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<!--  gp3obj.xml            XMOD documentation            Chris Wensley   -->
<!--                                                        & Murat Alp   -->
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<?xml version="1.0" encoding="ISO-8859-15"?>
  <!-- $Id: gp3obj.xml,v 2.11  Exp $ -->

<Chapter Label="chap-obj3">

<Heading>Crossed squares and their morphisms</Heading>

<Index>crossed module</Index>
<Index>cat2-group</Index>
<Index>3d-object</Index>

Crossed squares were introduced by Guin-Wal\'ery and Loday 
(see, for example, <Cite Key="brow:lod"/>)
as fundamental crossed squares of commutative squares of spaces,
but are also of purely algebraic interest.
We denote by <M>[n]</M> the set <M>\{1,2,\ldots,n\}</M>.
We use the <M>n=2</M> version of the definition of crossed <M>n</M>-cube
as given by Ellis and Steiner <Cite Key="ell:st"/>.
<P/>
A <E>crossed square</E> <M>\mathcal{R}</M> consists of the following:
<List>
<Item>
Groups <M>R_J</M> for each of the four subsets <M>J \subseteq [2]</M>;
</Item>
<Item>
a commutative diagram of group homomorphisms:
  <Display>
  \ddot{\partial}_1 : R_{[2]} \to R_{\{2\}}, \quad 
  \ddot{\partial}_2 : R_{[2]} \to R_{\{1\}}, \quad 
  \dot{\partial}_1 : R_{\{1\}} \to R_{\emptyset}, \quad 
  \dot{\partial}_2 : R_{\{2\}} \to R_{\emptyset}; 
  </Display>
</Item>
<Item>
actions of <M>R_{\emptyset}</M> on <M>R_{\{1\}}, R_{\{2\}}</M> 
and <M>R_{[2]}</M> which determine actions of 
<M>R_{\{1\}}</M> on <M>R_{\{2\}}</M> and <M>R_{[2]}</M> 
via <M>\dot{\partial}_1</M> 
and actions of 
<M>R_{\{2\}}</M> on <M>R_{\{1\}}</M> and <M>R_{[2]}</M> 
via <M>\dot{\partial}_2</M>~;
</Item>
<Item>
a function ~ <M>\boxtimes : R_{\{1\}} \times R_{\{2\}} \to R_{[2]}</M>~.
</Item>
</List>
The following axioms must be satisfied for all 
<M>l \in R_{[2]},\; m,m_1,m_2 \in R_{\{1\}},\; 
n,n_1,n_2 \in R_{\{2\}},\; p \in R_{\emptyset}</M>~:
<List>
<Item>
the homomorphisms <M>\ddot{\partial}_1, \ddot{\partial}_2</M> 
preserve the action of <M>R_{\emptyset}</M>~;
</Item>
<Item>
each of 
<Display>
 \ddot{\mathcal{R}}_1 = (\ddot{\partial}_1 : R_{[2]} \to R_{\{2\}}),~
 \ddot{\mathcal{R}}_2 = (\ddot{\partial}_2 : R_{[2]} \to R_{\{1\}}), ~
  \dot{\mathcal{R}}_1  = (\dot{\partial}_1 : R_{\{1\}} \to R_{\emptyset}),~
  \dot{\mathcal{R}}_2  = (\dot{\partial}_2 : R_{\{2\}} \to R_{\emptyset}),
</Display>
and the diagonal  
<Display>
\mathcal{R}_{12} = (\partial_{12} := \dot{\partial}_1\ddot{\partial}_2 
                  = \dot{\partial}_2\ddot{\partial}_1 
                  : R_{[2]} \to R_{\emptyset})
</Display>
are crossed modules (with actions via <M>R_{\emptyset}</M>);
</Item>
<Item>
<Index>crossed pairing</Index>
<M>\boxtimes</M> is a \emph{crossed pairing}:
  <List>
  <Item>
  <M>(m_1m_2 \boxtimes n)\;=\;{(m_1 \boxtimes n)}^{m_2}\;(m_2 \boxtimes n)</M>~,
  </Item>
  <Item>
  <M>(m \boxtimes n_1n_2) \;=\; (m \boxtimes n_2)\;{(m \boxtimes n_1)}^{n_2}</M>~,
  </Item>
  <Item> 
  <M>(m \boxtimes n)^{p} \;=\; (m^p \boxtimes n^p)</M>~;
  </Item>
  </List>
</Item>
<Item>
<M>\ddot{\partial}_1 (m \boxtimes n) \;=\; (n^{-1})^{m}\;n</M>
\quad \mbox{and} \quad
<M>\ddot{\partial}_2 (m \boxtimes n) \;=\; m^{-1}\;m^{n}</M>~,
</Item>
<Item>
<M>(m \boxtimes \ddot{\partial}_1 l) \;=\; (l^{-1})^{m}\;l</M>
\quad \mbox{and} \quad
<M>(\ddot{\partial}_2 l \boxtimes n) \;=\; l^{-1}\;l^n</M>~.
</Item>
</List>
Note that the actions of  <M>R_{\{1\}}</M>  on  <M>R_{\{2\}}</M>  
and  <M>R_{\{2\}}</M>  on  <M>R_{\{1\}}</M>  via  <M>R_{\emptyset}</M>
are compatible since
<Display>
{m_1}^{(n^m)} \;=\; {m_1}^{\dot{\partial}_2(n^m)} 
              \;=\; {m_1}^{m^{-1}(\dot{\partial}_2 n)m}
\;=\; (({m_1}^{m^{-1}})^n)^m~.
</Display>




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

Analogously to the data structure used for crossed modules, 
crossed squares are implemented as <C>3d-objects</C>. 
When times allows, cat2-groups will also be implemented, 
with conversion between the two types of structure.

Some standard constructions of crossed squares are listed below. 
At present, a limited number of constructions are implemented.
Morphisms of crossed squares have also been implemented, 
though there is a lot still to do.

<ManSection>
   <Func Name="XSq"
         Arg="args" />
   <Oper Name="XSqByNormalSubgroups"
         Arg="P N M L" />
   <Oper Name="ActorXSq" 
         Arg="X0" />
   <Attr Name="Transpose3dObject" 
         Arg="S0" />
   <Attr Name="Name" 
         Arg="S0" />
<!-- %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% -->
<!--   <Oper Name="XSqByAutomorphismGroup" -->
<!--         Arg="grp" />                  -->
<!--   <Oper Name="XSqByXMods"             -->
<!--         Arg="X1 X2 X3 X4 action pairing" /> -->
<!--   <Oper Name="AsXSq"                        -->
<!--         Arg="obj" />                        -->
<Description>
Here are some standard examples of crossed squares.
<List>
<Item>
If <M>M, N</M> are normal subgroups of a group <M>P</M>, 
and <M>L = M \cap N</M>, then the four inclusions, 
<M>L \to N,~ L \to M,~ M \to P,~ N \to P</M>, 
together with the actions of <M>P</M> on <M>M, N</M> 
and <M>L</M> given by conjugation, and the crossed pairing  
<Display>
\boxtimes \;:\; M \times N \to M\cap N, \quad 
(m,n) \mapsto [m,n] \,=\, m^{-1}n^{-1}mn \,=\,(n^{-1})^mn \,=\, m^{-1}m^n
</Display>
is a crossed square. 
This construction is implemented as <C>XSqByNormalSubgroups(P,N,M,L);</C>.
</Item>
<Item>
The actor <M>\mathcal{A}(\mathcal{X}_0)</M> of a crossed module 
<M>\mathcal{X}_0</M> has been described in Chapter 5.
The crossed pairing is given by 
<Display>
\boxtimes \;:\;  R \times W \,\to\, S, \quad (r,\chi) \,\mapsto\, \chi r~.
</Display>
This is implemented as <C>ActorXSq( X0 );</C>.
</Item>
<Item>
The <E>transpose</E> of <M>\mathcal{R}</M> is the crossed square  
<M>\tilde{\mathcal{R}}</M> obtained by interchanging 
<M>R_{\{1\}}</M> with <M>R_{\{2\}}</M>, 
<M>\ddot{\partial}_1</M> with <M>\ddot{\partial}_2</M>, and 
<M>\dot{\partial}_1</M> with <M>\dot{\partial}_2</M>.  
The crossed pairing is given by 
<Display>
\tilde{\boxtimes} \;:\; R_{\{2\}} \times R_{\{1\}} \to R_{[2]}, \quad 
(n,m) \;\mapsto\; n\,\tilde{\boxtimes}\,m := (m \boxtimes n)^{-1}~.
</Display>
</Item>
</List>
The following constructions will be implemented in the next release.
<List>
<Item>
If <M>M, N</M> are ordinary <M>P</M>-modules 
and <M>A</M> is an arbitrary abelian group on which <M>P</M> acts trivially, 
then there is a crossed square with sides 
<Display>
0 : A \to N,\quad 0 : A \to M,\quad
0 : M \to P,\quad 0 : N \to P. 
</Display>
</Item>
<Item>
For a group <M>L</M>, the automorphism crossed module 
<M>{\rm Act}\ L = (\iota : L \to {\rm Aut}\ L)</M> 
splits to form the square 
with <M>(\iota_1 : L \to {\rm Inn}\ L)</M> on two sides, 
and <M>(\iota_2 : {\rm Inn}\ L \to {\rm Aut}\ L)</M> on the other two sides, 
where <M>\iota_1</M> maps <M>l \in L</M> to the inner automorphism  
<M>\beta_l : L \to L,\; l^{\prime} \mapsto l^{-1}l^{\prime}l</M>,  
and $\iota_2$ is the inclusion of <M>{\rm Inn}\ L</M> in  <M>{\rm Aut}\ L</M>. 
The actions are standard, and the crossed pairing is 
<Display>
\boxtimes \;:\; {\rm Inn}\ L \times {\rm Inn}\ L \to L, \quad
(\beta_l, \beta_{l^{\prime}}) \;\mapsto\; [l, l^{\prime}]~.
</Display>
</Item>
</List>
</Description>
</ManSection>

<Example>
<![CDATA[
gap> c := (11,12,13,14,15,16);;
gap> d := (12,16)(13,15);;
gap> cd := c*d;;
gap> d12 := Group( [ c, d ] );;
gap> s3a := Subgroup( d12, [ c^2, d ] );;
gap> s3b := Subgroup( d12, [ c^2, cd ] );;
gap> c3 := Subgroup( d12, [ c^2 ] );;
gap> SetName( d12, "d12");  SetName( s3a, "s3a" );
gap> SetName( s3b, "s3b" );  SetName( c3, "c3" );
gap> XSconj := XSqByNormalSubgroups( d12, s3b, s3a, c3 );
[  c3 -> s3b ]
[  |      |  ]
[ s3a -> d12 ]
gap> Name( XSconj );
"[c3->s3b,s3a->d12]"
gap> XStrans := Transpose3dObject( XSconj );
[  c3 -> s3a ]
[  |      |  ]
[ s3b -> d12 ]
gap> X12 := XModByNormalSubgroup( d12, s3a );
[s3a->d12]
gap> XSact := ActorXSq( X12 );
crossed square with:
      up = Whitehead[s3a->d12]
    left = [s3a->d12]
   right = Actor[s3a->d12]
    down = Norrie[s3a->d12]
]]>
</Example>

<ManSection>
   <Prop Name="IsXSq"
         Arg="obj" />
<!--   <Prop Name="IsCat2"   -->
<!--         Arg="obj" />    -->
   <Prop Name="Is3dObject"
         Arg="obj" />
   <Prop Name="IsPerm3dObject"
         Arg="obj" />
   <Prop Name="IsPc3dObject"
         Arg="obj" />
   <Prop Name="IsFp3dObject"
         Arg="obj" />
   <Prop Name="IsPreXSq"
         Arg="obj" />
<Description>
These are the basic properties for 3dobjects, 
and crossed squares in particular.
</Description>
</ManSection>

<ManSection>
   <Attr Name="Up2dObject"
         Arg="XS" />
   <Attr Name="Left2dObject"
         Arg="XS" />
   <Attr Name="Down2dObject"
         Arg="XS" />
   <Attr Name="Right2dObject"
         Arg="XS" />
   <Attr Name="DiagonalAction"
         Arg="XS" />
   <Attr Name="XPair"
         Arg="XS" />
   <Oper Name="ImageElmXPair"
         Arg="XS pair" />
<Description>
In this implementation the attributes used in the construction of
a crossed square <C>XS</C> are the four crossed modules (2d-objects) 
on the sides of the square; 
the diagonal action of <M>P</M> on <M>L</M>,
and the crossed pairing.
<P/>
The &GAP; development team have suggested that crossed pairings should be 
implemented as a special case of <C>BinaryMappings</C> -- a structure 
which does not yet exist in &GAP;.
As a temporary measure, crossed pairings have been implemented using 
<C>Mapping2ArgumentsByFunction</C>.
</Description>
</ManSection>
<P/>
<Example>
<![CDATA[
gap> Up2dObject( XSconj );
[c3->s3b]
gap> Right2dObject( XSact );
Actor[s3a->d12]
gap> xpconj := XPair( XSconj );;
gap> ImageElmXPair( xpconj, [ (1,6)(2,5)(3,4), (2,6)(3,5) ] );
(1,3,5)(2,4,6)
gap> diag := DiagonalAction( XSact );
[ (2,3)(6,8)(7,9), (1,2)(4,6)(5,7) ] ->
[ [ (11,13,15)(12,14,16), (12,16)(13,15) ] ->
    [ (11,15,13)(12,16,14), (12,16)(13,15) ],
  [ (11,13,15)(12,14,16), (12,16)(13,15) ] ->
    [ (11,15,13)(12,16,14), (11,13)(14,16) ] ]
]]>
</Example>

</Section>



<Section><Heading>Morphisms of crossed squares</Heading>

<Index>morphism of 3d-object</Index>
<Index>crossed square morphism</Index>
<Index>3d-mapping</Index>
This section describes an initial implementation of morphisms of 
(pre-)crossed squares.

<ManSection>
   <Attr Name="Source"
         Arg="map" />
   <Attr Name="Range"
         Arg="map" />
   <Attr Name="Up2dMorphism"
         Arg="map" />
   <Attr Name="Left2dMorphism"
         Arg="map" />
   <Attr Name="Down2dMorphism"
         Arg="map" />
   <Attr Name="Right2dMorphism"
         Arg="map" />
<Description>
Morphisms of <C>3dObjects</C> are implemented as <C>3dMappings</C>.
These have a pair of 3d-objects as source and range, together with four 
2d-morphisms mapping between the four pairs of crossed modules  
on the four sides of the squares.
These functions return <C>fail</C> when invalid data is supplied.
</Description>
</ManSection>


<ManSection>
   <Prop Name="IsXSqMorphism"
         Arg="map" />
<!--   <Prop Name="IsCat2Morphism"    -->
<!--         Arg="map" />             -->
   <Prop Name="IsPreXSqMorphism"
         Arg="map" />
<!--   <Prop Name="IsPreCat2Morphism" -->
<!--         Arg="map" />             -->
   <Prop Name="IsBijective"
         Arg="mor" />
   <Prop Name="IsAutomorphism3dObject"
         Arg="mor" />
<Description>
A morphism <C>mor</C> between two pre-crossed squares 
<M>\mathcal{R}_{1}</M> and  <M>\mathcal{R}_{2}</M> 
consists of four crossed module morphisms  
<C>Up2dMorphism( mor )</C>, 
mapping the <C>Up2dObject</C> of <M>\mathcal{R}_1</M> 
to that of <M>\mathcal{R}_2</M>, 
<C>Left2dMorphism( mor )</C>, <C>Down2dMorphism( mor )</C> 
and <C>Right2dMorphism( mor )</C>. 
These four morphisms are required to commute with the four boundary maps
and to preserve the rest of the structure. 
The current version of <C>IsXSqMorphism</C> does not perform all the 
required checks.
</Description>
</ManSection>

<Example>
<![CDATA[
gap> ad12 := GroupHomomorphismByImages( d12, d12, [c,d], [c,d^c] );;
gap> as3a := GroupHomomorphismByImages( s3a, s3a, [c^2,d], [c^2,d^c] );;
gap> as3b := GroupHomomorphismByImages( s3b, s3b, [c^2,cd], [c^2,cd^c] );;
gap> idc3 := IdentityMapping( c3 );;
gap> upconj := Up2dObject( XSconj );;
gap> leftconj := Left2dObject( XSconj );; 
gap> downconj := Down2dObject( XSconj );; 
gap> rightconj := Right2dObject( XSconj );; 
gap> up := XModMorphismByHoms( upconj, upconj, idc3, as3b );
[[c3->s3b] => [c3->s3b]]
gap> left := XModMorphismByHoms( leftconj, leftconj, idc3, as3a );
[[c3->s3a] => [c3->s3a]]
gap> down := XModMorphismByHoms( downconj, downconj, as3a, ad12 );
[[s3a->d12] => [s3a->d12]]
gap> right := XModMorphismByHoms( rightconj, rightconj, as3b, ad12 );
[[s3b->d12] => [s3b->d12]]
gap> autoconj := XSqMorphism( XSconj, XSconj, up, left, down, right );; 
gap> ord := Order( autoconj );;
gap> Display( autoconj );
Morphism of crossed squares :-
:    Source = [c3->s3b,s3a->d12]
:     Range = [c3->s3b,s3a->d12]
:     order = 3
:    up-left: [ [ (11,13,15)(12,14,16) ], [ (11,13,15)(12,14,16) ] ]
:   up-right: [ [ (11,13,15)(12,14,16), (11,16)(12,15)(13,14) ],
  [ (11,13,15)(12,14,16), (11,12)(13,16)(14,15) ] ]
:  down-left: [ [ (11,13,15)(12,14,16), (12,16)(13,15) ],
  [ (11,13,15)(12,14,16), (11,13)(14,16) ] ]
: down-right: [ [ (11,12,13,14,15,16), (12,16)(13,15) ],
  [ (11,12,13,14,15,16), (11,13)(14,16) ] ]
gap> KnownPropertiesOfObject( autoconj );
[ "IsTotal", "IsSingleValued", "IsInjective", "IsSurjective", "Is3dMapping",
  "IsPreXSqMorphism", "IsXSqMorphism", "IsEndomorphism3dObject" ]
gap> IsAutomorphism3dObject( autoconj );
true
]]>
</Example>


</Section>


</Chapter>