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<div class="ChapSects"><a href="chap2.html#X7967FE8E7BBDF485">2 Examples and Tests</a>
<div class="ContSect"> <a href="chap2.html#X8104A77D7B5CCD4F">2.1 Basic Commands</a>

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<div class="ContSect"> <a href="chap2.html#X78D1062D78BE08C1">2.2 Functors</a>

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<div class="ContSect"> <a href="chap2.html#X85B1F5077BE75340">2.3 Solving (Homogeneous) Linear Systems</a>

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<div class="ContSect"> <a href="chap2.html#X852F59737EC57965">2.4 Homology object</a>

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<div class="ContSect"> <a href="chap2.html#X7CDB7FF27D734D2D">2.5 Liftable</a>

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<div class="ContSect"> <a href="chap2.html#X797080CE873BB56D">2.6 Monoidal structure</a>

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<div class="ContSect"> <a href="chap2.html#X8218390A785F1EDB">2.7 MorphismFromSourceToPushout and MorphismFromFiberProductToSink</a>

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<div class="ContSect"> <a href="chap2.html#X84410C637B3C1D0E">2.8 Opposite category</a>

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<div class="ContSect"> <a href="chap2.html#X7F34E29281DF3FCE">2.9 PreComposeList and PostComposeList</a>

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<div class="ContSect"> <a href="chap2.html#X87F5609A7B33C58C">2.10 Split epi summand</a>

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<div class="ContSect"> <a href="chap2.html#X7DCD99628504B810">2.11 Kernel</a>

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<div class="ContSect"> <a href="chap2.html#X7DE20941803BFBD9">2.12 FiberProduct</a>

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<div class="ContSect"> <a href="chap2.html#X8245BF297DF9A3E7">2.13 WrapperCategory</a>

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<h3>2 Examples and Tests</h3>

<a id="X8104A77D7B5CCD4F" name="X8104A77D7B5CCD4F"></a>

<h4>2.1 Basic Commands</h4>

<div class="example"><pre>
gap> LoadPackage( "LinearAlgebraForCAP", false );
true
gap> Q := HomalgFieldOfRationals();;
gap> vec := MatrixCategory( Q );;
gap> a := MatrixCategoryObject( vec, 3 );
<A vector space object over Q of dimension 3>
gap> IsProjective( a );
true
gap> ap := 3/vec;;
gap> IsEqualForObjects( a, ap );
true
gap> b := MatrixCategoryObject( vec, 4 );
<A vector space object over Q of dimension 4>
gap> homalg_matrix := HomalgMatrix( [ [ 1, 0, 0, 0 ],
> [ 0, 1, 0, -1 ],
> [ -1, 0, 2, 1 ] ], 3, 4, Q );;
gap> alpha := VectorSpaceMorphism( a, homalg_matrix, b );
<A morphism in Category of matrices over Q>
</pre></div>

<div class="example"><pre>
gap> # @drop_example_in_Julia: view/print/display strings of matrices differ between GAP and Julia, see https://github.com/homalg-project/MatricesForHomalg.jl/issues/41
> Display( alpha );
[ [ 1, 0, 0, 0 ],
 [ 0, 1, 0, -1 ],
 [ -1, 0, 2, 1 ] ]

A morphism in Category of matrices over Q
</pre></div>

<div class="example"><pre>
gap> alphap := homalg_matrix/vec;;
gap> IsCongruentForMorphisms( alpha, alphap );
true
gap> homalg_matrix := HomalgMatrix( [ [ 1, 1, 0, 0 ],
> [ 0, 1, 0, -1 ],
> [ -1, 0, 2, 1 ] ], 3, 4, Q );;
gap> beta := VectorSpaceMorphism( a, homalg_matrix, b );
<A morphism in Category of matrices over Q>
gap> CokernelObject( alpha );
<A vector space object over Q of dimension 1>
gap> c := CokernelProjection( alpha );;
gap> Display( EntriesOfHomalgMatrixAsListList( UnderlyingMatrix( c ) ) );
[ [ 0 ], [ 1 ], [ -1/2 ], [ 1 ] ]
gap> gamma := UniversalMorphismIntoDirectSum( [ c, c ] );;
gap> Display( EntriesOfHomalgMatrixAsListList( UnderlyingMatrix( gamma ) ) );
[ [ 0, 0 ], [ 1, 1 ], [ -1/2, -1/2 ], [ 1, 1 ] ]
gap> colift := CokernelColift( alpha, gamma );;
gap> IsEqualForMorphisms( PreCompose( c, colift ), gamma );
true
gap> FiberProduct( alpha, beta );
<A vector space object over Q of dimension 2>
gap> F := FiberProduct( alpha, beta );
<A vector space object over Q of dimension 2>
gap> p1 := ProjectionInFactorOfFiberProduct( [ alpha, beta ], 1 );
<A morphism in Category of matrices over Q>
gap> Display( EntriesOfHomalgMatrixAsListList( UnderlyingMatrix( PreCompose( p1, alpha ) ) ) );
[ [ 0, 1, 0, -1 ], [ -1, 0, 2, 1 ] ]
gap> Pushout( alpha, beta );
<A vector space object over Q of dimension 5>
gap> i1 := InjectionOfCofactorOfPushout( [ alpha, beta ], 1 );
<A morphism in Category of matrices over Q>
gap> i2 := InjectionOfCofactorOfPushout( [ alpha, beta ], 2 );
<A morphism in Category of matrices over Q>
gap> u := UniversalMorphismFromDirectSum( [ b, b ], [ i1, i2 ] );
<A morphism in Category of matrices over Q>
</pre></div>

<div class="example"><pre>
gap> # @drop_example_in_Julia: differences in the output of SyzygiesOfRows, see https://github.com/homalg-project/MatricesForHomalg.jl/issues/50
> Display( EntriesOfHomalgMatrixAsListList( UnderlyingMatrix( u ) ) );
[ [ 0, 1, 1, 0, 0 ],\
[ 1, 0, 1, 0, -1 ],\
[ -1/2, 0, 1/2, 1, 1/2 ],\
[ 1, 0, 0, 0, 0 ],\
[ 0, 1, 0, 0, 0 ],\
[ 0, 0, 1, 0, 0 ],\
[ 0, 0, 0, 1, 0 ],\
[ 0, 0, 0, 0, 1 ] ]
</pre></div>

<div class="example"><pre>
gap> KernelObjectFunctorial( u, IdentityMorphism( Source( u ) ), u ) = IdentityMorphism( MatrixCategoryObject( vec, 3 ) );
true
gap> IsZeroForMorphisms( CokernelObjectFunctorial( u, IdentityMorphism( Range( u ) ), u ) );
true
gap> DirectProductFunctorial( [ u, u ] ) = DirectSumFunctorial( [ u, u ] );
true
gap> CoproductFunctorial( [ u, u ] ) = DirectSumFunctorial( [ u, u ] );
true
gap> IsCongruentForMorphisms(
> FiberProductFunctorial( [ u, u ], [ IdentityMorphism( Source( u ) ), IdentityMorphism( Source( u ) ) ], [ u, u ] ),
> IdentityMorphism( FiberProduct( [ u, u ] ) )
> );
true
gap> IsCongruentForMorphisms(
> PushoutFunctorial( [ u, u ], [ IdentityMorphism( Range( u ) ), IdentityMorphism( Range( u ) ) ], [ u, u ] ),
> IdentityMorphism( Pushout( [ u, u ] ) )
> );
true
gap> IsCongruentForMorphisms( ((1/2) / Q) * alpha, alpha * ((1/2) / Q) );
true
gap> Dimension( HomomorphismStructureOnObjects( a, b ) ) = Dimension( a ) * Dimension( b );
true
gap> IsCongruentForMorphisms(
> PreCompose( [ u, DualOnMorphisms( i1 ), DualOnMorphisms( alpha ) ] ),
> InterpretMorphismFromDistinguishedObjectToHomomorphismStructureAsMorphism( Source( u ), Source( alpha ),
> PreCompose(
> InterpretMorphismAsMorphismFromDistinguishedObjectToHomomorphismStructure( DualOnMorphisms( i1 ) ),
> HomomorphismStructureOnMorphisms( u, DualOnMorphisms( alpha ) )
> )
> )
> );
true
gap> op := Opposite( vec );;
gap> alpha_op := Opposite( op, alpha );
<A morphism in Opposite( Category of matrices over Q )>
gap> basis := BasisOfExternalHom( Source( alpha_op ), Range( alpha_op ) );;
gap> coeffs := CoefficientsOfMorphism( alpha_op );;
gap> Display( coeffs );
[ 1, 0, 0, 0, 0, 1, 0, -1, -1, 0, 2, 1 ]
gap> IsEqualForMorphisms( alpha_op, LinearCombinationOfMorphisms( Source( alpha_op ), coeffs, basis, Range( alpha_op ) ) );
true
gap> vec := CapCategory( alpha );;
gap> t := TensorUnit( vec );;
gap> z := ZeroObject( vec );;
gap> IsCongruentForMorphisms(
> ZeroObjectFunctorial( vec ),
> InterpretMorphismFromDistinguishedObjectToHomomorphismStructureAsMorphism( z, z, ZeroMorphism( t, z ) )
> );
true
gap> IsCongruentForMorphisms(
> ZeroObjectFunctorial( vec ),
> InterpretMorphismFromDistinguishedObjectToHomomorphismStructureAsMorphism(
> z, z,
> InterpretMorphismAsMorphismFromDistinguishedObjectToHomomorphismStructure( ZeroObjectFunctorial( vec ) )
> )
> );
true
gap> right_side := PreCompose( [ i1, DualOnMorphisms( u ), u ] );;
gap> x := SolveLinearSystemInAbCategory( [ [ i1 ] ], [ [ u ] ], [ right_side ] )[1];;
gap> IsCongruentForMorphisms( PreCompose( [ i1, x, u ] ), right_side );
true
gap> a_otimes_b := TensorProductOnObjects( a, b );
<A vector space object over Q of dimension 12>
gap> hom_ab := InternalHomOnObjects( a, b );
<A vector space object over Q of dimension 12>
gap> cohom_ab := InternalCoHomOnObjects( a, b );
<A vector space object over Q of dimension 12>
gap> hom_ab = cohom_ab;
true
gap> unit_ab := VectorSpaceMorphism(
> a_otimes_b,
> HomalgIdentityMatrix( Dimension( a_otimes_b ), Q ),
> a_otimes_b
> );
<A morphism in Category of matrices over Q>
gap> unit_hom_ab := VectorSpaceMorphism(
> hom_ab,
> HomalgIdentityMatrix( Dimension( hom_ab ), Q ),
> hom_ab
> );
<A morphism in Category of matrices over Q>
gap> unit_cohom_ab := VectorSpaceMorphism(
> cohom_ab,
> HomalgIdentityMatrix( Dimension( cohom_ab ), Q ),
> cohom_ab
> );
<A morphism in Category of matrices over Q>
gap> ev_ab := ClosedMonoidalLeftEvaluationMorphism( a, b );
<A morphism in Category of matrices over Q>
gap> coev_ab := ClosedMonoidalLeftCoevaluationMorphism( a, b );
<A morphism in Category of matrices over Q>
gap> coev_ba := ClosedMonoidalLeftCoevaluationMorphism( b, a );
<A morphism in Category of matrices over Q>
gap> cocl_ev_ab := CoclosedMonoidalLeftEvaluationMorphism( a, b );
<A morphism in Category of matrices over Q>
gap> cocl_ev_ba := CoclosedMonoidalLeftEvaluationMorphism( b, a );
<A morphism in Category of matrices over Q>
gap> cocl_coev_ab := CoclosedMonoidalLeftCoevaluationMorphism( a, b );
<A morphism in Category of matrices over Q>
gap> cocl_coev_ba := CoclosedMonoidalLeftCoevaluationMorphism( b, a );
<A morphism in Category of matrices over Q>
gap> UnderlyingMatrix( ev_ab ) = TransposedMatrix( UnderlyingMatrix( cocl_ev_ab ) );
true
gap> UnderlyingMatrix( coev_ab ) = TransposedMatrix( UnderlyingMatrix( cocl_coev_ab ) );
true
gap> UnderlyingMatrix( coev_ba ) = TransposedMatrix( UnderlyingMatrix( cocl_coev_ba ) );
true
gap> tensor_hom_adj_1_hom_ab := InternalHomToTensorProductLeftAdjunctMorphism( a, b, unit_hom_ab );
<A morphism in Category of matrices over Q>
gap> cohom_tensor_adj_1_cohom_ab := InternalCoHomToTensorProductLeftAdjunctMorphism( a, b, unit_cohom_ab );
<A morphism in Category of matrices over Q>
gap> tensor_hom_adj_1_ab := TensorProductToInternalHomLeftAdjunctMorphism( a, b, unit_ab );
<A morphism in Category of matrices over Q>
gap> cohom_tensor_adj_1_ab := TensorProductToInternalCoHomLeftAdjunctMorphism( a, b, unit_ab );
<A morphism in Category of matrices over Q>
gap> ev_ab = tensor_hom_adj_1_hom_ab;
true
gap> cocl_ev_ba = cohom_tensor_adj_1_cohom_ab;
true
gap> coev_ba = tensor_hom_adj_1_ab;
true
gap> cocl_coev_ba = cohom_tensor_adj_1_ab;
true
gap> c := MatrixCategoryObject( vec, 2 );
<A vector space object over Q of dimension 2>
gap> d := MatrixCategoryObject( vec, 1 );
<A vector space object over Q of dimension 1>
</pre></div>

<div class="example"><pre>
gap> pre_compose := MonoidalPreComposeMorphism( a, b, c );
<A morphism in Category of matrices over Q>
gap> post_compose := MonoidalPostComposeMorphism( a, b, c );
<A morphism in Category of matrices over Q>
gap> pre_cocompose := MonoidalPreCoComposeMorphism( c, b, a );
<A morphism in Category of matrices over Q>
gap> post_cocompose := MonoidalPostCoComposeMorphism( c, b, a );
<A morphism in Category of matrices over Q>
gap> UnderlyingMatrix( pre_compose ) = TransposedMatrix( UnderlyingMatrix( pre_cocompose ) );
true
gap> UnderlyingMatrix( post_compose ) = TransposedMatrix( UnderlyingMatrix( post_cocompose ) );
true
gap> tp_hom_comp := TensorProductInternalHomCompatibilityMorphism( [ a, b, c, d ] );
<A morphism in Category of matrices over Q>
gap> cohom_tp_comp := InternalCoHomTensorProductCompatibilityMorphism( [ b, d, a, c ] );
<A morphism in Category of matrices over Q>
gap> UnderlyingMatrix( tp_hom_comp ) = TransposedMatrix( UnderlyingMatrix( cohom_tp_comp ) );
true
gap> lambda := LambdaIntroduction( alpha );
<A morphism in Category of matrices over Q>
gap> lambda_elim := LambdaElimination( a, b, lambda );
<A morphism in Category of matrices over Q>
gap> alpha = lambda_elim;
true
gap> alpha_op := VectorSpaceMorphism( b, TransposedMatrix( UnderlyingMatrix( alpha ) ), a );
<A morphism in Category of matrices over Q>
gap> colambda := CoLambdaIntroduction( alpha_op );
<A morphism in Category of matrices over Q>
gap> colambda_elim := CoLambdaElimination( b, a, colambda );
<A morphism in Category of matrices over Q>
gap> alpha_op = colambda_elim;
true
gap> UnderlyingMatrix( lambda ) = TransposedMatrix( UnderlyingMatrix( colambda ) );
true
gap> delta := PreCompose( colambda, lambda);
<A morphism in Category of matrices over Q>
gap> Display( EntriesOfHomalgMatrixAsListList( UnderlyingMatrix( TraceMap( delta ) ) ) );
[ [ 9 ] ]
gap> Display( EntriesOfHomalgMatrixAsListList( UnderlyingMatrix( CoTraceMap( delta ) ) ) );
[ [ 9 ] ]
gap> TraceMap( delta ) = CoTraceMap( delta );
true
gap> RankMorphism( a ) = CoRankMorphism( a );
true
</pre></div>

<a id="X78D1062D78BE08C1" name="X78D1062D78BE08C1"></a>

<h4>2.2 Functors</h4>

<div class="example"><pre>
gap> LoadPackage( "LinearAlgebraForCAP", false );
true
gap> ring := HomalgFieldOfRationals( );;
gap> vec := MatrixCategory( ring );;
gap> F := CapFunctor( "CohomForVec", [ vec, [ vec, true ] ], vec );;
gap> obj_func := function( A, B ) return TensorProductOnObjects( A, DualOnObjects( B ) ); end;;
gap> mor_func := function( source, alpha, beta, range ) return TensorProductOnMorphismsWithGivenTensorProducts( source, alpha, DualOnMorphisms( beta ), range ); end;;
gap> AddObjectFunction( F, obj_func );;
gap> AddMorphismFunction( F, mor_func );;
gap> Display( InputSignature( F ) );
[ [ Category of matrices over Q, false ], [ Category of matrices over Q, true ] ]
gap> V1 := TensorUnit( vec );;
gap> V3 := DirectSum( V1, V1, V1 );;
gap> pi1 := ProjectionInFactorOfDirectSum( [ V1, V1 ], 1 );;
gap> pi2 := ProjectionInFactorOfDirectSum( [ V3, V1 ], 1 );;
gap> value1 := ApplyFunctor( F, pi1, pi2 );;
gap> input := ProductCategoryMorphism( SourceOfFunctor( F ), [ pi1, Opposite( pi2 ) ] );;
gap> value2 := ApplyFunctor( F, input );;
gap> IsCongruentForMorphisms( value1, value2 );
true
gap> InstallFunctor( F, "F_installation" );;
gap> F_installation( pi1, pi2 );;
gap> F_installation( input );;
gap> F_installationOnObjects( V1, V1 );;
gap> F_installationOnObjects( ProductCategoryObject( SourceOfFunctor( F ), [ V1, Opposite( V1 ) ] ) );;
gap> F_installationOnMorphisms( pi1, pi2 );;
gap> F_installationOnMorphisms( input );;
gap> F2 := CapFunctor( "CohomForVec2", ProductCategory( [ vec, Opposite( vec ) ] ), vec );;
gap> AddObjectFunction( F2, a -> obj_func( a[1], Opposite( a[2] ) ) );;
gap> AddMorphismFunction( F2, function( source, datum, range ) return mor_func( source, datum[1], Opposite( datum[2] ), range ); end );;
gap> input := ProductCategoryMorphism( SourceOfFunctor( F2 ), [ pi1, Opposite( pi2 ) ] );;
gap> value3 := ApplyFunctor( F2, input );;
gap> IsCongruentForMorphisms( value1, value3 );
true
gap> Display( InputSignature( F2 ) );
[ [ Product of: Category of matrices over Q, Opposite( Category of matrices over Q ), false ] ]
gap> InstallFunctor( F2, "F_installation2" );;
gap> F_installation2( input );;
gap> F_installation2OnObjects( ProductCategoryObject( SourceOfFunctor( F2 ), [ V1, Opposite( V1 ) ] ) );;
gap> F_installation2OnMorphisms( input );;
</pre></div>

<a id="X85B1F5077BE75340" name="X85B1F5077BE75340"></a>

<h4>2.3 Solving (Homogeneous) Linear Systems</h4>

<div class="example"><pre>
gap> LoadPackage( "LinearAlgebraForCAP", false );
true
gap> QQ := HomalgFieldOfRationals();;
gap> QQ_mat := MatrixCategory( QQ );
Category of matrices over Q
gap> t := TensorUnit( QQ_mat );
<A vector space object over Q of dimension 1>
gap> id_t := IdentityMorphism( t );
<An identity morphism in Category of matrices over Q>
gap> 1*(11)*7 + 2*(12)*8 + 3*(13)*9;
620
gap> 4*(11)*3 + 5*(12)*4 + 6*(13)*1;
450
gap> alpha := [ [ 1/QQ * id_t, 2/QQ * id_t, 3/QQ * id_t ], [ 4/QQ * id_t, 5/QQ * id_t, 6/QQ * id_t ] ];;
gap> beta := [ [ 7/QQ * id_t, 8/QQ * id_t, 9/QQ * id_t ], [ 3/QQ * id_t, 4/QQ * id_t, 1/QQ * id_t ] ];;
gap> gamma := [ 620/QQ * id_t, 450/QQ * id_t ];;
gap> MereExistenceOfSolutionOfLinearSystemInAbCategory(
> QQ_mat, alpha, beta, gamma );
true
gap> MereExistenceOfUniqueSolutionOfLinearSystemInAbCategory(
> QQ_mat, alpha, beta, gamma );
false
gap> x := SolveLinearSystemInAbCategory( QQ_mat, alpha, beta, gamma );;
gap> (1*7)/QQ * x[1] + (2*8)/QQ * x[2] + (3*9)/QQ * x[3] = gamma[1];
true
gap> (4*3)/QQ * x[1] + (5*4)/QQ * x[2] + (6*1)/QQ * x[3] = gamma[2];
true
gap> MereExistenceOfUniqueSolutionOfHomogeneousLinearSystemInAbCategory(
> QQ_mat, alpha, beta );
false
gap> B := BasisOfSolutionsOfHomogeneousLinearSystemInLinearCategory(
> QQ_mat, alpha, beta );;
gap> Length( B );
1
gap> (1*7)/QQ * B[1][1] + (2*8)/QQ * B[1][2] + (3*9)/QQ * B[1][3] = 0/QQ * id_t;
true
gap> (4*3)/QQ * B[1][1] + (5*4)/QQ * B[1][2] + (6*1)/QQ * B[1][3] = 0/QQ * id_t;
true
gap> 2*(11)*5 + 3*(12)*7 + 9*(13)*2;
596
gap> Add( alpha, [ 2/QQ * id_t, 3/QQ * id_t, 9/QQ * id_t ] );;
gap> Add( beta, [ 5/QQ * id_t, 7/QQ * id_t, 2/QQ * id_t ] );;
gap> Add( gamma, 596/QQ * id_t );;
gap> MereExistenceOfSolutionOfLinearSystemInAbCategory(
> QQ_mat, alpha, beta, gamma );
true
gap> MereExistenceOfUniqueSolutionOfLinearSystemInAbCategory(
> QQ_mat, alpha, beta, gamma );
true
gap> x := SolveLinearSystemInAbCategory( QQ_mat, alpha, beta, gamma );;
gap> (1*7)/QQ * x[1] + (2*8)/QQ * x[2] + (3*9)/QQ * x[3] = gamma[1];
true
gap> (4*3)/QQ * x[1] + (5*4)/QQ * x[2] + (6*1)/QQ * x[3] = gamma[2];
true
gap> (2*5)/QQ * x[1] + (3*7)/QQ * x[2] + (9*2)/QQ * x[3] = gamma[3];
true
gap> MereExistenceOfUniqueSolutionOfHomogeneousLinearSystemInAbCategory(
> QQ_mat, alpha, beta );
true
gap> B := BasisOfSolutionsOfHomogeneousLinearSystemInLinearCategory(
> QQ_mat, alpha, beta );;
gap> Length( B );
0
gap> alpha := [ [ 2/QQ * id_t, 3/QQ * id_t ] ];;
gap> delta := [ [ 3/QQ * id_t, 3/QQ * id_t ] ];;
gap> B := BasisOfSolutionsOfHomogeneousDoubleLinearSystemInLinearCategory( alpha, delta );;
gap> Length( B );
1
gap> mor1 := PreCompose( alpha[1][1], B[1][1] ) + PreCompose( alpha[1][2], B[1][2] );
<A morphism in Category of matrices over Q>
gap> mor2 := PreCompose( B[1][1], delta[1][1] ) + PreCompose( B[1][2], delta[1][2] );
<A morphism in Category of matrices over Q>
gap> mor1 = mor2;
true
</pre></div>

<a id="X852F59737EC57965" name="X852F59737EC57965"></a>

<h4>2.4 Homology object</h4>

<div class="example"><pre>
gap> field := HomalgFieldOfRationals( );;
gap> vec := MatrixCategory( field );;
gap> A := MatrixCategoryObject( vec, 1 );;
gap> B := MatrixCategoryObject( vec, 2 );;
gap> C := MatrixCategoryObject( vec, 3 );;
gap> alpha := VectorSpaceMorphism( A, HomalgMatrix( [ [ 1, 0, 0 ] ], 1, 3, field ), C );;
gap> beta := VectorSpaceMorphism( C, HomalgMatrix( [ [ 1, 0 ], [ 1, 1 ], [ 1, 2 ] ], 3, 2, field ), B );;
gap> IsZeroForMorphisms( PreCompose( alpha, beta ) );
false
gap> IsCongruentForMorphisms(
> IdentityMorphism( HomologyObject( alpha, beta ) ),
> HomologyObjectFunctorial( alpha, beta, IdentityMorphism( C ), alpha, beta )
> );
true
gap> kernel_beta := KernelEmbedding( beta );;
gap> K := Source( kernel_beta );;
gap> IsIsomorphism(
> HomologyObjectFunctorial( 
> MorphismFromZeroObject( K ), 
> MorphismIntoZeroObject( K ),
> kernel_beta,
> MorphismFromZeroObject( Source( beta ) ),
> beta
> )
> );
true
gap> cokernel_alpha := CokernelProjection( alpha );;
gap> Co := Range( cokernel_alpha );;
gap> IsIsomorphism(
> HomologyObjectFunctorial( 
> alpha,
> MorphismIntoZeroObject( Range( alpha ) ),
> cokernel_alpha,
> MorphismFromZeroObject( Co ),
> MorphismIntoZeroObject( Co )
> )
> );
true
gap> op := Opposite( vec );;
gap> alpha_op := Opposite( op, alpha );;
gap> beta_op := Opposite( op, beta );;
gap> IsCongruentForMorphisms(
> IdentityMorphism( HomologyObject( beta_op, alpha_op ) ),
> HomologyObjectFunctorial( beta_op, alpha_op, IdentityMorphism( Opposite( C ) ), beta_op, alpha_op )
> );
true
gap> kernel_beta := KernelEmbedding( beta_op );;
gap> K := Source( kernel_beta );;
gap> IsIsomorphism(
> HomologyObjectFunctorial( 
> MorphismFromZeroObject( K ), 
> MorphismIntoZeroObject( K ),
> kernel_beta,
> MorphismFromZeroObject( Source( beta_op ) ),
> beta_op
> )
> );
true
gap> cokernel_alpha := CokernelProjection( alpha_op );;
gap> Co := Range( cokernel_alpha );;
gap> IsIsomorphism(
> HomologyObjectFunctorial( 
> alpha_op,
> MorphismIntoZeroObject( Range( alpha_op ) ),
> cokernel_alpha,
> MorphismFromZeroObject( Co ),
> MorphismIntoZeroObject( Co )
> )
> );
true
</pre></div>

<a id="X7CDB7FF27D734D2D" name="X7CDB7FF27D734D2D"></a>

<h4>2.5 Liftable</h4>

<div class="example"><pre>
gap> field := HomalgFieldOfRationals( );;
gap> vec := MatrixCategory( field );;
gap> V := MatrixCategoryObject( vec, 1 );;
gap> W := MatrixCategoryObject( vec, 2 );;
gap> alpha := VectorSpaceMorphism( V, HomalgMatrix( [ [ 1, -1 ] ], 1, 2, field ), W );;
gap> beta := VectorSpaceMorphism( W, HomalgMatrix( [ [ 1, 2 ], [ 3, 4 ] ], 2, 2, field ), W );;
gap> IsLiftable( alpha, beta );
true
gap> IsLiftable( beta, alpha );
false
gap> IsLiftableAlongMonomorphism( beta, alpha );
true
gap> gamma := VectorSpaceMorphism( W, HomalgMatrix( [ [ 1 ], [ 1 ] ], 2, 1, field ), V );;
gap> IsColiftable( beta, gamma );
true
gap> IsColiftable( gamma, beta );
false
gap> IsColiftableAlongEpimorphism( beta, gamma );
true
gap> PreCompose( PreInverseForMorphisms( gamma ), gamma ) = IdentityMorphism( V );
true
gap> PreCompose( alpha, PostInverseForMorphisms( alpha ) ) = IdentityMorphism( V );
true
</pre></div>

<a id="X797080CE873BB56D" name="X797080CE873BB56D"></a>

<h4>2.6 Monoidal structure</h4>

<div class="example"><pre>
gap> LoadPackage( "LinearAlgebraForCAP", false );
true
gap> Q := HomalgFieldOfRationals();;
gap> vec := MatrixCategory( Q );;
gap> a := MatrixCategoryObject( vec, 1 );
<A vector space object over Q of dimension 1>
gap> b := MatrixCategoryObject( vec, 2 );
<A vector space object over Q of dimension 2>
gap> c := MatrixCategoryObject( vec, 3 );
<A vector space object over Q of dimension 3>
gap> z := ZeroObject( vec );
<A vector space object over Q of dimension 0>
gap> alpha := VectorSpaceMorphism( a, [ [ 1, 0 ] ], b );
<A morphism in Category of matrices over Q>
gap> beta := VectorSpaceMorphism( b,
> [ [ 1, 0, 0 ], [ 0, 1, 0 ] ], c );
<A morphism in Category of matrices over Q>
gap> gamma := VectorSpaceMorphism( c,
> [ [ 0, 1, 1 ], [ 1, 0, 1 ], [ 1, 1, 0 ] ], c );
<A morphism in Category of matrices over Q>
gap> IsCongruentForMorphisms(
> TensorProductOnMorphisms( alpha, beta ),
> TensorProductOnMorphisms( beta, alpha )
> );
false
gap> IsCongruentForMorphisms(
> AssociatorRightToLeft( a, b, c ),
> IdentityMorphism( TensorProductOnObjects( a, TensorProductOnObjects( b, c ) ) )
> );
true
gap> IsCongruentForMorphisms(
> gamma,
> LambdaElimination( c, c, LambdaIntroduction( gamma ) )
> );
true
gap> IsZeroForMorphisms( TraceMap( gamma ) );
true
gap> IsCongruentForMorphisms(
> RankMorphism( DirectSum( a, b ) ),
> RankMorphism( c )
> );
true
gap> IsCongruentForMorphisms(
> Braiding( b, c ),
> IdentityMorphism( TensorProductOnObjects( b, c ) )
> );
false
gap> IsCongruentForMorphisms(
> PreCompose( Braiding( b, c ), Braiding( c, b ) ),
> IdentityMorphism( TensorProductOnObjects( b, c ) )
> );
true
</pre></div>

<a id="X8218390A785F1EDB" name="X8218390A785F1EDB"></a>

<h4>2.7 MorphismFromSourceToPushout and MorphismFromFiberProductToSink</h4>

<div class="example"><pre>
gap> field := HomalgFieldOfRationals( );;
gap> vec := MatrixCategory( field );;
gap> A := MatrixCategoryObject( vec, 3 );;
gap> B := MatrixCategoryObject( vec, 2 );;
gap> alpha := VectorSpaceMorphism( B, HomalgMatrix( [ [ 1, -1, 1 ], [ 1, 1, 1 ] ], 2, 3, field ), A );;
gap> beta := VectorSpaceMorphism( B, HomalgMatrix( [ [ 1, 2, 1 ], [ 2, 1, 1 ] ], 2, 3, field ), A );;
gap> m := MorphismFromFiberProductToSink( [ alpha, beta ] );;
gap> IsCongruentForMorphisms(
> m,
> PreCompose( ProjectionInFactorOfFiberProduct( [ alpha, beta ], 1 ), alpha )
> );
true
gap> IsCongruentForMorphisms(
> m,
> PreCompose( ProjectionInFactorOfFiberProduct( [ alpha, beta ], 2 ), beta )
> );
true
gap> IsCongruentForMorphisms(
> MorphismFromKernelObjectToSink( alpha ),
> PreCompose( KernelEmbedding( alpha ), alpha )
> );
true
gap> alpha_p := DualOnMorphisms( alpha );;
gap> beta_p := DualOnMorphisms( beta );;
gap> m_p := MorphismFromSourceToPushout( [ alpha_p, beta_p ] );;
gap> IsCongruentForMorphisms(
> m_p,
> PreCompose( alpha_p, InjectionOfCofactorOfPushout( [ alpha_p, beta_p ], 1 ) )
> );
true
gap> IsCongruentForMorphisms(
> m_p,
> PreCompose( beta_p, InjectionOfCofactorOfPushout( [ alpha_p, beta_p ], 2 ) )
> );
true
gap> IsCongruentForMorphisms(
> MorphismFromSourceToCokernelObject( alpha_p ),
> PreCompose( alpha_p, CokernelProjection( alpha_p ) )
> );
true
</pre></div>

<a id="X84410C637B3C1D0E" name="X84410C637B3C1D0E"></a>

<h4>2.8 Opposite category</h4>

<div class="example"><pre>
gap> LoadPackage( "LinearAlgebraForCAP", ">= 2024.01-04", false );
true
gap> QQ := HomalgFieldOfRationals();;
gap> vec := MatrixCategory( QQ );;
gap> op := Opposite( vec );;
gap> Perform( ListKnownCategoricalProperties( op ), Display );
IsAbCategory
IsAbelianCategory
IsAbelianCategoryWithEnoughInjectives
IsAbelianCategoryWithEnoughProjectives
IsAdditiveCategory
IsAdditiveMonoidalCategory
IsBraidedMonoidalCategory
IsCategoryWithCoequalizers
IsCategoryWithCokernels
IsCategoryWithEqualizers
IsCategoryWithInitialObject
IsCategoryWithKernels
IsCategoryWithTerminalObject
IsCategoryWithZeroObject
IsClosedMonoidalCategory
IsCoclosedMonoidalCategory
IsEnrichedOverCommutativeRegularSemigroup
IsEquippedWithHomomorphismStructure
IsLinearCategoryOverCommutativeRing
IsLinearCategoryOverCommutativeRingWithFinitelyGeneratedFreeExternalHoms
IsMonoidalCategory
IsPreAbelianCategory
IsRigidSymmetricClosedMonoidalCategory
IsRigidSymmetricCoclosedMonoidalCategory
IsSkeletalCategory
IsStrictMonoidalCategory
IsSymmetricClosedMonoidalCategory
IsSymmetricCoclosedMonoidalCategory
IsSymmetricMonoidalCategory
gap> V1 := Opposite( TensorUnit( vec ) );;
gap> V2 := DirectSum( V1, V1 );;
gap> V3 := DirectSum( V1, V2 );;
gap> V4 := DirectSum( V1, V3 );;
gap> V5 := DirectSum( V1, V4 );;
gap> IsWellDefined( MorphismBetweenDirectSums( op, [ ], [ ], [ V1 ] ) );
true
gap> IsWellDefined( MorphismBetweenDirectSums( op, [ V1 ], [ [ ] ], [ ] ) );
true
gap> alpha13 := InjectionOfCofactorOfDirectSum( [ V1, V2 ], 1 );;
gap> alpha14 := InjectionOfCofactorOfDirectSum( [ V1, V2, V1 ], 3 );;
gap> alpha15 := InjectionOfCofactorOfDirectSum( [ V2, V1, V2 ], 2 );;
gap> alpha23 := InjectionOfCofactorOfDirectSum( [ V2, V1 ], 1 );;
gap> alpha24 := InjectionOfCofactorOfDirectSum( [ V1, V2, V1 ], 2 );;
gap> alpha25 := InjectionOfCofactorOfDirectSum( [ V2, V2, V1 ], 1 );;
gap> mat := [
> [ alpha13, alpha14, alpha15 ],
> [ alpha23, alpha24, alpha25 ]
> ];;
gap> mor := MorphismBetweenDirectSums( mat );;
gap> IsWellDefined( mor );
true
gap> IsWellDefined( Opposite( mor ) );
true
gap> IsCongruentForMorphisms(
> UniversalMorphismFromImage( mor, [ CoastrictionToImage( mor ), ImageEmbedding( mor ) ] ),
> IdentityMorphism( ImageObject( mor ) )
> );
true
</pre></div>

<a id="X7F34E29281DF3FCE" name="X7F34E29281DF3FCE"></a>

<h4>2.9 PreComposeList and PostComposeList</h4>

<div class="example"><pre>
gap> field := HomalgFieldOfRationals( );;
gap> vec := MatrixCategory( field );;
gap> A := MatrixCategoryObject( vec, 1 );;
gap> B := MatrixCategoryObject( vec, 2 );;
gap> C := MatrixCategoryObject( vec, 3 );;
gap> alpha := VectorSpaceMorphism( A, HomalgMatrix( [ [ 1, 0, 0 ] ], 1, 3, field ), C );;
gap> beta := VectorSpaceMorphism( C, HomalgMatrix( [ [ 1, 0 ], [ 1, 1 ], [ 1, 2 ] ], 3, 2, field ), B );;
gap> IsCongruentForMorphisms( PreCompose( alpha, beta ), PostCompose( beta, alpha ) );
true
gap> IsCongruentForMorphisms( PreComposeList( A, [ ], A ), IdentityMorphism( A ) );
true
gap> IsCongruentForMorphisms( PreComposeList( A, [ alpha ], C ), alpha );
true
gap> IsCongruentForMorphisms( PreComposeList( A, [ alpha, beta ], B ), PreCompose( alpha, beta ) );
true
gap> IsCongruentForMorphisms( PostComposeList( A, [ ], A ), IdentityMorphism( A ) );
true
gap> IsCongruentForMorphisms( PostComposeList( A, [ alpha ], C ), alpha );
true
gap> IsCongruentForMorphisms( PostComposeList( A, [ beta, alpha ], B ), PostCompose( beta, alpha ) );
true
</pre></div>

<a id="X87F5609A7B33C58C" name="X87F5609A7B33C58C"></a>

<h4>2.10 Split epi summand</h4>

<div class="example"><pre>
gap> LoadPackage( "LinearAlgebraForCAP", false );
true
gap> Q := HomalgFieldOfRationals();;
gap> Qmat := MatrixCategory( Q );;
gap> a := MatrixCategoryObject( Qmat, 3 );;
gap> b := MatrixCategoryObject( Qmat, 4 );;
gap> homalg_matrix := HomalgMatrix( [ [ 1, 0, 0, 0 ],
> [ 0, 1, 0, -1 ],
> [ -1, 0, 2, 1 ] ], 3, 4, Q );;
gap> alpha := VectorSpaceMorphism( a, homalg_matrix, b );;
gap> beta := SomeReductionBySplitEpiSummand( alpha );;
gap> IsWellDefinedForMorphisms( beta );
true
gap> Dimension( Source( beta ) );
0
gap> Dimension( Range( beta ) );
1
gap> gamma := SomeReductionBySplitEpiSummand_MorphismFromInputRange( alpha );;
gap> Display( EntriesOfHomalgMatrixAsListList( UnderlyingMatrix( gamma ) ) );
[ [ 0 ], [ 1 ], [ -1/2 ], [ 1 ] ]
</pre></div>

<div class="example"><pre>
gap> # @drop_example_in_Julia: differences in the output of (Safe)RightDivide, see https://github.com/homalg-project/MatricesForHomalg.jl/issues/50
> delta := SomeReductionBySplitEpiSummand_MorphismToInputRange( alpha );;
gap> Display( EntriesOfHomalgMatrixAsListList( UnderlyingMatrix( delta ) ) );
[ [ 0, 1, 0, 0 ] ]
</pre></div>

<a id="X7DCD99628504B810" name="X7DCD99628504B810"></a>

<h4>2.11 Kernel</h4>

<div class="example"><pre>
gap> Q := HomalgFieldOfRationals();;
gap> vec := MatrixCategory( Q );;
gap> V := MatrixCategoryObject( vec, 2 );
<A vector space object over Q of dimension 2>
gap> W := MatrixCategoryObject( vec, 3 );
<A vector space object over Q of dimension 3>
gap> alpha := VectorSpaceMorphism( V, [ [ 1, 1, 1 ], [ -1, -1, -1 ] ], W );
<A morphism in Category of matrices over Q>
gap> k := KernelObject( alpha );
<A vector space object over Q of dimension 1>
gap> T := MatrixCategoryObject( vec, 2 );
<A vector space object over Q of dimension 2>
gap> tau := VectorSpaceMorphism( T, [ [ 2, 2 ], [ 2, 2 ] ], V );
<A morphism in Category of matrices over Q>
gap> k_lift := KernelLift( alpha, tau );
<A morphism in Category of matrices over Q>
gap> HasKernelEmbedding( alpha );
false
gap> KernelEmbedding( alpha );
<A split monomorphism in Category of matrices over Q>
</pre></div>

<div class="example"><pre>
gap> Q := HomalgFieldOfRationals();;
gap> vec := MatrixCategory( Q );;
gap> V := MatrixCategoryObject( vec, 2 );
<A vector space object over Q of dimension 2>
gap> W := MatrixCategoryObject( vec, 3 );
<A vector space object over Q of dimension 3>
gap> alpha := VectorSpaceMorphism( V, [ [ 1, 1, 1 ], [ -1, -1, -1 ] ], W );
<A morphism in Category of matrices over Q>
gap> k := KernelObject( alpha );
<A vector space object over Q of dimension 1>
gap> T := MatrixCategoryObject( vec, 2 );
<A vector space object over Q of dimension 2>
gap> tau := VectorSpaceMorphism( T, [ [ 2, 2 ], [ 2, 2 ] ], V );
<A morphism in Category of matrices over Q>
gap> k_lift := KernelLift( alpha, tau );
<A morphism in Category of matrices over Q>
gap> HasKernelEmbedding( alpha );
false
</pre></div>

<div class="example"><pre>
gap> Q := HomalgFieldOfRationals();;
gap> vec := MatrixCategory( Q );;
gap> V := MatrixCategoryObject( vec, 2 );
<A vector space object over Q of dimension 2>
gap> W := MatrixCategoryObject( vec, 3 );
<A vector space object over Q of dimension 3>
gap> alpha := VectorSpaceMorphism( V, [ [ 1, 1, 1 ], [ -1, -1, -1 ] ], W );
<A morphism in Category of matrices over Q>
gap> k := KernelObject( alpha );
<A vector space object over Q of dimension 1>
gap> k_emb := KernelEmbedding( alpha );
<A split monomorphism in Category of matrices over Q>
gap> IsEqualForObjects( Source( k_emb ), k );
true
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