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<!--  gp2obj.xml           XMOD documentation             Chris Wensley   -->
<!--                                                        & Murat Alp   -->
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<?xml version="1.0" encoding="ISO-8859-15"?>
  <!-- $Id: gp2obj.xml,v 2.11  Exp $ -->

<Chapter Label="chap-obj2">

<Heading>2d-objects</Heading>

<Section><Heading>Constructions for crossed modules</Heading>

<Index>crossed module</Index>
<Index>2d-object</Index>
A crossed module  <M>\mathcal{X} = (\partial : S \to R )</M> 
consists of a group homomorphism <M>\partial </M>, 
called the <E>boundary</E> of <M>\mathcal{X}</M>, 
with <E>source</E> <M>S</M>  and <E>range</E> <M>R</M>. 
The Group <M>R</M> acts on itself by conjugation, and on <M>S</M> by an action 
<M>\alpha : R \to {\rm Aut}(S)</M> such that,  
for all <M>s,s_1,s_2 \in S</M>  and  <M>r \in R</M>,
<Display>
{\bf XMod\ 1} ~:~  \partial(s^r) 
   = r^{-1} (\partial s) r
   = (\partial s)^r,
\qquad
{\bf XMod\ 2} ~:~  s_1^{\partial s_2} 
   = s_2^{-1}s_1 s_2
   = {s_1}^{s_2}.
</Display>
<P/>
The kernel of  <M>\partial</M>  is abelian.
<P/>
There are a variety of constructors for crossed modules:

<ManSection>
   <Func Name="XMod"
         Arg="args" />
   <Oper Name="XModByBoundaryAndAction"
         Arg="bdy act" />
   <Oper Name="XModByTrivialAction"
         Arg="bdy" />
   <Oper Name="XModByNormalSubgroup"
         Arg="G N" />
   <Oper Name="XModByCentralExtension"
         Arg="bdy" />
   <Oper Name="XModByAutomorphismGroup"
         Arg="grp" />
   <Oper Name="XModByInnerAutomorphismGroup"
         Arg="grp" />
   <Oper Name="XModByGroupOfAutomorphisms"
         Arg="G A" />
   <Oper Name="XModByAbelianModule"
         Arg="abgrp" />
   <Oper Name="DirectProduct"
         Arg="X1 X2" />
<Description>
Here are the standard constructions which these implement:
<List>
<Item>
A <E>conjugation crossed module</E> is an inclusion of a normal subgroup  
<M>S \unlhd R</M>, where <M>R</M> acts on <M>S</M> by conjugation.
</Item>
<Item>
A <E>central extension crossed module</E> has as boundary a surjection
<M>\partial : S \to R</M> with central kernel,
where <M>r \in R</M> acts on <M>S</M> by conjugation 
with <M>\partial^{-1}r</M>.
</Item>
<Item>
An <E>automorphism crossed module</E> has as range a subgroup <M>R</M>
of the automorphism group  Aut<M>(S)</M>  of  <M>S</M>
which contains the inner automorphism group of <M>S</M>.
The boundary maps <M>s \in S</M> to the inner automorphism of <M>S</M> 
by <M>s</M>.
</Item>
<Item>
A <E>trivial action crossed module</E>  <M>\partial : S \to R</M>
has  <M>s^r = s</M>  for all  <M>s \in S, \; r \in R</M>,  
the source is abelian and the image lies in the centre of the range.
</Item>
<Item>
A <E>crossed abelian module</E> has an abelian module as source
and the zero map as boundary.
</Item>
<Item>
The direct product  <M>\mathcal{X}_{1} \times \mathcal{X}_{2}</M>
of two crossed modules has source  <M>S_1 \times S_2</M>,
range  <M>R_1 \times R_2</M>  and boundary
<M>\partial_1 \times \partial_2</M>,  with  <M>R_1,\ R_2</M>  acting
trivially on  <M>S_2,\ S_1</M>  respectively.
</Item>
</List>
</Description>
</ManSection>

<ManSection>
   <Attr Name="Source"
         Arg="X0" />
   <Attr Name="Range"
         Arg="X0" />
   <Attr Name="Boundary"
         Arg="X0" />
   <Attr Name="AutoGroup"
         Arg="X0" />
   <Attr Name="XModAction"
         Arg="X0" />
<Description>
In this implementation the attributes used in the construction of
a crossed module <C>X0</C> are as follows.
<List>
<Item>
<C>Source(X0)</C> and <C>Range(X0)</C> are the source <M>S</M> 
and range <M>R</M> of <M>\partial</M>, the boundary <C>Boundary(X0)</C>;
</Item>
<Item>
<C>AutoGroup(X0)</C> is a group of automorphisms of <M>S</M>;
</Item>
<Item>
<C>XModAction(X0)</C> is a homomorphism from <M>R</M> to <C>AutoGroup(X0)</C>.
</Item>
</List>
</Description>
</ManSection>

<ManSection>
   <Attr Name="Size"
         Arg="X0" />
   <Attr Name="Name"
         Arg="X0" />
<Description>
More familiar attributes are <C>Size</C> and <C>Name</C>, 
the latter formed by concatenating the names of the source and range 
(if these exist).
An <C>Enumerator</C> function has not been implemented.
<Index>display a 2d-object</Index>
The <C>Display</C> function is used to print details of <C>2d-objects</C>.
</Description>
</ManSection>

Here is a simple example of an automorphism crossed module, 
using a cyclic group of size five.

<Example>
<![CDATA[
gap> c5 := Group( (5,6,7,8,9) );;
gap> SetName( c5, "c5" );
gap> X1 := XModByAutomorphismGroup( c5 );
[c5 -> PAut(c5)] 
gap> Display( X1 );
Crossed module [c5 -> PAut(c5)] :-
: Source group c5 has generators:
  [ (5,6,7,8,9) ]
: Range group PAut(c5) has generators:
  [ (1,2,4,3) ]
: Boundary homomorphism maps source generators to:
  [ () ]
: Action homomorphism maps range generators to automorphisms:
  (1,2,4,3) --> { source gens --> [ (5,7,9,6,8) ] }
  This automorphism generates the group of automorphisms.
gap> Size( X1 );
[ 5, 4 ]
gap> Print( RepresentationsOfObject(X1), "\n" );
[ "IsComponentObjectRep", "IsAttributeStoringRep", "IsPreXModObj" ]
gap> Print( KnownPropertiesOfObject(X1), "\n" );
[ "Is2dObject", "IsPerm2dObject", "IsPreXMod", "IsXMod",
  "IsTrivialAction2dObject", "IsAutomorphismGroup2dObject" ]
gap> Print( KnownAttributesOfObject(X1), "\n" );
[ "Name", "Size", "Range", "Source", "Boundary", "AutoGroup", "XModAction" ]
]]>
</Example>

<ManSection>
   <Oper Name="SubXMod"
         Arg="X0 src rng" />
   <Attr Name="IdentitySubXMod"
         Arg="X0" />
   <Attr Name="NormalSubXMods"
         Arg="X0" />
<Description>
With the standard crossed module constructors listed above as building blocks, 
sub-crossed modules, normal sub-crossed modules 
<M>\mathcal{N} \lhd \mathcal{X}</M>,
and also quotients <M>\mathcal{X}/\mathcal{N}</M> may be constructed.
A sub-crossed module  <M>\mathcal{S} = (\delta : N \to M)</M>
is <E>normal</E> in <M>\mathcal{X} = (\partial : S \to R)</M>  if
<List>
<Item>
<M>N,M</M> are normal subgroups of <M>S,R</M> respectively,
</Item>
<Item>
<M>\delta</M> is the restriction of <M>\partial</M>,
</Item>
<Item>
<M>n^r \in N</M>  for all  <M>n \in N,~r \in R</M>,
</Item>
<Item>
<M>s^{-1}s^m \in N</M>  for all  <M>m \in M,~s \in S</M>.
</Item>
</List>
These conditions ensure that  <M>M \ltimes N</M>  is normal in  
the semidirect product <M>R \ltimes S</M>.
</Description>
</ManSection>

</Section>


<Section><Heading>Pre-crossed modules</Heading>

<Index>pre-crossed module</Index>
<ManSection>
   <Oper Name="PreXModByBoundaryAndAction"
         Arg="bdy act" />
   <Oper Name="SubPreXMod"
         Arg="X0 src rng" />
<Description>
When axiom  {\bf XMod\ 2}  is <E>not</E> satisfied, 
the corresponding structure is known as a <E>pre-crossed module</E>.
</Description>
</ManSection>

<Example>
<![CDATA[
gap> c := (11,12,13,14,15,16,17,18);;  d := (12,18)(13,17)(14,16);;
gap> d16 := Group( c, d );;
gap> sk4 := Subgroup( d16, [ c^4, d ] );;
gap> SetName( d16, "d16" );  SetName( sk4, "sk4" );
gap> bdy16 := GroupHomomorphismByImages( d16, sk4, [c,d], [c^4,d] );;
gap> h1 := GroupHomomorphismByImages( d16, d16, [c,d], [c^5,d] );;
gap> h2 := GroupHomomorphismByImages( d16, d16, [c,d], [c,c^4*d] );;
gap> aut16 := Group( [ h1, h2 ] );;
gap> act16 := GroupHomomorphismByImages( sk4, aut16, [c^4,d], [h1,h2] );;
gap> P16 := PreXModByBoundaryAndAction( bdy16 );
[d16->sk4]
]]>
</Example>

<Index>Peiffer subgroup</Index>
<ManSection>
   <Attr Name="PeifferSubgroup"
         Arg="X0" />
   <Attr Name="XModByPeifferQuotient"
         Arg="prexmod" />
<Description>
The <E>Peiffer subgroup</E> of a pre-crossed module <M>P</M> of <M>S</M>
is the subgroup of  <M>{\rm ker}(\partial)</M>
generated by  <E>Peiffer commutators</E>
<Display>
\langle s_1,s_2 \rangle \quad=\quad 
(s_1^{-1})^{\partial s_2}~s_2^{-1}~s_1~s_2~.
</Display>
Then  <M>\mathcal{P} = (0 : P \to \{1_R\})</M>
is a normal sub-pre-crossed module of  <M>\mathcal{X}</M>
and  <M>\mathcal{X}/\mathcal{P} = (\partial : S/P \to R)</M>
is a crossed module.
<P/>
In the following example the Peiffer subgroup is cyclic of size <M>4</M>.
</Description>
</ManSection>

<Example>
<![CDATA[
gap> P := PeifferSubgroup( P16 );
Group( [ (11,15)(12,16)(13,17)(14,18), (11,17,15,13)(12,18,16,14) ] )
gap> X16 := XModByPeifferQuotient( P16 );
[d16/P->sk4]
gap> Display( X16 );
Crossed module [d16/P->sk4] :-
: Source group has generators:
  [ f1, f2 ]
: Range group has generators:
  [ (11,15)(12,16)(13,17)(14,18), (12,18)(13,17)(14,16) ]
: Boundary homomorphism maps source generators to:
  [ (12,18)(13,17)(14,16), (11,15)(12,16)(13,17)(14,18) ]
  The automorphism group is trivial
gap> iso16 := IsomorphismPermGroup( Source( X16 ) );;
gap> S16 := Image( iso16 );
Group([ (1,3)(2,4), (1,2)(3,4) ])   
]]>
</Example>

<ManSection>
   <Prop Name="IsPermXMod"
         Arg="X0" />
   <Prop Name="IsPcPreXMod"
         Arg="X0" />
<Description>
When both source and range groups are of the same type,
corresponding properties are assigned to the crossed module.
</Description>
</ManSection>
</Section>


<Section Label="sect-cat1">
<Heading>Cat1-groups and pre-cat1-groups</Heading>
<Index>cat1-group</Index>

<ManSection>
   <Attr Name="Source"
         Arg="C" />
   <Attr Name="Range"
         Arg="C" />
   <Attr Name="TailMap"
         Arg="C" />
   <Attr Name="HeadMap"
         Arg="C" />
   <Attr Name="RangeEmbedding"
         Arg="C" />
   <Attr Name="KernelEmbedding"
         Arg="C" />
   <Attr Name="Boundary"
         Arg="C" />
   <Attr Name="Name"
         Arg="C" />
   <Attr Name="Size"
         Arg="C" />
<Description> 
These are the attributes of a cat1-group <M>\mathcal{C}</M> 
in this implementation.
<P/>
In <Cite Key="L1" />, Loday reformulated the notion of a 
crossed module as a cat1-group, 
namely a group <M>G</M> with a pair of homomorphisms <M>t,h : G \to G</M>
having  a common image <M>R</M> and satisfying certain axioms.
We find it convenient to define a cat1-group 
<M>\mathcal{C} = (e;t,h : G \to R )</M>  as having source group  <M>G</M>,
range group <M>R</M>,  and three homomorphisms:  two surjections  
<M>t,h : G \to R</M>  and an embedding  <M>e : R \to G</M>  satisfying:
<Display>
{\bf Cat\ 1} ~:~ te = he = {\rm id}_R,
\qquad
{\bf Cat\ 2} ~:~ [\ker t, \ker h] = \{ 1_G \}.
</Display>
It follows that  
<M>teh = h</M>, <M>het = t</M>, <M>tet = t</M>, <M>heh = h</M>.
<P/>
The maps  <M>t,h</M>  are often referred to as the 
<E>source</E> and <E>target</E>,
but we choose to call them the
<E>tail</E> and <E>head</E> of  <M>\mathcal{C}</M>, 
because <E>source</E> is the &GAP; term for the domain of a function.
The <C>RangeEmbedding</C> is the embedding of <C>R</C> in <C>G</C>,
the <C>KernelEmbedding</C> is the inclusion of 
the kernel of <C>t</C> in <C>G</C>,
and the <C>Boundary</C> is the restriction of <C>h</C> 
to the kernel of <C>t</C>.
</Description>
</ManSection>

<ManSection>
   <Attr Name="Cat1"
         Arg="args" />
   <Attr Name="PreCat1ByTailHeadEmbedding"
         Arg="t h e" />
   <Attr Name="PreCat1ByEndomorphisms"
         Arg="t h" />
   <Attr Name="PreCat1ByNormalSubgroup"
         Arg="G N" />
   <Attr Name="Cat1ByPeifferQuotient"
         Arg="P" />
   <Attr Name="Reverse"
         Arg="C0" />
<Description>
These are some of the constructors for pre-cat1-groups and cat1-groups.
<P/>
The following listing shows an example of a cat1-group of pc-groups:
</Description>
</ManSection>

<Example>
<![CDATA[
gap> s3 := SymmetricGroup(IsPcGroup,3);;   
gap> gens3 := GeneratorsOfGroup(s3);
[ f1, f2 ]
gap> pc4 := CyclicGroup(4);; 
gap> SetName(s3,"s3");  SetName( pc4, "pc4" );
gap> s3c4 := DirectProduct( s3, pc4 );; 
gap> SetName( s3c4, "s3c4" );  
gap> gens3c4 := GeneratorsOfGroup( s3c4 );
[ f1, f2, f3, f4 ]
gap> a := gens3[1];;  b := gens3[2];;  one := One(s3);;
gap> t2 := GroupHomomorphismByImages( s3c4, s3, gens3c4, [a,b,one,one] );
[ f1, f2, f3, f4 ] -> [ f1, f2, &lt;identity&gt; of ..., &lt;identity&gt; of ... ]
gap> e2 := Embedding( s3c4, 1 );
[ f1, f2 ] -> [ f1, f2 ]
gap> C2 := Cat1( t2, t2, e2 );
[s3c4=>s3]
gap> Display( C2 );
Cat1-group [s3c4=>s3] :-
: source group has generators:
  [ f1, f2, f3, f4 ]
:  range group has generators:
  [ f1, f2 ]
: tail homomorphism maps source generators to:
  [ f1, f2, &lt;identity&gt; of ..., &lt;identity&gt; of ... ]
: head homomorphism maps source generators to:
  [ f1, f2, &lt;identity&gt; of ..., &lt;identity&gt; of ... ]
: range embedding maps range generators to:
  [ f1, f2 ]
: kernel has generators:
  [ f3, f4 ]
: boundary homomorphism maps generators of kernel to:
  [ &lt;identity&gt; of ..., &lt;identity&gt; of ... ]
: kernel embedding maps generators of kernel to:
  [ f3, f4 ]
gap> IsPcCat1( C2 );
true
gap> Size( C2 );
[ 24, 6 ]
]]>
</Example>

<ManSection>
   <Attr Name="Cat1OfXMod"
         Arg="X0" />
   <Attr Name="XModOfCat1"
         Arg="C0" />
   <Attr Name="PreCat1OfPreXMod"
         Arg="P0" />
   <Attr Name="PreXModOfPreCat1"
         Arg="P0" />
<Description>
The category of crossed modules is equivalent to the category of cat1-groups,
and the functors between these two categories may be described as follows.

Starting with the crossed module 
<M>\mathcal{X} = (\partial : S \to R)</M> the group <M>G</M> is defined 
as the semidirect product <M>G = R \ltimes S</M>
using the action from  <M>\mathcal{X}</M>,
with multiplication rule
<Display>
(r_1,s_1)(r_2,s_2) ~=~ (r_1r_2,{s_1}^{r_2}s_2).
</Display>
The structural morphisms are given by
<Display>
t(r,s) = r, \quad h(r,s) = r (\partial s), \quad er = (r,1).
</Display>
On the other hand, starting with a cat1-group 
<M> \mathcal{C} = (e;t,h : G \to R)</M>,  we define 
<M> S = \ker t</M>, the range <M>R</M> remains unchanged, and 
<M> \partial = h\!\mid_S </M>.
The action of  <M>R</M>  on  <M>S</M>  is conjugation in  <M>G</M>  via the embedding
of  <M>R</M>  in  <M>G</M>.
</Description>
</ManSection>


<Example>
<![CDATA[
gap> SetName( Kernel(t2), "ker(t2)" );;
gap> X2 := XModOfCat1( C2 );
[Group( [ f3, f4 ] )->s3]
gap> Display( X2 );
Crossed module [ker(t2)->s3] :-
: Source group has generators:
  [ f3, f4 ]
: Range group s3 has generators:
  [ f1, f2 ]
: Boundary homomorphism maps source generators to:
  [ &lt;identity&gt; of ..., &lt;identity&gt; of ... ]
  The automorphism group is trivial
: associated cat1-group is [s3c4=>s3]
]]>
</Example>
</Section>


<Section><Heading>Selection of a small cat1-group</Heading>
<Index>selection of a small cat1-group</Index>

The <C>Cat1</C> function may also be used to select a cat1-group 
from a data file.
All cat1-structures on groups of size up to <M>47</M> are stored in a list
in file <F>cat1data.g</F>.
Global variables <C>CAT1&uscore;LIST&uscore;MAX&uscore;SIZE := 47</C> and
<C>CAT1&uscore;LIST&uscore;CLASS&uscore;SIZES</C> are also stored.
The &XMod;~2 version of the database orders the groups of size up to <M>47</M>
according to the &GAP;~4 numbering of small groups.
The data is read into the list <C>CAT1&uscore;LIST</C> 
only when this function is called.

<ManSection>
   <Attr Name="Cat1Select"
         Arg="size gpnum num" />
<Description>
This function may be used in three ways. 
<C>Cat1Select( size )</C> returns the names of the groups with this size. 
<C>Cat1Select( size, gpnum )</C> prints a list of cat1-structures 
for this chosen group. 
<C>Cat1Select( size, gpnum, num )</C> 
(or just <C>Cat1( size, gpnum, num )</C>) 
returns the chosen cat1-group.
<P/>
The example below is the first case in which <M>t \neq h</M>
and the associated conjugation crossed module
is given by the normal subgroup <C>c3</C> of <C>s3</C>.
</Description>
</ManSection>
<Example>
<![CDATA[
gap> L18 := Cat1Select( 18 );
#I  Loading cat1-group data into CAT1_LIST
Usage:  Cat1( size, gpnum, num )
[ "d18", "c18", "s3c3", "c3^2|Xc2", "c6c3" ]
gap> Cat1Select( 18, 4 );
There are 4 cat1-structures for the group c3^2|Xc2.
[ [range gens], source & range names, [tail genimages], [head genimages] ] :-
[ [ (1,2,3), (4,5,6), (2,3)(5,6) ],  tail = head = identity mapping ]
[ [ (2,3)(5,6) ], "c3^2", "c2", [ (), (), (2,3)(5,6) ],
  [ (), (), (2,3)(5,6) ] ]
[ [ (4,5,6), (2,3)(5,6) ], "c3", "s3", [ (), (4,5,6), (2,3)(5,6) ],
  [ (), (4,5,6), (2,3)(5,6) ] ]
[ [ (4,5,6), (2,3)(5,6) ], "c3", "s3", [ (4,5,6), (4,5,6), (2,3)(5,6) ],
  [ (), (4,5,6), (2,3)(5,6) ] ]
Usage:  Cat1( size, gpnum, num );
Group has generators [ (1,2,3), (4,5,6), (2,3)(5,6) ]
4
gap> C4 := Cat1( 18, 4, 4 );
[c3^2|Xc2=>s3]
gap> Display( C4 );
Cat1-group [c3^2|Xc2=>s3] :-
: source group has generators:
  [ (1,2,3), (4,5,6), (2,3)(5,6) ]
:  range group has generators:
  [ (4,5,6), (4,5,6), (2,3)(5,6) ]
: tail homomorphism maps source generators to:
  [ (4,5,6), (4,5,6), (2,3)(5,6) ]
: head homomorphism maps source generators to:
  [ (), (4,5,6), (2,3)(5,6) ]
: range embedding maps range generators to:
  [ (4,5,6), (4,5,6), (2,3)(5,6) ]
: kernel has generators:
  [ ( 1, 2, 3)( 4, 6, 5) ]
: boundary homomorphism maps generators of kernel to:
  [ (4,6,5) ]
: kernel embedding maps generators of kernel to:
  [ (1,2,3)(4,6,5) ]
gap> XC4 := XModOfCat1( C4 );
[Group( [ ( 1, 2, 3)( 4, 6, 5) ] )->s3]
]]>
</Example>

</Section>

</Chapter>
