Magma - Modules over Basic Algebras

Modules over Basic Algebras

A module M over a basic algebra B is presented as a sequence of matrices, one for each generator of the algebra.

Indecomposable Projective Modules

The indecomposable projective modules are defined from the structure of the algebra and have associated path trees that solve the homomorphism lifting problem.

ProjectiveModule(B, i) : AlgBas, RngIntElt -> ModRng

The i^th projective module of the basic algebra B.

PathTree(B, i) : AlgBas, RngIntElt -> ModRng

The path tree of the i^th projective module of the basic algebra B.

ActionGenerator(B, i) : AlgBas, RngIntElt -> SeqEnum

The sequence of matrices for the generators of the basic algebra B acting on the i^th projective module of B.

IdempotentActionGenerators(B, i) : AlgBas, RngIntElt -> SeqEnum

The sequence of matrices for the idempotent generators of the basic algebra B acting on the i^th projective module of B.

NonIdempotentActionGenerators(B, i) : AlgBas, RngIntElt -> SeqEnum

The sequence of matrices for the nonidempotent generators of the basic algebra B acting on the i^th projective module of B.

Injection(B, i, v) : AlgBas, RngIntElt, ModRngElt -> AlgBasElt

Given a vector v in in the i^th projective module of the basic algebra B, the function returns the image of inclusion of v into B.

Creation

AModule(B, Q) : AlgBas, SeqEnum[AlgMatElt] -> ModRng

Given a basic algebra B and a sequence Q of elements in a matrix algebra the function returns the B-module M on which the generators of B act by multiplication by the corresponding elements of Q.

ProjectiveModule(B, S) : AlgBas, SeqEnum[RngIntElt] -> ModAlg, SeqEnum, SeqEnum

Given a sequence S = [s₁, s₂, ... ], the function returns a projective module which is the direct sum of s₁ copies of the first projective of the algebra B, s₂ copies of the second, etc. It also returns the sequence of inclusions and projections from and to the indecomposable projective modules.

IrreducibleModule(B, i) : AlgBas, RngIntElt -> ModAlg

SimpleModule(B, i) : AlgBas, RngIntElt -> ModAlg

The i^th irreducible module of the algebra B. The module is the quotient of the i^th projective module by its radical.

ZeroModule(B) : AlgBas -> ModAlg

The zero B-module.

RightRegularModule(B) : AlgBas -> ModAlg

The algebra B as a right module over itself. The module is the direct sum of the projectives modules of B.

RegularRepresentation(v) : AlgBasElt -> AlgMatElt

If v is an element of a basic algebra given as a vector in the underlying space, then the function computes the matrix of the action by right multiplication of the element on the algebra.

Restriction(M, B, xi) : ModAlgBas, AlgBas, ModMatFldElt -> ModAlgBas

If B is a subalgebra of the basic algebra A, ξ is the embedding of B into A, and M is an A-module, then the function returns the restriction of M to a B-module.

ChangeAlgebra(M, B, xi) : ModAlgBas , AlgBas, Map -> ModAlgBas

ChangeAlgebra(M, B, xi) : ModAlgBas , AlgBas, ModMatFldElt -> ModAlgBas

Given a module M over an algebra A and an algebra homomorphism ξ from B to A, the function returns the module M as a B-module.

JacobsonRadical(M) : ModAlg -> ModAlg

The Jacobson radical of the module M.

Socle(M) : ModAlg -> ModAlg

The socle of the module M. The sum of the simple submodules of M.

Access Functions

Algebra(M) : ModAlg -> AlgBas

Given a module M over a basic algebra B, the function returns B.

Dimension(M) : ModAlg -> RngIntElt

The dimension of the module M over its base ring.

Action(M) : ModAlg -> AlgMat

The matrix algebra of the action of the algebra of M on M.

IsomorphismTypesOfRadicalLayers(M) : ModAlgBas -> SeqEnum

Given a module M over a basic algebra, returns the sequence of isomorphism types of simple composition factors in each layer of the radical filtration of M.

IsomorphismTypesOfSocleLayers(M) : ModAlgBas -> SeqEnum

Given a module M over a basic algebra, returns a sequence of isomorphism types of simple composition factors in each socle layer with reversed order, i. e. isomorphism types of the socle of M will appear last.

IsomorphismTypesOfBasicAlgebraSequence(S) : SeqEnum -> SeqEnum

Given a sequence of irreducible modules S for a basic algebra A, return a sequence of isomorphism types comparing with the simple modules of A.

Example `AlgBas_restriction-to-center (H94E13)`

We show the restriction of a module over an algebra A to a subalgebra of A.

> G := SmallGroup(32,7);
> A := BasicAlgebra(G);
> C, mu := Center(A);
> X := RightRegularModule(A);
> Z := JacobsonRadical(X);
> L := Restriction(Z,C,mu);
> L;
AModule L of dimension 31 over GF(2)
> A eq Algebra(L);
True
> IndecomposableSummands(L);
[
    AModule of dimension 1 over GF(2),
    AModule of dimension 30 over GF(2)
]
> Dimension(Socle(L));
16

Next we show how to pull back modules along a quotient map. We use the same algebra A.

> U := ideal<A|[A.13 +A.17]>;
> Q, theta := quo<A|U>;
> X := ProjectiveModule(Q,1);
> Y := ChangeAlgebras(X,A,theta);
> Y;
AModule Y of dimension 16 over GF(2)

Example `AlgBas_ChangeAlgebras-2 (H94E14)`

Here is another example of pulling back a module along a quotient map. This one involves algebras with more than one idempotent.

> load m11;
Loading "/usr/local/dmagma/libs/pergps/m11"
M11 - Mathieu group on 11 letters - degree 11
Order 7 920 = 2^4 * 3^2 * 5 * 11;  Base 1,2,3,4
Group: G
> A:= BasicAlgebraOfPrincipalBlock(G,GF(2));
> A;
Basic algebra of dimension 22 over GF(2)
Number of projective modules: 3
Number of generators: 9
> DimensionsOfProjectiveModules(A);
[ 8, 8, 6 ]
> I := ideal<A|[A.9]>;
> B, mu := quo<A|I>;
> B;
Basic algebra of dimension 6 over GF(2)
Number of projective modules: 2
Number of generators: 5
> P := ProjectiveModule(B,1);
> P;
AModule P of dimension 3 over GF(2)
> Q := ChangeAlgebras(P,A,mu);
> Algebra(Q) eq A;
true

Example `AlgBas_RadicalLayers (H94E15)`

In this example, we investigate the structure of the projective modules of a basic algebra.

> G := PSL(3,3);
> N := Normalizer(G,Sylow(G,2));
> A := BasicAlgebraOfHeckeAlgebra(G,N,GF(2));
> DimensionsOfProjectiveModules(A);
[ 1, 2, 3, 9, 9, 1, 1, 1, 1 ]
> IsomorphismTypesOfRadicalLayers(ProjectiveModule(A,4));
[
    [ 4 ],
    [ 2, 3, 4, 5 ],
    [ 4, 4, 5 ],
    [ 5 ]
]
> IsomorphismTypesOfSocleLayers(ProjectiveModule(A,4));  
[
    [ 4 ],
    [ 3, 4, 5 ],
    [ 2, 4, 5 ],
    [ 4, 5 ]
]

So we see that, unlike a group algebra, a Hecke algebra can have indecomposable projective modules whose socles are not simple.

Predicates

The following functions return a boolean value.

IsSemisimple(M) : ModAlg -> BoolElt, SeqEnum

Returns true if the module M is a semisimple module and false otherwise. If true, then the function also returns a list of the ranks of the primitive idempotents of the algebra. This is also a list of the multiplicities of the simple modules of the algebra as composition factors in a composition series for the module.

IsProjective(M) : ModAlg -> BoolElt, SeqEnum

Returns true if the module M is projective. The function also returns a sequence of multiplicities of the standard projective modules as direct summands of the projective cover of M.

IsInjective(M) : ModAlg -> BoolElt, SeqEnum

Returns true if the module M is injective. The function also returns a sequence of multiplicities of the standard injective modules as direct summands of the injective hull of M.

Elementary Operations

m * b : ModAlgElt, AlgBasElt -> ModAlgElt

Given an element b in a basic algebra B and an element m in a module M over B, m * b is the product.

Example `AlgBas_AModules (H94E16)`

We obtain the dimensions of the radical layers of the group algebra of an extra special group of order 243 over a field of characteristic 3.

> G := ExtraSpecialGroup(3,2);
> G;
Permutation group G acting on a set of cardinality 243
> ff := GF(3);
> A := BasicAlgebra(G,ff);
> A;
Basic algebra of dimension 243 over GF(3)
Number of projective modules: 1
Number of generators: 6
> P := ProjectiveModule(A,1);
> P;
AModule P of dimension 243 over GF(3)
> R := JacobsonRadical(P);
> R;
AModule R of dimension 242 over GF(3)
> while Dimension(R) ne 0 do
>     T := JacobsonRadical(R);
>     print Dimension(R) - Dimension(T);
>     R := T;
> end while;
4
11
20
30
36
39
36
30
20
11
4
1

Example `AlgBas_AModules-2 (H94E17)`

We consider the mod-2 group algebra of an extraspecial group of order 128, and construct the module induced from the trivial module on the subgroups of order 4 generated by the first generator of the group.

> G := ExtraSpecialGroup(2,3);
> G;
Permutation group G acting on a set of cardinality 128
> F := GF(2);
> A := BasicAlgebra(G,F);
> A;
Basic algebra of dimension 128 over GF(2)
Number of projective modules: 1
Number of generators: 8
> A.1;
(1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
    0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
    0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
    0 0 0 0 0 0 0 0 0 0 0 0)

Note that cs{A.1} is the unique idempotent (identity element) in the group algebra, whereas cs{A.2} is cs{G.1-1} where cs{G.1} is the first generator of the group.

> A.2;
(0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
    0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
    0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
    0 0 0 0 0 0 0 0 0 0 0 0)
> g := A.1+A.2;

Now we check the order of cs g.

> g^2;
(1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
    0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0
    0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
    0 0 0 0 0 0 0 0 0 0 0 0)
> g^4;
(1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
    0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
    0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
    0 0 0 0 0 0 0 0 0 0 0 0)
> g^4 eq A!1;
true

So cs{g} has order 4.

> P := ProjectiveModule(A,1);
> P;
AModule P of dimension 128 over GF(2)

Note that cs{P} is generated by cs{P.1} which corresponds to the identity element of cs{A} if we think of cs{P} as the algebra cs{A} as a module over itself. Now we create the induced module as the submodule generated by (cs(g) - 1)³, since (cs(g) - 1)⁴ = 0.

> U := sub<P|P.1*A.6>;
> U;
AModule U of dimension 32 over GF(2)

Because the dimension is a quarter of the order of the group we can be sure that we have the right thing by just checking that cs{U} is generated by a cs{g} fixed point.

> U.1*g eq U.1;
true

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Version: V2.29 of Fri Nov 28 15:14:01 AEDT 2025

Contents

SimpleModule(B, i) : AlgBas, RngIntElt -> ModAlg

ChangeAlgebra(M, B, xi) : ModAlgBas , AlgBas, ModMatFldElt -> ModAlgBas