3 Quotient groups by homomorphisms

Given a group homomorphism, the cosets of its kernel correspond to elements in the image. Our hom coset representation stores the homomorphism and the element in the source group. The image is an attribute which is computed as necessary. Two cosets are equal if their images are the same. Where ever practical a coset is identified with its image. For example, if the homomorphism maps into a permutation group, the cosets are considered to be permutations. Since cosets can be multiplied, we can use them to form a quotient group. Any computation in this quotient group will be shadowed in the source group.

The functions and operations described in this chapter have been added very recently and are still undergoing development. It is conceivable that names of variants of the functionality might change in future versions. If you plan to use these functions in your own code, please contact us.

  • IsHomCoset( obj ) C

    IsHomCoset has one category for each kind of image (and corresponding representations).

  • IsHomCosetToPerm( obj ) C

  • IsHomCosetToMatrix( obj ) C

    gdc - We need HomCosetToMatrix to be in same family as Matrix, so that GAP allows vector * HomCosetToMatrix and other algorithms that take elements of the HomCosetToMatrix Unfortunately, I don't know how to set the family correctly for compatibility.

  • IsHomCosetToFp( obj ) C

  • IsHomCosetToTuple( obj ) C

  • IsHomCosetToAdditiveElt( obj ) C

    Here the image is an ADDITIVE group of matrices.

  • IsHomCosetToObjectRep( obj ) R

    The generic representation

    It also has one property for each kind of source.

  • IsHomCosetOfPerm P

  • IsHomCosetOfMatrix P

  • IsHomCosetOfFp P

  • IsHomCosetOfTuple( ) P

  • IsHomCosetOfAdditiveElt P

    Sections

    1. Creating hom cosets and quotient groups
    2. Operations on hom cosets

    3.1 Creating hom cosets and quotient groups

  • HomCoset( hom, elt ) F

    Create a hom coset. It is better to use one of the QuotientGroupBy... functions.

  • HomCosetWithImage( hom, srcElt, imgElt ) F

    Create a hom coset with given homomorphism, source and image elements. It is better to use one of the QuotientGroupBy... functions.

  • QuotientGroupHom( hom ) A

    The quotient group associated with the homomorphism. It is better to use one of the QuotientGroupBy... functions.

  • QuotientGroupByHomomorphism( hom ) F

    The quotient group associated with the homomorphism.

  • QuotientGroupByImages( srcGroup, rangeGroup, srcGens, imgGens ) F

    Create a quotient group from the homomorphism which takes maps srcGens[i] in srcGroup to imgGens[i] in rangeGroup.

  • QuotientGroupByImagesNC( srcGroup, rangeGroup, srcGens, imgGens ) F

    Same as QuotientGroupByImages but without checking that the homomorphism makes sense.

    3.2 Operations on hom cosets

  • Homomorphism( hcoset ) O
  • Homomorphism( Q ) O

    The homomorphism of a hom coset, respectively a hom quotient group.

  • SourceElt( hcoset ) O

    The source element of a hom coset.

  • ImageElt( hcoset ) A

    The image element of a hom coset.

  • CanonicalElt( hcoset ) A

    A canonical elt of a hom coset. Note that SourceElt may be different for non-identical equal cosets. CanonicalElt gives the same element for different representation of a coset. This will compute a chain for the range group if one does not already exist.

  • Source( Q ) A

    Source group of a hom quotient group.

  • Range( Q ) A

    Range group of a hom quotient group.

  • ImagesSource( Q ) A

    Image group of a hom quotient group.

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    GAP 4 manual
    September 2000