10 Functors
  
  
  10.1 Functors: Category and Representations
  
  
  10.2 Functors: Constructors
  
  
  10.3 Functors: Attributes
  
  
  10.4 Basic Functors
  
  10.4-1 functor_Cokernel
  
  functor_Cokernel global variable
  
  The functor that associates to a map its cokernel.
  
    Code  
    InstallValue( functor_Cokernel_for_fp_modules,
            CreateHomalgFunctor(
                    [ "name", "Cokernel" ],
                    [ "category", HOMALG_MODULES.category ],
                    [ "operation", "Cokernel" ],
                    [ "natural_transformation", "CokernelEpi" ],
                    [ "special", true ],
                    [ "number_of_arguments", 1 ],
                    [ "1", [ [ "covariant" ],
                            [ IsMapOfFinitelyGeneratedModulesRep,
                              [ IsHomalgChainMorphism, IsImageSquare ] ] ] ],
                    [ "OnObjects", _Functor_Cokernel_OnModules ]
                    )
            );
  
  
  10.4-2 Cokernel
  
  Cokernel( phi )  operation
  
  The   following  example  also  makes  use  of  the  natural  transformation
  CokernelEpi.
  
    Example  
    gap> ZZ := HomalgRingOfIntegers( );
    Z
    gap> M := HomalgMatrix( "[ 2, 3, 4,   5, 6, 7 ]", 2, 3, ZZ );;
    gap> M := LeftPresentation( M );
    <A non-torsion left module presented by 2 relations for 3 generators>
    gap> N := HomalgMatrix( "[ 2, 3, 4, 5,   6, 7, 8, 9 ]", 2, 4, ZZ );;
    gap> N := LeftPresentation( N );
    <A non-torsion left module presented by 2 relations for 4 generators>
    gap> mat := HomalgMatrix( "[ \
    > 1, 0, -3, -6, \
    > 0, 1,  6, 11, \
    > 1, 0, -3, -6  \
    > ]", 3, 4, ZZ );;
    gap> phi := HomalgMap( mat, M, N );;
    gap> IsMorphism( phi );
    true
    gap> phi;
    <A homomorphism of left modules>
    gap> coker := Cokernel( phi );
    <A left module presented by 5 relations for 4 generators>
    gap> ByASmallerPresentation( coker );
    <A rank 1 left module presented by 1 relation for 2 generators>
    gap> Display( coker );
    Z/< 8 > + Z^(1 x 1)
    gap> nu := CokernelEpi( phi );
    <An epimorphism of left modules>
    gap> Display( nu );
    [ [  -5,   0 ],
      [  -6,   1 ],
      [   1,  -2 ],
      [   0,   1 ] ]
    
    the map is currently represented by the above 4 x 2 matrix
    gap> DefectOfExactness( phi, nu );
    <A zero left module>
    gap> ByASmallerPresentation( nu );
    <A non-zero epimorphism of left modules>
    gap> Display( nu );
    [ [   2,   0 ],
      [   1,  -2 ],
      [   0,   1 ] ]
    
    the map is currently represented by the above 3 x 2 matrix
    gap> PreInverse( nu );
    false
  
  
  10.4-3 functor_ImageObject
  
  functor_ImageObject global variable
  
  The functor that associates to a map its image.
  
    Code  
    InstallValue( functor_ImageObject_for_fp_modules,
            CreateHomalgFunctor(
                    [ "name", "ImageObject for modules" ],
                    [ "category", HOMALG_MODULES.category ],
                    [ "operation", "ImageObject" ],
                    [ "natural_transformation", "ImageObjectEmb" ],
                    [ "number_of_arguments", 1 ],
                    [ "1", [ [ "covariant" ],
                            [ IsMapOfFinitelyGeneratedModulesRep and
                              AdmissibleInputForHomalgFunctors ] ] ],
                    [ "OnObjects", _Functor_ImageObject_OnModules ]
                    )
            );
  
  
  10.4-4 ImageObject
  
  ImageObject( phi )  operation
  
  The  following  example  also  makes  use  of  the  natural  transformations
  ImageObjectEpi and ImageObjectEmb.
  
    Example  
    gap> ZZ := HomalgRingOfIntegers( );
    Z
    gap> M := HomalgMatrix( "[ 2, 3, 4,   5, 6, 7 ]", 2, 3, ZZ );;
    gap> M := LeftPresentation( M );
    <A non-torsion left module presented by 2 relations for 3 generators>
    gap> N := HomalgMatrix( "[ 2, 3, 4, 5,   6, 7, 8, 9 ]", 2, 4, ZZ );;
    gap> N := LeftPresentation( N );
    <A non-torsion left module presented by 2 relations for 4 generators>
    gap> mat := HomalgMatrix( "[ \
    > 1, 0, -3, -6, \
    > 0, 1,  6, 11, \
    > 1, 0, -3, -6  \
    > ]", 3, 4, ZZ );;
    gap> phi := HomalgMap( mat, M, N );;
    gap> IsMorphism( phi );
    true
    gap> phi;
    <A homomorphism of left modules>
    gap> im := ImageObject( phi );
    <A left module presented by yet unknown relations for 3 generators>
    gap> ByASmallerPresentation( im );
    <A free left module of rank 1 on a free generator>
    gap> pi := ImageObjectEpi( phi );
    <A non-zero split epimorphism of left modules>
    gap> epsilon := ImageObjectEmb( phi );
    <A monomorphism of left modules>
    gap> phi = pi * epsilon;
    true
  
  
  10.4-5 Kernel
  
  Kernel( phi )  operation
  
  The   following  example  also  makes  use  of  the  natural  transformation
  KernelEmb.
  
    Example  
    gap> ZZ := HomalgRingOfIntegers( );
    Z
    gap> M := HomalgMatrix( "[ 2, 3, 4,   5, 6, 7 ]", 2, 3, ZZ );;
    gap> M := LeftPresentation( M );
    <A non-torsion left module presented by 2 relations for 3 generators>
    gap> N := HomalgMatrix( "[ 2, 3, 4, 5,   6, 7, 8, 9 ]", 2, 4, ZZ );;
    gap> N := LeftPresentation( N );
    <A non-torsion left module presented by 2 relations for 4 generators>
    gap> mat := HomalgMatrix( "[ \
    > 1, 0, -3, -6, \
    > 0, 1,  6, 11, \
    > 1, 0, -3, -6  \
    > ]", 3, 4, ZZ );;
    gap> phi := HomalgMap( mat, M, N );;
    gap> IsMorphism( phi );
    true
    gap> phi;
    <A homomorphism of left modules>
    gap> ker := Kernel( phi );
    <A cyclic left module presented by yet unknown relations for a cyclic generato\
    r>
    gap> Display( ker );
    Z/< -3 >
    gap> ByASmallerPresentation( last );
    <A cyclic torsion left module presented by 1 relation for a cyclic generator>
    gap> Display( ker );
    Z/< 3 >
    gap> iota := KernelEmb( phi );
    <A monomorphism of left modules>
    gap> Display( iota );
    [ [  0,  2,  4 ] ]
    
    the map is currently represented by the above 1 x 3 matrix
    gap> DefectOfExactness( iota, phi );
    <A zero left module>
    gap> ByASmallerPresentation( iota );
    <A non-zero monomorphism of left modules>
    gap> Display( iota );
    [ [  2,  0 ] ]
    
    the map is currently represented by the above 1 x 2 matrix
    gap> PostInverse( iota );
    fail
  
  
  10.4-6 DefectOfExactness
  
  DefectOfExactness( phi, psi )  operation
  
  We follow the associative convention for applying maps. For left modules phi
  is  applied first and from the right. For right modules psi is applied first
  and from the left.
  
  The   following  example  also  makes  use  of  the  natural  transformation
  KernelEmb.
  
    Example  
    gap> ZZ := HomalgRingOfIntegers( );
    Z
    gap> M := HomalgMatrix( "[ 2, 3, 4, 0,   5, 6, 7, 0 ]", 2, 4, ZZ );;
    gap> M := LeftPresentation( M );
    <A non-torsion left module presented by 2 relations for 4 generators>
    gap> N := HomalgMatrix( "[ 2, 3, 4, 5,   6, 7, 8, 9 ]", 2, 4, ZZ );;
    gap> N := LeftPresentation( N );
    <A non-torsion left module presented by 2 relations for 4 generators>
    gap> mat := HomalgMatrix( "[ \
    > 1, 3,  3,  3, \
    > 0, 3, 10, 17, \
    > 1, 3,  3,  3, \
    > 0, 0,  0,  0  \
    > ]", 4, 4, ZZ );;
    gap> phi := HomalgMap( mat, M, N );;
    gap> IsMorphism( phi );
    true
    gap> phi;
    <A homomorphism of left modules>
    gap> iota := KernelEmb( phi );
    <A monomorphism of left modules>
    gap> DefectOfExactness( iota, phi );
    <A zero left module>
    gap> hom_iota := Hom( iota );	## a shorthand for Hom( iota, ZZ );
    <A homomorphism of right modules>
    gap> hom_phi := Hom( phi );	## a shorthand for Hom( phi, ZZ );
    <A homomorphism of right modules>
    gap> DefectOfExactness( hom_iota, hom_phi );
    <A cyclic right module on a cyclic generator satisfying yet unknown relations>
    gap> ByASmallerPresentation( last );
    <A cyclic torsion right module on a cyclic generator satisfying 1 relation>
    gap> Display( last );
    Z/< 2 >
  
  
  10.4-7 Functor_Hom
  
  Functor_Hom global variable
  
  The bifunctor Hom.
  
    Code  
    InstallValue( Functor_Hom_for_fp_modules,
            CreateHomalgFunctor(
                    [ "name", "Hom" ],
                    [ "category", HOMALG_MODULES.category ],
                    [ "operation", "Hom" ],
                    [ "number_of_arguments", 2 ],
                    [ "1", [ [ "contravariant", "right adjoint", "distinguished" ] ] ],
                    [ "2", [ [ "covariant", "left exact" ] ] ],
                    [ "OnObjects", _Functor_Hom_OnModules ],
                    [ "OnMorphisms", _Functor_Hom_OnMaps ],
                    [ "MorphismConstructor", HOMALG_MODULES.category.MorphismConstructor ]
                    )
            );
  
  
  10.4-8 Hom
  
  Hom( o1, o2 )  operation
  
  o1  resp.  o2 could be a module, a map, a complex (of modules or of again of
  complexes), or a chain morphism.
  
  Each  generator  of  a  module  of homomorphisms is displayed as a matrix of
  appropriate dimensions.
  
    Example  
    gap> ZZ := HomalgRingOfIntegers( );
    Z
    gap> M := HomalgMatrix( "[ 2, 3, 4,   5, 6, 7 ]", 2, 3, ZZ );;
    gap> M := LeftPresentation( M );
    <A non-torsion left module presented by 2 relations for 3 generators>
    gap> N := HomalgMatrix( "[ 2, 3, 4, 5,   6, 7, 8, 9 ]", 2, 4, ZZ );;
    gap> N := LeftPresentation( N );
    <A non-torsion left module presented by 2 relations for 4 generators>
    gap> mat := HomalgMatrix( "[ \
    > 1, 0, -3, -6, \
    > 0, 1,  6, 11, \
    > 1, 0, -3, -6  \
    > ]", 3, 4, ZZ );;
    gap> phi := HomalgMap( mat, M, N );;
    gap> IsMorphism( phi );
    true
    gap> phi;
    <A homomorphism of left modules>
    gap> psi := Hom( phi, M );
    <A homomorphism of right modules>
    gap> ByASmallerPresentation( psi );
    <A non-zero homomorphism of right modules>
    gap> Display( psi );
    [ [   1,   1,   0,   1 ],
      [   2,   2,   0,   0 ],
      [   0,   0,   6,  10 ] ]
    
    the map is currently represented by the above 3 x 4 matrix
    gap> homNM := Source( psi );
    <A rank 2 right module on 4 generators satisfying 2 relations>
    gap> IsIdenticalObj( homNM, Hom( N, M ) );	## the caching at work
    true
    gap> homMM := Range( psi );
    <A rank 1 right module on 3 generators satisfying 2 relations>
    gap> IsIdenticalObj( homMM, Hom( M, M ) );	## the caching at work
    true
    gap> Display( homNM );
    Z/< 3 > + Z/< 3 > + Z^(2 x 1)
    gap> Display( homMM );
    Z/< 3 > + Z/< 3 > + Z^(1 x 1)
    gap> IsMonomorphism( psi );
    false
    gap> IsEpimorphism( psi );
    false
    gap> GeneratorsOfModule( homMM );
    <A set of 3 generators of a homalg right module>
    gap> Display( last );
    [ [  0,  0,  0 ],
      [  0,  1,  2 ],
      [  0,  0,  0 ] ]
    
    the map is currently represented by the above 3 x 3 matrix
    
    [ [  0,  2,  4 ],
      [  0,  0,  0 ],
      [  0,  2,  4 ] ]
    
    the map is currently represented by the above 3 x 3 matrix
    
    [ [   0,   1,   3 ],
      [   0,   0,  -2 ],
      [   0,   1,   3 ] ]
    
    the map is currently represented by the above 3 x 3 matrix
    
    a set of 3 generators given by the the above matrices
    gap> GeneratorsOfModule( homNM );
    <A set of 4 generators of a homalg right module>
    gap> Display( last );
    [ [  0,  1,  2 ],
      [  0,  1,  2 ],
      [  0,  1,  2 ],
      [  0,  0,  0 ] ]
    
    the map is currently represented by the above 4 x 3 matrix
    
    [ [  0,  1,  2 ],
      [  0,  0,  0 ],
      [  0,  0,  0 ],
      [  0,  2,  4 ] ]
    
    the map is currently represented by the above 4 x 3 matrix
    
    [ [   0,   0,  -3 ],
      [   0,   0,   7 ],
      [   0,   0,  -5 ],
      [   0,   0,   1 ] ]
    
    the map is currently represented by the above 4 x 3 matrix
    
    [ [   0,   1,  -3 ],
      [   0,   0,  12 ],
      [   0,   0,  -9 ],
      [   0,   2,   6 ] ]
    
    the map is currently represented by the above 4 x 3 matrix
    
    a set of 4 generators given by the the above matrices
  
  
  If for example the source N gets a new presentation, you will see the effect
  on the generators:
  
    Example  
    gap> ByASmallerPresentation( N );
    <A rank 2 left module presented by 1 relation for 3 generators>
    gap> GeneratorsOfModule( homNM );
    <A set of 4 generators of a homalg right module>
    gap> Display( last );
    [ [  0,  3,  6 ],
      [  0,  1,  2 ],
      [  0,  0,  0 ] ]
    
    the map is currently represented by the above 3 x 3 matrix
    
    [ [   0,   9,  18 ],
      [   0,   0,   0 ],
      [   0,   2,   4 ] ]
    
    the map is currently represented by the above 3 x 3 matrix
    
    [ [   0,   0,   0 ],
      [   0,   0,  -5 ],
      [   0,   0,   1 ] ]
    
    the map is currently represented by the above 3 x 3 matrix
    
    [ [   0,   9,  18 ],
      [   0,   0,  -9 ],
      [   0,   2,   6 ] ]
    
    the map is currently represented by the above 3 x 3 matrix
    
    a set of 4 generators given by the the above matrices
  
  
  Now we compute a certain natural filtration on Hom(M,M):
  
    Example  
    gap> dM := Resolution( M );
    <A non-zero right acyclic complex containing a single morphism of left modules\
     at degrees [ 0 .. 1 ]>
    gap> hMM := Hom( dM, dM );
    <A non-zero acyclic cocomplex containing a single morphism of right complexes \
    at degrees [ 0 .. 1 ]>
    gap> BMM := HomalgBicomplex( hMM );
    <A non-zero bicocomplex containing right modules at bidegrees [ 0 .. 1 ]x
    [ -1 .. 0 ]>
    gap> II_E := SecondSpectralSequenceWithFiltration( BMM );
    <A stable cohomological spectral sequence with sheets at levels 
    [ 0 .. 2 ] each consisting of right modules at bidegrees [ -1 .. 0 ]x
    [ 0 .. 1 ]>
    gap> Display( II_E );
    The associated transposed spectral sequence:
    
    a cohomological spectral sequence at bidegrees
    [ [ 0 .. 1 ], [ -1 .. 0 ] ]
    ---------
    Level 0:
    
     * *
     * *
    ---------
    Level 1:
    
     * *
     . .
    ---------
    Level 2:
    
     s s
     . .
    
    Now the spectral sequence of the bicomplex:
    
    a cohomological spectral sequence at bidegrees
    [ [ -1 .. 0 ], [ 0 .. 1 ] ]
    ---------
    Level 0:
    
     * *
     * *
    ---------
    Level 1:
    
     * *
     * *
    ---------
    Level 2:
    
     s s
     . s
    gap> filt := FiltrationBySpectralSequence( II_E );
    <A descending filtration with degrees [ -1 .. 0 ] and graded parts:
      
    -1:	<A non-zero cyclic torsion right module on a cyclic generator satisfying
         yet unknown relations>
       0:	<A rank 1 right module on 3 generators satisfying 2 relations>
    of
    <A right module on 4 generators satisfying yet unknown relations>>
    gap> ByASmallerPresentation( filt );
    <A descending filtration with degrees [ -1 .. 0 ] and graded parts:
      
    -1:	<A non-zero cyclic torsion right module on a cyclic generator satisfying 1\
     relation>
       0:	<A rank 1 right module on 2 generators satisfying 1 relation>
    of
    <A rank 1 right module on 3 generators satisfying 2 relations>>
    gap> Display( filt );
    Degree -1:
    
    Z/< 3 >
    ----------
    Degree 0:
    
    Z/< 3 > + Z^(1 x 1)
    gap> Display( homMM );
    Z/< 3 > + Z/< 3 > + Z^(1 x 1)
  
  
  10.4-9 Functor_TensorProduct
  
  Functor_TensorProduct global variable
  
  The tensor product bifunctor.
  
    Code  
    InstallValue( Functor_TensorProduct_for_fp_modules,
            CreateHomalgFunctor(
                    [ "name", "TensorProduct" ],
                    [ "category", HOMALG_MODULES.category ],
                    [ "operation", "TensorProductOp" ],
                    [ "number_of_arguments", 2 ],
                    [ "1", [ [ "covariant", "left adjoint", "distinguished" ] ] ],
                    [ "2", [ [ "covariant", "left adjoint" ] ] ],
                    [ "OnObjects", _Functor_TensorProduct_OnModules ],
                    [ "OnMorphisms", _Functor_TensorProduct_OnMaps ],
                    [ "MorphismConstructor", HOMALG_MODULES.category.MorphismConstructor ]
                    )
            );
  
  
  10.4-10 TensorProduct
  
  TensorProduct( o1, o2 )  operation
  \*( o1, o2 )  operation
  
  o1  resp.  o2 could be a module, a map, a complex (of modules or of again of
  complexes), or a chain morphism.
  
  The  symbol  *  is  a  shorthand  for several operations associated with the
  functor   Functor_TensorProduct_for_fp_modules   installed  under  the  name
  TensorProduct.
  
    Example  
    gap> ZZ := HomalgRingOfIntegers( );
    Z
    gap> M := HomalgMatrix( "[ 2, 3, 4,   5, 6, 7 ]", 2, 3, ZZ );
    <A 2 x 3 matrix over an internal ring>
    gap> M := LeftPresentation( M );
    <A non-torsion left module presented by 2 relations for 3 generators>
    gap> N := HomalgMatrix( "[ 2, 3, 4, 5,   6, 7, 8, 9 ]", 2, 4, ZZ );
    <A 2 x 4 matrix over an internal ring>
    gap> N := LeftPresentation( N );
    <A non-torsion left module presented by 2 relations for 4 generators>
    gap> mat := HomalgMatrix( "[ \
    > 1, 0, -3, -6, \
    > 0, 1,  6, 11, \
    > 1, 0, -3, -6  \
    > ]", 3, 4, ZZ );
    <A 3 x 4 matrix over an internal ring>
    gap> phi := HomalgMap( mat, M, N );
    <A "homomorphism" of left modules>
    gap> IsMorphism( phi );
    true
    gap> phi;
    <A homomorphism of left modules>
    gap> L := Hom( ZZ, M );
    <A rank 1 right module on 3 generators satisfying yet unknown relations>
    gap> ByASmallerPresentation( L );
    <A rank 1 right module on 2 generators satisfying 1 relation>
    gap> Display( L );
    Z/< 3 > + Z^(1 x 1)
    gap> L;
    <A rank 1 right module on 2 generators satisfying 1 relation>
    gap> psi := phi * L;
    <A homomorphism of right modules>
    gap> ByASmallerPresentation( psi );
    <A non-zero homomorphism of right modules>
    gap> Display( psi );
    [ [   0,   0,   1,   1 ],
      [   0,   0,   8,   1 ],
      [   0,   0,   0,  -2 ],
      [   0,   0,   0,   2 ] ]
    
    the map is currently represented by the above 4 x 4 matrix
    gap> ML := Source( psi );
    <A rank 1 right module on 4 generators satisfying 3 relations>
    gap> IsIdenticalObj( ML, M * L );	## the caching at work
    true
    gap> NL := Range( psi );
    <A rank 2 right module on 4 generators satisfying 2 relations>
    gap> IsIdenticalObj( NL, N * L );	## the caching at work
    true
    gap> Display( ML );
    Z/< 3 > + Z/< 3 > + Z/< 3 > + Z^(1 x 1)
    gap> Display( NL );
    Z/< 3 > + Z/< 12 > + Z^(2 x 1)
  
  
  Now we compute a certain natural filtration on the tensor product M*L:
  
    Example  
    gap> P := Resolution( M );
    <A non-zero right acyclic complex containing a single morphism of left modules\
     at degrees [ 0 .. 1 ]>
    gap> GP := Hom( P );
    <A non-zero acyclic cocomplex containing a single morphism of right modules at\
     degrees [ 0 .. 1 ]>
    gap> CE := Resolution( GP );
    <An acyclic cocomplex containing a single morphism of right complexes at degre\
    es [ 0 .. 1 ]>
    gap> FCE := Hom( CE, L );
    <A non-zero acyclic complex containing a single morphism of left cocomplexes a\
    t degrees [ 0 .. 1 ]>
    gap> BC := HomalgBicomplex( FCE );
    <A non-zero bicomplex containing left modules at bidegrees [ 0 .. 1 ]x
    [ -1 .. 0 ]>
    gap> II_E := SecondSpectralSequenceWithFiltration( BC );
    <A stable homological spectral sequence with sheets at levels 
    [ 0 .. 2 ] each consisting of left modules at bidegrees [ -1 .. 0 ]x
    [ 0 .. 1 ]>
    gap> Display( II_E );
    The associated transposed spectral sequence:
    
    a homological spectral sequence at bidegrees
    [ [ 0 .. 1 ], [ -1 .. 0 ] ]
    ---------
    Level 0:
    
     * *
     * *
    ---------
    Level 1:
    
     * *
     . .
    ---------
    Level 2:
    
     s s
     . .
    
    Now the spectral sequence of the bicomplex:
    
    a homological spectral sequence at bidegrees
    [ [ -1 .. 0 ], [ 0 .. 1 ] ]
    ---------
    Level 0:
    
     * *
     * *
    ---------
    Level 1:
    
     * *
     . s
    ---------
    Level 2:
    
     s s
     . s
    gap> filt := FiltrationBySpectralSequence( II_E );
    <An ascending filtration with degrees [ -1 .. 0 ] and graded parts:
       0:	<A rank 1 left module presented by 1 relation for 2 generators>
      -1:	<A non-zero left module presented by 2 relations for 2 generators>
    of
    <A non-zero left module presented by 10 relations for 6 generators>>
    gap> ByASmallerPresentation( filt );
    <An ascending filtration with degrees [ -1 .. 0 ] and graded parts:
       0:	<A rank 1 left module presented by 1 relation for 2 generators>
      -1:	<A non-zero torsion left module presented by 2 relations
                 for 2 generators>
    of
    <A rank 1 left module presented by 3 relations for 4 generators>>
    gap> Display( filt );
    Degree 0:
    
    Z/< 3 > + Z^(1 x 1)
    ----------
    Degree -1:
    
    Z/< 3 > + Z/< 3 >
    gap> Display( ML );
    Z/< 3 > + Z/< 3 > + Z/< 3 > + Z^(1 x 1)
  
  
  10.4-11 Functor_Ext
  
  Functor_Ext global variable
  
  The bifunctor Ext.
  
  Below    is    the   only   specific   line   of   code   used   to   define
  Functor_Ext_for_fp_modules and all the different operations Ext in homalg.
  
    Code  
    RightSatelliteOfCofunctor( Functor_Hom_for_fp_modules, "Ext" );
  
  
  10.4-12 Ext
  
  Ext( [c, ]o1, o2[, str] )  operation
  
  Compute  the  c-th  extension  object of o1 with o2 where c is a nonnegative
  integer  and  o1 resp. o2 could be a module, a map, a complex (of modules or
  of   again   of   complexes),  or  a  chain  morphism.  If  str=a  then  the
  (cohomologically)  graded  object  Ext^i(o1,o2) for 0 ≤ i ≤c is computed. If
  neither  c  nor  str  is  specified  then  the cohomologically graded object
  Ext^i(o1,o2)  for  0  ≤  i  ≤  d  is  computed, where d is the length of the
  internally computed free resolution of o1.
  
  Each  generator  of  a  module  of  extensions  is  displayed as a matrix of
  appropriate dimensions.
  
    Example  
    gap> ZZ := HomalgRingOfIntegers( );
    Z
    gap> M := HomalgMatrix( "[ 2, 3, 4,   5, 6, 7 ]", 2, 3, ZZ );;
    gap> M := LeftPresentation( M );
    <A non-torsion left module presented by 2 relations for 3 generators>
    gap> N := TorsionObject( M );
    <A cyclic torsion left module presented by yet unknown relations for a cyclic \
    generator>
    gap> iota := TorsionObjectEmb( M );
    <A monomorphism of left modules>
    gap> psi := Ext( 1, iota, N );
    <A homomorphism of right modules>
    gap> ByASmallerPresentation( psi );
    <A non-zero homomorphism of right modules>
    gap> Display( psi );
    [ [  2 ] ]
    
    the map is currently represented by the above 1 x 1 matrix
    gap> extNN := Range( psi );
    <A non-zero cyclic torsion right module on a cyclic generator satisfying 1 rel\
    ation>
    gap> IsIdenticalObj( extNN, Ext( 1, N, N ) );	## the caching at work
    true
    gap> extMN := Source( psi );
    <A non-zero cyclic torsion right module on a cyclic generator satisfying 1 rel\
    ation>
    gap> IsIdenticalObj( extMN, Ext( 1, M, N ) );	## the caching at work
    true
    gap> Display( extNN );
    Z/< 3 >
    gap> Display( extMN );
    Z/< 3 >
  
  
  10.4-13 Functor_Tor
  
  Functor_Tor global variable
  
  The bifunctor Tor.
  
  Below    is    the   only   specific   line   of   code   used   to   define
  Functor_Tor_for_fp_modules and all the different operations Tor in homalg.
  
    Code  
    LeftSatelliteOfFunctor( Functor_TensorProduct_for_fp_modules, "Tor" );
  
  
  10.4-14 Tor
  
  Tor( [c, ]o1, o2[, str] )  operation
  
  Compute  the  c-th  torsion  object  of  o1 with o2 where c is a nonnegative
  integer  and  o1 resp. o2 could be a module, a map, a complex (of modules or
  of   again   of   complexes),  or  a  chain  morphism.  If  str=a  then  the
  (cohomologically)  graded  object  Tor_i(o1,o2) for 0 ≤ i ≤c is computed. If
  neither  c  nor  str  is  specified  then  the cohomologically graded object
  Tor_i(o1,o2)  for  0  ≤  i  ≤  d  is  computed, where d is the length of the
  internally computed free resolution of o1.
  
    Example  
    gap> ZZ := HomalgRingOfIntegers( );
    Z
    gap> M := HomalgMatrix( "[ 2, 3, 4,   5, 6, 7 ]", 2, 3, ZZ );;
    gap> M := LeftPresentation( M );
    <A non-torsion left module presented by 2 relations for 3 generators>
    gap> N := TorsionObject( M );
    <A cyclic torsion left module presented by yet unknown relations for a cyclic \
    generator>
    gap> iota := TorsionObjectEmb( M );
    <A monomorphism of left modules>
    gap> psi := Tor( 1, iota, N );
    <A homomorphism of left modules>
    gap> ByASmallerPresentation( psi );
    <A non-zero homomorphism of left modules>
    gap> Display( psi );
    [ [  1 ] ]
    
    the map is currently represented by the above 1 x 1 matrix
    gap> torNN := Source( psi );
    <A non-zero cyclic torsion left module presented by 1 relation for a cyclic ge\
    nerator>
    gap> IsIdenticalObj( torNN, Tor( 1, N, N ) );	## the caching at work
    true
    gap> torMN := Range( psi );
    <A non-zero cyclic torsion left module presented by 1 relation for a cyclic ge\
    nerator>
    gap> IsIdenticalObj( torMN, Tor( 1, M, N ) );	## the caching at work
    true
    gap> Display( torNN );
    Z/< 3 >
    gap> Display( torMN );
    Z/< 3 >
  
  
  10.4-15 Functor_RHom
  
  Functor_RHom global variable
  
  The bifunctor RHom.
  
  Below    is    the   only   specific   line   of   code   used   to   define
  Functor_RHom_for_fp_modules and all the different operations RHom in homalg.
  
    Code  
    RightDerivedCofunctor( Functor_Hom_for_fp_modules );
  
  
  10.4-16 RHom
  
  RHom( [c, ]o1, o2[, str] )  operation
  
  Compute  the  c-th  extension  object of o1 with o2 where c is a nonnegative
  integer  and  o1 resp. o2 could be a module, a map, a complex (of modules or
  of  again  of  complexes),  or  a  chain  morphism.  The string str may take
  different values:
  
      If str=a then R^i Hom(o1,o2) for 0 ≤ i ≤c is computed.
  
      If  str=c  then  the  c-th connecting homomorphism with respect to the
        short exact sequence o1 is computed.
  
      If  str=t  then  the  exact  triangle upto cohomological degree c with
        respect to the short exact sequence o1 is computed.
  
  If neither c nor str is specified then the cohomologically graded object R^i
  Hom(o1,o2)  for  0  ≤  i  ≤  d  is  computed,  where  d is the length of the
  internally computed free resolution of o1.
  
  Each generator of a module of derived homomorphisms is displayed as a matrix
  of appropriate dimensions.
  
    Example  
    gap> ZZ := HomalgRingOfIntegers( );
    Z
    gap> m := HomalgMatrix( [ [ 8, 0 ], [ 0, 2 ] ], ZZ );;
    gap> M := LeftPresentation( m );
    <A left module presented by 2 relations for 2 generators>
    gap> Display( M );
    Z/< 8 > + Z/< 2 >
    gap> M;
    <A torsion left module presented by 2 relations for 2 generators>
    gap> a := HomalgMatrix( [ [ 2, 0 ] ], ZZ );;
    gap> alpha := HomalgMap( a, "free", M );
    <A homomorphism of left modules>
    gap> pi := CokernelEpi( alpha );
    <An epimorphism of left modules>
    gap> Display( pi );
    [ [  1,  0 ],
      [  0,  1 ] ]
    
    the map is currently represented by the above 2 x 2 matrix
    gap> iota := KernelEmb( pi );
    <A monomorphism of left modules>
    gap> Display( iota );
    [ [  2,  0 ] ]
    
    the map is currently represented by the above 1 x 2 matrix
    gap> N := Kernel( pi );
    <A cyclic torsion left module presented by yet unknown relations for a cyclic \
    generator>
    gap> Display( N );
    Z/< 4 >
    gap> C := HomalgComplex( pi );
    <A left acyclic complex containing a single morphism of left modules at degree\
    s [ 0 .. 1 ]>
    gap> Add( C, iota );
    gap> ByASmallerPresentation( C );
    <A non-zero short exact sequence containing
    2 morphisms of left modules at degrees [ 0 .. 2 ]>
    gap> Display( C );
    -------------------------
    at homology degree: 2
    Z/< 4 >
    -------------------------
    [ [  0,  6 ] ]
    
    the map is currently represented by the above 1 x 2 matrix
    ------------v------------
    at homology degree: 1
    Z/< 2 > + Z/< 8 >
    -------------------------
    [ [  0,  1 ],
      [  1,  1 ] ]
    
    the map is currently represented by the above 2 x 2 matrix
    ------------v------------
    at homology degree: 0
    Z/< 2 > + Z/< 2 >
    -------------------------
    gap> T := RHom( C, N );
    <An exact cotriangle containing 3 morphisms of right cocomplexes at degrees
    [ 0, 1, 2, 0 ]>
    gap> ByASmallerPresentation( T );
    <A non-zero exact cotriangle containing
    3 morphisms of right cocomplexes at degrees [ 0, 1, 2, 0 ]>
    gap> L := LongSequence( T );
    <A cosequence containing 5 morphisms of right modules at degrees [ 0 .. 5 ]>
    gap> Display( L );
    -------------------------
    at cohomology degree: 5
    Z/< 4 >
    ------------^------------
    [ [  0,  3 ] ]
    
    the map is currently represented by the above 1 x 2 matrix
    -------------------------
    at cohomology degree: 4
    Z/< 2 > + Z/< 4 >
    ------------^------------
    [ [  0,  1 ],
      [  0,  0 ] ]
    
    the map is currently represented by the above 2 x 2 matrix
    -------------------------
    at cohomology degree: 3
    Z/< 2 > + Z/< 2 >
    ------------^------------
    [ [  1 ],
      [  0 ] ]
    
    the map is currently represented by the above 2 x 1 matrix
    -------------------------
    at cohomology degree: 2
    Z/< 4 >
    ------------^------------
    [ [  0,  2 ] ]
    
    the map is currently represented by the above 1 x 2 matrix
    -------------------------
    at cohomology degree: 1
    Z/< 2 > + Z/< 4 >
    ------------^------------
    [ [  0,  1 ],
      [  2,  0 ] ]
    
    the map is currently represented by the above 2 x 2 matrix
    -------------------------
    at cohomology degree: 0
    Z/< 2 > + Z/< 2 >
    -------------------------
    gap> IsExactSequence( L );
    true
    gap> L;
    <An exact cosequence containing 5 morphisms of right modules at degrees
    [ 0 .. 5 ]>
  
  
  10.4-17 Functor_LTensorProduct
  
  Functor_LTensorProduct global variable
  
  The bifunctor LTensorProduct.
  
  Below    is    the   only   specific   line   of   code   used   to   define
  Functor_LTensorProduct_for_fp_modules   and  all  the  different  operations
  LTensorProduct in homalg.
  
    Code  
    LeftDerivedFunctor( Functor_TensorProduct_for_fp_modules );
  
  
  10.4-18 LTensorProduct
  
  LTensorProduct( [c, ]o1, o2[, str] )  operation
  
  Compute  the  c-th  torsion  object  of  o1 with o2 where c is a nonnegative
  integer  and  o1 resp. o2 could be a module, a map, a complex (of modules or
  of  again  of  complexes),  or  a  chain  morphism.  The string str may take
  different values:
  
      If str=a then L_i TensorProduct(o1,o2) for 0 ≤ i ≤c is computed.
  
      If  str=c  then  the  c-th connecting homomorphism with respect to the
        short exact sequence o1 is computed.
  
      If  str=t  then  the  exact  triangle upto cohomological degree c with
        respect to the short exact sequence o1 is computed.
  
  If neither c nor str is specified then the cohomologically graded object L_i
  TensorProduct(o1,o2) for 0 ≤ i ≤ d is computed, where d is the length of the
  internally computed free resolution of o1.
  
  Each generator of a module of derived homomorphisms is displayed as a matrix
  of appropriate dimensions.
  
    Example  
    gap> ZZ := HomalgRingOfIntegers( );
    Z
    gap> m := HomalgMatrix( [ [ 8, 0 ], [ 0, 2 ] ], ZZ );;
    gap> M := LeftPresentation( m );
    <A left module presented by 2 relations for 2 generators>
    gap> Display( M );
    Z/< 8 > + Z/< 2 >
    gap> M;
    <A torsion left module presented by 2 relations for 2 generators>
    gap> a := HomalgMatrix( [ [ 2, 0 ] ], ZZ );;
    gap> alpha := HomalgMap( a, "free", M );
    <A homomorphism of left modules>
    gap> pi := CokernelEpi( alpha );
    <An epimorphism of left modules>
    gap> Display( pi );
    [ [  1,  0 ],
      [  0,  1 ] ]
    
    the map is currently represented by the above 2 x 2 matrix
    gap> iota := KernelEmb( pi );
    <A monomorphism of left modules>
    gap> Display( iota );
    [ [  2,  0 ] ]
    
    the map is currently represented by the above 1 x 2 matrix
    gap> N := Kernel( pi );
    <A cyclic torsion left module presented by yet unknown relations for a cyclic \
    generator>
    gap> Display( N );
    Z/< 4 >
    gap> C := HomalgComplex( pi );
    <A left acyclic complex containing a single morphism of left modules at degree\
    s [ 0 .. 1 ]>
    gap> Add( C, iota );
    gap> ByASmallerPresentation( C );
    <A non-zero short exact sequence containing
    2 morphisms of left modules at degrees [ 0 .. 2 ]>
    gap> Display( C );
    -------------------------
    at homology degree: 2
    Z/< 4 >
    -------------------------
    [ [  0,  6 ] ]
    
    the map is currently represented by the above 1 x 2 matrix
    ------------v------------
    at homology degree: 1
    Z/< 2 > + Z/< 8 >
    -------------------------
    [ [  0,  1 ],
      [  1,  1 ] ]
    
    the map is currently represented by the above 2 x 2 matrix
    ------------v------------
    at homology degree: 0
    Z/< 2 > + Z/< 2 >
    -------------------------
    gap> T := LTensorProduct( C, N );
    <An exact triangle containing 3 morphisms of left complexes at degrees
    [ 1, 2, 3, 1 ]>
    gap> ByASmallerPresentation( T );
    <A non-zero exact triangle containing
    3 morphisms of left complexes at degrees [ 1, 2, 3, 1 ]>
    gap> L := LongSequence( T );
    <A sequence containing 5 morphisms of left modules at degrees [ 0 .. 5 ]>
    gap> Display( L );
    -------------------------
    at homology degree: 5
    Z/< 4 >
    -------------------------
    [ [  1,  3 ] ]
    
    the map is currently represented by the above 1 x 2 matrix
    ------------v------------
    at homology degree: 4
    Z/< 2 > + Z/< 4 >
    -------------------------
    [ [  0,  1 ],
      [  0,  1 ] ]
    
    the map is currently represented by the above 2 x 2 matrix
    ------------v------------
    at homology degree: 3
    Z/< 2 > + Z/< 2 >
    -------------------------
    [ [  2 ],
      [  0 ] ]
    
    the map is currently represented by the above 2 x 1 matrix
    ------------v------------
    at homology degree: 2
    Z/< 4 >
    -------------------------
    [ [  0,  2 ] ]
    
    the map is currently represented by the above 1 x 2 matrix
    ------------v------------
    at homology degree: 1
    Z/< 2 > + Z/< 4 >
    -------------------------
    [ [  0,  1 ],
      [  1,  1 ] ]
    
    the map is currently represented by the above 2 x 2 matrix
    ------------v------------
    at homology degree: 0
    Z/< 2 > + Z/< 2 >
    -------------------------
    gap> IsExactSequence( L );
    true
    gap> L;
    <An exact sequence containing 5 morphisms of left modules at degrees
    [ 0 .. 5 ]>
  
  
  10.4-19 Functor_HomHom
  
  Functor_HomHom global variable
  
  The bifunctor HomHom.
  
  Below    is    the   only   specific   line   of   code   used   to   define
  Functor_HomHom_for_fp_modules  and  all  the  different operations HomHom in
  homalg.
  
    Code  
    Functor_Hom_for_fp_modules * Functor_Hom_for_fp_modules;
  
  
  10.4-20 Functor_LHomHom
  
  Functor_LHomHom global variable
  
  The bifunctor LHomHom.
  
  Below    is    the   only   specific   line   of   code   used   to   define
  Functor_LHomHom_for_fp_modules  and  all the different operations LHomHom in
  homalg.
  
    Code  
    LeftDerivedFunctor( Functor_HomHom_for_fp_modules );
  
  
  
  10.5 Tool Functors
  
  
  10.6 Other Functors
  
  
  10.7 Functors: Operations and Functions