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<?xml version="1.0" encoding="UTF-8"?>

<Section Label="sect-cat2-definitions">
<Heading>Definitions and constructions for cat<M>^2</M>-groups and their morphisms
</Heading>

<Index>cat<M>^2</M>-group</Index>

We give here three equivalent definitions of cat<M>^2</M>-groups.
When we come to define cat<M>^n</M>-groups we shall give a similar set of
definitions.

Firstly, we take the definition of a cat<M>^2</M>-group from
Section 5 of Brown and Loday <Cite Key="brow:lod"/>, suitably modified.
A cat<M>^2</M>-group <M>\calC = (C_{[2]},C_{\{2\}},C_{\{1\}},C_{\emptyset})</M>
comprises four groups (one for each of the subsets of <M>[2]</M>)
and <M>15</M> homomorphisms, as shown in the following diagram:
<Display>
<![CDATA[
\vcenter{\xymatrix{
& C_{[2]} \ar[ddd] <-1.2ex> \ar[ddd] <-2.0ex>_{\ddot{t}_2,\ddot{h}_2}
 \ar[rrr] <+1.2ex> \ar[rrr] <+2.0ex>^{\ddot{t}_1,\ddot{h}_1}
 \ar[dddrrr] <-0.2ex> \ar[dddrrr] <-1.0ex>_(0.55){t_{[2]},h_{[2]}}
 &&& C_{\{2\}} \ar[lll]^{\ddot{e}_1}
 \ar[ddd]<+1.2ex> \ar[ddd] <+2.0ex>^{\dot{t}_2,\dot{h}_2} \\
\calC \quad = \quad
& &&& \\
& &&& \\
& C_{\{1\}} \ar[uuu]_{\ddot{e}_2}
 \ar[rrr] <-1.2ex> \ar[rrr] <-2.0ex>_{\dot{t}_1,\dot{h}_1}
 &&& C_{\emptyset} \ar[uuu]^{\dot{e}_2} \ar[lll]_{\dot{e}_1}
 \ar[uuulll] <-1.0ex>_{e_{[2]}}
\\
}}
]]></Display>
The following axioms are satisfied by these homomorphisms:
<List>
<Item>
the four sides of the square (up, left, right, down) are cat<M>^1</M>-groups,
denoted <M>\ddot{\calC}_1, \ddot{\calC}_2, \dot{\calC}_1, \dot{\calC}_2</M>;
</Item>
<Item>
<M>
\dot{t}_1\circ\ddot{h}_2 = \dot{h}_2\circ\ddot{t}_1, ~
\dot{t}_2\circ\ddot{h}_1 = \dot{h}_1\circ\ddot{t}_2, ~
\dot{e}_1\circ\dot{t}_2 = \ddot{t}_2\circ\ddot{e}_1, ~
\dot{e}_2\circ\dot{t}_1 = \ddot{t}_1\circ\ddot{e}_2, ~
\dot{e}_1\circ\dot{h}_2 = \ddot{h}_2\circ\ddot{e}_1, ~
\dot{e}_2\circ\dot{h}_1 = \ddot{h}_1\circ\ddot{e}_2;
</M>
</Item>
<Item>
<M>
\dot{t}_1\circ\ddot{t}_2 = \dot{t}_2\circ\ddot{t}_1 = t_{[2]}, ~
\dot{h}_1\circ\ddot{h}_2 = \dot{h}_2\circ\ddot{h}_1 = h_{[2]}, ~
\dot{e}_1\circ\ddot{e}_2 = \dot{e}_2\circ\ddot{e}_1 = e_{[2]},
</M>
making the diagonal a pre-cat<M>^1</M>-group
<M>(e_{[2]}; t_{[2]}, h_{[2]} : C_{[2]} \to C_{\emptyset})</M>.
</Item>
</List>
It follows from these identities that
<M>(\ddot{t}_1,\dot{t}_1),\,(\ddot{h}_1,\dot{h}_1)</M>
and <M>(\ddot{e}_1,\dot{e}_1)</M> are morphisms of cat<M>^1</M>-groups,
and similarly in the vertical direction.

Secondly, we give the simplest of the three definitions,
adapted from Ellis-Steiner <Cite Key="ell:st"/>.
A cat<M>^2</M>-group <M>\calC</M> consists of groups <M>G, R_1,R_2</M>
and six homomorphisms
<M>t_1,h_1 : G \to R_2,~ e_1 : R_2 \to G,~
t_2,h_2 : G \to R_1,~ e_2 : R_1 \to G</M>,
satisfying the following axioms for all <M>1 \leqslant i \leqslant 2</M>,
<List>
<Item>
<M>
(t_i \circ e_i)r = r,~ (h_i \circ e_i)r = r,~
\forall r \in R_{[2] \setminus \{i\}}, \quad
[\ker t_i, \ker h_i] = 1,
</M>
</Item>
<Item>
<M>
(e_1 \circ t_1) \circ (e_2 \circ t_2) = (e_2 \circ t_2) \circ (e_1 \circ t_1), \quad
(e_1 \circ h_1) \circ (e_2 \circ h_2) = (e_2 \circ h_2) \circ (e_1 \circ h_1),
</M>
</Item>
<Item>
<M>
(e_1 \circ t_1) \circ (e_2 \circ h_2) = (e_2 \circ h_2) \circ (e_1 \circ t_1), \quad
(e_2 \circ t_2) \circ (e_1 \circ h_1) = (e_1 \circ h_1) \circ (e_2 \circ t_2).
</M>
</Item>
</List>

Our third definition defines a cat<M>^2</M>-group as a
"cat<M>^1</M>-group of cat<M>^1</M>-groups".
A cat<M>^2</M>-group <M>\calC</M> consists of two cat<M>^1</M>-groups
<M>\calC_1 = (e_1;t_1,h_1 : G_1 \to R_1)</M> and
<M>\calC_2 = (e_2;t_2,h_2 : G_2 \to R_2)</M> and cat<M>^1</M>-morphisms
<M>t = (\ddot{t},\dot{t}),\; h = (\ddot{h},\dot{h}) :
\calC_1 \to \calC_2,\;
e = (\ddot{e},\dot{e}) : \calC_2 \to \calC_1</M>,
subject to the following conditions:
<Display>
(t \circ e) ~\mbox{and}~ (h \circ e)
~\mbox{are the identity mapping on}~ \calC_2,
\qquad
[\ker t, \ker h] = \{ 1_{\calC_1} \},
</Display>
where <M>\ker t = (\ker \ddot{t},\ \ker \dot{t})</M>,
and similarly for <M>\ker h</M>.

In a recent paper
<E>Computing 3-Dimensional Groups : Crossed Squares and Cat2-Groups</E>,
by Arvasi, Odabas and Wensley <Cite Key="AOW"/>,
there are tables listing the numbers of isomorphism classes of
cat<M>^2</M>-groups on groups of order at most <M>30</M> –
a total of <M>1007</M> cat<M>^2</M>-groups.

As with cat<M>^1</M>-groups, there is no requirement for the various
tail and head maps to be endomorphisms, but most of the cat<M>^2</M>-groups
constructed by this package are of this type, so that groups
<M>C_{\{2\}},C_{\{1\}}</M> and <M>C_{\emptyset}</M>
are all subgroups of <M>C_{[2]}</M>.

<ManSection Label="cat2-group">
 <Func Name="Cat2Group"
 Arg="args" />
 <Func Name="PreCat2Group"
 Arg="args" />
 <Prop Name="IsCat2Group"
 Arg="C" />
 <Oper Name="PreCat2GroupByPreCat1Groups"
 Arg="L" />
<Description>
Following the second definition above, the global functions <C>Cat2Group</C>
and <C>PreCat2Group</C> are normally called with two arguments -
the generating up and left cat<M>^1</M>-groups <M>\calC_1</M>
and <M>\calC_2</M>.
In this case the groups <M>C_{[2]}, C_{\{2\}}, C_{\{1\}}</M>
are the common source and the ranges of <M>\calC_1,\calC_2</M>,
while <M>C_{\emptyset}</M> is the range of
<M>(e_1 \circ t_1) \circ (e_2 \circ t_2) = (e_2 \circ t_2) \circ (e_1 \circ t_1)</M>.

Alternatively, they may be called with a single argument which is a
crossed square.

The operation <C>PreCat2GroupByPreCat1Groups</C> has five arguments -
the up, left, right, down and diagonal cat<M>^1</M>-groups.

The two cat<M>^2</M>-groups <C>C2a, C2b</C> constructed in the
following example are isomorphic.
They differ in the down-right group <C>P</C>.
</Description>
</ManSection>

<Example>
<![CDATA[
gap> a := (1,2,3,4,5,6);; b := (2,6)(3,5);;
gap> G := Group( a, b );; SetName( G, "d12" );
gap> t1 := GroupHomomorphismByImages( G, G, [a,b], [a^3,b] );;
gap> up := PreCat1GroupByEndomorphisms( t1, t1 );;
gap> t2 := GroupHomomorphismByImages( G, G, [a,b], [a^4,b] );;
gap> left := PreCat1GroupByEndomorphisms( t2, t2 );;
gap> C2a := Cat2Group( up, left );
(pre-)cat2-group with generating (pre-)cat1-groups:
1 : [d12 => Group( [ (1,4)(2,5)(3,6), (2,6)(3,5) ] )]
2 : [d12 => Group( [ (1,5,3)(2,6,4), (2,6)(3,5) ] )]
gap> IsCat2Group( C2a );
true
gap> genR := [ (1,4)(2,5)(3,6), (2,6)(3,5) ];;
gap> R := Subgroup( G, genR );;
gap> genQ := [ (1,3,5)(2,4,6), (2,6)(3,5) ];;
gap> Q := Subgroup( G, genQ );;
gap> Pa := Group( b );; SetName( Pa, "c2a" );
gap> Pb := Group( (7,8) );; SetName( Pb, "c2b" );
gap> t3 := GroupHomomorphismByImages( R, P, genR, [(),(7,8)] );;
gap> e3 := GroupHomomorphismByImages( P, R, [(7,8)], [(2,6)(3,5)] );;
gap> right := PreCat1GroupByTailHeadEmbedding( t3, t3, e3 );;
gap> t4 := GroupHomomorphismByImages( Q, P, genQ, [(),(7,8)] );;
gap> e4 := GroupHomomorphismByImages( P, Q, [(7,8)], [(2,6)(3,5)] );;
gap> down := PreCat1GroupByTailHeadEmbedding( t4, t4, e4 );;
gap> t0 := t1 * t3;;
gap> e0 := GroupHomomorphismByImages( P, G, [(7,8)], [(2,6)(3,5)] );;
gap> diag := PreCat1GroupByTailHeadEmbedding( t0, t0, e0 );;
gap> C2b := PreCat2GroupByPreCat1Groups( up, left, right, down, diag );
(pre-)cat2-group with generating (pre-)cat1-groups:
1 : [d12 => Group( [ (1,4)(2,5)(3,6), (2,6)(3,5) ] )]
2 : [d12 => Group( [ (1,5,3)(2,6,4), (2,6)(3,5) ] )]
gap> C2a = C2b;
false
gap> GroupsOfHigherDimensionalGroup( C2a )[4];
Group([ (), (2,6)(3,5) ])
gap> GroupsOfHigherDimensionalGroup( C2b )[4];
Group([ (7,8) ])
]]>
</Example>

<ManSection>
 <Attr Name="Up2DimensionalGroup"
 Arg="C2G" Label="for cat2-groups" />
 <Attr Name="Left2DimensionalGroup"
 Arg="C2G" Label="for cat2-groups" />
 <Attr Name="Down2DimensionalGroup"
 Arg="C2G" Label="for cat2-groups" />
 <Attr Name="Right2DimensionalGroup"
 Arg="C2G" Label="for cat2-groups" />
 <Attr Name="Diagonal2DimensionalGroup"
 Arg="C2G" Label="for cat2-groups" />
<Description>
These six attributes were defined for crossed squares
but apply equally to cat<M>^2</M>-groups.
</Description>
</ManSection>

<Example>
<![CDATA[
gap> Diagonal2DimensionalGroup( C2a );
[d12 => Group( [ (), (2,6)(3,5) ] )]
gap> Diagonal2DimensionalGroup( C2b );
[d12 => Group( [ (7,8) ] )]
]]>
</Example>

<ManSection>
 <Oper Name="DirectProduct" Arg="C2a,C2b" />
<Description>
The direct product <M>\calC_{1} \times \calC_{2}</M>
has as its four up, left, right and down cat<M>^1</M>-groups
the direct products of those in <M>\calC_{1}</M> and <M>\calC_{2}</M>.
The embeddings and projections are constructed automatically,
and placed in the <C>DirectProductInfo</C> attribute,
together with the two <E>objects</E> <M>\calC_{1}</M> and <M>\calC_{2}</M>.
</Description>
</ManSection>

<Example>
<![CDATA[
gap> C2ab := DirectProductOp( [ C2a, C2b ], C2a );
(pre-)cat2-group with generating (pre-)cat1-groups:
1 : [Group( [ (1,2,3,4,5,6), (2,6)(3,5), ( 7, 8, 9,10,11,12), ( 8,12)( 9,11)
] ) => Group( [ (1,4)(2,5)(3,6), (2,6)(3,5), ( 7,10)( 8,11)( 9,12),
 ( 8,12)( 9,11) ] )]
2 : [Group( [ (1,2,3,4,5,6), (2,6)(3,5), ( 7, 8, 9,10,11,12), ( 8,12)( 9,11)
] ) => Group( [ (1,5,3)(2,6,4), (2,6)(3,5), ( 7, 9,11)( 8,10,12),
 ( 8,12)( 9,11) ] )]
gap> StructureDescription( C2ab );
[ "C2 x C2 x S3 x S3", "C2 x C2 x C2 x C2", "S3 x S3", "C2 x C2" ]
gap> SetName( C2ab, "C2ab" );
gap> Embedding( C2ab, 1 );
<mapping: (pre-)cat2-group with generating (pre-)cat1-groups:
1 : [d12 => Group( [ (1,4)(2,5)(3,6), (2,6)(3,5) ] )]
2 : [d12 => Group( [ (1,5,3)(2,6,4), (2,6)(3,5) ] )] -> C2ab >
gap> Projection( C2ab, 2 );
<mapping: C2ab -> (pre-)cat2-group with generating (pre-)cat1-groups:
1 : [d12 => Group( [ (1,4)(2,5)(3,6), (2,6)(3,5) ] )]
2 : [d12 => Group( [ (1,5,3)(2,6,4), (2,6)(3,5) ] )] >
]]>
</Example>

<ManSection>
 <Oper Name="DisplayLeadMaps" Arg="C0" />
<Description>
This operation provides an alternative to <C>Display</C>
giving a shorter output. Generators of the up-left group are output,
together with their images under the up and left tail and head maps.
</Description>
</ManSection>

<Example>
<![CDATA[
gap> DisplayLeadMaps( C2b );
(pre-)cat2-group with up-left group: [ (1,2,3,4,5,6), (2,6)(3,5) ]
 up tail=head images: [ (1,4)(2,5)(3,6), (2,6)(3,5) ]
left tail=head images: [ (1,5,3)(2,6,4), (2,6)(3,5) ]
]]>
</Example>

<ManSection>
 <Attr Name="Transpose3DimensionalGroup"
 Arg="S0" Label="for cat2-groups" />
<Description>
The <E>transpose</E> of a cat<M>^2</M>-group <M>\calC</M> with groups
<M>[G,R,Q,P]</M> is the cat<M>^2</M>-group <M>\tilde{\calC}</M>
with groups <M>[G,Q,R,P]</M>.
</Description>
</ManSection>

<Example>
<![CDATA[
gap> TC2a := Transpose3DimensionalGroup( C2a );
(pre-)cat2-group with generating (pre-)cat1-groups:
1 : [d12 => Group( [ (1,5,3)(2,6,4), (2,6)(3,5) ] )]
2 : [d12 => Group( [ (1,4)(2,5)(3,6), (2,6)(3,5) ] )]
]]>
</Example>

<ManSection Label="cat2-mor">
 <Func Name="Cat2GroupMorphism"
 Arg="args" />
 <Oper Name="Cat2GroupMorphismByCat1GroupMorphisms"
 Arg="src, rng, upmor, ltmor" />
 <Oper Name="Cat2GroupMorphismByGroupHomomorphisms"
 Arg="src, rng, homs" />
 <Func Name="PreCat2GroupMorphism"
 Arg="args" />
 <Oper Name="PreCat2GroupMorphismByPreCat1GroupMorphisms"
 Arg="src, rng, upmor, ltmor" />
 <Oper Name="PreCat2GroupMorphismByGroupHomomorphisms"
 Arg="src, rng, homs" />
<Description>
A (pre-)cat<M>^2</M>-group morphism
<M>\mu : \calC = (G,R,Q,P) \to \calC' = (G',R',Q',P')</M>
is a list of four group homomorphisms
<M>\gamma : G \to G',~ \rho : R \to R',~ \xi : Q \to Q'</M>
and <M>\pi : P \to P'</M>
which commute with all the tail, head and embedding maps so that
<M>(\gamma,\rho), (\gamma,\xi), (\rho,\pi)</M> and <M>(\xi,\pi)</M>
are all (pre-)cat<M>^1</M>-group morphisms.

For the operations <C>(Pre)Cat2GroupMorphismByPreCat1GroupMorphisms</C>
the third and fourth parameters <C>upmor, ltmor</C>
are two cat<M>^1</M>-group morphisms
with source the up and left cat<M>^1</M>-groups in <M>\calC</M>.

For the operations <C>(Pre)Cat2GroupMorphismByGroupHomomorphisms</C>
the third parameter <C>mors</C> is the list <M>[\gamma,\rho,\xi,\pi]</M>.

The example constructs an automorphism of <C>c2a</C> is two ways,
using the two methods described above,
an d verifies that the result is the same in each case.
</Description>
</ManSection>
<Example>
<![CDATA[
gap> gamma := GroupHomomorphismByImages( G, G, [a,b], [a^-1,b] );;
gap> rho := IdentityMapping( R );;
gap> xi := GroupHomomorphismByImages( Q, Q, [a^2,b], [a^-2,b] );;
gap> pi := IdentityMapping( Pa );;
gap> homs := [ gamma, rho, xi, pi ];;
gap> mor1 := Cat2GroupMorphismByGroupHomomorphisms( C2a, C2a, homs );
<mapping: (pre-)cat2-group with generating (pre-)cat1-groups:
1 : [d12 => Group( [ (1,4)(2,5)(3,6), (2,6)(3,5) ] )]
2 : [d12 => Group( [ (1,5,3)(2,6,4), (2,6)(3,5) ] )] -> (pre-)cat
2-group with generating (pre-)cat1-groups:
1 : [d12 => Group( [ (1,4)(2,5)(3,6), (2,6)(3,5) ] )]
2 : [d12 => Group( [ (1,5,3)(2,6,4), (2,6)(3,5) ] )] >
gap> upmor := Cat1GroupMorphism( up, up, gamma, rho );;
gap> ltmor := Cat1GroupMorphism( left, left, gamma, xi );;
gap> mor2 := Cat2GroupMorphismByCat1GroupMorphisms( C2a, C2a, upmor, ltmor );;
gap> mor1 = mor2;
true
]]>
</Example>

<ManSection Label="cat2-xsq">
 <Attr Name="Cat2GroupOfCrossedSquare"
 Arg="xsq" />
 <Attr Name="CrossedSquareOfCat2Group"
 Arg="CC" />
<Description>
These functions provide for conversion between crossed squares and
cat<M>^2</M>-groups.
(They are the 3-dimensional equivalents of
<Ref Oper="Cat1GroupOfXMod"/> and <Ref Oper="XModOfCat1Group"/>.)
The actor crossed square <C>XSact</C> was constructed in section
<Ref Func="ActorCrossedSquare" />.
</Description>
</ManSection>

<Example>
<![CDATA[
gap> xsC2a := CrossedSquareOfCat2Group( C2a );
crossed square with crossed modules:
 up = [Group( () ) -> Group( [ (1,4)(2,5)(3,6) ] )]
 left = [Group( () ) -> Group( [ (1,3,5)(2,4,6) ] )]
 right = [Group( [ (1,4)(2,5)(3,6) ] ) -> Group( [ (2,6)(3,5) ] )]
 down = [Group( [ (1,3,5)(2,4,6) ] ) -> Group( [ (2,6)(3,5) ] )]

gap> IdGroup( xsC2a );
[ [ 1, 1 ], [ 2, 1 ], [ 3, 1 ], [ 2, 1 ] ]

gap> SetName( Source( Right2DimensionalGroup( XSact ) ), "c5:c4" );
gap> SetName( Range( Right2DimensionalGroup( XSact ) ), "c5:c4" );
gap> Name( XSact );
"[d10a->c5:c4,d20->c5:c4]"

gap> C2act := Cat2GroupOfCrossedSquare( XSact );
(pre-)cat2-group with generating (pre-)cat1-groups:
1 : [((c5:c4 |X c5:c4) |X (d20 |X d10a))=>(c5:c4 |X c5:c4)]
2 : [((c5:c4 |X c5:c4) |X (d20 |X d10a))=>(c5:c4 |X d20)]
gap> Size3d( C2act );
[ 80000, 400, 400, 20 ]
]]>
</Example>

<ManSection Label="subdiag-cat1">
 <Attr Name="Subdiagonal2DimensionalGroup"
 Arg="obj" />
<Description>
The diagonal of a crossed square is always a crossed module,
but the diagonal of a cat<M>^2</M>-group need only be a pre-cat<M>^1</M>-group.
There is, however, a sub-cat<M>^1</M>-group of this diagonal which,
in the case of a cat<M>^2</M>-group constructed from a crossed square,
is <M>(P \ltimes L => P)</M>.
(The name of this operation is very provisional.)

</Description>
</ManSection>

<Example>
<![CDATA[
gap> G24 := SmallGroup( 24, 10 );;
gap> w := G24.1;; x := G24.2;; y := G24.3;; z := G24.4;; o := One(G24);;
gap> R := Subgroup( G24, [x,y] );;
gap> txy := GroupHomomorphismByImages( G24, R, [w,x,y,z], [o,x,y,o] );;
gap> exy := GroupHomomorphismByImages( R, G24, [x,y], [x,y] );;
gap> C1xy := PreCat1GroupByTailHeadEmbedding( txy, txy, exy );;
gap> Q := Subgroup( G24, [w,y] );;
gap> twy := GroupHomomorphismByImages( G24, Q, [w,x,y,z], [w,o,y,o] );;
gap> ewy := GroupHomomorphismByImages( Q, G24, [w,y], [w,y] );;
gap> C1wy := PreCat1GroupByTailHeadEmbedding( twy, twy, ewy );;
gap> C2wxy := PreCat2Group( C1xy, C1xy );;
gap> dg := Diagonal2DimensionalGroup( C2wxy );;
gap> C1sub := Subdiagonal2DimensionalGroup( C2wxy );;
gap> [ IsCat1Group(dg), IsCat1Group(C1sub), IsSub2DimensionalGroup(dg,C1sub) ];
[ false, true, true ]
]]>
</Example>

<ManSection>
 <Oper Name="SubCat2Group"
 Arg="C2G, ul, ur, dl" />
 <Oper Name="IsSubCat2Group"
 Arg="C2G, subC2G" />
 <Oper Name="SubPreCat2Group"
 Arg="C2G, ul, ur, dl" />
 <Oper Name="IsSubPreCat2Group"
 Arg="C2G, subC2G" />
<Description>
As with crossed squares, we may construct sub-structures of cat<M>^2</M>-groups.
In this case we need only prescribe three subgroups, rather than four,
so that the up and left sub-cat<M>^1</M>-groups can be specified.
The remaining sub-cat<M>^1</M>-groups are then calculated automatically.
</Description>
</ManSection>

<Example>
<![CDATA[
gap> gps := GroupsOfHigherDimensionalGroup( C2ab );;
gap> c6c2 := Subgroup( gps[1], [ (1,2,3,4,5,6), (8,12)(9,11) ] );;
gap> c2c2 := Subgroup( gps[2], [ (1,4)(2,5)(3,6), (8,12)(9,11) ] );;
gap> c3c2 := Subgroup( gps[3], [ (1,5,3)(2,6,4), (8,12)(9,11) ] );;
gap> SC2ab := SubCat2Group( C2ab, c6c2, c2c2, c3c2 );;
gap> Display( SC2ab );
(pre-)cat2-group with groups: [ Group( [ (1,2,3,4,5,6), ( 8,12)( 9,11) ] ),
 Group( [ (1,4)(2,5)(3,6), ( 8,12)( 9,11) ] ),
 Group( [ (1,5,3)(2,6,4), ( 8,12)( 9,11) ] ),
 Group( [ (), ( 8,12)( 9,11) ] ) ]
 up tail=head: [ [ (1,2,3,4,5,6), ( 8,12)( 9,11) ],
 [ (1,4)(2,5)(3,6), ( 8,12)( 9,11) ] ]
left tail=head: [ [ (1,2,3,4,5,6), ( 8,12)( 9,11) ],
 [ (1,5,3)(2,6,4), ( 8,12)( 9,11) ] ]
right tail=head: [ [ (1,4)(2,5)(3,6), ( 8,12)( 9,11) ],
 [ (), ( 8,12)( 9,11) ] ]
down tail=head: [ [ (1,5,3)(2,6,4), ( 8,12)( 9,11) ], [ (), ( 8,12)( 9,11) ]
]
]]>
</Example>

<ManSection>
 <Attr Name="TrivialSubCat2Group"
 Arg="C2G" />
 <Attr Name="TrivialSubPreCat2Group"
 Arg="C2G" />
<Description>
A special case of the previous operation is when all the subgroups are trivial.
</Description>
</ManSection>

<Example>
<![CDATA[
gap> TC2ab := TrivialSubCrossedSquare( C2ab );
crossed square with crossed modules:
 up = [Group( () ) -> Group( () )]
 left = [Group( () ) -> Group( () )]
 right = [Group( () ) -> Group( () )]
 down = [Group( () ) -> Group( () )]
]]>
</Example>

</Section>

<Section Label="sect-allcat2">
<Heading>Enumerating cat<M>^2</M>-groups with a given source</Heading>

This section mirrors that for cat<M>^1</M>-groups (<Ref Sect="sect-allcat1" />).
As the size of a group <M>G</M> increases,
the number of cat<M>^2</M>-groups with source <M>G</M> increases rapidly.
However, one is usually only interested in the isomorphism classes
of cat<M>^2</M>-groups with source <M>G</M>.
An iterator <C>AllCat2GroupsIterator</C> is provided,
which runs through the various cat<M>^2</M>-groups.
This iterator finds, for each unordered pair of subgroups <M>R,Q</M> of
<M>G</M>, the cat<M>^2</M>-groups whose <C>Up2DimensionalGroup</C>
has range <M>R</M>, and whose <C>Left2DimensionalGroup</C> has range <M>Q</M>.
It does this by running through
<C>UnoderedPairsIterator(AllSubgroupsIterator(G))</C>
provided by the <Package>Utils</Package> package,
and then using the iterator <C>AllCat2GroupsWithImagesIterator(G,R,Q)</C>.

<ManSection>
 <Oper Name="AllCat2GroupsWithImagesIterator"
 Arg="G R Q" />
 <Attr Name="AllCat2GroupsWithImagesNumber"
 Arg="G R Q" />
 <Oper Name="AllCat2GroupsWithImages"
 Arg="G R Q" />
 <Oper Name="AllCat2GroupsWithImagesUpToIsomorphism"
 Arg="G R Q" />
<Description>
The iterator <C>AllCat2GroupsWithImagesIterator(G)</C> iterates through all
the cat<M>^2</M>-groups with source <C>G</C> and generating cat<M>^1</M>-groups
<C>(G=>R)</C> and <C>(G=>Q)</C>.
The attribute <C>AllCat2GroupsWithImagesNumber(G)</C> runs through this iterator
to determine the number <M>n</M> of these cat<M>^2</M>-groups.
The operation <C>AllCat2GroupsWithImages(G)</C> returns a list containing these
<M>n</M> cat<M>^2</M>-groups.
Since these lists can get very long, this operation should only be used
for simple cases.
The operation <C>AllCat2GroupsWithImagesUpToIsomorphism(G)</C> returns
representatives of the isomorphism classes of these cat<M>^2</M>-groups.
</Description>
</ManSection>

<Example>
<![CDATA[
gap> G8 := Group( (1,2), (3,4), (5,6) );;
gap> A := Subgroup( G8, [ (1,2) ] );;
gap> B := Subgroup( G8, [ (3,4) ] );;
gap> AllCat2GroupsWithImagesNumber( G8, A, A );
4
gap> all := AllCat2GroupsWithImages( G8, A, A );;
gap> for C2 in all do DisplayLeadMaps( C2 ); od;
(pre-)cat2-group with up-left group: [ (1,2), (3,4), (5,6) ]
 up tail=head images: [ (1,2), (1,2), () ]
left tail=head images: [ (1,2), (1,2), () ]
(pre-)cat2-group with up-left group: [ (1,2), (3,4), (5,6) ]
 up tail=head images: [ (1,2), (), () ]
left tail=head images: [ (1,2), (), () ]
(pre-)cat2-group with up-left group: [ (1,2), (3,4), (5,6) ]
 up tail=head images: [ (1,2), (), (1,2) ]
left tail=head images: [ (1,2), (), (1,2) ]
(pre-)cat2-group with up-left group: [ (1,2), (3,4), (5,6) ]
 up tail=head images: [ (1,2), (1,2), (1,2) ]
left tail=head images: [ (1,2), (1,2), (1,2) ]
gap> AllCat2GroupsWithImagesNumber( G8, A, B );
16
gap> iso := AllCat2GroupsWithImagesUpToIsomorphism( G8, A, B );;
gap> for C2 in iso do DisplayLeadMaps( C2 ); od;
(pre-)cat2-group with up-left group: [ (1,2), (3,4), (5,6) ]
 up tail=head images: [ (1,2), (), () ]
left tail=head images: [ (), (3,4), () ]
(pre-)cat2-group with up-left group: [ (1,2), (3,4), (5,6) ]
 up tail=head images: [ (1,2), (), () ]
left tail/head images: [ (), (3,4), () ], [ (), (3,4), (3,4) ]
(pre-)cat2-group with up-left group: [ (1,2), (3,4), (5,6) ]
 up tail/head images: [ (1,2), (), () ], [ (1,2), (), (1,2) ]
left tail/head images: [ (), (3,4), () ], [ (), (3,4), (3,4) ]
]]>
</Example>

<ManSection>
 <Oper Name="AllCat2GroupsWithFixedUp"
 Arg="C" />
 <Oper Name="AllCat2GroupsWithFixedUpAndLeftRange"
 Arg="C R" />
<Description>
The operation <C>AllCat2GroupsWithFixedUp(C)</C> constructs all the
cat<M>^2</M>-groups with a fixed <C>Up2DimensionalGroup</C> <M>C</M>.
In the second operation the user may also specify the range of the
<C>Left2DimensionalGroup</C>.
</Description>
</ManSection>

<Example>
<![CDATA[
gap> up := Up2DimensionalGroup( iso[1] );
[Group( [ (1,2), (3,4), (5,6) ] )=>Group( [ (1,2), (), () ] )]
gap> AllCat2GroupsWithFixedUp( up );;
gap> Length(last);
28
gap> L := AllCat2GroupsWithFixedUpAndLeftRange( up, B );;
gap> for C in L do DisplayLeadMaps( C ); od;
(pre-)cat2-group with up-left group: [ (1,2), (3,4), (5,6) ]
 up tail=head images: [ (1,2), (), () ]
left tail=head images: [ (), (3,4), () ]
(pre-)cat2-group with up-left group: [ (1,2), (3,4), (5,6) ]
 up tail=head images: [ (1,2), (), () ]
left tail/head images: [ (), (3,4), () ], [ (), (3,4), (3,4) ]
(pre-)cat2-group with up-left group: [ (1,2), (3,4), (5,6) ]
 up tail=head images: [ (1,2), (), () ]
left tail/head images: [ (), (3,4), (3,4) ], [ (), (3,4), () ]
(pre-)cat2-group with up-left group: [ (1,2), (3,4), (5,6) ]
 up tail=head images: [ (1,2), (), () ]
left tail=head images: [ (), (3,4), (3,4) ]
]]>
</Example>

<ManSection>
 <Attr Name="AllCat2GroupsMatrix"
 Arg="G" />
<Description>
The operation <C>AllCat2GroupsMatrix(G)</C> constructs a symmetric matrix
<M>M</M> with rows and columns labelled by the cat<M>^1</M>-groups
<M>C_i</M> on <M>G</M>, where <M>M_{ij}</M> is
<M>1</M> if <M>C_i,C_j</M> combine to form a cat<M>^2</M>-group,
and <M>0</M> otherwise. The matrix is automatically printed out
with dots in place of zeroes.

In the example we see that the dihedral group <M>D_{12}</M>
has <M>12</M> cat<M>^1</M>-groups and <M>41</M> cat<M>^2</M>-groups,
<M>12</M> of which are symmetric.
This operation is intended to be used to illustrate how cat<M>^2</M>-groups
are formed, and should only be used with groups of low order.

The attribute <C>AllCat2GroupsNumber(G)</C> returns the
number <M>n</M> of these cat<M>^2</M>-groups.
</Description>
</ManSection>

<Example>
<![CDATA[
gap> AllCat2GroupsMatrix(d12);;
number of cat2-groups found = 41
1.....1..1.1
.1.....1.1.1
..1.....11.1
...1....1.11
....1.1...11
.....1.1..11
1...1.1..111
.1...1.1.111
..11....1111
111...1111.1
...111111.11
111111111111
gap> AllCat2GroupsNumber(d12);
41
]]>
</Example>

<ManSection>
 <Oper Name="AllCat2GroupsIterator"
 Arg="G" />
 <Oper Name="AllCat2Groups"
 Arg="G" />
 <Oper Name="AllCat2GroupsUpToIsomorphism"
 Arg="G" />
 <Oper Name="AllCat2GroupFamilies"
 Arg="G" />
 <Attr Name="CatnGroupNumbers"
 Arg="G" Label="for cat2-groups" />
 <Attr Name="CatnGroupLists"
 Arg="G" Label="for cat2-groups" />
<Description>
The iterator <C>AllCat2GroupsIterator(G)</C> iterates through all the
cat<M>^2</M>-groups with source <C>G</C>.
The operation <C>AllCat2Groups(G)</C> returns a list containing these <M>n</M>
cat<M>^2</M>-groups.
Since these lists can get very long, this operation should only be used
for simple cases.
The operation <C>AllCat2GroupsUpToIsomorphism(G)</C> returns representatives
of the isomorphism classes of these subgroups.
The operation <C>AllCat2GroupFamilies(G)</C> returns a list of lists.
The <M>k</M>-th list contains the positions of the cat<M>^2</M>-groups in
<C>AllCat2Groups(G)</C> which are isomorphic to the <M>k</M>-th representative.
So, for <C>d12</C>, the <M>41</M> cat<M>^2</M>-groups form <M>10</M> classes,
and the sizes of these classes are <C>[6,6,6,6,3,6,3,2,2,1]</C>.
Four of these classes contain symmetric cat<M>^2</M>-groups.

The field <C>CatnGroupNumbers(G).cat2</C> is the number of cat<M>^2</M>-groups
on <M>G</M>, while <C>CatnGroupNumbers(G).iso2</C> is the number of
isomorphism classes of these cat<M>^2</M>-groups.
Also <C>CatnGroupNumbers(G).symm</C> is the number of cat<M>^2</M>-groups
whose <C>Up2DimensionalGroup</C> is the same as the
<C>Left2DimensionalGroup</C>, while <C>CatnGroupNumbers(G).siso</C> is the
number of isomorphism classes of these symmetric cat<M>^2</M>-groups.

Provided that <C>CatnGroupLists(G).omit</C> is not set to <C>true</C>,
<E>sorted</E> lists of generating pairs, and of the classes they belong to,
are added to the record <C>CatnGroupLists</C>.
For example <C>[5,7]</C> in these lists for <C>d12</C> indicates that there is
a cat<M>^2</M>-group generated by the fifth and seventh cat<M>^1</M>-groups
and that this is in the second class whose representative is <C>[1,7]</C>.
Classes <C>[1,5,8,10]</C> contain symmetric cat<M>^2</M>-groups.
</Description>
</ManSection>

<Example>
<![CDATA[
gap> AllCat2GroupsNumber( d12 );
41
gap> reps2 := AllCat2GroupsUpToIsomorphism( d12 );;
gap> Length( reps2 );
10
gap> List( reps2, C -> StructureDescription( C ) );
[ [ "D12", "C2", "C2", "C2" ], [ "D12", "C2", "C2 x C2", "C2" ],
 [ "D12", "C2", "S3", "C2" ], [ "D12", "C2", "D12", "C2" ],
 [ "D12", "C2 x C2", "C2 x C2", "C2 x C2" ], [ "D12", "C2 x C2", "S3", "C2" ]
 , [ "D12", "C2 x C2", "D12", "C2 x C2" ], [ "D12", "S3", "S3", "S3" ],
 [ "D12", "S3", "D12", "S3" ], [ "D12", "D12", "D12", "D12" ] ]
gap> fams := AllCat2GroupFamilies( d12 );
[ [ 1, 2, 3, 4, 5, 6 ], [ 7, 8, 10, 11, 13, 14 ], [ 16, 17, 18, 23, 24, 25 ],
 [ 30, 31, 32, 33, 34, 35 ], [ 9, 12, 15 ], [ 19, 20, 21, 26, 27, 28 ],
 [ 36, 37, 38 ], [ 22, 29 ], [ 39, 40 ], [ 41 ] ]
gap> CatnGroupNumbers( d12 );
rec( cat1 := 12, cat2 := 41, idem := 21, iso1 := 4, iso2 := 10,
 isopredg := 0, predg := 0, siso := 4, symm := 12 )
gap> CatnGroupLists( d12 );
rec( allcat2pos := [ 1, 7, 9, 16, 19, 22, 30, 36, 39, 41 ],
 cat2classes :=
 [ [ [ 1, 1 ], [ 2, 2 ], [ 3, 3 ], [ 4, 4 ], [ 5, 5 ], [ 6, 6 ] ],
 [ [ 1, 7 ], [ 5, 7 ], [ 2, 8 ], [ 6, 8 ], [ 3, 9 ], [ 4, 9 ] ],
 [ [ 1, 10 ], [ 2, 10 ], [ 3, 10 ], [ 4, 11 ], [ 5, 11 ], [ 6, 11 ] ],
 [ [ 1, 12 ], [ 2, 12 ], [ 3, 12 ], [ 4, 12 ], [ 5, 12 ], [ 6, 12 ] ],
 [ [ 7, 7 ], [ 8, 8 ], [ 9, 9 ] ],
 [ [ 7, 10 ], [ 8, 10 ], [ 9, 10 ], [ 7, 11 ], [ 8, 11 ], [ 9, 11 ] ],
 [ [ 7, 12 ], [ 8, 12 ], [ 9, 12 ] ], [ [ 10, 10 ], [ 11, 11 ] ],
 [ [ 10, 12 ], [ 11, 12 ] ], [ [ 12, 12 ] ] ],
 cat2pairs := [ [ 1, 1 ], [ 1, 7 ], [ 1, 10 ], [ 1, 12 ], [ 2, 2 ],
 [ 2, 8 ], [ 2, 10 ], [ 2, 12 ], [ 3, 3 ], [ 3, 9 ], [ 3, 10 ],
 [ 3, 12 ], [ 4, 4 ], [ 4, 9 ], [ 4, 11 ], [ 4, 12 ], [ 5, 5 ],
 [ 5, 7 ], [ 5, 11 ], [ 5, 12 ], [ 6, 6 ], [ 6, 8 ], [ 6, 11 ],
 [ 6, 12 ], [ 7, 7 ], [ 7, 10 ], [ 7, 11 ], [ 7, 12 ], [ 8, 8 ],
 [ 8, 10 ], [ 8, 11 ], [ 8, 12 ], [ 9, 9 ], [ 9, 10 ], [ 9, 11 ],
 [ 9, 12 ], [ 10, 10 ], [ 10, 12 ], [ 11, 11 ], [ 11, 12 ], [ 12, 12 ] ],
 omit := false, pisopos := [ ], sisopos := [ 1, 5, 8, 10 ] )
]]>
</Example>

</Section>

</Chapter>

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