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# SPDX-License-Identifier: GPL-2.0-or-later
# Modules: A homalg based package for the Abelian category of finitely presented modules over computable rings
#
# Implementations
#
## Implementation stuff for homalg maps ( = module homomorphisms ).
## <#GAPDoc Label="Maps:intro">
## A &homalg; map is a data structures for maps (module homomorphisms) between finitely generated modules.
## Each map in &homalg; knows its source (&see; <Ref BookName="homalg" Attr="Source"/>) and its target
## (&see; <Ref BookName="homalg" Attr="Range"/>). A map is represented by a &homalg; matrix relative to the
## current set of generators of the source resp. target &homalg; module. As with modules
## (&see; Chapter <Ref Chap="Modules"/>), maps in &homalg; are realized in an intrinsic manner:
## If the presentations of the source or/and target module are altered after the map was constructed,
## a new adapted representation matrix of the map is automatically computed whenever needed.
## For this the internal transition matrices of the modules are used. &homalg; uses the so-called
## <E>associative</E> convention for maps. This means that maps of left modules are applied
## from the right, whereas maps of right modules from the left.
## <#/GAPDoc>
####################################
#
# representations:
#
####################################
## <#GAPDoc Label="IsMapOfFinitelyGeneratedModulesRep">
## <ManSection>
## <Filt Type="Representation" Arg="phi" Name="IsMapOfFinitelyGeneratedModulesRep"/>
## <Returns><C>true</C> or <C>false</C></Returns>
## <Description>
## The &GAP; representation of maps between finitley generated &homalg; modules. <P/>
## (It is a representation of the &GAP; category <Ref BookName="homalg" Filt="IsHomalgChainMorphism"/>,
## which is a subrepresentation of the &GAP; representation <C>IsStaticMorphismOfFinitelyGeneratedObjectsRep</C>.)
## </Description>
## </ManSection>
## <#/GAPDoc>
##
DeclareRepresentation( "IsMapOfFinitelyGeneratedModulesRep",
IsHomalgMap and IsStaticMorphismOfFinitelyGeneratedObjectsRep,
[ "source", "target", "matrices", "index_pairs_of_presentations" ] );
####################################
#
# families and types:
#
####################################
# a new family:
BindGlobal( "TheFamilyOfHomalgMaps",
NewFamily( "TheFamilyOfHomalgMaps" ) );
# four new types:
BindGlobal( "TheTypeHomalgMapOfLeftModules",
NewType( TheFamilyOfHomalgMaps,
AdmissibleInputForHomalgFunctors and
IsMapOfFinitelyGeneratedModulesRep and
IsHomalgLeftObjectOrMorphismOfLeftObjects ) );
BindGlobal( "TheTypeHomalgMapOfRightModules",
NewType( TheFamilyOfHomalgMaps,
AdmissibleInputForHomalgFunctors and
IsMapOfFinitelyGeneratedModulesRep and
IsHomalgRightObjectOrMorphismOfRightObjects ) );
BindGlobal( "TheTypeHomalgSelfMapOfLeftModules",
NewType( TheFamilyOfHomalgMaps,
AdmissibleInputForHomalgFunctors and
IsMapOfFinitelyGeneratedModulesRep and
IsHomalgSelfMap and
IsHomalgLeftObjectOrMorphismOfLeftObjects ) );
BindGlobal( "TheTypeHomalgSelfMapOfRightModules",
NewType( TheFamilyOfHomalgMaps,
AdmissibleInputForHomalgFunctors and
IsMapOfFinitelyGeneratedModulesRep and
IsHomalgSelfMap and
IsHomalgRightObjectOrMorphismOfRightObjects ) );
####################################
#
# methods for operations:
#
####################################
## <#GAPDoc Label="HomalgRing:map">
## <ManSection>
## <Oper Arg="phi" Name="HomalgRing"/>
## <Returns>a &homalg; ring</Returns>
## <Description>
## The &homalg; ring of the &homalg; map <A>phi</A>.
## <Example><![CDATA[
## gap> zz := HomalgRingOfIntegers( );
## Z
## gap> phi := HomalgIdentityMap( 2 * zz );
## <The identity morphism of a non-zero left module>
## gap> R := HomalgRing( phi );
## Z
## gap> IsIdenticalObj( R, zz );
## true
## ]]></Example>
## </Description>
## </ManSection>
## <#/GAPDoc>
##
InstallMethod( HomalgRing,
"for homalg maps",
[ IsHomalgMap ],
function( phi )
return HomalgRing( Source( phi ) );
end );
##
InstallMethod( homalgResetFilters,
"for homalg maps",
[ IsHomalgMap ],
function( phi )
local property;
for property in LIHOM.intrinsic_properties do
ResetFilterObj( phi, ValueGlobal( property ) );
od;
end );
## provided to avoid branching in the code and always returns fail
InstallMethod( PositionOfTheDefaultPresentation,
"for homalg maps",
[ IsMapOfFinitelyGeneratedModulesRep ],
function( M )
return fail;
end );
##
InstallMethod( PairOfPositionsOfTheDefaultPresentations,
"for homalg maps",
[ IsMapOfFinitelyGeneratedModulesRep ],
function( phi )
local pos_s, pos_t;
pos_s := PositionOfTheDefaultPresentation( Source( phi ) );
pos_t := PositionOfTheDefaultPresentation( Range( phi ) );
if IsHomalgLeftObjectOrMorphismOfLeftObjects( phi ) then
return [ pos_s, pos_t ];
else
return [ pos_t, pos_s ];
fi;
end );
##
InstallMethod( MatrixOfMap, ## FIXME: make this optimal by finding shortest ways
"for homalg maps",
[ IsMapOfFinitelyGeneratedModulesRep, IsInt, IsInt ],
function( phi, _pos_s, _pos_t )
local pos_s, pos_t, index_pair, l, dist, min, pos, matrix;
if _pos_s < 1 then
pos_s := PositionOfTheDefaultPresentation( Source( phi ) );
else
pos_s := _pos_s;
fi;
if _pos_t < 1 then
pos_t := PositionOfTheDefaultPresentation( Range( phi ) );
else
pos_t := _pos_t;
fi;
if IsHomalgLeftObjectOrMorphismOfLeftObjects( phi ) then
index_pair := [ pos_s, pos_t ];
else
index_pair := [ pos_t, pos_s ];
fi;
l := phi!.index_pairs_of_presentations;
if not index_pair in l then
dist := List( l, a -> AbsInt( index_pair[1] - a[1] ) + AbsInt( index_pair[2] - a[2] ) );
min := Minimum( dist );
pos := PositionProperty( dist, a -> a = min );
if IsHomalgLeftObjectOrMorphismOfLeftObjects( phi ) then
matrix :=
TransitionMatrix( Source( phi ), pos_s, l[pos][1] )
* phi!.matrices.( String( l[pos] ) )
* TransitionMatrix( Range( phi ), l[pos][2], pos_t );
else
matrix :=
TransitionMatrix( Range( phi ), pos_t, l[pos][1] )
* phi!.matrices.( String( l[pos] ) )
* TransitionMatrix( Source( phi ), l[pos][2], pos_s );
fi;
phi!.matrices.( String( index_pair ) ) := matrix;
Add( l, index_pair );
fi;
if IsBound( phi!.reduced_matrices.( String( index_pair ) ) ) then
return phi!.reduced_matrices.( String( index_pair ) );
else
return phi!.matrices.( String( index_pair ) );
fi;
end );
##
InstallMethod( MatrixOfMap,
"for homalg maps",
[ IsMapOfFinitelyGeneratedModulesRep and IsHomalgSelfMap, IsPosInt ],
function( phi, pos_s_t )
return MatrixOfMap( phi, pos_s_t, pos_s_t );
end );
##
InstallMethod( MatrixOfMap,
"for homalg maps",
[ IsMapOfFinitelyGeneratedModulesRep ],
function( phi )
return MatrixOfMap( phi, 0, 0 );
end );
##
InstallMethod( ZeroMutable,
"for homalg maps",
[ IsMapOfFinitelyGeneratedModulesRep ],
function( phi )
return TheZeroMorphism( Source( phi ), Range( phi ) );
end );
## a synonym of `-<elm>':
InstallMethod( AdditiveInverseMutable,
"of homalg maps",
[ IsMapOfFinitelyGeneratedModulesRep ],
function( phi )
local psi;
psi := MinusOne( HomalgRing( phi ) ) * phi;
SetPropertiesOfAdditiveInverse( psi, phi ); # not needed because of "*" ?
return psi;
end );
##
InstallMethod( BasisOfModule,
"for homalg maps",
[ IsMapOfFinitelyGeneratedModulesRep ],
function( phi )
BasisOfModule( Source( phi ) );
BasisOfModule( Range( phi ) );
return phi;
end );
##
InstallMethod( DecideZero,
"for homalg maps",
[ IsMapOfFinitelyGeneratedModulesRep ],
function( phi )
local rel, index_pair, matrix, reduced, zero;
## for compatibility with GradedModules:
## do not use RelationsOfModule( Range( phi ) ) here
## as it will be non-graded even if phi is graded
rel := MatrixOfRelations( Range( phi ) );
index_pair := PairOfPositionsOfTheDefaultPresentations( phi );
matrix := MatrixOfMap( phi );
if IsHomalgLeftObjectOrMorphismOfLeftObjects( phi ) then
reduced := DecideZeroRows( matrix, BasisOfRowModule( rel ) );
else
reduced := DecideZeroColumns( matrix, BasisOfColumnModule( rel ) );
fi;
reduced := DecideZero( reduced );
zero := IsZero( reduced );
SetIsZero( phi, zero );
if not zero then
SetIsZero( Source( phi ), false );
SetIsZero( Range( phi ), false );
fi;
## replace the original matrix by the reduced one; this is important
## since we want to keep the reduced form of a matrix over a
## residue class ring, although they are ``equal'' when compared using =
phi!.matrices.(String( index_pair )) := reduced;
phi!.reduced_matrices.(String( index_pair )) := reduced;
return phi;
end );
##
InstallMethod( OnLessGenerators,
"for homalg maps",
[ IsMapOfFinitelyGeneratedModulesRep ],
function( phi )
OnLessGenerators( Source( phi ) );
OnLessGenerators( Range( phi ) );
return phi;
end );
##
InstallMethod( UnionOfRelations,
"for homalg maps",
[ IsMapOfFinitelyGeneratedModulesRep ],
function( phi )
return UnionOfRelations( MatrixOfMap( phi ), Range( phi ) );
end );
##
InstallMethod( SyzygiesGenerators,
"for homalg maps",
[ IsMapOfFinitelyGeneratedModulesRep ],
function( phi )
local syz;
syz := SyzygiesGenerators( MatrixOfMap( phi ), Range( phi ) );
if NrRelations( syz ) = 0 then
SetIsMonomorphism( phi, true );
fi;
return syz;
end );
##
InstallMethod( ReducedSyzygiesGenerators,
"for homalg maps",
[ IsMapOfFinitelyGeneratedModulesRep ],
function( phi )
local syz;
syz := ReducedSyzygiesGenerators( MatrixOfMap( phi ), Range( phi ) );
if NrRelations( syz ) = 0 then
SetIsMonomorphism( phi, true );
fi;
return syz;
end );
##
InstallMethod( Preimage,
"for a matrix and a homalg map",
[ IsHomalgMatrix, IsMapOfFinitelyGeneratedModulesRep ],
function( m, phi )
local M, rel, mat;
M := Range( phi );
rel := MatrixOfRelations( M );
mat := MatrixOfMap( phi );
if IsHomalgLeftObjectOrMorphismOfLeftObjects( phi ) then
return RightDivide( m, mat, rel );
else
return LeftDivide( m, mat, rel );
fi;
end );
##
InstallMethod( SuccessivePreimages,
"for a matrix and a homalg selfmap",
[ IsHomalgMatrix, IsHomalgSelfMap and IsMapOfFinitelyGeneratedModulesRep ],
function( m, phi )
local preimages, pre, n;
preimages := [ m ];
pre := Preimage( m, phi );
while IsHomalgMatrix( pre ) do
Add( preimages, pre );
n := Length( preimages );
pre := Preimage( preimages[n], phi );
od;
return preimages;
end );
## <#GAPDoc Label="PreInverse">
## <ManSection>
## <Oper Arg="phi" Name="PreInverse"/>
## <Returns>a &homalg; map, <C>false</C>, or <C>fail</C></Returns>
## <Description>
## Compute a pre-inverse of the morphism <A>phi</A> in case one exists.
## For a pre-inverse to exist <A>phi</A> must be an epimorphism. For <E>commutative</E> rings
## &homalg; has an algorithm installed which decides the existence and returns
## a pre-inverse in case one exists. If a pre-inverse does not exist then <C>false</C>
## is returned. The algorithm finds a particular solution of a two-side inhomogeneous linear system
## over <M>R := </M><C>HomalgRing</C><M>( <A>phi</A> )</M>.
## For <E>non</E>commutative rings a heuristic method is installed. If it
## finds a pre-inverse it returns it, otherwise it returns <C>fail</C>
## (&see; <Ref Label="Modules-limitation" Text="Principal limitation"/>).
## The operation <C>PreInverse</C> is used to install a method for the property
## <Ref BookName="homalg" Prop="IsSplitEpimorphism"/>. <P/>
## <C>PreInverse</C> checks if it can decide the projectivity of <C>Range</C><M>( <A>phi</A> )</M>.
## </Description>
## </ManSection>
## <#/GAPDoc>
##
InstallMethod( PreInverse,
"for homalg maps",
[ IsMapOfFinitelyGeneratedModulesRep ],
function( phi )
local R, S, T, M, p, Ib, Ic, b, c, P, d, Id, PI, B, A, L, sigma;
## no need to search for phi!.PreInverse
## as this is not the highest priority method
R := HomalgRing( phi );
if not ( HasIsCommutative( R ) and IsCommutative( R ) ) then
TryNextMethod( );
fi;
S := Source( phi );
T := Range( phi );
M := MatrixOfRelations( ReducedBasisOfModule( S ) );
p := MatrixOfMap( phi );
#=====# begin of the core procedure #=====#
Ib := MatrixOfMap( TheIdentityMorphism( S ) );
Ic := MatrixOfMap( TheIdentityMorphism( T ) );
b := NumberRows( Ib );
c := NumberRows( Ic );
P := ReducedBasisOfModule( T );
d := NrRelations( P );
Id := HomalgIdentityMatrix( d, R );
P := MatrixOfRelations( P );
PI := Involution( P );
B := EntriesOfHomalgMatrix( Ic );
B := Concatenation( ListWithIdenticalEntries( b * d, Zero( R ) ), B );
if IsHomalgLeftObjectOrMorphismOfLeftObjects( phi ) then
B := HomalgMatrix( B, 1, b * d + c * c, R );
A := UnionOfColumns( KroneckerMat( PI, Ib ), KroneckerMat( Ic, p ) );
L := DiagMat( [ KroneckerMat( Id, M ), KroneckerMat( Ic, P ) ] );
sigma := RightDivide( B, A, L );
else
B := HomalgMatrix( B, b * d + c * c, 1, R );
A := UnionOfRows( KroneckerMat( Ib, PI ), KroneckerMat( p, Ic ) );
L := DiagMat( [ KroneckerMat( M, Id ), KroneckerMat( P, Ic ) ] );
sigma := LeftDivide( A, B, L );
fi;
if IsBool( sigma ) then ## no split
## from a method below we already know that phi is an epimorphism
if IsEpimorphism( phi ) then ## to be sure ;)
## so T is not projective since phi is not split
SetIsProjective( T, false );
fi;
phi!.PreInverse := false;
return phi!.PreInverse;
fi;
## we already have every thing to build the (matrix of the) split sigma
sigma := EntriesOfHomalgMatrix( sigma );
if IsHomalgLeftObjectOrMorphismOfLeftObjects( phi ) then
sigma := HomalgMatrix( sigma, c, b, R );
else
sigma := HomalgMatrix( sigma, b, c, R );
fi;
sigma := HomalgMap( sigma, T, S );
DecideZero( sigma );
Assert( 3, IsMonomorphism( sigma ) );
SetIsSplitEpimorphism( phi, true );
SetIsSplitMonomorphism( sigma, true );
## a direct summand of a projective module is again projective [HS, I.4.5]
if HasIsProjective( S ) and IsProjective( S ) then
SetIsProjective( T, true );
fi;
phi!.PreInverse := sigma;
sigma!.PostInverse := phi;
return sigma;
end );
##
InstallMethod( PreInverse,
"for homalg maps",
[ IsMapOfFinitelyGeneratedModulesRep ],
function( phi )
local S, T, Id, A, L, sigma;
if IsBound(phi!.PreInverse) then
return phi!.PreInverse;
fi;
if not IsEpimorphism( phi ) then
return false;
elif IsIsomorphism( phi ) then
## only in case the standard method for IsIsomorphism wasn't triggered:
UpdateObjectsByMorphism( phi );
sigma := phi ^ -1;
phi!.PreInverse := sigma;
phi!.PostInverse := sigma;
sigma!.PreInverse := phi;
sigma!.PostInverse := phi;
return sigma;
fi;
DecideZero( phi );
S := Source( phi );
T := Range( phi );
Id := HomalgIdentityMatrix( NrGenerators( T ), HomalgRing( T ) );
A := MatrixOfMap( phi );
L := MatrixOfRelations( T );
if IsHomalgLeftObjectOrMorphismOfLeftObjects( phi ) then
sigma := RightDivide( Id, A, L );
else
sigma := LeftDivide( A, Id, L );
fi;
if not IsHomalgMatrix( sigma ) then ## no split even on the level of matrices
## from above we already know that phi is an epimorphism,
## so T is not projective since phi is not split [HS, I.4.7.(3)]
SetIsProjective( T, false );
phi!.PreInverse := false;
return phi!.PreInverse;
elif HasIsZero( sigma ) and IsZero( sigma ) then
## if the matrix is nonzero we still don't know if a split exists
SetIsZero( T, true );
SetIsZero( phi, true );
sigma := TheZeroMorphism( T, S );
phi!.PreInverse := sigma;
sigma!.PostInverse := phi;
return sigma;
fi;
sigma := HomalgMap( sigma, T, S );
if IsMorphism( sigma ) then
DecideZero( sigma );
Assert( 3, IsMonomorphism( sigma ) );
SetIsSplitEpimorphism( phi, true );
SetIsSplitMonomorphism( sigma, true );
## a direct summand of a projective module is again projective [HS, I.4.5]
if HasIsProjective( S ) and IsProjective( S ) then
SetIsProjective( T, true );
fi;
phi!.PreInverse := sigma;
sigma!.PostInverse := phi;
return sigma;
fi;
TryNextMethod( );
end );
##
InstallMethod( PostInverse,
"for homalg maps",
[ IsMapOfFinitelyGeneratedModulesRep ],
function( phi )
local S, T, A, chi;
if IsBound(phi!.PostInverse) then
return phi!.PostInverse;
fi;
if not IsMonomorphism( phi ) then
return false;
elif IsIsomorphism( phi ) then
## only in case the standard method for IsIsomorphism wasn't triggered:
UpdateObjectsByMorphism( phi );
chi := phi ^ -1;
phi!.PostInverse := chi;
phi!.PreInverse := chi;
chi!.PostInverse := phi;
chi!.PreInverse := phi;
return chi;
fi;
DecideZero( phi );
S := Source( phi );
T := Range( phi );
A := MatrixOfMap( phi );
if IsHomalgLeftObjectOrMorphismOfLeftObjects( phi ) then
chi := RightInverse( A );
else
chi := LeftInverse( A );
fi;
## a post-inverse might still exist even if Right/LeftInverse fails
## (this is fundamentally different from the situation in PreInverse)
if IsBool( chi ) then
TryNextMethod( );
fi;
if HasIsZero( chi ) and IsZero( chi ) then
## if the matrix is nonzero we still don't know if a split exists
SetIsZero( S, true );
SetIsZero( phi, true );
chi := TheZeroMorphism( T, S );
phi!.PostInverse := chi;
chi!.PreInverse := phi;
return chi;
return TheZeroMorphism( T, S );
fi;
chi := HomalgMap( chi, T, S );
if IsMorphism( chi ) then
DecideZero( chi );
Assert( 3, IsEpimorphism( chi ) );
SetIsSplitMonomorphism( phi, true );
SetIsSplitEpimorphism( chi, true );
## a direct factor of an injective module is again injective [HS, I.6.3]
if HasIsInjective( T ) and IsInjective( T ) then
SetIsInjective( S, true );
fi;
phi!.PostInverse := chi;
chi!.PreInverse := phi;
return chi;
fi;
TryNextMethod( );
end );
##
InstallMethod( \/,
"for a homalg map and a module of homomorphisms",
[ IsMapOfFinitelyGeneratedModulesRep, IsFinitelyPresentedModuleRep ],
function( phi, H )
local S_T, left, R, n, gen, proc, rel, map;
if not FunctorOfGenesis( H ) = Functor_Hom_for_fp_modules then
TryNextMethod( );
fi;
S_T := Genesis( H )[1][1].arguments_of_functor;
if not IsIdenticalObj( Source( phi ), S_T[1] ) or
not IsIdenticalObj( Range( phi ), S_T[2] ) then
return false;
fi;
if not IsMorphism( phi ) then
return false;
fi;
left := IsHomalgLeftObjectOrMorphismOfLeftObjects( H );
if IsZero( phi ) then
R := HomalgRing( phi );
n := NrGenerators( H );
if left then
return HomalgZeroMatrix( 1, n, R );
else
return HomalgZeroMatrix( n, 1, R );
fi;
fi;
gen := GeneratorsOfModule( H );
DecideZero( gen );
proc := ProcedureToNormalizeGenerators( gen );
rel := RelationsOfHullModule( gen );
BasisOfModule( rel );
gen := MatrixOfGenerators( H );
DecideZero( phi );
map := MatrixOfMap( phi );
map := CallFuncList( proc[1], Concatenation( [ map ], proc{[ 2 .. Length( proc ) ]} ) );
if left then ## H not phi !!!
return RightDivide( map, gen, rel );
else
return LeftDivide( gen, map, rel );
fi;
end );
####################################
#
# constructor functions and methods:
#
####################################
## <#GAPDoc Label="HomalgMap">
## <ManSection>
## <Func Arg="mat, M, N" Name="HomalgMap" Label="constructor for maps"/>
## <Func Arg="mat[, string]" Name="HomalgMap" Label="constructor for maps between free modules"/>
## <Returns>a &homalg; map</Returns>
## <Description>
## This constructor returns a map (homomorphism) of finitely presented modules. It is represented by the
## &homalg; matrix <A>mat</A> relative to the current set of generators of the source &homalg; module <A>M</A>
## and target module <A>N</A> (&see; <Ref Sect="Modules:Constructors"/>). Unless the source module is free
## <E>and</E> given on free generators the returned map will cautiously be indicated using
## parenthesis: <Q>homomorphism</Q>. To verify if the result is indeed a well defined map use
## <Ref BookName="homalg" Prop="IsMorphism"/>. If the presentations of the source or/and
## target module are altered after the map was constructed, a new adapted representation matrix of the map is
## automatically computed whenever needed. For this the internal transition matrices of the modules are used.
## If source and target are identical objects, and only then, the map is created as a selfmap (endomorphism).
## &homalg; uses the so-called <E>associative</E> convention for maps. This means that maps of left modules are
## applied from the right, whereas maps of right modules from the left.
## <Example><![CDATA[
## gap> zz := HomalgRingOfIntegers( );;
## 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, -2, -4, \
## > 0, 1, 4, 7, \
## > 1, 0, -2, -4 \
## > ]", 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> Display( phi );
## [ [ 1, 0, -2, -4 ],
## [ 0, 1, 4, 7 ],
## [ 1, 0, -2, -4 ] ]
##
## the map is currently represented by the above 3 x 4 matrix
## gap> ByASmallerPresentation( M );
## <A rank 1 left module presented by 1 relation for 2 generators>
## gap> Display( last );
## Z/< 3 > + Z^(1 x 1)
## gap> Display( phi );
## [ [ 2, 1, 0, -1 ],
## [ 1, 0, -2, -4 ] ]
##
## the map is currently represented by the above 2 x 4 matrix
## gap> ByASmallerPresentation( N );
## <A rank 2 left module presented by 1 relation for 3 generators>
## gap> Display( N );
## Z/< 4 > + Z^(1 x 2)
## gap> Display( phi );
## [ [ -8, 0, 0 ],
## [ -3, -1, -2 ] ]
##
## the map is currently represented by the above 2 x 3 matrix
## gap> ByASmallerPresentation( phi );
## <A non-zero homomorphism of left modules>
## gap> Display( phi );
## [ [ 0, 0, 0 ],
## [ 1, -1, -2 ] ]
##
## the map is currently represented by the above 2 x 3 matrix
## ]]></Example>
## To construct a map with source being a not yet specified free module
## <Example><![CDATA[
## gap> N;
## <A rank 2 left module presented by 1 relation for 3 generators>
## gap> SetPositionOfTheDefaultSetOfGenerators( N, 1 );
## gap> N;
## <A rank 2 left module presented by 2 relations for 4 generators>
## gap> psi := HomalgMap( mat, "free", N );
## <A homomorphism of left modules>
## gap> Source( psi );
## <A free left module of rank 3 on free generators>
## ]]></Example>
## To construct a map between not yet specified free left modules
## <Example><![CDATA[
## gap> chi := HomalgMap( mat ); ## or chi := HomalgMap( mat, "l" );
## <A homomorphism of left modules>
## gap> Source( chi );
## <A free left module of rank 3 on free generators>
## gap> Range( chi );
## <A free left module of rank 4 on free generators>
## ]]></Example>
## To construct a map between not yet specified free right modules
## <Example><![CDATA[
## gap> kappa := HomalgMap( mat, "r" );
## <A homomorphism of right modules>
## gap> Source( kappa );
## <A free right module of rank 4 on free generators>
## gap> Range( kappa );
## <A free right module of rank 3 on free generators>
## ]]></Example>
## </Description>
## </ManSection>
## <#/GAPDoc>
##
InstallGlobalFunction( _HomalgMap,
function( arg )
local nargs, source, pos_s, target, pos_t, R, type, matrix, left, matrices, reduced_matrices,
mat, nr_rows, nr_columns, index_pair, morphism, option;
nargs := Length( arg );
if IsHomalgRelations( arg[1] ) then
mat := MatrixOfRelations( arg[1] );
fi;
if nargs > 1 then
if IsHomalgModule( arg[2] ) then
source := arg[2];
pos_s := PositionOfTheDefaultPresentation( source );
elif arg[2] = "free" and nargs > 2 and IsHomalgModule( arg[3] )
and ( IsHomalgMatrix( arg[1] ) or IsHomalgRelations( arg[1] ) ) then
if IsHomalgMatrix( arg[1] ) then
mat := arg[1];
fi;
if IsHomalgLeftObjectOrMorphismOfLeftObjects( arg[3] ) then
nr_rows := NumberRows( mat );
source := HomalgFreeLeftModule( nr_rows, HomalgRing( arg[3] ) );
else
nr_columns := NumberColumns( mat );
source := HomalgFreeRightModule( nr_columns, HomalgRing( arg[3] ) );
fi;
pos_s := PositionOfTheDefaultPresentation( source );
elif IsHomalgRing( arg[2] ) and not ( IsList( arg[1] ) and nargs = 2 ) then
source := "ring";
elif IsList( arg[2] ) and IsHomalgModule( arg[2][1] ) and IsPosInt( arg[2][2] ) then
source := arg[2][1];
pos_s := arg[2][2];
if not IsBound( SetsOfRelations( source )!.( pos_s ) ) then
Error( "the source module does not possess a ", arg[2][2], ". set of relations (this positive number is given as the second entry of the list provided as the second argument)\n" );
fi;
fi;
fi;
if not IsBound( source ) then
if IsHomalgMatrix( arg[1] ) then
ResetFilterObj( arg[1], IsMutable );
matrix := arg[1];
elif IsHomalgRelations( arg[1] ) then
matrix := MatrixOfRelations( arg[1] );
left := IsHomalgRelationsOfLeftModule( arg[1] );
elif IsHomalgRing( arg[nargs] ) then
matrix := HomalgMatrix( arg[1], arg[nargs] );
else
Error( "The second argument must be the source module or the last argument should be an IsHomalgRing\n" );
fi;
R := HomalgRing( matrix );
if nargs > 1 and IsStringRep( arg[2] ) and Length( arg[2] ) > 0
and LowercaseString( arg[2]{[1..1]} ) = "r" then
left := false; ## we explicitly asked for a morphism of right modules
elif not IsBound( left ) then
left := true;
fi;
if left then
source := HomalgFreeLeftModule( NumberRows( matrix ), R );
target := HomalgFreeLeftModule( NumberColumns( matrix ), R );
type := TheTypeHomalgMapOfLeftModules;
else
source := HomalgFreeRightModule( NumberColumns( matrix ), R );
target := HomalgFreeRightModule( NumberRows( matrix ), R );
type := TheTypeHomalgMapOfRightModules;
fi;
matrices := rec( );
morphism := rec(
matrices := matrices,
reduced_matrices := rec( ),
index_pairs_of_presentations := [ [ 1, 1 ] ]);
matrices.( String( [ 1, 1 ] ) ) := matrix;
## Objectify:
ObjectifyWithAttributes(
morphism, type,
Source, source,
Range, target );
if ( HasNrRelations( source ) = true and NrRelations( source ) = 0 ) then
SetIsMorphism( morphism, true );
fi;
if HasIsZero( source ) and IsZero( source ) then
SetIsGeneralizedMonomorphism( morphism, true ); ## we don't know yet if IsMorhphism( morphism ) = true
fi;
if HasIsZero( target ) and IsZero( target ) then
SetIsGeneralizedEpimorphism( morphism, true ); ## we don't know yet if IsMorhphism( morphism ) = true
fi;
return morphism;
fi;
if nargs > 2 then
if IsHomalgModule( arg[3] ) then
target := arg[3];
pos_t := PositionOfTheDefaultPresentation( target );
elif arg[3] = "free" and IsHomalgModule ( source )
and ( IsHomalgMatrix( arg[1] ) or IsHomalgRelations( arg[1] ) ) then
if IsHomalgLeftObjectOrMorphismOfLeftObjects( source ) then
if IsHomalgMatrix( arg[1] ) then
nr_columns := NumberColumns( arg[1] );
elif IsHomalgRelations( arg[1] ) then
nr_columns := NumberColumns( MatrixOfRelations( arg[1] ) );
fi;
target := HomalgFreeLeftModule( nr_columns, HomalgRing( arg[1] ) );
else
if IsHomalgMatrix( arg[1] ) then
nr_rows := NumberRows( arg[1] );
elif IsHomalgRelations( arg[1] ) then
nr_rows := NumberRows( MatrixOfRelations( arg[1] ) );
fi;
target := HomalgFreeRightModule( nr_rows, HomalgRing( arg[1] ) );
fi;
pos_t := PositionOfTheDefaultPresentation( target );
elif IsHomalgRing( arg[3] ) then
if source = "ring" then
source := HomalgFreeLeftModule( 1, arg[2] );
if not IsIdenticalObj( arg[2], arg[3] ) then
Error( "the source and target modules must be defined over the same ring\n" );
fi;
target := source; ## we get an endomorphism
pos_s := PositionOfTheDefaultPresentation( source );
pos_t := pos_s;
else
target := HomalgFreeLeftModule( 1, arg[3] );
pos_t := PositionOfTheDefaultPresentation( target );
fi;
elif IsList( arg[3] ) and IsHomalgModule( arg[3][1] ) and IsPosInt( arg[3][2] ) then
target := arg[3][1];
pos_t := arg[3][2];
if not IsBound( SetsOfRelations( target )!.( pos_t ) ) then
Error( "the target module does not possess a ", arg[3][2], ". set of relations (this positive number is given as the second entry of the list provided as the third argument)\n" );
fi;
fi;
elif source = "ring" then
source := HomalgFreeLeftModule( 1, arg[2] );
target := source; ## we get an endomorphism
pos_s := PositionOfTheDefaultPresentation( source );
pos_t := pos_s;
else
pos_t := pos_s;
fi;
R := HomalgRing( source );
if IsBound( target ) and not IsIdenticalObj( source, target ) then
if not IsIdenticalObj( R, HomalgRing( target ) ) then
Error( "the source and target modules must be defined over the same ring\n" );
elif IsHomalgLeftObjectOrMorphismOfLeftObjects( source ) and IsHomalgLeftObjectOrMorphismOfLeftObjects( target ) then
type := TheTypeHomalgMapOfLeftModules;
elif IsHomalgRightObjectOrMorphismOfRightObjects( source ) and IsHomalgRightObjectOrMorphismOfRightObjects( target ) then
type := TheTypeHomalgMapOfRightModules;
else
Error( "the source and target modules of a morphism must either both be left or both be right modules\n" );
fi;
else
target := source;
if IsHomalgLeftObjectOrMorphismOfLeftObjects( source ) then
type := TheTypeHomalgSelfMapOfLeftModules;
else
type := TheTypeHomalgSelfMapOfRightModules;
fi;
fi;
if IsHomalgLeftObjectOrMorphismOfLeftObjects( source ) then
nr_rows := NrGenerators( source, pos_s );
nr_columns := NrGenerators( target, pos_t );
index_pair := [ pos_s, pos_t ];
else
nr_columns := NrGenerators( source, pos_s );
nr_rows := NrGenerators( target, pos_t );
index_pair := [ pos_t, pos_s ];
fi;
matrices := rec( );
reduced_matrices := rec( );
morphism := rec(
matrices := matrices,
reduced_matrices := reduced_matrices,
index_pairs_of_presentations := [ index_pair ]);
if IsList( arg[1] ) and Length( arg[1] ) = 1 and IsString( arg[1][1] ) and Length( arg[1][1] ) > 0 then
option := arg[1][1];
if Length( option ) > 3 and LowercaseString( option{[1..4]} ) = "zero" then
## the zero map:
matrix := HomalgZeroMatrix( nr_rows, nr_columns, R );
matrices.( String( index_pair ) ) := matrix;
reduced_matrices.( String( index_pair ) ) := matrix;
## Objectify:
ObjectifyWithAttributes(
morphism, type,
Source, source,
Range, target,
IsZero, true );
if HasIsZero( source ) and IsZero( source ) then
SetIsSplitMonomorphism( morphism, true );
fi;
if HasIsZero( target ) and IsZero( target ) then
SetIsSplitEpimorphism( morphism, true );
fi;
elif Length( option ) > 7 and LowercaseString( option{[1..8]} ) = "identity" then
## the identity map:
if nr_rows <> nr_columns then
Error( "for a matrix of a morphism to be the identity matrix the number of generators of the source and target module must coincide\n" );
fi;
matrix := HomalgIdentityMatrix( nr_rows, R );
matrices.( String( index_pair ) ) := matrix;
if IsIdenticalObj( source, target ) then
if pos_s = pos_t then
## Objectify:
ObjectifyWithAttributes(
morphism, type,
Source, source,
Range, target,
IsMorphism, true,
IsOne, true );
else
## Objectify:
ObjectifyWithAttributes(
morphism, type,
Source, source,
Range, target );
fi;
else
## Objectify:
ObjectifyWithAttributes(
morphism, type,
Source, source,
Range, target,
IsMorphism, true,
IsEpimorphism, true );
fi;
else
Error( "wrong first argument: ", arg[1], "\n" );
fi;
else
if IsHomalgMatrix( arg[1] ) then
if not IsIdenticalObj( HomalgRing( arg[1] ), R ) then
Error( "the matrix and the modules are not defined over identically the same ring\n" );
fi;
ResetFilterObj( arg[1], IsMutable );
matrix := arg[1];
elif IsHomalgRelations( arg[1] ) then
if not IsIdenticalObj( HomalgRing( arg[1] ), R ) then
Error( "the matrix and the modules are not defined over identically the same ring\n" );
fi;
matrix := MatrixOfRelations( arg[1] );
elif IsString( arg[1] ) then
if IsHomalgLeftObjectOrMorphismOfLeftObjects( source ) then
nr_rows := NrGenerators( source, pos_s );
nr_columns := NrGenerators( target, pos_t );
elif IsHomalgRightObjectOrMorphismOfRightObjects( source ) then
nr_rows := NrGenerators( target, pos_t );
nr_columns := NrGenerators( source, pos_s );
fi;
matrix := HomalgMatrix( arg[1], nr_rows, nr_columns, R );
elif IsList( arg[1] ) then
matrix := HomalgMatrix( arg[1], R );
else
Error( "the first argument must be in { IsHomalgMatrix, IsHomalgRelations, IsMatrix, IsString, IsList } but received: ", arg[1], "\n" );
fi;
if IsHomalgLeftObjectOrMorphismOfLeftObjects( source )
and ( NrGenerators( source, pos_s ) <> NumberRows( matrix )
or NrGenerators( target, pos_t ) <> NumberColumns( matrix ) ) then
Error( "the dimensions of the matrix do not match the numbers of generators of the modules\n" );
elif IsHomalgRightObjectOrMorphismOfRightObjects( source )
and ( NrGenerators( source, pos_s ) <> NumberColumns( matrix )
or NrGenerators( target, pos_t ) <> NumberRows( matrix ) ) then
Error( "the dimensions of the matrix do not match the numbers of generators of the modules\n" );
fi;
matrices.( String( index_pair ) ) := matrix;
## Objectify:
ObjectifyWithAttributes(
morphism, type,
Source, source,
Range, target );
fi;
if ( HasNrRelations( source ) = true and NrRelations( source ) = 0 ) then
SetIsMorphism( morphism, true );
fi;
if HasIsZero( source ) and IsZero( source ) then
SetIsGeneralizedMonomorphism( morphism, true ); ## we don't know yet if IsMorhphism( morphism ) = true
fi;
if HasIsZero( target ) and IsZero( target ) then
SetIsGeneralizedEpimorphism( morphism, true ); ## we don't know yet if IsMorhphism( morphism ) = true
fi;
return morphism;
end );
##
InstallMethod( HomalgMap,
"for three objects",
[ IsObject, IsObject, IsObject ],
_HomalgMap );
##
InstallMethod( HomalgMap,
"for a list and two homalg modules",
[ IsList, IsHomalgModule, IsHomalgModule ],
_HomalgMap );
##
InstallMethod( HomalgMap,
"for a homalg matrix and two homalg modules",
[ IsHomalgMatrix, IsHomalgModule, IsHomalgModule ],
_HomalgMap );
##
InstallMethod( HomalgMap,
"for two objects",
[ IsObject, IsObject ],
_HomalgMap );
##
InstallMethod( HomalgMap,
"for an object",
[ IsObject ],
_HomalgMap );
## <#GAPDoc Label="HomalgZeroMap">
## <ManSection>
## <Func Arg="M, N" Name="HomalgZeroMap" Label="constructor for zero maps"/>
## <Returns>a &homalg; map</Returns>
## <Description>
## The constructor returns the zero map between the source &homalg; module <A>M</A>
## and the target &homalg; module <A>N</A>.
## <Example><![CDATA[
## gap> zz := HomalgRingOfIntegers( );;
## 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> HomalgZeroMap( M, N );
## <The zero morphism of left modules>
## ]]></Example>
## </Description>
## </ManSection>
## <#/GAPDoc>
##
InstallGlobalFunction( HomalgZeroMap,
function( arg )
return CallFuncList( HomalgMap, Concatenation( [ [ "zero" ] ], arg ) );
end );
## <#GAPDoc Label="HomalgIdentityMap">
## <ManSection>
## <Func Arg="M, N" Name="HomalgIdentityMap" Label="constructor for identity maps"/>
## <Returns>a &homalg; map</Returns>
## <Description>
## The constructor returns the identity map of the &homalg; module <A>M</A>.
## <Example><![CDATA[
## gap> zz := HomalgRingOfIntegers( );;
## 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> HomalgIdentityMap( M );
## <The identity morphism of a non-zero left module>
## ]]></Example>
## </Description>
## </ManSection>
## <#/GAPDoc>
##
InstallGlobalFunction( HomalgIdentityMap,
function( arg )
return CallFuncList( HomalgMap, Concatenation( [ [ "identity" ] ], arg ) );
end );
##
InstallMethod( OnAFreeSource,
"for homalg maps",
[ IsMapOfFinitelyGeneratedModulesRep ],
function( phi )
local psi;
psi := HomalgMap( MatrixOfMap( phi ), "free", Range( phi ) );
if HasIsMorphism( phi ) and IsMorphism( phi ) or HasIsGeneralizedMorphismWithFullDomain( phi ) and IsGeneralizedMorphismWithFullDomain( phi ) then
Assert( 3, IsMorphism( psi ) );
SetIsMorphism( psi, true );
fi;
if HasIsEpimorphism( phi ) and IsEpimorphism( phi ) then
Assert( 3, IsEpimorphism( psi ) );
SetIsEpimorphism( psi, true );
fi;
return psi;
end );
## works without side effects
InstallMethod( RemoveMorphismAid,
"for homalg maps",
[ IsMapOfFinitelyGeneratedModulesRep ],
function( phi )
return HomalgMap( MatrixOfMap( phi ), Source( phi ), Range( phi ) );
end );
## works without side effects
InstallMethod( GeneralizedMorphism,
"for homalg maps",
[ IsMapOfFinitelyGeneratedModulesRep, IsObject ],
function( phi, morphism_aid_map )
local morphism_aid_map1, psi;
if not IsHomalgMap( morphism_aid_map ) and not ( IsList( morphism_aid_map ) and Length( morphism_aid_map ) = 1 and IsHomalgMap( morphism_aid_map[1] ) ) then
return phi;
fi;
if not IsList( morphism_aid_map ) and not IsIdenticalObj( Range( phi ), Range( morphism_aid_map ) ) then
Error( "the targets of the two morphisms must coincide or the aid must be a list\n" );
fi;
if IsList( morphism_aid_map ) then
morphism_aid_map1 := morphism_aid_map;
else
## we don't need the source of the morphism aid map
morphism_aid_map1 := OnAFreeSource( morphism_aid_map );
fi;
## prepare a copy of phi
psi := HomalgMap( MatrixOfMap( phi ), Source( phi ), Range( phi ) );
SetMorphismAid( psi, morphism_aid_map1 );
## some properties of the morphism phi imply
## properties for the generalized morphism psi
SetPropertiesOfGeneralizedMorphism( psi, phi );
return psi;
end );
## works without side effects
InstallMethod( AddToMorphismAid,
"for homalg maps",
[ IsHomalgMap, IsObject ],
function( phi, morphism_aid_map )
local morphism_aid_map1, morphism_aid_map0;
if not IsHomalgMap( morphism_aid_map ) then
return phi;
fi;
if not IsIdenticalObj( Range( phi ), Range( morphism_aid_map ) ) then
Error( "the targets of the two morphisms must coincide\n" );
fi;
## we don't need the source of the new morphism aid map
morphism_aid_map1 := OnAFreeSource( morphism_aid_map );
if HasMorphismAid( phi ) then
## we don't need the source of the old morphism aid map
morphism_aid_map0 := OnAFreeSource( MorphismAid( phi ) );
morphism_aid_map1 := CoproductMorphism( morphism_aid_map0, morphism_aid_map1 );
fi;
return GeneralizedMorphism( phi, morphism_aid_map1 );
end );
##
InstallMethod( ShallowCopy,
"for homalg maps",
[ IsMapOfFinitelyGeneratedModulesRep ],
function( phi )
local psi;
if HasMorphismAid( phi ) then
TryNextMethod( );
fi;
if IsHomalgEndomorphism( phi ) then
psi := HomalgMap( MatrixOfMap( phi ), ShallowCopy( Source( phi ) ) );
else
psi := HomalgMap( MatrixOfMap( phi ), ShallowCopy( Source( phi ) ), ShallowCopy( Range( phi ) ) );
fi;
MatchPropertiesAndAttributes( phi, psi, LIHOM.intrinsic_properties, LIHOM.intrinsic_attributes );
return psi;
end );
##
InstallMethod( UpdateObjectsByMorphism,
"for homalg maps",
[ IsHomalgMap and IsIsomorphism ],
function( phi )
if HasIsZero( Source( phi ) ) or HasIsZero( Range( phi ) ) then
IsZero( phi );
fi;
MatchPropertiesAndAttributes( Source( phi ), Range( phi ), LIMOD.intrinsic_properties, LIMOD.intrinsic_attributes );
end );
##
InstallMethod( AnIsomorphism,
"for homalg modules",
[ IsFinitelyPresentedModuleRep ],
function( M )
local rel, N, iso;
rel := RelationsOfModule( M );
## important since each set of relations knows the module it represents
rel := ShallowCopy( rel );
N := Presentation( GeneratorsOfModule( M ), rel );
## define the obvious isomorphism between N an M
iso := HomalgIdentityMatrix( NrGenerators( M ), HomalgRing( M ) );
iso := HomalgMap( iso, N, M );
SetIsIsomorphism( iso, true );
## copy the known properties and attributes of im to def
UpdateObjectsByMorphism( iso );
return iso;
end );
##
InstallMethod( Pullback,
"for a ring map and a module map",
[ IsHomalgRingMap, IsMapOfFinitelyGeneratedModulesRep ],
function( phi, f )
local Sf, Tf, S, T, map;
Sf := Source( f );
Tf := Range( f );
if IsBound( Sf!.distinguished ) and Sf!.distinguished = true then
if IsHomalgLeftObjectOrMorphismOfLeftObjects( Sf ) then
S := 1 * Range( phi );
else
S := Range( phi ) * 1;
fi;
else
S := Pullback( phi, Sf );
fi;
if IsBound( Tf!.distinguished ) and Tf!.distinguished = true then
if IsHomalgLeftObjectOrMorphismOfLeftObjects( Tf ) then
T := 1 * Range( phi );
else
T := Range( phi ) * 1;
fi;
else
T := Pullback( phi, Tf );
fi;
map := Pullback( phi, MatrixOfMap( f ) );
map := HomalgMap( map, S, T );
return map;
end );
####################################
#
# View, Print, and Display methods:
#
####################################
##
InstallMethod( Display,
"for homalg maps",
[ IsMapOfFinitelyGeneratedModulesRep ],
function( o )
Display( o, "" );
end );
##
InstallMethod( Display,
"for homalg maps",
[ IsMapOfFinitelyGeneratedModulesRep, IsString ],
function( o, extra_information )
local T, mat;
T := Range( o );
mat := MatrixOfMap( o );
Display( mat );
if extra_information <> "" then
Print( "\nthe ", extra_information, " map is currently represented by the above ", NumberRows( mat ), " x ", NumberColumns( mat ), " matrix\n" );
else
Print( "\nthe map is currently represented by the above ", NumberRows( mat ), " x ", NumberColumns( mat ), " matrix\n" );
fi;
end );
[ 0.48Quellennavigators
Projekt
]
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