73 Vector Enumeration

This chapter describes the VE (Version 3) share library package for computing matrix representations of finitely presented algebras. See Installing the Vector Enumeration Package for the installation of the package, and the VE manual Lin93 for details of the implementation.

The default application of VE, namely the function `Operation` for finitely presented algebras (see chapter Finitely Presented Algebras), is described in Operation for Finitely Presented Algebras.

The interface between GAP3 and VE is described in More about Vector Enumeration.

In Examples of Vector Enumeration the examples given in the VE manual serve as examples for the use of VE with GAP3.

Finally, section Using Vector Enumeration with the MeatAxe shows how the MeatAxe share library (see chapter The MeatAxe) and VE can work hand in hand.

The functions of the package can be used after loading the package with

` gap> RequirePackage( "ve" ); `

The package is also loaded automatically when `Operation` is called for the action of a finitely presented algebra on a quotient module.

73.1 Operation for Finitely Presented Algebras

`Operation( F, Q )`

This is the default application of VE. F is a finitely presented algebra (see chapter Finitely Presented Algebras), Q is a quotient of a free F-module, and the result is a matrix algebra representing a faithful action on Q.

If Q is the zero module then the matrices have dimension zero, so the result is a null algebra (see NullAlgebra) consisting only of a zero element.

The algebra homomorphism, the isomorphic module for the matrix algebra, and the module homomorphism can be constructed as described in chapters Algebras and Modules.

```    gap> a:= FreeAlgebra( GF(2), 2 );
UnitalAlgebra( GF(2), [ a.1, a.2 ] )
gap> a:= a / [ a.1^2 - a.one, # group algebra of V4 over GF(2)
>              a.2^2 - a.one,
>              a.1*a.2 - a.2*a.1 ];
UnitalAlgebra( GF(2), [ a.1, a.2 ] )
gap> op:= Operation( a, a^1 );
UnitalAlgebra( GF(2),
[ [ [ 0*Z(2), 0*Z(2), Z(2)^0, 0*Z(2) ], [ 0*Z(2), 0*Z(2), 0*Z(2),
Z(2)^0 ], [ Z(2)^0, 0*Z(2), 0*Z(2), 0*Z(2) ],
[ 0*Z(2), Z(2)^0, 0*Z(2), 0*Z(2) ] ],
[ [ 0*Z(2), Z(2)^0, 0*Z(2), 0*Z(2) ],
[ Z(2)^0, 0*Z(2), 0*Z(2), 0*Z(2) ],
[ 0*Z(2), 0*Z(2), 0*Z(2), Z(2)^0 ],
[ 0*Z(2), 0*Z(2), Z(2)^0, 0*Z(2) ] ] ] )
gap> Size( op );
16 ```

As stated in the introduction to this chapter, VE is a share library package. The computations are done by standalone programs written in C.

The interface between VE and GAP3 consists essentially of two parts, namely the global variable `VE`, and the function `FpAlgebraOps.OperationQuotientModule`.

The 'VE' record

`VE` is a record with components

`Path`:

the full path name of the directory that contains the executables of the standalones `me`, `qme`, `zme`,

`options`:

a string with command line options for VE; it will be appended to the command string of `CallVE` (see below), so the default options chosen there can be overwritten. This may be useful for example in case of the `-v` option to enable the printing of comments (see section 4.3 of Lin93), but you should not change the output file (using `-o`) when you simply call `Operation` for a finitely presented algebra. `options` is defaulted to the empty string.

FpAlgebraOps.OperationQuotientModule

This function is called automatically by `FpAlgebraOps.Operation` (see Operation for Finitely Presented Algebras), it can also be called directly as follows.

`FpAlgebraOps.OperationQuotientModule( A, Q, opr )`
`FpAlgebraOps.OperationQuotientModule( A, Q, "mtx" )`

It takes a finitely presented algebra A and a list of submodule generators Q, that is, the entries of Q are list of equal length, with entries in A, and returns the matrix representation computed by the VE program.

The third argument must be either one of the operations `OnPoints`, `OnRight`, or the string `"mtx"`. In the latter case the output will be an algebra of MeatAxe matrices, see Using Vector Enumeration with the MeatAxe for further explanation.

Accessible Subroutines

The following three functions are used by `FpAlgebraOps.OperationQuotientModule`. They are the real interface that allows to access VE from GAP3.

`PrintVEInput( A, Q, names )`

takes a finitely presented algebra A, a list of submodule generators Q, and a list names of names the generators shall have in the presentation that is passed to VE, and prints a string that represents the input presentation for VE. See section 3.1 of the VE manual Lin93 for a description of the syntax.

```    gap> PrintVEInput( a, [ [ a.zero ] ], [ "A", "B" ] );
2.
A B .
.
.
{1}(0).
A*A, B*B, :
A*B+B*A = 0, .  ```

`CallVE( commandstr, infile, outfile, options )`

calls VE with command string commandstr, presentation file infile, and command line options options, and prescribes the output file outfile.

If not overwritten in the string options, the default options `"-i -P -v0 -Y VE.out -L`# `"` are chosen.

Of course it is not necessary that infile was produced using `PrintVEInput`, and also the output is independent of GAP3.

```    gap> PrintTo( "infile.pres",
>             PrintVEInput( a, [ [ a.zero ] ], [ "A", "B" ] ) );
gap> CallVE( "me", "infile", "outfile", " -G -vs2" ); ```

(The option `-G` sets the output format to GAP3, `-vs2` chooses a more verbose mode.)

`VEOutput( A, Q, names, outfile )`
`VEOutput( A, Q, names, outfile, "mtx" )`

returns the output record produced by VE that was written to the file outfile. A component `operation` is added that contains the information for the construction of the operation homomorphisms.

The arguments A, Q, names describe the finitely presented algebra, the quotient module it acts on, and the chosen generators names, i.e., the original structures for that VE was called.

```    gap> out:= VEOutput( a, [ [ a.zero ] ], [ "A", "B" ], "outfile" );;
gap> out.dim; out.operation.moduleinfo.preimagesBasis;
4
[ [ a.one ], [ a.2 ], [ a.1 ], [ a.1*a.2 ] ] ```

If the optional fifth argument `"mtx"` is present, the output is regarded as an algebra of MeatAxe matrices (see section Using Vector Enumeration with the MeatAxe). For that, an appropriate command string had to be passed to `CallVE`.

73.3 Examples of Vector Enumeration

We consider those of the examples given in chapter 8 of the VE manual that can be used in GAP3.

8.1 The natural permutation representation of S3

The symmetric group S3 is also the dihedral group D6, and so is presented by two involutions with product of order 3. Taking the permutation action on the cosets of the cyclic group generated by one of the involutions we obtain the following presentation.

```    gap> a:= FreeAlgebra( Rationals, 2 );;
gap> a:= a / [ a.1^2 - a.one, a.2^2 - a.one,
>              (a.1*a.2)^3 - a.one ];
UnitalAlgebra( Rationals, [ a.1, a.2 ] )
gap> a.name:= "a";; ```

We choose as module `q` the quotient of the regular module for `a` by the submodule generated by `a.1 - 1`, and compute the action of `a` on `q`.

```    gap> m:= a^1;;
gap> q:= m / [ [ a.1 - a.one ] ];
Module( a, [ [ a.one ] ] ) / [ [ -1*a.one+a.1 ] ]
gap> op:= Operation( a, q );
UnitalAlgebra( Rationals,
[ [ [ 1, 0, 0 ], [ 0, 0, 1 ], [ 0, 1, 0 ] ],
[ [ 0, 1, 0 ], [ 1, 0, 0 ], [ 0, 0, 1 ] ] ] )
gap> op.name:= "op";; ```

8.2 A Quotient of a Permutation Representation

The permutation representation constructed in example 8.1 fixes the all-ones vector (as do all permutation representations). This is the image of the module element `[ a.one + a.2 + a.2*a.1 ]` in the corresponding module for the algebra `op`.

```    gap> ophom:= OperationHomomorphism( a, op );;
gap> opmod:= OperationModule( op );
Module( op, [ [ 1, 0, 0 ], [ 0, 1, 0 ], [ 0, 0, 1 ] ] )
gap> modhom:= OperationHomomorphism( q, opmod );;
gap> pre:= PreImagesRepresentative( modhom, [ 1, 1, 1 ] );;
gap> pre:= pre.representative;
[ a.one+a.2+a.2*a.1 ] ```

We could have computed such a preimage also by computing a matrix that maps the image of the submodule generator of `q` to the all-ones vector, and applying a preimage to the submodule generator. Of course the we do not necessarily get the same representatives.

```    gap> images:= List( Generators( q ), x -> Image( modhom, x ) );
[ [ 1, 0, 0 ] ]
gap> rep:= RepresentativeOperation( op, images, [ 1, 1, 1 ] );
[ [ 1, 1, 1 ], [ 1, 1, 1 ], [ 1, 1, 1 ] ]
gap> PreImagesRepresentative( ophom, rep );
a.one+a.1*a.2+a.2*a.1 ```

Now we factor out the fixed submodule by enlarging the denominator of the module `q`. (Note that we could also compute the action of the matrix algebra if we were only interested in the 2-dimensional representation.)

Accordingly we can write down the following presentation for the quotient module.

```    gap> q:= m / [ [ a.1 - a.one ], pre ];;
gap> op:= Operation( a, q );
UnitalAlgebra( Rationals,
[ [ [ 1, 0 ], [ -1, -1 ] ], [ [ 0, 1 ], [ 1, 0 ] ] ] ) ```

8.3 A Non-cyclic Module

If we take the direct product of two copies of the permutation representation constructed in example 8.1, we can identify the fixed vectors in the two copies in the following presentation.

```    gap> m:= a^2;;
gap> q:= m / [ [ a.zero, a.1 - a.one ], [ a.1 - a.one, a.zero ],
>              [ a.one+a.2+a.2*a.1, -a.one-a.2-a.2*a.1 ] ];
Module( a, [ [ a.one, a.zero ], [ a.zero, a.one ] ] ) /
[ [ a.zero, -1*a.one+a.1 ], [ -1*a.one+a.1, a.zero ],
[ a.one+a.2+a.2*a.1, -1*a.one+-1*a.2+-1*a.2*a.1 ] ] ```

We compute the matrix representation.

```    gap> op:= Operation( a, q );
UnitalAlgebra( Rationals,
[ [ [ 1, 0, 0, 0, 0 ], [ 0, 1, 0, 0, 0 ], [ 0, 0, 0, 1, 0 ],
[ 0, 0, 1, 0, 0 ], [ 1, -1, 1, 1, -1 ] ],
[ [ 0, 0, 1, 0, 0 ], [ 0, 0, 0, 0, 1 ], [ 1, 0, 0, 0, 0 ],
[ 0, 0, 0, 1, 0 ], [ 0, 1, 0, 0, 0 ] ] ] ) ```

In this case it is interesting to look at the images of the module generators and pre-images of the basis vectors. Note that these preimages are elements of a factor module, corresponding elements of the free module are again found as representatives.

```    gap> ophom:= OperationHomomorphism( a, op );;
gap> opmod:= OperationModule( op );;
gap> opmod.name:= "opmod";;
gap> modhom:= OperationHomomorphism( q, opmod );;
gap> List( Generators( q ), x -> Image( modhom, x ) );
[ [ 1, 0, 0, 0, 0 ], [ 0, 1, 0, 0, 0 ] ]
gap> basis:= Basis( opmod );
CanonicalBasis( opmod )
gap> preim:= List( basis.vectors, x ->
>               PreImagesRepresentative( modhom, x ) );;
gap> preim:= List( preim, Representative );
[ [ a.one, a.zero ], [ a.zero, a.one ], [ a.2, a.zero ],
[ a.2*a.1, a.zero ], [ a.zero, a.2 ] ] ```

8.4 A Monoid Representation

The Coxeter monoid of type B2 has a transformation representation on four points. This can be constructed as a matrix representation over GF(3), from the following presentation.

```    gap> a:= FreeAlgebra( GF(3), 2 );;
gap> a:= a / [ a.1^2 - a.1, a.2^2 - a.2,
> (a.1*a.2)^2 - (a.2*a.1)^2 ];;
gap> q:= a^1 / [ [ a.1 - a.one ] ];;
gap> op:= Operation( a, q );
UnitalAlgebra( GF(3),
[ [ [ Z(3)^0, 0*Z(3), 0*Z(3), 0*Z(3) ], [ 0*Z(3), 0*Z(3), Z(3)^0,
0*Z(3) ], [ 0*Z(3), 0*Z(3), Z(3)^0, 0*Z(3) ],
[ 0*Z(3), 0*Z(3), 0*Z(3), Z(3)^0 ] ],
[ [ 0*Z(3), Z(3)^0, 0*Z(3), 0*Z(3) ],
[ 0*Z(3), Z(3)^0, 0*Z(3), 0*Z(3) ],
[ 0*Z(3), 0*Z(3), 0*Z(3), Z(3)^0 ],
[ 0*Z(3), 0*Z(3), 0*Z(3), Z(3)^0 ] ] ] ) ```

8.7 A Quotient of a Polynomial Ring

The quotient of a polynomial ring by the ideal generated by some polynomials will be finite-dimensional just when the polynomials have finitely many common roots in the algebraic closure of the ground ring. For example, three polynomials in three variables give us the following presentation for the quotient of their ideal.

Define `a` to be the polynomial algebra on three variables.

```    gap> a:= FreeAlgebra( Rationals, 3 );;
gap> a:= a / [ a.1 * a.2 - a.2 * a.1,
>              a.1 * a.3 - a.3 * a.1,
>              a.2 * a.3 - a.3 * a.2 ];; ```

Define the quotient module by the polynomials `A+B+C`, `AB+BC+CA`, `ABC-1`.

```    gap> q:= a^1 / [ [ a.1+a.2+a.3 ],
>                [ a.1*a.2+a.2*a.3+a.3*a.1 ],
>                [ a.1*a.2*a.3-a.one ]        ];; ```

Compute the representation.

```    gap> op:= Operation( a, q );
UnitalAlgebra( Rationals,
[ [ [ 0, 1, 0, 0, 0, 0 ], [ 0, 0, 0, 0, 1, 0 ],
[ -1, 0, 0, 0, 0, -1 ], [ 0, 0, 1, 0, 0, 0 ],
[ 1, 0, 0, 0, 0, 0 ], [ 0, -1, 0, -1, 0, 0 ] ],
[ [ 0, 0, 0, 1, 0, 0 ], [ 0, 0, 1, 0, 0, 0 ], [ 0, 0, 0, 0, 0, 1 ],
[ 0, 0, -1, 0, -1, 0 ], [ -1, 0, 0, 0, 0, -1 ],
[ 0, 1, 0, 0, 0, 0 ] ],
[ [ 0, -1, 0, -1, 0, 0 ], [ 0, 0, -1, 0, -1, 0 ],
[ 1, 0, 0, 0, 0, 0 ], [ 0, 0, 0, 0, 1, 0 ],
[ 0, 0, 0, 0, 0, 1 ], [ 0, 0, 0, 1, 0, 0 ] ] ] ) ```

73.4 Using Vector Enumeration with the MeatAxe

One can deal with the matrix representation constructed by VE also using the MeatAxe share library. This way the matrices are not read into GAP3 but written to files and converted into internal MeatAxe format. See chapter The MeatAxe for details.

```    gap> a:= FreeAlgebra( GF(2), 2 );;
gap> a:= a / [ a.1^2 - a.one, a.2^2 - a.one,
>              (a.1*a.2)^3 - a.one ];;
gap> RequirePackage("meataxe");
#I  The MeatAxe share library functions are available now.
#I  All files will be placed in the directory
#I     '/var/tmp/tmp.006103'
#I  Use 'MeatAxe.SetDirectory( <path> )' if you want to change.
gap> op:= Operation( a, a^1, "mtx" );
UnitalAlgebra( GF(2),
[ MeatAxeMat( "/var/tmp/tmp.006103/a/g.1", GF(2), [ 6, 6 ], a.1 ),
MeatAxeMat( "/var/tmp/tmp.006103/a/g.2", GF(2), [ 6, 6 ], a.2 ) ] )
gap> Display( op.1 );
#I  calling 'maketab' for field of size 2
MeatAxe.Matrix := [
[0,0,1,0,0,0],
[0,0,0,1,0,0],
[1,0,0,0,0,0],
[0,1,0,0,0,0],
[0,0,0,0,0,1],
[0,0,0,0,1,0]
]*Z(2);
gap> MeatAxe.Unbind(); ```

gap3-jm
11 Mar 2019