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# 7 Utility functions

### Sections

1. Inclusion and Restriction Mappings
2. Endomorphism Classes and Automorphisms
3. Abelian Modules
4. Distinct and Common Representatives

## 7.1 Inclusion and Restriction Mappings

- `InclusionMappingGroups(` G`,` H`) O`
- `RestrictionMappingGroups(` hom`,` src`,` rng`) O`
- `MappingToOne(` G`,` H`) O`

  The first set of utilities concerns inclusion and restriction mappings. Restriction may apply to both the source and the range of the map. The map `incd8` is the inclusion of `d8` in `d16` used in Section 3.3.

      gap> Print( incd8, "\n" );
      [ (11,13,15,17)(12,14,16,18), (11,18)(12,17)(13,16)(14,15) ] ->
      [ (11,13,15,17)(12,14,16,18), (11,18)(12,17)(13,16)(14,15) ]
      gap> imd8 := Image( incd8 );;
      gap> resd8 := RestrictionMappingGroups( incd8, c4, imd8 );;
      gap> Source( res8 );  Range( res8 );
      c4
      Group([ (11,13,15,17)(12,14,16,18), (11,18)(12,17)(13,16)(14,15) ])
      gap> MappingToOne( c4, imd8 );
      [ (11,13,15,17)(12,14,16,18) ] -> [ () ]

  ## 7.2 Endomorphism Classes and Automorphisms

- `EndomorphismClasses(` grp`,` case`) F`
- `EndoClassNaturalHom(` class`) A`
- `EndoClassIsomorphism(` class`) A`
- `EndoClassConjugators(` class`) A`
- `AutoGroup(` class`) A`

  The monoid of endomorphisms of a group is used when calculating the monoid of derivations of a crossed module and when determining all the cat1-structures on a group.

  An endomorphism e of R with image H¢ is determined by

  ·
    a normal subgroup N of R and a permutation representation q: R/N ® Q of the quotient, giving a projection q°n: R ® Q, where n: R ® R/N is the natural homomorphism;
  ·
    an automorphism a of Q;
  ·
    a subgroup H¢ in a conjugacy class [H] of subgroups of R isomorphic to Q having representative H, an isomorphism f: Q @ H, and a conjugating element c Î R such that Hc = H¢.

  Then e takes values

  e r = (f a q n r)c ·

  Endomorphisms are placed in the same class if they have the same choice of N and [H], and so the number of endomorphisms is

  |End(R)| = | å classes | |Aut(Q)|·|[H]| ·

  The function `EndomorphismClasses(` grp`,` case`)` may be called in three ways:

  ·
    case 1 includes automorphisms and the zero map,
  ·
    case 2 excludes automorphisms and the zero map,
  ·
    case 3 is when `N` intersects `H` trivially.

      gap> end8 := EndomorphismClasses( d8, 1 );;
      gap> Length( end8 );
      13
      gap> e4 := end8[4];
      <enumerator>
      gap> EndoClassNaturalHom( e4 );
      GroupHomomorphismByImages( d8, Group( [ f1 ] ),
      [ (11,13,15,17)(12,14,16,18), (12,18)(13,17)(14,16) ], [ f1, f1 ] )
      gap> EndoClassIsomorphism( e4 );
      Pcgs([ f1 ]) -> [ (11,13)(14,18)(15,17) ]
      gap> EndoClassConjugators( e4 );
      [ (), (12,18)(13,17)(14,16) ]
      gap> AutoGroup( e4 );
      Group( [ Pcgs([ f1 ]) -> [ f1 ] ] )
      gap> L := List( end8, e -> Length(EndoClassConjugators(e)) * Size(AutoGroup(e)) );
      [ 8, 1, 2, 2, 1, 2, 2, 1, 2, 2, 6, 6, 1 ]
      gap> Sum( L );
      36

- `InnerAutomorphismsByNormalSubgroup(` G`,` N`) O`
- `IsGroupOfAutomorphisms(` A`) P`

  Inner automorphisms of a group `G` by the elements of a normal subgroup `N` are calculated with the first of these functions, usually with `G` = `N`.

      gap> autd8 := AutomorphismGroup( d8 );;
      gap> innd8 := InnerAutomorphismsByNormalSubgroup( d8, d8 );;
      gap> GeneratorsOfGroup( innd8 );
      [ InnerAutomorphism( d8, (11,13,15,17)(12,14,16,18) ),
        InnerAutomorphism( d8, (12,18)(13,17)(14,16) ) ]
      gap> IsGroupOfAutomorphisms( innd8 );
      true

  ## 7.3 Abelian Modules

- `IsAbelianModule(` obj`) P`
- `AbelianModuleGroup(` obj`) A`
- `AbelianModuleAction(` obj`) A`
- `AbelianModuleObject(` grp`,` act`) O`

  An abelian module is an abelian group together with a group action. These are used by the crossed module constructor `XModByAbelianModule`.

      gap> x := (6,7)(8,9);;  y := (6,8)(7,9);;  z := (6,9)(7,8);;
      gap> k4 := Group( x, y );  SetName( k4, "k4" );
      gap> s3 := Group( (1,2), (2,3) );;  SetName( s3, "s3" );
      gap> alpha := GroupHomomorphismByImages( k4, k4, [x,y], [y,x] );
      gap> beta := GroupHomomorphismByImages( k4, k4, [x,y], [x,z] );
      gap> aut := Group( alpha, beta );
      gap> act := GroupHomomorphismByImages( s3, aut, [(1,2),(2,3)], [alpha,beta] );
      gap> abmod := AbelianModuleObject( k4, act );
      <enumerator>
      gap> Xabmod := XModByAbelianModule( abmod );
      [k4->s3]

  The resulting `Xabmod` is isomorphic to the output from `XModByAutomorphismGroup( k4 );`.

  ## 7.4 Distinct and Common Representatives

- `DistinctRepresentatives(` list`) O`
- `CommonRepresentatives(` list1`,` list2`) O`
- `CommonTransversal(` grp`,` subgrp`) O`
- `IsCommonTransversal(` grp`,` subgrp`,` list`) O`

  The final set of utilities deal with lists of subsets of [1 ¼ n] and construct systems of distinct and common representatives using simple, non-recursive, combinatorial algorithms.

  When L is a set of n subsets of [1 ¼ n] and the Hall condition is satisfied (the union of any k subsets has at least k elements), a set of distinct representatives exists.

  When J,K are both lists of n sets, the function `CommonRepresentatives` returns two lists: the set of representatives, and a permutation of the subsets of the second list. It may also be used to provide a common transversal for sets of left and right cosets of a subgroup H of a group G, although a greedy algorithm is usually quicker.

      gap> J := [ [1,2,3], [3,4], [3,4], [1,2,4] ];;
      gap> DistinctRepresentatives( J );
      [ 1, 3, 4, 2 ]
      gap> K := [ [3,4], [1,2], [2,3], [2,3,4] ];;
      gap> CommonRepresentatives( J, K );
      [ [ 3, 3, 3, 1 ], [ 1, 3, 4, 2 ] ]
      gap> CommonTransversal( d16, c4 );
      [ (), (12,18)(13,17)(14,16), (11,12,13,14,15,16,17,18),
        (11,12)(13,18)(14,17)(15,16) ]
      gap> IsCommonTransversal( d16, c4, [ (), c, d, c*d ] );
      true

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