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8 Crossed squares and Cat^2-groups
 8.1 Definition of a crossed square and a crossed n-cube of groups
 8.2 Constructions for crossed squares
 8.3 Morphisms of crossed squares
 8.4 Definitions and constructions for cat^2-groups and their morphisms
 8.5 Definition and constructions for cat^n-groups and their morphisms

8 Crossed squares and Cat^2-groups

The term 3d-group refers to a set of equivalent categories of which the most common are the categories of crossed squares and cat^2-groups. A 3d-mapping is a function between two 3d-groups which preserves all the structure.

The material in this chapter should be considered experimental. A major overhaul took place in time for XMod version 2.73, with the names of a number of operations being changed.

8.1 Definition of a crossed square and a crossed n-cube of groups

Crossed squares were introduced by Guin-Waléry and Loday (see, for example, [BL87]) as fundamental crossed squares of commutative squares of spaces, but are also of purely algebraic interest. We denote by [n] the set {1,2,...,n}. We use the n=2 version of the definition of crossed n-cube as given by Ellis and Steiner [ES87].

A crossed square mathcalS consists of the following:

Here is a picture of the situation:

\vcenter{\xymatrix{ & & S_{[2]} \ar[rr]^{\ddot{\partial}_1} \ar[dd]_{\ddot{\partial}_2} && S_{\{2\}} \ar[dd]^{\dot{\partial}_2} && L \ar[rr]^{\kappa} \ar[dd]_{\lambda} && M \ar[dd]^{\mu} & \\ \mathcal{S} & = & && & = && \\ & & S_{\{1\}} \ar[rr]_{\dot{\partial}_1} && S_{\emptyset} && N \ar[rr]_{\nu} && P }}

The following axioms must be satisfied for all l ∈ L, m,m_1,m_2 ∈ M, n,n_1,n_2 ∈ N, p ∈ P.

Note that the actions of M on N and N on M via P are compatible since

{n_1}^{(m^n)} \;=\; {n_1}^{\dot{\partial}_2(m^n)} \;=\; {n_1}^{n^{-1}(\dot{\partial}_2 m)n} \;=\; (({n_1}^{n^{-1}})^m)^n.

(A precrossed square is a similar structure which satisfies some subset of these axioms. This notion needs to be clarified.)

Crossed squares are the k=2 case of a crossed k-cube of groups, defined as follows. (This is an attempt to translate Definition 2.1 in Ronnie Brown's Computing homotopy types using crossed n-cubes of groups into right actions -- but this definition is not yet completely understood!)

A crossed k-cube of groups consists of the following:

There is then a long list of axioms which must be satisfied.

8.2 Constructions for crossed squares

Analogously to the data structure used for crossed modules, crossed squares are implemented as 3d-groups. There are also experimental implementations of cat^2-groups, with conversion between the two types of structure. Some standard constructions of crossed squares are listed below. At present, a limited number of constructions is implemented. Morphisms of crossed squares have also been implemented, though there is still a lot to be done.

8.2-1 CrossedSquareByNormalSubgroups
‣ CrossedSquareByNormalSubgroups( L, M, N, P )( operation )

If L, M, N are normal subgroups of a group P, and [M,N] leqslant L leqslant M ∩ N, then the four inclusions L -> M,~ L -> N,~ M -> P,~ N -> P, together with the actions of P on M, N and L given by conjugation, form a crossed square with crossed pairing

\boxtimes \;:\; N \times M \to L, \quad (n,m) \mapsto [n,m] \,=\, n^{-1}m^{-1}nm \,=\,(m^{-1})^nm \,=\, n^{-1}n^m\,.

This construction is implemented as CrossedSquareByNormalSubgroups(L,M,N,P); (note that the parent group come last).


gap> d20 := DihedralGroup( IsPermGroup, 20 );;
gap> gend20 := GeneratorsOfGroup( d20 ); 
[ (1,2,3,4,5,6,7,8,9,10), (2,10)(3,9)(4,8)(5,7) ]
gap> p1 := gend20[1];;  p2 := gend20[2];;  p12 := p1*p2; 
(1,10)(2,9)(3,8)(4,7)(5,6)
gap> d10a := Subgroup( d20, [ p1^2, p2 ] );;
gap> d10b := Subgroup( d20, [ p1^2, p12 ] );;
gap> c5d := Subgroup( d20, [ p1^2 ] );;
gap> SetName( d20, "d20" );  SetName( d10a, "d10a" ); 
gap> SetName( d10b, "d10b" );  SetName( c5d, "c5d" ); 
gap> XSconj := CrossedSquareByNormalSubgroups( c5d, d10a, d10b, d20 );
[  c5d -> d10a ]
[   |      |   ]
[ d10b -> d20  ]

8.2-2 CrossedSquareByNormalSubXMod
‣ CrossedSquareByNormalSubXMod( X0, X1 )( operation )

If mathcalX_1 = (∂_1 : S_1 -> R_1) is a normal sub-crossed module of mathcalX_0 = (∂_0 : S_0 -> R_0) then the inclusion morphism gives a crossed square with crossed pairing

\boxtimes \;:\; R_1 \times S_0 \to S_1, \quad (r_1,s_0) \mapsto (s_0^{-1})^{r_1} s_0.

The example constructs the same crossed square as in the previous subsection.


gap> X20 := XModByNormalSubgroup( d20, d10a );; 
gap> X10 := XModByNormalSubgroup( d10b, c5d );; 
gap> ok := IsNormalSub2DimensionalDomain( X20, X10 ); 
true 
gap> XS20 := CrossedSquareByNormalSubXMod( X20, X10 ); 
[  c5d -> d10a ]
[   |      |   ]
[ d10b -> d20  ]

8.2-3 ActorCrossedSquare
‣ ActorCrossedSquare( X0 )( operation )

The actor mathcalA(mathcalX_0) of a crossed module mathcalX_0 has been described in Chapter 5 (see ActorXMod (6.1-2)). The crossed pairing is given by

\boxtimes \;:\; R \times W \,\to\, S, \quad (\chi,r) \,\mapsto\, \chi r~.

This is implemented as ActorCrossedSquare(X0);.


gap> XSact := ActorCrossedSquare( X20 );
crossed square with:
      up = Whitehead[d10a->d20]
    left = [d10a->d20]
   right = Actor[d10a->d20]
    down = Norrie[d10a->d20]

8.2-4 CrossedSquareByAutomorphismGroup
‣ CrossedSquareByAutomorphismGroup( G )( operation )

For G a group let Inn(G) be its inner automorphism group and Aut(G) its full automorphism group. Then there is a crossed square with groups [G,Inn(G),Inn(G),Aut(G)] where the upper and left boundaries are the maps g ↦ ι_g, where ι_g is conjugation of G by g, and the right and down boundaries are inclusions. The crossed pairing is gived by ι_g ⊠ ι_h = [g,h].


gap> AXS20 := CrossedSquareByAutomorphismGroup( d20 );
[      d20 -> Inn(d20) ]
[     |          |     ]
[ Inn(d20) -> Aut(d20) ]

gap> StructureDescription( AXS20 );
[ "D20", "D10", "D10", "C2 x (C5 : C4)" ]

8.2-5 CrossedSquareByPullback
‣ CrossedSquareByPullback( X1, X2 )( operation )

If crossed modules mathcalX_1 = (ν : N -> P) and mathcalX_2 = (μ : M -> P) have a common range P, let L be the pullback of {ν,μ}. Then N acts on L by (n,m)^n' = (n^n',m^ν n'), and M acts on L by (n,m)^m' = (n^μ m', m^m'). So (π_1 : L -> N) and (π_2 : L -> M) are crossed modules, where π_1,π_2 are the two projections. The crossed pairing is given by:

\boxtimes \;:\; N \times M \to L, \quad (n,m) \mapsto (n^{-1}n^{\mu m}, (m^{-1})^{\nu n}m) .

The second example uses the central extension crossed module X12=(D12->S3) constructed in subsection (XModByCentralExtension (2.1-5)), with pullback group D12xC2.


gap> dn := Down2DimensionalGroup( XSconj );;
gap> rt := Right2DimensionalGroup( XSconj );;
gap> XSP := CrossedSquareByPullback( dn, rt ); 
gap> StructureDescription( XSP );                  
[ "C5", "D10", "D10", "D20" ]
gap> XS12 := CrossedSquareByPullback( X12, X12 );; 
gap> StructureDescription( XS12 );                  
[ "C2 x C2 x S3", "D12", "D12", "S3" ]

8.2-6 CrossedSquareByXModSplitting
‣ CrossedSquareByXModSplitting( X0 )( attribute )

For mathcalX = (∂ : S -> R) let Q be the image of . Then ∂ = ∂' ∘ ι where ∂' : S -> Q and ι is the inclusion of Q in R. The diagonal of the square is then the initial mathcalX, and the crossed pairing is given by commutators.

A particular case is when S is an R-module A and is the zero map.

\vcenter{\xymatrix{ & & S \ar[rr]^{\partial'} \ar[dd]_{\partial'} && Q \ar[dd]^{\iota} && A \ar[rr]^0 \ar[dd]_0 && 1 \ar[dd]^{\iota} & \\ & & && & && \\ & & Q \ar[rr]_{\iota} && R && 1 \ar[rr]_{\iota} && R }}


gap> k4 := Group( (1,2), (3,4) );;
gap> AX4 := XModByAutomorphismGroup( k4 );;
gap> X4 := Image( IsomorphismPermObject( AX4 ) );;
gap> XSS4 := CrossedSquareByXModSplitting( X4 );;
gap> StructureDescription( XSS4 );
[ "C2 x C2", "1", "1", "S3" ]
gap> XSS20 := CrossedSquareByXModSplitting( X20 );;
gap> up20 := Up2DimensionalGroup( XSS20 );; 
gap> Range( up20 ) = d10a; 
true
gap> SetName( Range( up20 ), "d10a" ); 
gap> Name( XSS20 ); 
"[d10a->d10a,d10a->d20]"

8.2-7 CrossedSquare
‣ CrossedSquare( args )( function )

The function CrossedSquare may be used to call some of the constructions described in the previous subsections.


gap> diag := Diagonal2DimensionalGroup( AXS20 );
[d20->Aut(d20)]
gap> XSdiag := CrossedSquare( diag );;      
gap> StructureDescription( XSdiag );  
[ "D20", "D10", "D10", "C2 x (C5 : C4)" ]

8.2-8 Transpose3DimensionalGroup
‣ Transpose3DimensionalGroup( S0 )( attribute )

The transpose of a crossed square mathcalS is the crossed square tildemathcalS} obtained by interchanging M with N, κ with λ, and ν with μ. The crossed pairing is given by

\tilde{\boxtimes} \;:\; M \times N \to L, \quad (m,n) \;\mapsto\; m\,\tilde{\boxtimes}\,n := (n \boxtimes m)^{-1}~.


gap> XStrans := Transpose3DimensionalGroup( XSconj ); 
[  c5d -> d10b ]
[   |      |   ]
[ d10a -> d20  ]


8.2-9 CentralQuotient
‣ CentralQuotient( X0 )( attribute )

The central quotient of a crossed module mathcalX = (∂ : S -> R) is the crossed square where:

This is the special case of an intended function CrossedSquareByCentralExtension which has not yet been implemented. In the example Xn7 X24, constructed in section 4.1.


gap> pos7 := Position( ids, [ [12,2], [24,5] ] );;
gap> Xn7 := nsx[pos7];; 
gap> IdGroup( Xn7 );
[ [ 12, 2 ], [ 24, 5 ] ]
gap> IdGroup( CentreXMod( Xn7 ) );  
[ [ 4, 1 ], [ 4, 1 ] ]
gap> CQXn7 := CentralQuotient( Xn7 );;
gap> StructureDescription( CQXn7 );
[ "C12", "C3", "C4 x S3", "S3" ]

8.2-10 IsCrossedSquare
‣ IsCrossedSquare( obj )( property )
‣ Is3dObject( obj )( property )
‣ IsPerm3dObject( obj )( property )
‣ IsPc3dObject( obj )( property )
‣ IsFp3dObject( obj )( property )
‣ IsPreCrossedSquare( obj )( property )

These are the basic properties for 3d-groups, and crossed squares in particular.

8.2-11 Up2DimensionalGroup
‣ Up2DimensionalGroup( XS )( attribute )
‣ Left2DimensionalGroup( XS )( attribute )
‣ Down2DimensionalGroup( XS )( attribute )
‣ Right2DimensionalGroup( XS )( attribute )
‣ DiagonalAction( XS )( attribute )
‣ CrossedPairing( XS )( attribute )
‣ Diagonal2DimensionalGroup( XS )( attribute )
‣ Name( S0 )( method )

These are the basic attributes of a crossed square mathcalS. The six objects used in the construction of mathcalS are the four crossed modules (2d-groups) on the sides of the square (up; left; right and down); the diagonal action of P on L; and the crossed pairing {M,N} -> L. The diagonal crossed module (L -> P) is an additional attribute.

Crossed pairings have been implemented using an operation Mapping2ArgumentsByFunction. This encodes a map {M,N} -> L as a map M × N -> L.


gap> Up2DimensionalGroup( XSconj );
[c5d->d10a]
gap> Right2DimensionalGroup( XSact );
Actor[d10a->d20]
gap> diact := DiagonalAction( XSact );;
gap> ImageElm( diact, (1,4)(2,3)(6,9)(7,8) );  
^(1,5,7,3)(2,8,6,10)
gap> Diagonal2DimensionalGroup( XSconj );
[c5d->d20]
gap> Name( XSconj ); 
"[c5d->d10a,d10b->d20]"

8.2-12 ImageElmCrossedPairing
‣ ImageElmCrossedPairing( XS, pair )( operation )

This operation returns the image when a crossed pairing {M,N} -> L is applied to the pair [m,n] with m ∈ M,~ n ∈ N.


gap> xp := CrossedPairing( XSconj );;                            
gap> ImageElmCrossedPairing( xp,                                 
>      [ (1,6)(2,5)(3,4)(7,10)(8,9), (1,5)(2,4)(6,9)(7,8) ] );
(1,7,8,5,3)(2,9,10,6,4)

8.3 Morphisms of crossed squares

This section describes an initial implementation of morphisms of (pre-)crossed squares.

8.3-1 CrossedSquareMorphism
‣ CrossedSquareMorphism( args )( function )
‣ CrossedSquareMorphismByXModMorphisms( src, rng, mors )( operation )
‣ CrossedSquareMorphismByGroupHomomorphisms( src, rng, homs )( operation )
‣ PreCrossedSquareMorphismByPreXModMorphisms( src, rng, mors )( operation )
‣ PreCrossedSquareMorphismByGroupHomomorphisms( src, rng, homs )( operation )

8.3-2 Source
‣ Source( map )( attribute )
‣ Range( map )( attribute )
‣ Up2DimensionalMorphism( map )( attribute )
‣ Left2DimensionalMorphism( map )( attribute )
‣ Down2DimensionalMorphism( map )( attribute )
‣ Right2DimensionalMorphism( map )( attribute )

Morphisms of 3dObjects are implemented as 3dMappings. These have a pair of 3d-groups 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 fail when invalid data is supplied.

8.3-3 IsCrossedSquareMorphism
‣ IsCrossedSquareMorphism( map )( property )
‣ IsPreCrossedSquareMorphism( map )( property )
‣ IsBijective( mor )( method )
‣ IsEndomorphism3dObject( mor )( property )
‣ IsAutomorphism3dObject( mor )( property )

A morphism mor between two pre-crossed squares mathcalS_1 and mathcalS_2 consists of four crossed module morphisms Up2DimensionalMorphism(mor), mapping the Up2DimensionalGroup of mathcalS_1 to that of mathcalS_2, Left2DimensionalMorphism(mor), Right2DimensionalMorphism(mor) and Down2DimensionalMorphism(mor). These four morphisms are required to commute with the four boundary maps and to preserve the rest of the structure. The current version of IsCrossedSquareMorphism does not perform all the required checks.


gap> ad20 := GroupHomomorphismByImages( d20, d20, [p1,p2], [p1,p2^p1] );;
gap> ad10a := GroupHomomorphismByImages( d10a, d10a, [p1^2,p2], [p1^2,p2^p1] );;
gap> ad10b := GroupHomomorphismByImages( d10b, d10b, [p1^2,p12], [p1^2,p12^p1] );;
gap> idc5d := IdentityMapping( c5d );;
gap> up := Up2DimensionalGroup( XSconj );;
gap> lt := Left2DimensionalGroup( XSconj );; 
gap> rt := Right2DimensionalGroup( XSconj );; 
gap> dn := Down2DimensionalGroup( XSconj );; 
gap> mup := XModMorphismByGroupHomomorphisms( up, up, idc5d, ad10a );
[[c5d->d10a] => [c5d->d10a]]
gap> mlt := XModMorphismByGroupHomomorphisms( lt, lt, idc5d, ad10b );
[[c5d->d10b] => [c5d->d10b]]
gap> mrt := XModMorphismByGroupHomomorphisms( rt, rt, ad10a, ad20 );
[[d10a->d20] => [d10a->d20]]
gap> mdn := XModMorphismByGroupHomomorphisms( dn, dn, ad10b, ad20 );
[[d10b->d20] => [d10b->d20]]
gap> autoconj := CrossedSquareMorphism( XSconj, XSconj, [mup,mlt,mrt,mdn] );; 
gap> ord := Order( autoconj );;
gap> Display( autoconj );
Morphism of crossed squares :- 
: Source = [c5d->d10a,d10b->d20]
: Range = [c5d->d10a,d10b->d20]
:     order = 5
:    up-left: [ [ ( 1, 3, 5, 7, 9)( 2, 4, 6, 8,10) ], 
  [ ( 1, 3, 5, 7, 9)( 2, 4, 6, 8,10) ] ]
:   up-right: 
[ [ ( 1, 3, 5, 7, 9)( 2, 4, 6, 8,10), ( 2,10)( 3, 9)( 4, 8)( 5, 7) ], 
  [ ( 1, 3, 5, 7, 9)( 2, 4, 6, 8,10), ( 1, 3)( 4,10)( 5, 9)( 6, 8) ] ]
:  down-left: 
[ [ ( 1, 3, 5, 7, 9)( 2, 4, 6, 8,10), ( 1,10)( 2, 9)( 3, 8)( 4, 7)( 5, 6) ], 
  [ ( 1, 3, 5, 7, 9)( 2, 4, 6, 8,10), ( 1, 2)( 3,10)( 4, 9)( 5, 8)( 6, 7) ] ]
: down-right: 
[ [ ( 1, 2, 3, 4, 5, 6, 7, 8, 9,10), ( 2,10)( 3, 9)( 4, 8)( 5, 7) ], 
  [ ( 1, 2, 3, 4, 5, 6, 7, 8, 9,10), ( 1, 3)( 4,10)( 5, 9)( 6, 8) ] ]
gap> IsAutomorphismHigherDimensionalDomain( autoconj );
true
gap> KnownPropertiesOfObject( autoconj );
[ "CanEasilyCompareElements", "CanEasilySortElements", "IsTotal", 
  "IsSingleValued", "IsInjective", "IsSurjective", 
  "IsPreCrossedSquareMorphism", "IsCrossedSquareMorphism", 
  "IsEndomorphismHigherDimensionalDomain", 
  "IsAutomorphismHigherDimensionalDomain" ]

8.3-4 InclusionMorphismHigherDimensionalDomains
‣ InclusionMorphismHigherDimensionalDomains( obj, sub )( operation )

8.4 Definitions and constructions for cat^2-groups and their morphisms

We shall give three definitions of cat^2-groups and show that they are equivalent. When we come to define cat^n-groups we shall give a similar set of three definitions.

Firstly, we take the definition of a cat^2-group from Section 5 of Brown and Loday [BL87], suitably modified. A cat^2-group mathcalC = (C_[2],C_{2},C_{1},C_∅) comprises four groups (one for each of the subsets of [2]) and 15 homomorphisms, as shown in the following diagram:

\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} \\ \mathcal{C} \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]}} \\ }}

The following axioms are satisfied by these homomorphisms:

It follows from these identities that (ddott_1,dott_1),(ddoth_1,doth_1) and (ddote_1,dote_1) are morphisms of cat^1-groups.

Secondly, we give the simplest of the three definitions, adapted from Ellis-Steiner [ES87]. A cat^2-group mathcalC consists of groups G, R_1,R_2 and six homomorphisms t_1,h_1 : G -> R_2,~ e_1 : R_2 -> G,~ t_2,h_2 : G -> R_1,~ e_2 : R_1 -> G, satisfying the following axioms for all 1 leqslant i leqslant 2,

Our third definition defines a cat^2-group as a "cat^1-group of cat^1-groups". A cat^2-group mathcalC consists of two cat^1-groups mathcalC_1 = (e_1;t_1,h_1 : G_1 -> R_1) and mathcalC_2 = (e_2;t_2,h_2 : G_2 -> R_2) and cat^1-morphisms t = (ddott,dott), h = (ddoth,doth) : mathcalC_1 -> mathcalC_2, e = (ddote,dote) : mathcalC_2 -> mathcalC_1, subject to the following conditions:

(t \circ e) ~\mbox{and}~ (h \circ e) ~\mbox{are the identity mapping on}~ \mathcal{C}_2, \qquad [\ker t, \ker h] = \{ 1_{\mathcal{C}_1} \},

where ker t = (ker ddott, ker dott), and similarly for ker h.

8.4-1 Cat2Group
‣ Cat2Group( args )( function )
‣ PreCat2Group( args )( function )
‣ IsCat2Group( C )( property )
‣ PreCat2GroupByPreCat1Groups( L )( operation )

The global functions Cat2Group and PreCat2Group are normally called with a single argument, a list of cat1-groups.


gap> a := (1,2,3,4)(5,6,7,8);;
gap> b := (1,5)(2,6)(3,7)(4,8);; 
gap> c := (2,6)(4,8);;
gap> G16 := Group( a, b, c );; 
gap> SetName( G16, "c4c2:c2" );
gap> t1a := GroupHomomorphismByImages( G16, G16, [a,b,c], [(),(),c] );; 
gap> C1a := PreCat1GroupByEndomorphisms( t1a, t1a );;
gap> t1b := GroupHomomorphismByImages( G16, G16, [a,b,c], [a,(),()] );; 
gap> C1b := PreCat1GroupByEndomorphisms( t1b, t1b );;
gap> C16 := Cat2Group( C1a, C1b );
cat2-group with generating (pre-)cat1-groups:
1 : [c4c2:c2 => Group( [ (), (), (2,6)(4,8) ] )]
2 : [c4c2:c2 => Group( [ (1,2,3,4)(5,6,7,8), (), () ] )]
gap> IsCat2Group( C16 );
true
gap> IsCat1Group( Diagonal2DimensionalGroup( C16 ) );
false

8.4-2 Cat2GroupMorphism
‣ Cat2GroupMorphism( args )( function )
‣ Cat2GroupMorphismByCat1GroupMorphisms( src, rng, mors )( operation )
‣ Cat2GroupMorphismByGroupHomomorphisms( src, rng, mors )( operation )
‣ PreCat2GroupMorphismByPreCat1GroupMorphisms( src, rng, mors )( operation )
‣ PreCat2GroupMorphismByGroupHomomorphisms( src, rng, mors )( operation )

8.4-3 Cat2GroupOfCrossedSquare
‣ Cat2GroupOfCrossedSquare( xsq )( attribute )
‣ CrossedSquareOfCat2Group( CC )( attribute )

These functions are very experimental!

These functions provide the conversion from crossed square to cat2-group, and conversely. (They are the 3-dimensional equivalents of Cat1GroupOfXMod (2.5-2) and XModOfCat1Group (2.5-2).)


gap> xsC16 := CrossedSquareOfCat2Group( C16 );
crossed square with crossed modules:
      up = [Group( [ (1,5)(2,6)(3,7)(4,8) ] ) -> Group( [ ( 2, 6)( 4, 8) ] )]
    left = [Group( [ (1,5)(2,6)(3,7)(4,8) ] ) -> Group( 
[ (1,2,3,4)(5,6,7,8), (), () ] )]
   right = [Group( [ ( 2, 6)( 4, 8) ] ) -> Group( () )]
    down = [Group( [ (1,2,3,4)(5,6,7,8), (), () ] ) -> Group( () )]

gap> IdGroup( xsC16 );
[ [ 2, 1 ], [ 2, 1 ], [ 4, 1 ], [ 1, 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 );             
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> Size( C2act );
[ 80000, 400, 400, 20 ]

8.5 Definition and constructions for cat^n-groups and their morphisms

In this chapter we are interested in cat^2-groups, but it is convenient in this section to give the more general definition. There are three equivalent description of a cat^n-group.

A cat^n-group consists of the following.

Note that, since the t_A,i, h_A,i and e_A,i commute, composite homomorphisms t_A,B, h_A,B : G_A -> G_A ∖ B and e_A,B : G_A ∖ B -> G_A are well defined for all B ⊆ A ⊆ [n].

Secondly, we give the simplest of the three descriptions, again adapted from Ellis-Steiner [ES87].

A cat^n-group mathcalC consists of 2^n groups G_A, one for each subset A of [n], and 3n homomorphisms

t_{[n],i}, h_{[n],i} : G_{[n]} \to G_{[n] \setminus \{i\}},~ e_{[n],i} : G_{[n] \setminus \{i\}} \to G_{[n]},

satisfying the following axioms for all 1 leqslant i leqslant n,}

Our third description defines a cat^n-group as a "cat^1-group of cat^(n-1)-groups".

A cat^n-group mathcalC consists of two cat^(n-1)-groups:

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