<Chapter Label="Methods for matrix groups">
<Heading>Methods for matrix groups</Heading>
<!-- %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% -->
<Section Label="Polycyclic presentations of matrix groups">
<Heading>Polycyclic presentations of matrix groups</Heading>
Groups defined by polycyclic presentations are called PcpGroups in
&GAP;.
We refer to the Polycyclic manual <Cite Key="Polycyclic"/> for further
background.
<P/>
Suppose that a collection <M>X</M> of
matrices of <M>GL(d,R)</M> is given, where the ring <M>R</M>
is either <M>&QQ;,&ZZ;</M> or a finite field. Let <M>G= \langle X \rangle</M>.
If the group <M>G</M> is polycyclic, then the
following functions determine a PcpGroup isomorphic to <M>G</M>.
<ManSection>
<Oper Name="PcpGroupByMatGroup" Arg="G"/>
<Description>
<A>G</A> is a subgroup of <M>GL(d,R)</M> where <M>R=&QQ;,&ZZ; </M> or <M>\mathbb{F}_q</M>.
If <A>G</A> is polycyclic, then
this function determines a PcpGroup isomorphic to <A>G</A>.
If <A>G</A> is not polycyclic, then
this function returns <C>fail</C>.
</Description>
</ManSection>
<ManSection>
<Meth Name="IsomorphismPcpGroup" Arg="G"/>
<Description>
<A>G</A> is a subgroup of <M>GL(d,R)</M> where <M>R=&QQ;,&ZZ; </M> or <M>\mathbb{F}_q</M>.
If <A>G</A> is polycyclic, then
this function determines an isomorphism
onto a PcpGroup.
If <A>G</A> is not polycyclic, then
this function returns <C>fail</C>.
<P/>
Note that the method <C>IsomorphismPcpGroup</C>,
installed in this package, cannot be
applied directly to a group given by the function <C>AlmostCrystallographicGroup</C>.
Please use <C>POL_AlmostCrystallographicGroup</C> (with the same
parameters as <C>AlmostCrystallographicGroup</C>) instead.
</Description>
</ManSection>
<ManSection>
<Meth Name="ImagesRepresentative" Arg="map, elm"/>
<Meth Name="ImageElm" Arg="map, elm"/>
<Meth Name="ImagesSet" Arg="map, elms"/>
<Description>
Here <A>map</A> is an isomorphism from a polycyclic matrix group <A>G</A>
onto a PcpGroup <A>H</A> calculated
by <Ref Attr="IsomorphismPcpGroup"/>.
These methods can be used to compute with such an isomorphism.
If the input <A>elm</A> is an element of <A>G</A>, then the function <C>ImageElm</C>
can be used to compute the image of <A>elm</A> under <A>map</A>.
If <A>elm</A> is not contained in <A>G</A>
then the function <C>ImageElm</C> returns <C>fail</C>.
The input <A>pcpelm</A> is an element
of <A>H</A>.
</Description>
</ManSection>
<ManSection>
<Meth Name="IsSolvableGroup" Arg="G"/>
<Description>
<A>G</A> is a subgroup of <M>GL(d,R)</M> where <M>R=&QQ;,&ZZ; </M> or <M>\mathbb{F}_q</M>.
This function tests if <A>G</A> is
solvable and returns <C>true</C> or <C>false</C>.
</Description>
</ManSection>
<ManSection>
<Prop Name="IsTriangularizableMatGroup" Arg="G"/>
<Description>
<A>G</A> is a subgroup of <M>GL(d,&QQ;)</M>.
This function tests if <A>G</A> is triangularizable
(possibly over a finite field extension)
and returns <C>true</C> or <C>false</C>.
</Description>
</ManSection>
<ManSection>
<Meth Name="IsPolycyclicGroup" Arg="G"/>
<Description>
<A>G</A> is a subgroup of <M>GL(d,R)</M> where <M>R=&QQ;,&ZZ; </M> or <M>\mathbb{F}_q</M>.
This function tests if <A>G</A> is
polycyclic and returns <C>true</C> or <C>false</C>.
</Description>
</ManSection>
Let <M>G</M> be a finitely generated solvable subgroup of <M>GL(d,&QQ;)</M>. The vector
space <M>&QQ;^d</M> is a module for the algebra <M>&QQ;[G]</M>. The following
functions provide the possibility to compute certain module series of
<M>&QQ;^d</M>. Recall that the radical <M>Rad_G(&QQ;^d)</M> is defined to be the
intersection of maximal <M>&QQ;[G]</M>-submodules of <M>&QQ;^d</M>. Also recall that the
radical series
<Display>
0=R_n < R_{n-1} < \dots < R_1 < R_0=&QQ;^d
</Display>
is defined by <M>R_{i+1}:= Rad_G(R_i)</M>.
<ManSection>
<Oper Name="RadicalSeriesSolvableMatGroup" Arg="G"/>
<Description>
This function returns a
radical series for the <M>&QQ;[G]</M>-module <M>&QQ;^d</M>, where <A>G</A> is a
solvable subgroup of <M>GL(d,&QQ;)</M>.
<P/>
A radical series of <M>&QQ;^d</M> can be refined to a homogeneous series.
</Description>
</ManSection>
<ManSection>
<Func Name="HomogeneousSeriesAbelianMatGroup" Arg="G"/>
<Description>
A module is said to be homogeneous if it is the direct sum of pairwise
irreducible isomorphic submodules. A homogeneous series of a module
is a submodule series such that the factors are homogeneous.
This function returns a
homogeneous series for the <M>&QQ;[G]</M>-module <M>&QQ;^d</M>, where <A>G</A> is an
abelian subgroup of <M>GL(d,&QQ;)</M>.
</Description>
</ManSection>
<ManSection>
<Func Name="HomogeneousSeriesTriangularizableMatGroup" Arg="G"/>
<Description>
A module is said to be homogeneous if it is the direct sum of pairwise
irreducible isomorphic submodules. A homogeneous series of a module
is a submodule series such that the factors are homogeneous.
This function returns a
homogeneous series for the <M>&QQ;[G]</M>-module <M>&QQ;^d</M>, where <A>G</A> is a
triangularizable subgroup of <M>GL(d,&QQ;)</M>.
<P/>
A homogeneous series can be refined to a composition series.
</Description>
</ManSection>
<ManSection>
<Func Name="CompositionSeriesAbelianMatGroup" Arg="G"/>
<Description>
A composition series of a module is a submodule series such that
the factors are irreducible. This function returns a
composition series for the <M>&QQ;[G]</M>-module <M>&QQ;^d</M>, where <A>G</A> is an
abelian subgroup of <M>GL(d,&QQ;)</M>.
</Description>
</ManSection>
<ManSection>
<Func Name="CompositionSeriesTriangularizableMatGroup" Arg="G"/>
<Description>
A composition series of a module is a submodule series such that
the factors are irreducible. This function returns a
composition series for the <M>&QQ;[G]</M>-module <M>&QQ;^d</M>, where <A>G</A> is a
triangularizable subgroup of <M>GL(d,&QQ;)</M>.
</Description>
</ManSection>
<!-- %\>TriangNormalSubgroupFiniteInd( <A>G</A> ) O --> <!-- % --> <!-- %<A>G</A> is a subgroup of <M>GL(d,&QQ;)</M>. If <M>G</M> is solvable then --> <!-- %this function computes a triangularizable normal subgroup of <A>G</A>, --> <!-- %which is of finite index in <A>G</A>. If <M>G</M> is not solvable, then the --> <!-- %function returns <C>fail</C>. -->
<ManSection>
<Oper Name="SubgroupsUnipotentByAbelianByFinite" Arg="G"/>
<Description>
<A>G</A> is a subgroup of <M>GL(d,R)</M> where <M>R=&QQ;</M> or <M>&ZZ;</M>.
If <A>G</A> is polycyclic, then
this function returns a record containing two normal subgroups
<M>T</M> and <M>U</M> of <M>G</M>.
The group <M>T</M> is unipotent-by-abelian
(and thus triangularizable) and
of finite index in <A>G</A>.
The group <M>U</M> is unipotent and is such that <M>T/U</M> is abelian.
If <A>G</A> is not polycyclic,
then the algorithm returns <C>fail</C>.
</Description>
</ManSection>
<ManSection>
<Func Name="PolExamples" Arg="l"/>
<Description>
Returns some examples for polycyclic rational matrix groups, where <A>l</A>
is an integer
between 1 and 24.
These can be used to test the functions in this package.
Some of the
properties of the examples are summarised in the following table.
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