Approximation by Complex Beta Operators of First Kind in Strips of Compact Disks

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  • Mediterr. J. Math. 10 (2013), 3139DOI 10.1007/s00009-011-0164-21660-5446/13/010031-9, published online November 30, 2011 2011 Springer Basel AG

    Mediterranean Journalof Mathematics

    Approximation by Complex Beta Operatorsof First Kind in Strips of Compact Disks

    Sorin G. Gal and Vijay Gupta

    Dedicated to the 60th birthday of professor Francesco Altomare

    Abstract. In this paper, the exact order of simultaneous approxima-tion and Voronovskaja kind results with quantitative estimate for thecomplex Beta operators of rst kind attached to analytic functions instrips of compact disks are obtained. In this way, we put in evidencethe overconvergence phenomenon for this operator, namely the exten-sions of approximation properties with upper and exact quantitativeestimates, from the real interval (0, 1) to strips in compact disks of thecomplex plane of the form SDr(0, 1) = {z C; |z| r, 0 < Re(z) < 1}and SDr[a, b] = {z C; |z| r, a Re(z) b}, with r 1 and0 < a < b < 1.Mathematics Subject Classication (2010). Primary 30E10 ; Secondary41A25.Keywords. Complex Beta operator of rst kind, strip of compact disk,simultaneous approximation, Voronovskaja-type result, exact degrees ofapproximation.

    1. Introduction

    If f : G C is an analytic function in the open set G C, with D1 G(where D1 = {z C : |z| < 1}), then S. N. Bernstein proved that the com-plex Bernstein polynomials converges uniformly to f in D1 (see e.g., Lorentz[7], p. 88). Exact quantitative estimates and quantitative Voronovskaja-typeresults for these polynomials (see Gal [3]), together with similar results forcomplex Bernstein-Stancu polynomials, complex Kantorovich-Stancu poly-nomials, complex Favard-Szasz-Mirakjan operators, Butzers linear combi-nations of complex Bernstein polynomials, complex Baskakov operators andcomplex Balazs-Szabados operators were obtained by the rst author in sev-eral recent papers collected by the recent book Gal [5].

    Corresponding author.

  • 32 S.G. Gal and V. Gupta Mediterr. J. Math.

    Furthermore, the approximation properties of the complex Durrmeyer-type operators were studied in Anastassiou-Gal [2], Gal [4] and Mahmudov[9, 10].

    The aim of the present article is to obtain approximation results forthe complex Beta operator of rst kind, rstly introduced in the case of realvariable in Muhlbach [11] and studied in the real case by e.g. Lupas [8], Khan[6] and Abel-Gupta-Mohapatra [1].

    The complex Beta operators will be dened for all n N and z Csatisfying 0 < Re(z) < 1, by

    Kn(f, z) =1

    B(nz, n(1 z)) 10

    tnz1(1 t)n(1z)1f(t)dt, (1.1)

    where B(, ) is the Eulers Beta function, dened by

    B(, ) =

    10

    t1(1 t)1, , C, Re(), Re() > 0.

    Our results put in evidence the overconvergence phenomenon for thiscomplex Beta operator of rst kind, that is the extensions of approximationproperties with upper and exact quantitative estimates, from the real interval(0, 1) to strips in compact disks of the complex plane of the form

    SDr(0, 1) = {z C; |z| r, 0 < Re(z) < 1}and

    SDr[a, b] = {z C; |z| r, a Re(z) b},with r 1 and 0 < a < b < 1.

    2. Auxiliary Results

    In the sequel, we shall need the following auxiliary results.

    Lemma 2.1. For all ep = tp, p N {0}, n N, z C with 0 < Re(z) < 1,

    we have Kn(e0, z) = 1 and

    Kn(ep+1, z) =nz + p

    n+ pKn(ep, z).

    Proof. By the relationship (1.1) of the Beta operators, it is obvious thatKn(e0, z) = 1. Next

    Kn(ep+1, z) =1

    B(nz, n(1 z))B(nz + p+ 1, n(1 z))

    =1

    B(nz, n(1 z))[(nz + p)

    (nz + p)(n(1 z))(n+ p)(n+ p)

    ]=

    nz + p

    n+ pKn(ep, z).

    This completes the proof of Lemma 2.1.

  • Vol. 10 (2013) Complex Beta Type Operators 33

    Lemma 2.2. If f is analytic in DR = {z C; |z| < R}, namely f(z) =k=0 ckz

    k, for all z DR, then for all n N, 1 r < R and z SDr(0, 1)we have

    Kn(f, z) =

    k=0

    ck Kn(ek, z).

    Moreover, for any 0 < a < b < 1, the convergence of the above series isuniform in SDr[a, b].

    Proof. Let |z| r and 0 < Re(z) < 1. Dening fm(z) =m

    k=0 ckek(z), by thelinearity of Kn given by (1.1), it follows that Kn(fm, z) =

    mk=0 ckKn(ek, z),

    for all |z| r, 0 < Re(z) < 1 and n,m N. It suces to prove that forany xed n N, we have limmKn(fm, z) = Kn(f, z), uniformly in anycompact strip SDr[a, b] with 0 < a < b < 1.

    Indeed, we get

    |Kn(fm, z)Kn(f, z)|

    =1

    |B(nz, n(1 z))| 10

    tnz1(1 t)n(1z)1[fm(t) f(t)]dt

    1|B(nz, n(1 z))| fm fr 10

    |tnz1(1 t)n(1z)1|dt.

    Because |B(nz, n(1 z))| > 0 for |z| r, 0 < Re(z) < 1 and n N, bythe continuity of |B(nz, n(1 z))| as function of z, it follows that for any0 < a < b < 1, there exist A,M > 0 both depending on a, b, r, such thatA |B(nz, n(1 z))| M , for all z SDr[a, b]. This immediately impliesthat there exists a positive constant Ca,b,r,A,M (f) > 0 (independent of m),such that for all z SDr[a, b] we have

    |Kn(fm, z)Kn(f, z)| Ca,b,r,A,M (f)fm fr, m N,which for m proves the lemma.

    3. Main Results

    The rst main result one refers to upper estimate.

    Corollary 3.1. Let R > 1 and f(z) =

    k=0 ckzk for all |z| < R. Take

    1 r < R. For all z SDr(0, 1) and n N, we have

    |Kn(f, z) f(z)| Cr(f)n

    ,

    where Cr(f) =1+r2

    k=2 |ck|k(k 1)rk1 < .

    Proof. Suppose that |z| r with 0 < Re(z) < 1. By Lemma 2.2 we haveKn(f, z) =

    k=0 ckKn(ek, z). Therefore we get

    |Kn(f, z)f(z)| k=0

    |ck| |Kn(ek, z)ek(z)| =k=2

    |ck| |Kn(ek, z)ek(z)|,

  • 34 S.G. Gal and V. Gupta Mediterr. J. Math.

    as Kn(e0, z) = e0(z) = 1 and Kn(e1, z) = e1(z) = z.By using now Lemma 2.1, for all |z| r, 0 < Re(z) < 1 and n N, we

    get

    |Kn(ek+1, z) ek+1(z)|

    =

    nz + kn+ k Kn(ek)(z) nz + kn+ k ek(z) + nz + kn+ k ek(z) ek+1(z)

    |nz + k|n+ k

    |Kn(ek)(z) ek(z)|+ |ek(z)| nz + kn+ k z

    r |Kn(ek)(z) ek(z)|+ rk |k(1 z)|

    n+ k

    r |Kn(ek)(z) ek(z)|+ rk(1 + r)kn,

    for all k = 0, 1, . . ..Taking above k = 1, 2, . . . , step by step we easily get by recurrence that

    |Kn(ek)(z) ek(z)| rk1(1 + r) 1n[1 + 2 + . . .+ (k 1)]

    = rk1(1 + r)k(k 1)

    2n,

    for all |z| r, 0 < Re(z) < 1 and n N, which immediately implies thecorollary.

    The following Voronovskaja-type result with a quantitative estimateholds.

    Theorem 3.2. Let R > 1 and suppose that f : DR C is analytic in DR ={z C : |z| < R} that is we can write f(z) = k=0 ckzk, for all z DR.For any xed r [1, R) and for all |z| r with 0 < Re(z) < 1 and n N,we have Kn(f, z) f(z) z(1 z)f (z)2n

    Mr(f)n2 ,where Mr(f) =

    k=2 |ck|(1 + r)k(k + 1)(k 1)2rk1 < .

    Proof. Denoting k,n(z) = Kn(ek)(z) and

    Ek,n(z) = k,n(z) ek(z) zk1(1 z)k(k 1)

    2n,

    rstly it is clear that E0,n(z) = E1,n(z) = 0. Then, we can writeKn(f, z) f(z) z(1 z)f (z)2n

    k=2

    |ck| |Ek,n(z)|,

    so it remains to estimate Ek,n(z) for k 2.In this sense, simple calculation based on Lemma 2.1 too, leads us to

    the formula

    Ek,n(z) =nz + k 1n+ k 1 Ek1,n(z) +

    zk2(1 z)(k 1)22n(n+ k 1) [k(1 z) 2].

  • Vol. 10 (2013) Complex Beta Type Operators 35

    This immediately implies, for all k 2 and |z| r with 0 < Re(z) < 1

    |Ek,n(z)| r|Ek1,n(z)|+ rk2(1 + r)(k 1)2

    2n2 [k(1 + r) + 2]

    r|Ek1,n(z)|+ rk1(1 + r)(k 1)2(k + 1)

    n2.

    Taking in the last inequality, k = 2, 3, . . . , and reasoning by recurrence, nallywe easily obtain

    |Ek,n(z)| rk1(1 + r)

    n2

    kj=1

    (j 1)2(j + 1) rk1(1 + r)

    n2k(k1)2(k+1).

    We conclude thatKn(f, z) f(z) z(1 z)f (z)2n

    k=2

    |ck| |Ek,n|

    1n2

    k=2

    |ck|(1 + r)k(k 1)2(k + 1)rk1.

    As f (4)(z) =

    k=4 ckk(k 1)(k 2)(k 3)zk4 and the series is absolutelyconvergent in |z| r, it easily follows that k=4 |ck|k(k 1)(k 2)(k 3)rk4 < , which implies that k=2 |ck|(1 + r)k(k 1)2(k + 1)rk1 < .This completes the proof of the theorem.

    In what follows, we obtain the exact order in approximation by thistype of complex Beta operators of rst kind and by their derivatives. In thissense, we present the following three results.

    Theorem 3.3. Let R > 1 and suppose that f : DR C is analytic in DR, thatis we can write f(z) =

    k=0 ckz

    k, for all z DR. If f is not a polynomialof degree 1, then for any r [1, R) and any 0 < a < b < 1, we have

    ||Kn(f, ) f ||SDr[a,b] Cr,a,b(f)n

    , n N,

    where SDr[a, b] = {z C : |z| r, a Re(z) b}, fSDr[a,b] =sup{|f(z)|; z SDr[a, b]} and Cr,a,b(f) depends only on f , a, b and r.Proof. For all |z| r with 0 < Re(z) < 1 and n N, we have

    Kn(f, z) f(z)

    =1

    n

    [z(1 z)f (z)

    2+

    1

    n

    {n2(Kn(f, z) f(z) z(1 z)f

    (z)2n

    )}].

    Also, we have

    ||F +G||SDr[a,b] ||F ||SDr[a,b] ||G||SDr[a,b] ||F ||SDr[a,b] ||G||SDr[a,b].

  • 36 S.G. Gal and V. Gupta Mediterr. J. Math.

    It follows

    ||Kn(f, ) f ||SDr[a,b] 1n

    [e1(1 e1)f 2SDr[a,b]

    1n

    {n2Kn(f, ) f e1(1 e1)f 2n

    SDr[a,b]

    }].

    Taking into account that by hypothesis f is not a polynomial of degree 1in DR, we get ||e1(1 e1)f ||SDr[a,b] > 0.

    Indeed, supposing the contrary it follows that z(1 z)f (z) = 0 for allSDr[a, b]. Therefore we get f (z) = 0, for all z SDr[a, b]. Because f isanalytic in DR, by the uniqueness of analytic functions we get f

    (z) = 0,for all z DR, that is f is a linear function in DR, which contradicts thehypothesis.

    Now by Theorem 3.2, we have

    n2Kn(f, ) f e1(1 e1)f 2n

    SDr[a,b]

    Mr(f).

    The