A homomorphism f from a magma with objects M to a magma with objects N consists of
a map f_O from the objects of M to those of N,
a map f_A from the arrows of M to those of N.
The map f_A is required to be compatible with tail and head and to preserve multiplication:
f_A(a : u \to v) * f_A(b : v \to w) ~=~ f_A(a*b : u \to w)
with tail f_O(u) and head f_O(w).
When M is a monoid or group, the map f_A is required to preserve object identities and inverses.
> MagmaWithObjectsHomomorphism( args ) | ( function ) |
> HomomorphismFromSinglePiece( src, rng, hom, imobs ) | ( operation ) |
> HomomorphismToSinglePiece( src, rng, images ) | ( operation ) |
> PieceImages( mwohom ) | ( attribute ) |
> HomsOfMapping( mwohom ) | ( attribute ) |
> PiecesOfMapping( mwohom ) | ( attribute ) |
> IsomorphismNewObjects( src, objlist ) | ( operation ) |
There are a variety of homomorphism constructors.
The simplest construction gives a homomorphism M -> N with both M and N connected. It is implemented as IsMappingToSinglePieceRep with attributes Source, Range and PieceImages. The operation requires the following information:
a magma homomorphism f from the underlying magma of M to the underlying magma of N,
a list imobs of the images of the objects of M.
In the first example we construct endomappings of m and M78.
gap> tup1 := [Tuple([m1,m2]), Tuple([m2,m1]), Tuple([m3,m4]), Tuple([m4,m3])]; gap> f1 := GeneralMappingByElements( m, m, tup1 ); gap> IsMagmaHomomorphism( f1 ); true gap> tup2 := [Tuple([m1,m1]), Tuple([m2,m1]), Tuple([m3,m1]), Tuple([m4,m1])];; gap> f2 := GeneralMappingByElements( m, m, tup2 );; gap> IsMagmaHomomorphism( f2 ); true gap> hom1 := MagmaWithObjectsHomomorphism( M78, M78, f1, [-8,-7] );; gap> Display( hom1 ); homomorphism to single piece magma: M78 -> M78 magma hom: <mapping: m -> m >, object map: [ -8, -7 ] -> [ -8, -7 ] gap> [ Source( hom1 ), Range( hom1 ) ]; [ M78, M78 ] gap> b87; [m3 : -8 -> -7] gap> im1 := ImageElm( hom1, b87 ); [m4 : -8 -> -7] gap> i56 := IsomorphismNewObjects( M78, [-5,-6] ); magma with objects homomorphism : [ [ IdentityMapping( m ), [ -5, -6 ] ] ] gap> M65 := Range( i56);; gap> SetName( M65, "M65" ); gap> j56 := InverseGeneralMapping( i56 );; gap> ImagesOfObjects( j56 ); [ -7, -8 ] gap> im2 := ImageElm( i56, b87 ); [m3 : -5 -> -6] gap> comp := j56 * hom1; magma with objects homomorphism : M65 -> M78 [ [ <mapping: m -> m >, [ -7, -8 ] ] ] gap> ImageElm( comp, im2 ); [m4 : -8 -> -7] |
A homomorphism to a connected magma with objects may have a source with several pieces, and so is a union of homomorphisms from single pieces.
gap> M4 := UnionOfPieces( [ M78, M65 ] );; gap> images := [ PieceImages( hom1 )[1], PieceImages( j56 )[1] ]; [ [ <mapping: m -> m >, [ -8, -7 ] ], [ IdentityMapping( m ), [ -7, -8 ] ] ] gap> hom4 := HomomorphismToSinglePiece( M4, M78, images ); magma with objects homomorphism : [ [ <mapping: m -> m >, [ -8, -7 ] ], [ IdentityMapping( m ), [ -7, -8 ] ] ] |
The next example exhibits a homomorphism between transformation semigroups with objects.
gap> t2 := Transformation( [2,2,4,1] );; gap> s2 := Transformation( [1,1,4,4] );; gap> r2 := Transformation( [4,1,3,3] );; gap> sgp2 := Semigroup( [ t2, s2, r2 ] );; gap> SetName( sgp2, "sgp<t2,s2,r2>" ); gap> ## apparently no method for transformation semigroups available for: gap> ## nat := NaturalHomomorphismByGenerators( sgp, sgp2 ); so we use: gap> flip := function( t ) ## where t is a transformation on [1..4] gap> local i, j, k, L, L2, n; gap> n := DegreeOfTransformation( t ); gap> L := ImageListOfTransformation( t ); gap> L2 := ShallowCopy( L ); gap> for i in [1..n] do gap> if IsOddInt(i) then j:=i+1; else j:=i-1; fi; gap> k := L[j]; gap> if IsOddInt(k) then L2[i]:=k+1; else L2[i]:=k-1; fi; gap> od; gap> return( Transformation( L2 ) ); gap> end;; gap> smap := MappingByFunction( sgp, sgp2, flip );; gap> ok := RespectsMultiplication( smap ); true gap> [ t, Image( smap, t ) ]; [ Transformation( [ 1, 1, 2, 3 ] ), Transformation( [ 2, 2, 4, 1 ] ) ] gap> [ s, Image( smap, s ) ]; [ Transformation( [ 2, 2, 3, 3 ] ), Transformation( [ 1, 1, 4, 4 ] ) ] gap> [ r, Image( smap, r ) ]; [ Transformation( [ 2, 3, 4, 4 ] ), Transformation( [ 4, 1, 3, 3 ] ) ] gap> SetName( smap, "smap" ); gap> T123 := SemigroupWithObjects( sgp2, [-13,-12,-11] ); gap> shom := MagmaWithObjectsHomomorphism( S123, T123, smap, [-11,-12,-13] );; gap> IsSemigroupWithObjectsHomomorphism( shom ); true gap> [ t12, ImageElm( shom, t12 ) ]; [ [Transformation( [ 1, 1, 2, 3 ] ) : -1 -> -2], [Transformation( [ 2, 2, 4, 1 ] ) : -13 -> -12] ] gap> [ s23, ImageElm( shom, s23 ) ]; [ [Transformation( [ 2, 2, 3, 3 ] ) : -2 -> -3], [Transformation( [ 1, 1, 4, 4 ] ) : -12 -> -11] ] gap> [ r31, ImageElm( shom, r31 ) ]; [ [Transformation( [ 2, 3, 4, 4 ] ) : -3 -> -1], [Transformation( [ 4, 1, 3, 3 ] ) : -11 -> -13] ] |
> HomomorphismByUnion( src, rng, homs ) | ( operation ) |
> IsInjectiveWithObjects( mwohom ) | ( property ) |
> IsSurjectiveWithObjects( mwohom ) | ( property ) |
> IsBijectiveWithObjects( mwohom ) | ( property ) |
> IsEndomorphismWithObjects( mwohom ) | ( property ) |
> IsAutomorphismWithObjects( mwohom ) | ( property ) |
When M has more than one connected component, a homomorphism is a union of homomorphisms, one for each piece.
gap> V1 := UnionOfPieces( [ M78, T123 ] );
magma with objects having 2 pieces :-
1: semigroup with objects :-
magma = sgp<t2,s2,r2>
objects = [ -13, -12, -11 ]
2: M78
gap> hUV := HomomorphismByUnionNC( U1, V1, [ hom1, shom ] );
magma with objects homomorphism :
[ magma with objects homomorphism :
[ [ <mapping: m -> m >, [ -8, -7 ] ] ]
, magma with objects homomorphism :
[ [ smap, [ -11, -12, -13 ] ] ]
]
gap> IsInjectiveWithObjects( hUV );
true
gap> IsSurjectiveWithObjects( hUV );
true
gap> IsBijectiveWithObjects( hUV );
true
gap> hVW := IsomorphismNewObjects( V1, [ [-103,-102,-101], [-108,-107] ] );
magma with objects homomorphism :
[ magma with objects homomorphism :
[ [ IdentityMapping( m ), [ -108, -107 ] ] ]
, magma with objects homomorphism :
[ [ IdentityMapping( sgp<t2,s2,r2> ), [ -103, -102, -101 ] ] ]
]
gap> hUW := hUV * hVW;
magma with objects homomorphism :
[ magma with objects homomorphism :
[ [ <mapping: m -> m >, [ -108, -107 ] ] ]
, magma with objects homomorphism :
[ [ smap, [ -101, -102, -103 ] ] ]
]
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