### 44 Matrix Groups

Matrix groups are groups generated by invertible square matrices.

In the following example we temporarily increase the line length limit from its default value 80 to 83 in order to get a nicer output format.

gap> m1 := [ [ Z(3)^0, Z(3)^0,   Z(3) ],
>            [   Z(3), 0*Z(3),   Z(3) ],
>            [ 0*Z(3),   Z(3), 0*Z(3) ] ];;
gap> m2 := [ [   Z(3),   Z(3), Z(3)^0 ],
>            [   Z(3), 0*Z(3),   Z(3) ],
>            [ Z(3)^0, 0*Z(3),   Z(3) ] ];;
gap> m := Group( m1, m2 );
Group(
[
[ [ Z(3)^0, Z(3)^0, Z(3) ], [ Z(3), 0*Z(3), Z(3) ],
[ 0*Z(3), Z(3), 0*Z(3) ] ],
[ [ Z(3), Z(3), Z(3)^0 ], [ Z(3), 0*Z(3), Z(3) ],
[ Z(3)^0, 0*Z(3), Z(3) ] ] ])


#### 44.1 IsMatrixGroup (Filter)

For most operations, GAP only provides methods for finite matrix groups. Many calculations in finite matrix groups are done via so-called "nice monomorphisms" (see Section 40.5) that represent a faithful action on vectors.

##### 44.1-1 IsMatrixGroup
 ‣ IsMatrixGroup( grp ) ( category )

The category of matrix groups.

#### 44.2 Attributes and Properties for Matrix Groups

##### 44.2-1 DimensionOfMatrixGroup
 ‣ DimensionOfMatrixGroup( mat-grp ) ( attribute )

The dimension of the matrix group.

##### 44.2-2 DefaultFieldOfMatrixGroup
 ‣ DefaultFieldOfMatrixGroup( mat-grp ) ( attribute )

Is a field containing all the matrix entries. It is not guaranteed to be the smallest field with this property.

##### 44.2-3 FieldOfMatrixGroup
 ‣ FieldOfMatrixGroup( matgrp ) ( attribute )

The smallest field containing all the matrix entries of all elements of the matrix group matgrp. As the calculation of this can be hard, this should only be used if one really needs the smallest field, use DefaultFieldOfMatrixGroup (44.2-2) to get (for example) the characteristic.

gap> DimensionOfMatrixGroup(m);
3
gap> DefaultFieldOfMatrixGroup(m);
GF(3)


##### 44.2-4 TransposedMatrixGroup
 ‣ TransposedMatrixGroup( matgrp ) ( attribute )

returns the transpose of the matrix group matgrp. The transpose of the transpose of matgrp is identical to matgrp.

gap> G := Group( [[0,-1],[1,0]] );
Group([ [ [ 0, -1 ], [ 1, 0 ] ] ])
gap> T := TransposedMatrixGroup( G );
Group([ [ [ 0, 1 ], [ -1, 0 ] ] ])
gap> IsIdenticalObj( G, TransposedMatrixGroup( T ) );
true


##### 44.2-5 IsFFEMatrixGroup
 ‣ IsFFEMatrixGroup( G ) ( category )

tests whether all matrices in G have finite field element entries.

#### 44.3 Actions of Matrix Groups

The basic operations for groups are described in Chapter 41, special actions for matrix groups mentioned there are OnLines (41.2-12), OnRight (41.2-2), and OnSubspacesByCanonicalBasis (41.2-15).

For subtleties concerning multiplication from the left or from the right, see 44.7.

##### 44.3-1 ProjectiveActionOnFullSpace
 ‣ ProjectiveActionOnFullSpace( G, F, n ) ( function )

Let G be a group of n by n matrices over a field contained in the finite field F. ProjectiveActionOnFullSpace returns the image of the projective action of G on the full row space $$\textit{F}^{\textit{n}}$$.

##### 44.3-2 ProjectiveActionHomomorphismMatrixGroup
 ‣ ProjectiveActionHomomorphismMatrixGroup( G ) ( function )

returns an action homomorphism for a faithful projective action of G on the underlying vector space. (Note: The action is not necessarily on the full space, if a smaller subset can be found on which the action is faithful.)

##### 44.3-3 BlowUpIsomorphism
 ‣ BlowUpIsomorphism( matgrp, B ) ( function )

For a matrix group matgrp and a basis B of a field extension $$L / K$$, say, such that the entries of all matrices in matgrp lie in $$L$$, BlowUpIsomorphism returns the isomorphism with source matgrp that is defined by mapping the matrix $$A$$ to BlownUpMat$$( A, \textit{B} )$$, see BlownUpMat (24.13-3).

gap> g:= GL(2,4);;
gap> B:= CanonicalBasis( GF(4) );;  BasisVectors( B );
[ Z(2)^0, Z(2^2) ]
gap> iso:= BlowUpIsomorphism( g, B );;
gap> Display( Image( iso, [ [ Z(4), Z(2) ], [ 0*Z(2), Z(4)^2 ] ] ) );
. 1 1 .
1 1 . 1
. . 1 1
. . 1 .
gap> img:= Image( iso, g );
<matrix group with 2 generators>
gap> Index( GL(4,2), img );
112


#### 44.4 GL and SL

##### 44.4-1 IsGeneralLinearGroup
 ‣ IsGeneralLinearGroup( grp ) ( property )
 ‣ IsGL( grp ) ( property )

The General Linear group is the group of all invertible matrices over a ring. This property tests, whether a group is isomorphic to a General Linear group. (Note that currently only a few trivial methods are available for this operation. We hope to improve this in the future.)

##### 44.4-2 IsNaturalGL
 ‣ IsNaturalGL( matgrp ) ( property )

This property tests, whether a matrix group is the General Linear group in the right dimension over the (smallest) ring which contains all entries of its elements. (Currently, only a trivial test that computes the order of the group is available.)

##### 44.4-3 IsSpecialLinearGroup
 ‣ IsSpecialLinearGroup( grp ) ( property )
 ‣ IsSL( grp ) ( property )

The Special Linear group is the group of all invertible matrices over a ring, whose determinant is equal to 1. This property tests, whether a group is isomorphic to a Special Linear group. (Note that currently only a few trivial methods are available for this operation. We hope to improve this in the future.)

##### 44.4-4 IsNaturalSL
 ‣ IsNaturalSL( matgrp ) ( property )

This property tests, whether a matrix group is the Special Linear group in the right dimension over the (smallest) ring which contains all entries of its elements. (Currently, only a trivial test that computes the order of the group is available.)

gap> IsNaturalGL(m);
false


##### 44.4-5 IsSubgroupSL
 ‣ IsSubgroupSL( matgrp ) ( property )

This property tests, whether a matrix group is a subgroup of the Special Linear group in the right dimension over the (smallest) ring which contains all entries of its elements.

#### 44.5 Invariant Forms

##### 44.5-1 InvariantBilinearForm
 ‣ InvariantBilinearForm( matgrp ) ( attribute )

This attribute describes a bilinear form that is invariant under the matrix group matgrp. The form is given by a record with the component matrix which is a matrix $$F$$ such that for every generator $$g$$ of matgrp the equation $$g \cdot F \cdot g^{tr} = F$$ holds.

##### 44.5-2 IsFullSubgroupGLorSLRespectingBilinearForm
 ‣ IsFullSubgroupGLorSLRespectingBilinearForm( matgrp ) ( property )

This property tests, whether a matrix group matgrp is the full subgroup of GL or SL (the property IsSubgroupSL (44.4-5) determines which it is) respecting the form stored as the value of InvariantBilinearForm (44.5-1) for matgrp.

##### 44.5-3 InvariantSesquilinearForm
 ‣ InvariantSesquilinearForm( matgrp ) ( attribute )

This attribute describes a sesquilinear form that is invariant under the matrix group matgrp over the field $$F$$ with $$q^2$$ elements, say. The form is given by a record with the component matrix which is is a matrix $$M$$ such that for every generator $$g$$ of matgrp the equation $$g \cdot M \cdot (g^{tr})^f = M$$ holds, where $$f$$ is the automorphism of $$F$$ that raises each element to its $$q$$-th power. ($$f$$ can be obtained as a power of the FrobeniusAutomorphism (59.4-1) value of $$F$$.)

##### 44.5-4 IsFullSubgroupGLorSLRespectingSesquilinearForm
 ‣ IsFullSubgroupGLorSLRespectingSesquilinearForm( matgrp ) ( property )

This property tests, whether a matrix group matgrp is the full subgroup of GL or SL (the property IsSubgroupSL (44.4-5) determines which it is) respecting the form stored as the value of InvariantSesquilinearForm (44.5-3) for matgrp.

 ‣ InvariantQuadraticForm( matgrp ) ( attribute )

For a matrix group matgrp, InvariantQuadraticForm returns a record containing at least the component matrix whose value is a matrix $$Q$$. The quadratic form $$q$$ on the natural vector space $$V$$ on which matgrp acts is given by $$q(v) = v Q v^{tr}$$, and the invariance under matgrp is given by the equation $$q(v) = q(v M)$$ for all $$v \in V$$ and $$M$$ in matgrp. (Note that the invariance of $$q$$ does not imply that the matrix $$Q$$ is invariant under matgrp.)

$$q$$ is defined relative to an invariant symmetric bilinear form $$f$$ (see InvariantBilinearForm (44.5-1)), via the equation $$q(\lambda x + \mu y) = \lambda^2 q(x) + \lambda \mu f(x,y) + \mu^2 q(y)$$, see [CCNPW85, Chapter 3.4]. If $$f$$ is represented by the matrix $$F$$ then this implies $$F = Q + Q^{tr}$$. In characteristic different from $$2$$, we have $$q(x) = f(x,x)/2$$, so $$Q$$ can be chosen as the strictly upper triangular part of $$F$$ plus half of the diagonal part of $$F$$. In characteristic $$2$$, $$F$$ does not determine $$Q$$ but still $$Q$$ can be chosen as an upper (or lower) triangular matrix.

Whenever the InvariantQuadraticForm value is set in a matrix group then also the InvariantBilinearForm (44.5-1) value can be accessed, and the two values are compatible in the above sense.

 ‣ IsFullSubgroupGLorSLRespectingQuadraticForm( matgrp ) ( property )

This property tests, whether the matrix group matgrp is the full subgroup of GL or SL (the property IsSubgroupSL (44.4-5) determines which it is) respecting the InvariantQuadraticForm (44.5-5) value of matgrp.

gap> g:= Sp( 2, 3 );;
gap> m:= InvariantBilinearForm( g ).matrix;
[ [ 0*Z(3), Z(3)^0 ], [ Z(3), 0*Z(3) ] ]
gap> [ 0, 1 ] * m * [ 1, -1 ];           # evaluate the bilinear form
Z(3)
gap> IsFullSubgroupGLorSLRespectingBilinearForm( g );
true
gap> g:= SU( 2, 4 );;
gap> m:= InvariantSesquilinearForm( g ).matrix;
[ [ 0*Z(2), Z(2)^0 ], [ Z(2)^0, 0*Z(2) ] ]
gap> [ 0, 1 ] * m * [ 1, 1 ];            # evaluate the bilinear form
Z(2)^0
gap> IsFullSubgroupGLorSLRespectingSesquilinearForm( g );
true
gap> g:= GO( 1, 2, 3 );;
gap> m:= InvariantBilinearForm( g ).matrix;
[ [ 0*Z(3), Z(3)^0 ], [ Z(3)^0, 0*Z(3) ] ]
gap> [ 0, 1 ] * m * [ 1, 1 ];            # evaluate the bilinear form
Z(3)^0
[ [ 0*Z(3), Z(3)^0 ], [ 0*Z(3), 0*Z(3) ] ]
gap> [ 0, 1 ] * q * [ 0, 1 ];            # evaluate the quadratic form
0*Z(3)
true


#### 44.6 Matrix Groups in Characteristic 0

Most of the functions described in this and the following section have implementations which use functions from the GAP package Carat. If Carat is not installed or not compiled, no suitable methods are available.

##### 44.6-1 IsCyclotomicMatrixGroup
 ‣ IsCyclotomicMatrixGroup( G ) ( category )

tests whether all matrices in G have cyclotomic entries.

##### 44.6-2 IsRationalMatrixGroup
 ‣ IsRationalMatrixGroup( G ) ( property )

tests whether all matrices in G have rational entries.

##### 44.6-3 IsIntegerMatrixGroup
 ‣ IsIntegerMatrixGroup( G ) ( property )

tests whether all matrices in G have integer entries.

##### 44.6-4 IsNaturalGLnZ
 ‣ IsNaturalGLnZ( G ) ( property )

tests whether G is $$GL_n(ℤ)$$ in its natural representation by $$n \times n$$ integer matrices. (The dimension $$n$$ will be read off the generating matrices.)

gap> IsNaturalGLnZ( GL( 2, Integers ) );
true


##### 44.6-5 IsNaturalSLnZ
 ‣ IsNaturalSLnZ( G ) ( property )

tests whether G is $$SL_n(ℤ)$$ in its natural representation by $$n \times n$$ integer matrices. (The dimension $$n$$ will be read off the generating matrices.)

gap> IsNaturalSLnZ( SL( 2, Integers ) );
true


##### 44.6-6 InvariantLattice
 ‣ InvariantLattice( G ) ( attribute )

returns a matrix $$B$$, whose rows form a basis of a $$ℤ$$-lattice that is invariant under the rational matrix group G acting from the right. It returns fail if the group is not unimodular. The columns of the inverse of $$B$$ span a $$ℤ$$-lattice invariant under G acting from the left.

##### 44.6-7 NormalizerInGLnZ
 ‣ NormalizerInGLnZ( G ) ( attribute )

is an attribute used to store the normalizer of G in $$GL_n(ℤ)$$, where G is an integer matrix group of dimension n. This attribute is used by Normalizer( GL( n, Integers ), G ).

##### 44.6-8 CentralizerInGLnZ
 ‣ CentralizerInGLnZ( G ) ( attribute )

is an attribute used to store the centralizer of G in $$GL_n(ℤ)$$, where G is an integer matrix group of dimension n. This attribute is used by Centralizer( GL( n, Integers ), G ).

##### 44.6-9 ZClassRepsQClass
 ‣ ZClassRepsQClass( G ) ( attribute )

The conjugacy class in $$GL_n(ℚ)$$ of the finite integer matrix group G splits into finitely many conjugacy classes in $$GL_n(ℤ)$$. ZClassRepsQClass( G ) returns representative groups for these.

##### 44.6-10 IsBravaisGroup
 ‣ IsBravaisGroup( G ) ( property )

test whether G coincides with its Bravais group (see BravaisGroup (44.6-11)).

##### 44.6-11 BravaisGroup
 ‣ BravaisGroup( G ) ( attribute )

returns the Bravais group of a finite integer matrix group G. If $$C$$ is the cone of positive definite quadratic forms $$Q$$ invariant under $$g \mapsto g Q g^{tr}$$ for all $$g \in \textit{G}$$, then the Bravais group of G is the maximal subgroup of $$GL_n(ℤ)$$ leaving the forms in that same cone invariant. Alternatively, the Bravais group of G can also be defined with respect to the action $$g \mapsto g^{tr} Q g$$ on positive definite quadratic forms $$Q$$. This latter definition is appropriate for groups G acting from the right on row vectors, whereas the former definition is appropriate for groups acting from the left on column vectors. Both definitions yield the same Bravais group.

##### 44.6-12 BravaisSubgroups
 ‣ BravaisSubgroups( G ) ( attribute )

returns the subgroups of the Bravais group of G, which are themselves Bravais groups.

##### 44.6-13 BravaisSupergroups
 ‣ BravaisSupergroups( G ) ( attribute )

returns the subgroups of $$GL_n(ℤ)$$ that contain the Bravais group of G and are Bravais groups themselves.

##### 44.6-14 NormalizerInGLnZBravaisGroup
 ‣ NormalizerInGLnZBravaisGroup( G ) ( attribute )

returns the normalizer of the Bravais group of G in the appropriate $$GL_n(ℤ)$$.

#### 44.7 Acting OnRight and OnLeft

In GAP, matrices by convention act on row vectors from the right, whereas in crystallography the convention is to act on column vectors from the left. The definition of certain algebraic objects important in crystallography implicitly depends on which action is assumed. This holds true in particular for quadratic forms invariant under a matrix group. In a similar way, the representation of affine crystallographic groups, as they are provided by the GAP package CrystGap, depends on which action is assumed. Crystallographers are used to the action from the left, whereas the action from the right is the natural one for GAP. For this reason, a number of functions which are important in crystallography, and whose result depends on which action is assumed, are provided in two versions, one for the usual action from the right, and one for the crystallographic action from the left.

For every such function, this fact is explicitly mentioned. The naming scheme is as follows: If SomeThing is such a function, there will be functions SomeThingOnRight and SomeThingOnLeft, assuming action from the right and from the left, respectively. In addition, there is a generic function SomeThing, which returns either the result of SomeThingOnRight or SomeThingOnLeft, depending on the global variable CrystGroupDefaultAction (44.7-1).

##### 44.7-1 CrystGroupDefaultAction
 ‣ CrystGroupDefaultAction ( global variable )

can have either of the two values RightAction and LeftAction. The initial value is RightAction. For functions which have variants OnRight and OnLeft, this variable determines which variant is returned by the generic form. The value of CrystGroupDefaultAction can be changed with with the function SetCrystGroupDefaultAction (44.7-2).

##### 44.7-2 SetCrystGroupDefaultAction
 ‣ SetCrystGroupDefaultAction( action ) ( function )

allows one to set the value of the global variable CrystGroupDefaultAction (44.7-1). Only the arguments RightAction and LeftAction are allowed. Initially, the value of CrystGroupDefaultAction (44.7-1) is RightAction.

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