905)
119889119905+ E [119884 (119905)]
119889120601 (119905)
119889119905
+ (1206012
(119905)1198622
119873+ 2
1198622
119873120593 (119905) 120601 (119905))E [119884 (119905)]
minus1
120593 (119905)(119863120601 (119905) + 119863120593 (119905) + 119863120601 (119905))
2
E [119884 (119905)]
minus 2119863 (119863120601 (119905) + 119863120593 (119905) + 119863120601 (119905))E [119884 (119905)]
(140)
Comparing the coefficients of 119884(119905) and E[119884(119905)] respec-tively we get
119889
119889119905120593 (119905) = 2119860
lowast120593 (119905) + 2119861120593 (119905) + 119863
2120593 (119905) minus
1198622
1198731205932
(119905)
120593 (0) = 1
(141)
119889
119889119905120601 (119905) = 2119860
lowast120601 (119905) + (
(119863 + 119863)2
120593 (119905)minus1198622
119873)120601
2
(119905)
+ 2119877 (119905) 120601 (119905) + (1198632+ 2119861 + 2119863119863)120593 (119905)
120601 (0) = 0
(142)
where 119877(119905) = 119861 + 119861 + (119863 + 119863)2
minus (1198622119873)120593(119905)
We solve (141) to get
120593 (119905) = (119890minus(2119860lowast+2119861+119863
2)119905(1 minus
1198622
119873(2119860lowast + 2119861 + 1198632))
+1198622
119873(2119860lowast + 2119861 + 1198632))
minus1
(143)
Then (142) exists a unique solution from the classical Riccatiequation theory
We now conclude the above discussions in the followingresult
Theorem 23 For onersquos linear quadratic stochastic partialdifferential control problem (131)-(132) the unique optimalcontrol 119906(sdot) isin 119880ad is given by
119906 (119905) = minus119862
119873(120593 (119905) 119884 (119905) + 120601 (119905)E [119884 (119905)]) (144)
with 120593(119905) satisfying (143) and 120601(119905) solving (142)
Conflict of Interests
The authors declare that they have no conflict of interestsregarding the publication of this paper
Acknowledgment
This research is partially supported by the Natural ScienceFoundation of China under Grant no 11301310
References
[1] Y Hu and S Peng ldquoAdapted solution of a backward semilinearstochastic evolution equationrdquo Stochastic Analysis and Applica-tions vol 9 no 4 pp 445ndash459 1991
[2] N IMahmudov andMAMcKibben ldquoOnbackward stochasticevolution equations in Hilbert spaces and optimal controlrdquoNonlinear Analysis vol 67 no 4 pp 1260ndash1274 2007
[3] M Fuhrman and G Tessitore ldquoNonlinear Kolmogorov equa-tions in infinite dimensional spaces the backward stochasticdifferential equations approach and applications to optimalcontrolrdquoThe Annals of Probability vol 30 no 3 pp 1397ndash14652002
[4] M Fuhrman and G Tessitore ldquoInfinite horizon backwardstochastic differential equations and elliptic equations inHilbert spacesrdquo Annals of Probability vol 32 no 1B pp 607ndash660 2004
[5] X Mao ldquoAdapted solutions of backward stochastic differentialequations with non-Lipschitz coefficientsrdquo Stochastic Processesand their Applications vol 58 no 2 pp 281ndash292 1995
[6] J Lasry and P Lions ldquoMean field gamesrdquo Japanese Journal ofMathematics vol 2 no 1 pp 229ndash260 2007
[7] R Buckdahn J Li and S Peng ldquoMean-field backward stochas-tic differential equations and related partial differential equa-tionsrdquo Stochastic Processes and their Applications vol 119 no 10pp 3133ndash3154 2009
[8] D Andersson and B Djehiche ldquoAmaximum principle for SDEsofmean-field typerdquoAppliedMathematics andOptimization vol63 no 3 pp 341ndash356 2011
[9] R Buckdahn B Djehiche J Li and S Peng ldquoMean-fieldbackward stochastic differential equations a limit approachrdquoThe Annals of Probability vol 37 no 4 pp 1524ndash1565 2009
[10] R Xu ldquoMean-field backward doubly stochastic differentialequations and related SPDEsrdquo Boundary Value Problems vol2012 article 114 2012
[11] R Buckdahn B Djehiche and J Li ldquoA general stochasticmaximum principle for SDEs of mean-field typerdquo AppliedMathematics and Optimization vol 64 no 2 pp 197ndash216 2011
[12] J Li ldquoStochasticmaximumprinciple in themean-field controlsrdquoAutomatica vol 48 no 2 pp 366ndash373 2012
[13] G Wang C Zhang and W Zhang ldquoStochastic maximumprinciple for mean-field type optimal control under partialinformationrdquo IEEE Transactions on Automatic Control vol 59no 2 pp 522ndash528 2014
[14] M Hafayed ldquoA mean-field necessary and sufficient conditionsfor optimal singular stochastic controlrdquo Communications inMathematics and Statistics vol 1 no 4 pp 417ndash435 2013
[15] M Hafayed ldquoA mean-field maximum principle for optimalcontrol of forward-backward stochastic differential equationswith Poisson jumpprocessesrdquo International Journal ofDynamicsand Control vol 1 no 4 pp 300ndash315 2013
[16] Y Hu and S Peng ldquoMaximum principle for semilinearstochastic evolution control systemsrdquo Stochastics and StochasticsReports vol 33 no 3-4 pp 159ndash180 1990
[17] M Fuhrman Y Hu and G Tessitore ldquoStochastic maximumprinciple for optimal control of SPDEsrdquo Comptes Rendus Math-ematique vol 350 no 13-14 pp 683ndash688 2012
Submit your manuscripts athttpwwwhindawicom
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
MathematicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Mathematical Problems in Engineering
Hindawi Publishing Corporationhttpwwwhindawicom
Differential EquationsInternational Journal of
Volume 2014
Applied MathematicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Probability and StatisticsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Mathematical PhysicsAdvances in
Complex AnalysisJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
OptimizationJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CombinatoricsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Operations ResearchAdvances in
Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Function Spaces
Abstract and Applied AnalysisHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of Mathematics and Mathematical Sciences
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Algebra
Discrete Dynamics in Nature and Society
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Decision SciencesAdvances in
Discrete MathematicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom
Volume 2014 Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Stochastic AnalysisInternational Journal of
18 Mathematical Problems in Engineering
= 120593 (119905) minus119861 +1198622
119873120593 (119905) minus 119863
2119884 (119905)
+ 119884 (119905)119889120593 (119905)
119889119905+ E [119884 (119905)]
119889120601 (119905)
119889119905
+ (1206012
(119905)1198622
119873+ 2
1198622
119873120593 (119905) 120601 (119905))E [119884 (119905)]
minus1
120593 (119905)(119863120601 (119905) + 119863120593 (119905) + 119863120601 (119905))
2
E [119884 (119905)]
minus 2119863 (119863120601 (119905) + 119863120593 (119905) + 119863120601 (119905))E [119884 (119905)]
(140)
Comparing the coefficients of 119884(119905) and E[119884(119905)] respec-tively we get
119889
119889119905120593 (119905) = 2119860
lowast120593 (119905) + 2119861120593 (119905) + 119863
2120593 (119905) minus
1198622
1198731205932
(119905)
120593 (0) = 1
(141)
119889
119889119905120601 (119905) = 2119860
lowast120601 (119905) + (
(119863 + 119863)2
120593 (119905)minus1198622
119873)120601
2
(119905)
+ 2119877 (119905) 120601 (119905) + (1198632+ 2119861 + 2119863119863)120593 (119905)
120601 (0) = 0
(142)
where 119877(119905) = 119861 + 119861 + (119863 + 119863)2
minus (1198622119873)120593(119905)
We solve (141) to get
120593 (119905) = (119890minus(2119860lowast+2119861+119863
2)119905(1 minus
1198622
119873(2119860lowast + 2119861 + 1198632))
+1198622
119873(2119860lowast + 2119861 + 1198632))
minus1
(143)
Then (142) exists a unique solution from the classical Riccatiequation theory
We now conclude the above discussions in the followingresult
Theorem 23 For onersquos linear quadratic stochastic partialdifferential control problem (131)-(132) the unique optimalcontrol 119906(sdot) isin 119880ad is given by
119906 (119905) = minus119862
119873(120593 (119905) 119884 (119905) + 120601 (119905)E [119884 (119905)]) (144)
with 120593(119905) satisfying (143) and 120601(119905) solving (142)
Conflict of Interests
The authors declare that they have no conflict of interestsregarding the publication of this paper
Acknowledgment
This research is partially supported by the Natural ScienceFoundation of China under Grant no 11301310
References
[1] Y Hu and S Peng ldquoAdapted solution of a backward semilinearstochastic evolution equationrdquo Stochastic Analysis and Applica-tions vol 9 no 4 pp 445ndash459 1991
[2] N IMahmudov andMAMcKibben ldquoOnbackward stochasticevolution equations in Hilbert spaces and optimal controlrdquoNonlinear Analysis vol 67 no 4 pp 1260ndash1274 2007
[3] M Fuhrman and G Tessitore ldquoNonlinear Kolmogorov equa-tions in infinite dimensional spaces the backward stochasticdifferential equations approach and applications to optimalcontrolrdquoThe Annals of Probability vol 30 no 3 pp 1397ndash14652002
[4] M Fuhrman and G Tessitore ldquoInfinite horizon backwardstochastic differential equations and elliptic equations inHilbert spacesrdquo Annals of Probability vol 32 no 1B pp 607ndash660 2004
[5] X Mao ldquoAdapted solutions of backward stochastic differentialequations with non-Lipschitz coefficientsrdquo Stochastic Processesand their Applications vol 58 no 2 pp 281ndash292 1995
[6] J Lasry and P Lions ldquoMean field gamesrdquo Japanese Journal ofMathematics vol 2 no 1 pp 229ndash260 2007
[7] R Buckdahn J Li and S Peng ldquoMean-field backward stochas-tic differential equations and related partial differential equa-tionsrdquo Stochastic Processes and their Applications vol 119 no 10pp 3133ndash3154 2009
[8] D Andersson and B Djehiche ldquoAmaximum principle for SDEsofmean-field typerdquoAppliedMathematics andOptimization vol63 no 3 pp 341ndash356 2011
[9] R Buckdahn B Djehiche J Li and S Peng ldquoMean-fieldbackward stochastic differential equations a limit approachrdquoThe Annals of Probability vol 37 no 4 pp 1524ndash1565 2009
[10] R Xu ldquoMean-field backward doubly stochastic differentialequations and related SPDEsrdquo Boundary Value Problems vol2012 article 114 2012
[11] R Buckdahn B Djehiche and J Li ldquoA general stochasticmaximum principle for SDEs of mean-field typerdquo AppliedMathematics and Optimization vol 64 no 2 pp 197ndash216 2011
[12] J Li ldquoStochasticmaximumprinciple in themean-field controlsrdquoAutomatica vol 48 no 2 pp 366ndash373 2012
[13] G Wang C Zhang and W Zhang ldquoStochastic maximumprinciple for mean-field type optimal control under partialinformationrdquo IEEE Transactions on Automatic Control vol 59no 2 pp 522ndash528 2014
[14] M Hafayed ldquoA mean-field necessary and sufficient conditionsfor optimal singular stochastic controlrdquo Communications inMathematics and Statistics vol 1 no 4 pp 417ndash435 2013
[15] M Hafayed ldquoA mean-field maximum principle for optimalcontrol of forward-backward stochastic differential equationswith Poisson jumpprocessesrdquo International Journal ofDynamicsand Control vol 1 no 4 pp 300ndash315 2013
[16] Y Hu and S Peng ldquoMaximum principle for semilinearstochastic evolution control systemsrdquo Stochastics and StochasticsReports vol 33 no 3-4 pp 159ndash180 1990
[17] M Fuhrman Y Hu and G Tessitore ldquoStochastic maximumprinciple for optimal control of SPDEsrdquo Comptes Rendus Math-ematique vol 350 no 13-14 pp 683ndash688 2012
Submit your manuscripts athttpwwwhindawicom
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
MathematicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Mathematical Problems in Engineering
Hindawi Publishing Corporationhttpwwwhindawicom
Differential EquationsInternational Journal of
Volume 2014
Applied MathematicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Probability and StatisticsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Mathematical PhysicsAdvances in
Complex AnalysisJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
OptimizationJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CombinatoricsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Operations ResearchAdvances in
Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Function Spaces
Abstract and Applied AnalysisHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of Mathematics and Mathematical Sciences
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Algebra
Discrete Dynamics in Nature and Society
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Decision SciencesAdvances in
Discrete MathematicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom
Volume 2014 Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Stochastic AnalysisInternational Journal of
Submit your manuscripts athttpwwwhindawicom
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
MathematicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Mathematical Problems in Engineering
Hindawi Publishing Corporationhttpwwwhindawicom
Differential EquationsInternational Journal of
Volume 2014
Applied MathematicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Probability and StatisticsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Mathematical PhysicsAdvances in
Complex AnalysisJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
OptimizationJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CombinatoricsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Operations ResearchAdvances in
Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Function Spaces
Abstract and Applied AnalysisHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of Mathematics and Mathematical Sciences
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Algebra
Discrete Dynamics in Nature and Society
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Decision SciencesAdvances in
Discrete MathematicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom
Volume 2014 Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Stochastic AnalysisInternational Journal of