Duals and Injectives

If k is the base ring for the algebra A then the k-dual Hom_k(M, k) for a right module M over A is a right module over the opposite algebra OA of A. Furthermore, the dual of a projective OA-module is an injective A-module and the dual of a projective OA-resolution of a module M is an A-injective resolution of the dual of M.

Dual(M) : ModAlg -> ModAlg

Given a module M defined over a basic algebra M, this function returns the dual of M as a module over the opposite of the algebra of M. Note that the opposite of the algebra of M is created if it does not already exist.

BaseChangeMatrix(A) : AlgBas -> ModAlg

Given a basic algebra A that has opposite algebra O, the function creates the change of basis matrix B from the vector space of A to the vector space of O, so that if x, y are in A then (xy)B is the same as (yB)(xB) ∈O.

Injective Modules

Injective hulls, and injective resolutions of a module are computed by taking the projective cover or projective resolution of the dual module over the opposite algebra and then again taking the dual to retrieve modules or complexes over the original algebra. If the opposite algebra of the module has not been computed then it will be created in the evaluation of any of the injective module functions.

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

The i^th injective module of the algebra B.

InjectiveHull(M) : ModAlg -> ModAlg, ModMatFldElt, SeqEnum[ModMatFldElt], SeqEnum[ModMatFldElt], SeqEnum[RngIntElt]

The injective hull of the module M is the injective module I of minimal dimension such that there is an inclusion ι: M -> I. The function returns I, ι, the sequences of inclusions and projections from and to the indecomposable injective summands of I, and the type of I as a sequence T := [t₁ ..., t_s] where I has t₁ summands of type 1, t₂ of type 2, etc.

InjectiveResolution(M, n) : ModAlg, RngIntElt -> ModCpx, ModMatFldElt

The complex giving the minimal injective resolution of the module M together with the inclusion homomorphism from M into its injective hull. Note that homomorphisms go from left to right so that the kernel of the first homomorphism in the complex is M. The function computes the compact injective resolution and creates the complex of the injective resolution from that.

CompactInjectiveResolution(M, n) : ModAlg, RngIntElt -> Rec

A minimal injective resolution for the module M out to n steps in compact form together with the coaugmentation map (M -> I₀). The compact form of the resolution is a list of the minimal pieces of information needed to reconstruct the boundary maps in the resolution. That is, the boundary map (I_{i - 1} mapright(∂_i) I_i) is recorded as a matrix whose entries are the images of the generators for indecomposable injective modules making up I_{i - 1} in the indecomposable projective modules making up I_i. The actual return of the function is the compact projective resolution of the dual module of M over the opposite algebra of the algebra of M. The return is a record with the fields:

(a)

The list of isomorphism types of the injective modules in the resolution, each given as a sequence of integers giving the number of direct summands of each indecomposable injective in the module (field name BettiNumbers).

(b)

The record of the boundary maps (field name ResolutionRecord).

(c)

The module M (field name Module).

(d)

The coaugmentation map (field name CoaugmentationMap).

(e)

The type of the resolution, whether projective or injective (field name Typ).

InjectiveSyzygyModule(M, n) : ModAlg, RngIntElt -> ModAlg

The n^th injective-syzygy module of M. The module is constructed from the compact injective resolution of M. The compact resolution is constructed if it does not already exist.

SimpleCohomologyDimensions(M) : ModAlg -> SeqEnum

The sequence of sequences of dimensions of the cohomology groups Ext^j(S_i, M) for simple modules S_i and module M, to the extent that they have been computed.

> ff := GF(8);
> FA<e1,e2,a,b,c> := FreeAlgebra(ff,5);
> rrr := [a*b*a*b*a, c*c*c*c, a*b*c - c*a*b];
> B := BasicAlgebra(FA,rrr,2,[<1,2>,<2,1>,<1,1>]);
> B;
Basic algebra of dimension 41 over GF(2^3)
Number of projective modules: 2
Number of generators: 5
> DimensionsOfProjectiveModules(B);
[ 20, 21 ]
> DimensionsOfInjectiveModules(B);
[ 24, 17 ]
> P1 := ProjectiveModule(B,1);
> Socle(P1);
AModule of dimension 1 over GF(2^3)

> time in1 := CompactInjectiveResolution(P1,10);
reverse trees
Time: 3.850

> SimpleCohomologyDimensions(P1);
[
    [ 1, 0 ],
    [ 0, 4 ],
    [ 4, 0 ],
    [ 4, 0 ],
    [ 4, 0 ],
    [ 4, 0 ],
    [ 4, 0 ],
    [ 4, 0 ],
    [ 4, 0 ],
    [ 4, 0 ]
]

> M := InjectiveSyzygyModule(P1,6);
> M;
AModule M of dimension 64 over GF(2^3)

> hh := AHom(M,M);
> hh;
KMatrixSpace of 64 by 64 matrices and dimension 128 over GF(2^3)
> [Rank(hh.i): i in [1 .. Dimension(hh)]];
[ 16, 16, 16, 16, 12, 12, 12, 12, 8, 8, 8, 8, 8, 8, 8, 8, 6, 6, 6, 6, 4, 4, 4, 
4, 4, 4, 4, 4, 2, 2, 2, 2, 4, 8, 12, 16, 16, 12, 8, 4, 8, 4, 12, 16, 4, 8, 12, 
16, 16, 16, 16, 16, 2, 4, 6, 8, 8, 6, 4, 2, 4, 2, 6, 8, 2, 4, 6, 8, 12, 12, 12, 
12, 8, 8, 8, 8, 8, 8, 8, 8, 6, 6, 6, 6, 4, 4, 4, 4, 4, 4, 4, 4, 2, 2, 2, 2, 16, 
16, 16, 16, 12, 12, 12, 12, 8, 8, 8, 8, 8, 8, 8, 8, 6, 6, 6, 6, 4, 4, 4, 4, 4, 
4, 4, 4, 2, 2, 2, 2 ]

> vv := Random(hh);
> Rank(vv);
64
> vv*vv eq vv;
false
> [Rank(vv*vv-u*vv):u in ff];
[ 60, 64, 64, 64, 64, 64, 64, 64 ]
> [u:u in ff];
[ 1, ff.1, ff.1^2, ff.1^3, ff.1^4, ff.1^5, ff.1^6, 0 ]
> Rank(vv*vv - vv);
60
> U := vv*vv - vv;
> Rank(U);
60
> Rank(U*U);
56
> Rank(U*U*U);
52
> Rank(U*U*U*U);
48
> Rank(U*U*U*U*U);
48
> Rank(U*U*U*U*U*U);
48
> T := U*U*U*U;
> N1 := Kernel(T);
> N2 := Image(T);
> Dimension(N1);
16
> Dimension(N2);
48
> Dimension(N1+N2);
64