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8 3d-groups and 3d-mappings : 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 3d-groups and 3d-mappings : 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.

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} & \\ \mathcal{S} & = & && \\ & & S_{\{1\}} \ar[rr]_{\dot{\partial}_1} && S_{\emptyset} }} }}

The following axioms must be satisfied for all l ∈ S_[2], m,m_1,m_2 ∈ S_{1}, n,n_1,n_2 ∈ S_{2}, p ∈ S_∅:

Note that the actions of S_{1} on S_{2} and S_{2} on S_{1} via S_∅ are compatible since

{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.

(A precrossed square is a similar structure which satisfies some subset of these axioms. [More needed here.])

In what follows we shall generally use the following notation for the S_J, namely L = S_[2];~ M = S_{1};~ N = S_{2} and P = S_∅.

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

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

The following axioms must be satisfied (long list to be added).

8.2 Constructions for crossed squares

Analogously to the data structure used for crossed modules, crossed squares are implemented as 3d-groups. When times allows, cat^2-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 be done.

8.2-1 CrossedSquare
‣ CrossedSquare( args )( function )
‣ CrossedSquareByNormalSubgroups( P, N, M, L )( operation )
‣ ActorCrossedSquare( X0 )( operation )
‣ Transpose3dGroup( S0 )( attribute )
‣ Name( S0 )( attribute )

Here are some standard examples of crossed squares.


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( d20, d10b, d10a, c5d );
[  c5d -> d10b ]
[   |      |   ]
[ d10a -> d20  ]

gap> Name( XSconj );
"[c5d->d10b,d10a->d20]"
gap> XStrans := Transpose3dGroup( XSconj ); 
[  c5d -> d10a ]
[   |      |   ]
[ d10b -> d20  ]

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

8.2-2 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]; 
[Group( [ f2, f3, f4 ] )->Group( [ f1, f2, f4, f5 ] )]
gap> IdGroup( CentreXMod(Xn7) );  
[ [ 4, 1 ], [ 4, 1 ] ]
gap> CQXn7 := CentralQuotient( Xn7 );
crossed square with:
      up = [Group( [ f2, f3, f4 ] )->Group( [ f1 ] )]
    left = [Group( [ f2, f3, f4 ] )->Group( [ f1, f2, f4, f5 ] )]
    down = [Group( [ f1, f2, f4, f5 ] )->Group( [ f1, f2 ] )]
   right = [Group( [ f1 ] )->Group( [ f1, f2 ] )]

gap> IdGroup( CQXn7 );
[ [ [ 12, 2 ], [ 3, 1 ] ], [ [ 24, 5 ], [ 6, 1 ] ] ]

8.2-3 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.

8.2-4 Up2DimensionalGroup
‣ Up2DimensionalGroup( XS )( attribute )
‣ Left2DimensionalGroup( XS )( attribute )
‣ Down2DimensionalGroup( XS )( attribute )
‣ Right2DimensionalGroup( XS )( attribute )
‣ DiagonalAction( XS )( attribute )
‣ CrossedPairing( XS )( attribute )
‣ ImageElmCrossedPairing( XS, pair )( operation )

In this implementation the attributes used in the construction of a crossed square XS are the four crossed modules (2d-groups) on the sides of the square (up; down, left; and right); the diagonal action of P on L; and the crossed pairing.

The GAP development team have suggested that crossed pairings should be implemented as a special case of BinaryMappings -- a structure which does not yet exist in GAP. As a temporary measure, crossed pairings have been implemented using Mapping2ArgumentsByFunction.


gap> Up2DimensionalGroup( XSconj );
[c5d->d10b]
gap> Right2DimensionalGroup( XSact );
Actor[d10a->d20]
gap> xpconj := CrossedPairing( XSconj );;
gap> ImageElmCrossedPairing( xpconj, [ p2, p12 ] );
(1,9,7,5,3)(2,10,8,6,4)
gap> diag := DiagonalAction( XSact );
[ (1,3,5,2,4)(6,10,14,8,12)(7,11,15,9,13), (1,2,5,4)(6,8,14,12)(7,11,13,9) 
 ] -> 
[ (1,3,5,2,4)(6,10,14,8,12)(7,11,15,9,13), (1,2,5,4)(6,8,14,12)(7,11,13,9) 
 ] -> [ ^(1,3,5,7,9)(2,4,6,8,10), ^(1,2,5,4)(3,8)(6,7,10,9) ]

8.3 Morphisms of crossed squares

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

8.3-1 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.

8.3-2 IsCrossedSquareMorphism
‣ IsCrossedSquareMorphism( map )( property )
‣ IsPreCrossedSquareMorphism( map )( property )
‣ IsBijective( mor )( property )
‣ 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), Down2DimensionalMorphism(mor) and Right2DimensionalMorphism(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> upconj := Up2DimensionalGroup( XSconj );;
gap> leftconj := Left2DimensionalGroup( XSconj );; 
gap> downconj := Down2DimensionalGroup( XSconj );; 
gap> rightconj := Right2DimensionalGroup( XSconj );; 
gap> up := XModMorphismByHoms( upconj, upconj, idc5d, ad10b );
[[c5d->d10b] => [c5d->d10b]]
gap> left := XModMorphismByHoms( leftconj, leftconj, idc5d, ad10a );
[[c5d->d10a] => [c5d->d10a]]
gap> down := XModMorphismByHoms( downconj, downconj, ad10a, ad20 );
[[d10a->d20] => [d10a->d20]]
gap> right := XModMorphismByHoms( rightconj, rightconj, ad10b, ad20 );
[[d10b->d20] => [d10b->d20]]
gap> autoconj := CrossedSquareMorphism( XSconj, XSconj, up, left, right, down );; 
gap> ord := Order( autoconj );;
gap> Display( autoconj );
Morphism of crossed squares :- 
:    Source = [c5d->d10b,d10a->d20]
:     Range = [c5d->d10b,d10a->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), ( 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-left: 
[ [ ( 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-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" ]

8.4 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-groups 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 )
‣ PreCat2GroupByPreCat1Groups( L )( operation )

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


gap> CC6 := Cat2Group( Cat1Group(6,2,2), Cat1Group(6,2,3) );
generating (pre-)cat1-groups:
1 : [C6=>Group( [ f1 ] )]
2 : [C6=>Group( [ f2 ] )]

gap> IsCat2Group( CC6 );
true

8.4-2 Cat2GroupOfCrossedSquare
‣ Cat2GroupOfCrossedSquare( xsq )( attribute )
‣ CrossedSquareOfCat2Group( CC )( attribute )

These functions are temporarily disabled.

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


gap> xsCC6 := CrossedSquareOfCat2Group( CC6 );
crossed square with:
      up = [Group( () )->Group( [ (1,2) ] )]
    left = [Group( () )->Group( [ (), (3,4,5) ] )]
    down = [Group( [ (), (3,4,5) ] )->Group( () )]
   right = [Group( [ (1,2) ] )->Group( () )]


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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