# Group Algebras¶

This module implements the category of group algebras for arbitrary groups over arbitrary commutative rings. For details, see sage.categories.algebra_functor.

AUTHOR:

• David Loeffler (2008-08-24): initial version
• Martin Raum (2009-08): update to use new coercion model – see trac ticket #6670.
• John Palmieri (2011-07): more updates to coercion, categories, etc., group algebras constructed using CombinatorialFreeModule – see trac ticket #6670.
• Nicolas M. Thiéry (2010-2017), Travis Scrimshaw (2017): generalization to a covariant functorial construction for monoid algebras, and beyond – see e.g. trac ticket #18700.
class sage.categories.group_algebras.GroupAlgebras(category, *args)

The category of group algebras over a given base ring.

EXAMPLES:

sage: C = Groups().Algebras(ZZ); C
Category of group algebras over Integer Ring
sage: C.super_categories()
[Category of hopf algebras with basis over Integer Ring,
Category of monoid algebras over Integer Ring]


We can also construct this category with:

sage: C is GroupAlgebras(ZZ)
True


Here is how to create the group algebra of a group $$G$$:

sage: G = DihedralGroup(5)
sage: QG = G.algebra(QQ); QG
Algebra of Dihedral group of order 10 as a permutation group over Rational Field


and an example of computation:

sage: g = G.an_element(); g
(1,4)(2,3)
sage: (QG.term(g) + 1)**3
4*() + 4*(1,4)(2,3)


Todo

• Check which methods would be better located in Monoid.Algebras or Groups.Finite.Algebras.
class ElementMethods
central_form()

Return self expressed in the canonical basis of the center of the group algebra.

INPUT:

• self – an element of the center of the group algebra

OUTPUT:

• A formal linear combination of the conjugacy class representatives representing its coordinates in the canonical basis of the center. See Groups.Algebras.ParentMethods.center_basis() for details.

Warning

• This method requires the underlying group to have a method conjugacy_classes_representatives (every permutation group has one, thanks GAP!).
• This method does not check that the element is indeed central. Use the method Monoids.Algebras.ElementMethods.is_central() for this purpose.
• This function has a complexity linear in the number of conjugacy classes of the group. One could easily implement a function whose complexity is linear in the size of the support of self.

EXAMPLES:

sage: QS3 = SymmetricGroup(3).algebra(QQ)
sage: A = QS3([2,3,1]) + QS3([3,1,2])
sage: A.central_form()
B[(1,2,3)]
sage: QS4 = SymmetricGroup(4).algebra(QQ)
sage: B = sum(len(s.cycle_type())*QS4(s) for s in Permutations(4))
sage: B.central_form()
4*B[()] + 3*B[(1,2)] + 2*B[(1,2)(3,4)] + 2*B[(1,2,3)] + B[(1,2,3,4)]

sage: QG = GroupAlgebras(QQ).example(PermutationGroup([[(1,2,3),(4,5)],[(3,4)]]))
sage: sum(i for i in QG.basis()).central_form()
B[()] + B[(4,5)] + B[(3,4,5)] + B[(2,3)(4,5)] + B[(2,3,4,5)] + B[(1,2)(3,4,5)] + B[(1,2,3,4,5)]


class ParentMethods
antipode_on_basis(g)

Return the antipode of the element g of the basis.

Each basis element g is group-like, and so has antipode $$g^{-1}$$. This method is used to compute the antipode of any element.

EXAMPLES:

sage: A = CyclicPermutationGroup(6).algebra(ZZ); A
Algebra of Cyclic group of order 6 as a permutation group over Integer Ring
sage: g = CyclicPermutationGroup(6).an_element();g
(1,2,3,4,5,6)
sage: A.antipode_on_basis(g)
(1,6,5,4,3,2)
sage: a = A.an_element(); a
() + (1,2,3,4,5,6) + 3*(1,3,5)(2,4,6) + 2*(1,5,3)(2,6,4)
sage: a.antipode()
() + 2*(1,3,5)(2,4,6) + 3*(1,5,3)(2,6,4) + (1,6,5,4,3,2)

center_basis()

Return a basis of the center of the group algebra.

The canonical basis of the center of the group algebra is the family $$(f_\sigma)_{\sigma\in C}$$, where $$C$$ is any collection of representatives of the conjugacy classes of the group, and $$f_\sigma$$ is the sum of the elements in the conjugacy class of $$\sigma$$.

OUTPUT:

• tuple of elements of self

Warning

• This method requires the underlying group to have a method conjugacy_classes (every permutation group has one, thanks GAP!).

EXAMPLES:

sage: SymmetricGroup(3).algebra(QQ).center_basis()
((), (2,3) + (1,2) + (1,3), (1,2,3) + (1,3,2))


coproduct_on_basis(g)

Return the coproduct of the element g of the basis.

Each basis element g is group-like. This method is used to compute the coproduct of any element.

EXAMPLES:

sage: A = CyclicPermutationGroup(6).algebra(ZZ); A
Algebra of Cyclic group of order 6 as a permutation group over Integer Ring
sage: g = CyclicPermutationGroup(6).an_element(); g
(1,2,3,4,5,6)
sage: A.coproduct_on_basis(g)
(1,2,3,4,5,6) # (1,2,3,4,5,6)
sage: a = A.an_element(); a
() + (1,2,3,4,5,6) + 3*(1,3,5)(2,4,6) + 2*(1,5,3)(2,6,4)
sage: a.coproduct()
() # () + (1,2,3,4,5,6) # (1,2,3,4,5,6) + 3*(1,3,5)(2,4,6) # (1,3,5)(2,4,6) + 2*(1,5,3)(2,6,4) # (1,5,3)(2,6,4)

counit(x)

Return the counit of the element x of the group algebra.

This is the sum of all coefficients of x with respect to the standard basis of the group algebra.

EXAMPLES:

sage: A = CyclicPermutationGroup(6).algebra(ZZ); A
Algebra of Cyclic group of order 6 as a permutation group over Integer Ring
sage: a = A.an_element(); a
() + (1,2,3,4,5,6) + 3*(1,3,5)(2,4,6) + 2*(1,5,3)(2,6,4)
sage: a.counit()
7

counit_on_basis(g)

Return the counit of the element g of the basis.

Each basis element g is group-like, and so has counit $$1$$. This method is used to compute the counit of any element.

EXAMPLES:

sage: A=CyclicPermutationGroup(6).algebra(ZZ);A
Algebra of Cyclic group of order 6 as a permutation group over Integer Ring
sage: g=CyclicPermutationGroup(6).an_element();g
(1,2,3,4,5,6)
sage: A.counit_on_basis(g)
1

group()

Return the underlying group of the group algebra.

EXAMPLES:

sage: GroupAlgebras(QQ).example(GL(3, GF(11))).group()
General Linear Group of degree 3 over Finite Field of size 11
sage: SymmetricGroup(10).algebra(QQ).group()
Symmetric group of order 10! as a permutation group

is_integral_domain(proof=True)

Return True if self is an integral domain.

This is false unless self.base_ring() is an integral domain, and even then it is false unless self.group() has no nontrivial elements of finite order. I don’t know if this condition suffices, but it obviously does if the group is abelian and finitely generated.

EXAMPLES:

sage: GroupAlgebra(SymmetricGroup(2)).is_integral_domain()
False
sage: GroupAlgebra(SymmetricGroup(1)).is_integral_domain()
True
sage: GroupAlgebra(SymmetricGroup(1), IntegerModRing(4)).is_integral_domain()
False
sage: GroupAlgebra(AbelianGroup(1)).is_integral_domain()
True
sage: GroupAlgebra(AbelianGroup(2, [0,2])).is_integral_domain()
False
sage: GroupAlgebra(GL(2, ZZ)).is_integral_domain() # not implemented
False

example(G=None)

Return an example of group algebra.

EXAMPLES:

sage: GroupAlgebras(QQ['x']).example()
Algebra of Dihedral group of order 8 as a permutation group over Univariate Polynomial Ring in x over Rational Field


An other group can be specified as optional argument:

sage: GroupAlgebras(QQ).example(AlternatingGroup(4))
Algebra of Alternating group of order 4!/2 as a permutation group over Rational Field

extra_super_categories()

Implement the fact that the algebra of a group is a Hopf algebra.

EXAMPLES:

sage: C = Groups().Algebras(QQ)
sage: C.extra_super_categories()
[Category of hopf algebras over Rational Field]
sage: sorted(C.super_categories(), key=str)
[Category of hopf algebras with basis over Rational Field,
Category of monoid algebras over Rational Field]