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Background Frobenius Algebras Examples of Frobenius Algebras Results and Properties
Frobenius Algebras
Geillan Aly
May 13, 2009
Background Frobenius Algebras Examples of Frobenius Algebras Results and Properties
Outline
Background
Frobenius Algebras
Examples of Frobenius Algebras
Results and Properties
Background Frobenius Algebras Examples of Frobenius Algebras Results and Properties
Background
We recall the following definitions, constructions and theorems.
Background Frobenius Algebras Examples of Frobenius Algebras Results and Properties
Theorem (1)Let K be a field, then the following are equivalent:
A is a ring and a K-vector space over K such that(ka)b = k(ab) = a(kb) for all k ∈ K, a, b ∈ A.
A is a K vector space with two K linear mapsµ : A⊗A → A (multiplication) η : K→ A (unit),such that the following diagrams commute:
AOO A⊗Aµoo
A⊗A
µ
OO
A⊗A ⊗Aµ⊗IAoo
IA⊗µ
OO
A A⊗Aµoo
A⊗A
µ
OO
K ⊗A � A � A⊗ Kη⊗IAoo
IA
jjUUUUUUUUUUUUUUUUUUUIA⊗η
OO
Background Frobenius Algebras Examples of Frobenius Algebras Results and Properties
DefinitionA structure A that satisfies the conditions of theorem 1 is an algebra. Wewill assume that all algebras A have a unique identity element 1A.
Background Frobenius Algebras Examples of Frobenius Algebras Results and Properties
Examples of commutative algebras
ExampleA field L such that L ⊂ K.
ExamplePolynomial algebra K[X1, . . . ,Xn].
Observe that a polynomial algebra satisfies the requirements of analgebra, but is an infinite dimensional vector space over K. Thus, there isno requirement that the algebra A be finite dimensional.
ExampleK valued functions on a nonempty set S.
Background Frobenius Algebras Examples of Frobenius Algebras Results and Properties
Examples of non-commutative algebras
ExampleMatrix algebra Mn(K) with matrix multiplication being the productoperation.
ExampleHom K(V ,V) for any vector space V with composition being the product
operation.
Observe that this example and example 4 are equivalent with respect toa choice of basis.
Background Frobenius Algebras Examples of Frobenius Algebras Results and Properties
DefinitionLet A be a K algebra, and M a vector space over K. Then M is aleft A-module if each a, a′ ∈ A, m, m′ ∈ M and k ∈ K a product am ∈ Mis defined such that
a(a + m′) = am + am′
(a + a′)m = am + a′m
(aa′)m = a(a′m)
1Am = m where 1A is the identity element in A
(ka)m = k(am)a(km)
A right A-module is defined analogously.
Background Frobenius Algebras Examples of Frobenius Algebras Results and Properties
RemarkA is itself a left A-module, denoted AA, with canonical left multiplication.This is called the left regular A-module. Again, we can define theright regular A-module, AA as a right A-module with the canonical rightmultiplication.
Background Frobenius Algebras Examples of Frobenius Algebras Results and Properties
DefinitionLet A be a K algebra and M a left A-module. Let M∗ be the dual spaceof M. Then M∗ becomes a right A-module if for ψ ∈ L∗, a ∈ A, l ∈ L ,
(ψa)(l) = ψ(al).
The right A module M∗ is the dual of M. Similarly, the dual of a rightA-module is a left A-module.
Background Frobenius Algebras Examples of Frobenius Algebras Results and Properties
Theorem (2)Let A be a finite dimensional algebra over a field K, then the followingare equivalent:
1 For AA and (AA)∗, two left A-modules, there exists an A algebraisomorphism λ :A A → (AA)∗.
2 There exists a non-degenerate linear form η : A⊗A → K which is“associative” in the following manner
η(ab ⊗ c) = η(a ⊗ bc) for a, b , c ∈ A.
3 There exists a linear form f ∈ A ∗ whose kernel contains nonon-trivial left or right ideals.
Background Frobenius Algebras Examples of Frobenius Algebras Results and Properties
DefinitionAn algebra A that satisfies any of the conditions of theorem 2 is aFrobenius algebra.
It is important to note that a Frobenius algebra is not a “type” of algebra,rather it is an algebra endowed with a given structure.
Background Frobenius Algebras Examples of Frobenius Algebras Results and Properties
Proof.Part 1 (1)⇒ (2):For λ : AA → (AA)∗, λ(ab) = aλ(b)∀a, b ∈ A. Thus ∀x ∈ A,
λ(ab)(x) = (aλ(b))(x) = λ(b)(xa).
Define the linear form η : A⊗A → K
η(x ⊗ y) B λ(y)(x).
η is non-degenerate since λ is a K isomorphism. If η(· ⊗ y) = 0 thenλ(y) = 0 and thus y = 0. If η(x ⊗ ·) = 0⇒ x = 0.
Associativity η(xy ⊗ z) = η(x ⊗ yz) follows fromλ(ab)(x) = (aλ(b))(x) = λ(b)(xa):
η(xy ⊗ z) B λ(z)(xy)= yλ(z)(x)= λ(yz)(x)= η(x ⊗ yz)
Background Frobenius Algebras Examples of Frobenius Algebras Results and Properties
Proof con’t.
Part 2 (2)⇒ (1):Let η be a non-degenerate linear form on A. Define λ : AA → AA as
λ(y)x B η(x ⊗ y).
λ is a K isomorphism since η is non-degenerate and is an A algebraisomorphism by the associativity of η.
Background Frobenius Algebras Examples of Frobenius Algebras Results and Properties
Proof con’t.
Part 3 (2)⇒ (3):Given a linear form η(x ⊗ y), define a linear function f ∈ A∗ as
f(x) B η(x ⊗ 1A).
Then f(xA) = 0 implies η(xA⊗ 1A) = η(x ⊗A) = 0.
Thus x = 0 since η is non-degenerate.
Likewise, f(Ax) = 0 implies x = 0.
Thus, the kernel of f contains no non-trivial left or right ideals.
Background Frobenius Algebras Examples of Frobenius Algebras Results and Properties
Proof con’t.
Part 4 (3)⇒ (2):Given a linear function f ∈ A∗ with no non-trivial left or right ideals, define
η(x ⊗ y) B f(xy).
It is clear that η satisfies the conditions of (2).
f does not have a non-trivial left or right ideal. If y is in an ideal Icontained in the kernel of f , then xy ∈ I implying thatf(xy) = 0 = η(x ⊗ y) and η is degenerate.
Background Frobenius Algebras Examples of Frobenius Algebras Results and Properties
DefinitionA symmetric Frobenius algebra is a Frobenius algebra such that thenon-degenerate linear form η defined in theorem 2 is actually a trace mapwhere η(ab) = η(ba). If A is a symmetric Frobenius algebra, the linearform will be denoted θ.
Note: The term symmetric Frobenius algebra is not the universal termused in defining this structure.
RemarkClearly if A is a commutative algebra, ie. ab = ba for all a, b ∈ A then acommutative Frobenius algebra is a symmetric Frobenius algebra.
Background Frobenius Algebras Examples of Frobenius Algebras Results and Properties
ExampleConsider the algebra Mn(K) where θ(ab) = tr(ab), the trace function.θ(a · b , c) = tr((ab)c) = tr(a(bc)) = θ(a, b · c)
Background Frobenius Algebras Examples of Frobenius Algebras Results and Properties
ExampleThe field C is a Frobenius algebra over R with form η(a + bi) B a.
Another form could be based on a different mapping:Consider the form η(2 + 3i) B 7; η(1 − i) B 4.[?]
Background Frobenius Algebras Examples of Frobenius Algebras Results and Properties
ExampleLet G be a finite group, R a ring and R[G] be the group ring.
θ(a · b) is the coefficient of the identity element of a · b.
This defines a Frobenius algebra:θ(a · b , c) is the coefficient of the identity element of(a · b) · c = a · b · c = a · (b · c). The coefficient of the identity element ofa · (b · c) is θ(a, b · c).
θ is non-degenerate:If θ(C[G]a) = 0 for a ∈ C[G]. Then θ(g−1a) = 0∀g ∈ G. Thus, θ(g−1a) isthe coefficient of g in a, implying that a = 0. Similarly, θ(aC[G]) = 0implies a = 0.
Background Frobenius Algebras Examples of Frobenius Algebras Results and Properties
Theorem ((3) Abrams)If A is a Frobenius algebra over a field K with form f, then every otherFrobenius form on A is given by u · f for u an invertible element in A.
Background Frobenius Algebras Examples of Frobenius Algebras Results and Properties
Proof.
Let u ∈ A be a unit. Then for any a ∈ A such that u · f(ax) = f(uax) = 0for all a ∈ A, ua = 0 and therefore a = 0.
Thus there are no non-trivial ideals in the kernel and u · f is a Frobeniusform.
Background Frobenius Algebras Examples of Frobenius Algebras Results and Properties
Proof con’t.
Consider g ∈ A∗ a Frobenius form not equal to f .
Then by the proof in theorem 2, g = λ(u) = u · f for some u ∈ A.
Since g is a Frobenius form, the map λ′ B gβ is an isomorphismA → A∗, where β : A → End (A), the map which takes an elementa ∈ A to the map “multiplication by a”.
Thus, there is a v ∈ A such that f = λ′(v) = v · g = vu · f . Thenλ(1A) = f = uv · f = λ(vu) implies that 1A = vu. Since λ is anisomorphism, u is a unit in A.
Background Frobenius Algebras Examples of Frobenius Algebras Results and Properties
Theorem (4)If A is a Frobenius algebra, then A∗ is a Frobenius algebra.
Proof.Let A be a Frobenius algebra with form f ∈ A∗. By theorem 2, allelements of A∗ are of the form a · f for a ∈ A.
As A and A∗ are isomorphic, define mutiplication in A∗ by(a · f)(b · f) B (ab · f).
Define τ : A∗ → K to be “evaluation at 1A.” Then the identityτ(ax · f) = f(ax) lets A∗ with structure τ be a Frobenius algebra.
Background Frobenius Algebras Examples of Frobenius Algebras Results and Properties
DefinitionA coalgebra A over a field K is a vector space A with two K-linear maps:
α : A → A⊗A and δ : A → K
such that the following diagrams commute
Aα //
α
��
A⊗A
IA⊗α��
A⊗Aα⊗IA // A⊗A ⊗A
AIA
**UUUUUUUUUUUUUUUUUUUα //
α
��
A⊗A
IA⊗δ��
A⊗Aδ⊗IA // K ⊗A � A � A⊗ K
The map α is the comultiplication map, δ is the counit map with axiomscoassociativity [(a ⊗ (b ⊗ c) = (a ⊗ b) ⊗ c] and the counit condition[A⊗ K � K ⊗A is a natural isomorphism].
Background Frobenius Algebras Examples of Frobenius Algebras Results and Properties
We can use the multiplication map µ on A:
µ : A⊗A → A∑ai ⊗ bj 7→
∑aibj
to define a comultiplication map β∗ on A∗:
β∗ : A∗ → A∗ ⊗A∗ � (A⊗A)∗∑f 7→ f ◦ µ
Background Frobenius Algebras Examples of Frobenius Algebras Results and Properties
As a Frobenius algebra, A and A∗ are isomorphic, thus A is prescribeda coalgebra structure by defining α, the comultiplication mapA → A⊗Aas (λ−1 ⊗ λ−1) ◦ β∗ ◦ λ:
Aα //
λ
��
A⊗A
A∗β∗ // A∗
⊗A∗
λ−1⊗λ−1
OO
A is coassociative and cocommutative by the definition of α.
Background Frobenius Algebras Examples of Frobenius Algebras Results and Properties
α can be also used to define the multiplication in A∗ since
(a · f)(b · f) = [a · f ⊗ b · f ] ◦ α = ab · f .
Let g : K→ A denote the unit map. The commutativity of
A∗
g∗
��>>>
>>>>
>>f ◦ β(a)
''PPPPPPPPPPPP
A
λ
OO
f // K a //
OO
f(a) = f ◦ β(a)(1A)
guarantees the commutativity of
A∗β∗ // A∗
⊗A∗
g∗⊗IA∗ // K⊗A∗
λ−1
��A
λ
OO
α // A⊗A
λ⊗λ
OO
f⊗IA // K⊗A
Since the top row is IA∗ , the bottom row is IA. Thus f is the counit in A.
Background Frobenius Algebras Examples of Frobenius Algebras Results and Properties
TheoremEvery finite dimensional semi-simple algebra admits a symmetricFrobenius structure.
Background Frobenius Algebras Examples of Frobenius Algebras Results and Properties
Proof.
Part 1 A matrix algebra over a Frobenius algebra is a Frobenius algebra:Let A be a Frobenius algebra over K with form η. Let Mn(A) be thealgebra of n × n matrices over A with the standard trace. Show that thecomposition
Mn(A)Tr // A
η // K
is a Frobenius form.
Recall that every finite dimensional simple algebra is a matrix algebraover a skew-field, showing that every finite-dimensional simple algebraadmits a Frobenius algebra structure.
Background Frobenius Algebras Examples of Frobenius Algebras Results and Properties
Proof con’t.
Part 2 Every finite-dimensional simple algebra is a matrix algebra admitsa symmetric Frobenius structure.
Part 3 Frobenius algebras are compatible over direct products.
Part 4 By Wedderburn, every finite dimensional semi-simple algebra isisomorphic to a direct sum of matrix algebras over skew-fields.