4 Arbitrary functions on groups: EndoMappings

The constructor function EndoMappingByPositionList returns the the endomapping that maps the i-th element of the group (in the ordering given by AsSortedList) to the i-th element of list.

    gap> G := GTW4_2;
    4/2
    gap> t1 := EndoMappingByPositionList ( G, [1, 2, 4, 4] );
    <mapping: 4/2 -> 4/2 >

The constructor function EndoMappingByFunction returns the function fun that maps elements of the group G into G as an endomapping.

    gap> t2 := EndoMappingByFunction ( GTW8_2, g -> g^-1 );
    <mapping: 8/2 -> 8/2 >
    gap> IsGroupHomomorphism ( t2 );
    true
    gap> t3 := EndoMappingByFunction ( GTW6_2, g -> g^-1 );
    <mapping: 6/2 -> 6/2 >
    gap> IsGroupHomomorphism ( t3 );
    false

EndoMappings and GroupGeneralMappings are different kinds of objects in GAP: GroupGeneralMappings model homomorphisms between two different groups, whereas EndoMappings model nonlinear functions on one group. However, GroupGeneralMappings can be transformed into Endomappings if they have equal source and range.

The constructor function AsEndoMapping returns the mapping map as an endomapping.

    gap> G1 := Group ((1,2,3), (1, 2));
    Group([ (1,2,3), (1,2) ])
    gap> G2 := Group ((2,3,4), (2, 3));
    Group([ (2,3,4), (2,3) ])
    gap> f1 := IsomorphismGroups ( G1, G2 );
    [ (1,2,3), (1,2) ] -> [ (2,3,4), (2,3) ]
    gap> f2 := IsomorphismGroups ( G2, G1 );
    [ (2,3,4), (2,3) ] -> [ (1,2,3), (1,2) ]
    gap> AsEndoMapping ( CompositionMapping ( f1, f2 ) );
    <mapping: Group( [ (2,3,4), (2,3) ] ) -> Group( [ (2,3,4), (2,3)
    ] ) >

EndoMappings and GroupGeneralMappings are two completely different kinds of objects in GAP, but they can be transformed into one another.

AsGroupGeneralMappingByImages returns the GroupGeneralMappingByImages that acts on the group the same way as the endomapping endomap. It only makes sense to use this function for endomappings that are group endomorphisms.

    gap> m := IdentityEndoMapping ( GTW6_2 );
    <mapping: 6/2 -> 6/2 >
    gap> AsGroupGeneralMappingByImages ( m );
    [ (1,2), (1,2,3) ] -> [ (1,2), (1,2,3) ]

IsEndoMapping returns true if the object obj is an endomapping and false otherwise.

    gap> IsEndoMapping ( InnerAutomorphisms ( GTW6_2 ) [3] );
    true

IdentitEndoMapping is the counterpart to the GAP standard library function IdentityMapping. It returns the identity transformation on the group G.

    gap> AsList ( UnderlyingRelation ( IdentityEndoMapping ( Group ((1,2,3,4)) ) ) );
    [ DirectProductElement( [ (), () ] ), DirectProductElement( [ (1,2,3,4), (1,2,
       3,4) ] ), DirectProductElement( [ (1,3)(2,4), (1,3)(2,4) ] ),
      DirectProductElement( [ (1,4,3,2), (1,4,3,2) ] ) ]

ConstantEndoMapping returns the endomapping on the group G which maps everything to the group element g of G.

    gap> C3 := CyclicGroup (3);
    <pc group of size 3 with 1 generator>
    gap> m := ConstantEndoMapping (C3, AsSortedList (C3) [2]);
    MappingByFunction( <pc group of size 3 with 
    1 generator>, <pc group of size 3 with 
    1 generator>, function( x ) ... end )
    gap> List (AsList (C3), x -> Image (m, x));
    [ f1, f1, f1 ]

4.2 Properties of endo mappings

IsIdentityEndoMapping returns true if endomap is the identity function on a group.

    gap> IsIdentityEndoMapping (EndoMappingByFunction ( 
    > AlternatingGroup ( [1..5] ), x -> x^31));
    true

IsConstantEndoMapping returns true if the endomapping endomap is constant and false otherwise.

    gap> C3 := CyclicGroup ( 3 );
    <pc group of size 3 with 1 generator>
    gap> IsConstantEndoMapping ( EndoMappingByFunction ( C3,  x -> x^3 ));
    true

A mapping t on an (additively written) group G is called distributive if for all elements x and y in G:\ t(x+y) = t(x) + t(y). The function IsDistributiveEndoMapping returns the according boolean value true or false.

    gap> G := Group ( (1,2,3), (1,2) );
    Group([ (1,2,3), (1,2) ])
    gap> IsDistributiveEndoMapping ( EndoMappingByFunction ( G, x -> x^3));
    false
    gap> IsDistributiveEndoMapping ( EndoMappingByFunction ( G, x -> x^7));
    true

4.3 Operations for endo mappings

While the composition operator * is applicable to mappings and transformations, the operation + (pointwise addition of the images) can only be applied to transformations.

The product operator * returns the transformation which is obtained from the transformations t1 and t2 by composition of t1 and t2 (i.e. performing t2 after t1).

    gap> t1 := ConstantEndoMapping ( GTW2_1, ());
    MappingByFunction( 2/1, 2/1, function( x ) ... end )
    gap> t2 := ConstantEndoMapping (GTW2_1, (1, 2));
    MappingByFunction( 2/1, 2/1, function( x ) ... end )
    gap> List ( AsList ( GTW2_1 ), x -> Image ( t1 * t2, x ));
    [ (1,2), (1,2) ]

The add operator + returns the endomapping which is obtained from the endomappings t1 and t2 by pointwise addition of t1 and t2. (Note that in this context addition means that for every place x in the source of t1 and t2, GAP performs the operation p * q, where p is the image of t1 at x and q is the image of t2 at x.)

The subtract operator - returns the endomapping which is obtained from the endomappings t1 and t2 by pointwise subtraction of t1 and t2. (Note that in this context subtraction means performing the GAP operation p * q^(-1), where p is the image of t1 at a place x and q is the image of t2 at x.)

    gap> G := SymmetricGroup ( 3 );
    Sym( [ 1 .. 3 ] )
    gap> invertingOnG := EndoMappingByFunction ( G, x -> x^-1 );
    <mapping: SymmetricGroup( [ 1 .. 3 ] ) -> SymmetricGroup(
    [ 1 .. 3 ] ) >
    gap> identityOnG := IdentityEndoMapping (G);
    <mapping: SymmetricGroup( [ 1 .. 3 ] ) -> SymmetricGroup(
    [ 1 .. 3 ] ) >
    gap> AsSortedList ( G );
    [ (), (2,3), (1,2), (1,2,3), (1,3,2), (1,3) ]
    gap> List ( AsSortedList (G), 
    >              x -> Image ( identityOnG * invertingOnG, x ));
    [ (), (2,3), (1,2), (1,3,2), (1,2,3), (1,3) ]
    gap> List ( AsSortedList (G),
    >              x -> Image ( identityOnG + invertingOnG, x ));
    [ (), (), (), (), (), () ]
    gap> IsIdentityEndoMapping ( - invertingOnG );
    true
    gap> - invertingOnG = identityOnG;
    true

4.4 Nicer ways to print a mapping

GraphOfMapping returns the set of all pairs (x,m(x)), where x lies in the source of the mapping. In particular, it returns List (Source (m), x -> [x, Image (m, x)]);

    gap> G := SymmetricGroup ( 3 );
    Sym( [ 1 .. 3 ] )
    gap> m := ConstantEndoMapping (G, (1,2,3)) + IdentityEndoMapping( G );
    MappingByFunction( Sym( [ 1 .. 3 ] ), Sym( [ 1 .. 3 ] ), function( g ) ... end )
    gap> PrintArray( GraphOfMapping( m ) );
    [ [       (),  (1,2,3) ],
      [    (2,3),    (1,3) ],
      [    (1,2),    (2,3) ],
      [  (1,2,3),  (1,3,2) ],
      [  (1,3,2),       () ],
      [    (1,3),    (1,2) ] ]

If mapping is a polynomial function on its source then PrintAsTerm prints a polynomial that induces the mapping mapping.

    gap> G := SymmetricGroup ( 3 );
    Sym( [ 1 .. 3 ] )
    gap> p := Random( PolynomialNearRing( G ) );
    <mapping: SymmetricGroup( [ 1 .. 3 ] ) -> SymmetricGroup( [ 1 .. 3 ] ) >
    gap> PrintAsTerm( p );
    g1 - x - 2 * g1 - g2 - x - g1 - g2 + g1 - x - 2 * g1 - 
    g2 - x - g1 - g2 - 3 * x + g1
    gap> GeneratorsOfGroup( G );
    [ (1,2,3), (1,2) ]

The expressions g1 and g2 stand for the first and secong generator of the group G respectively. The result is not necessarily a polynomial of minimal length.

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