Goto Chapter: Top 1 2 Ind

### 2 Examples and Tests

#### 2.1 Basic Commands

gap> Q := HomalgFieldOfRationals();;
gap> a := VectorSpaceObject( 3, Q );
<A vector space object over Q of dimension 3>
gap> b := VectorSpaceObject( 4, Q );
<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 );
<A 3 x 4 matrix over an internal ring>
gap> alpha := VectorSpaceMorphism( a, homalg_matrix, b );
<A morphism in Category of matrices over Q>
gap> Display( alpha );
[ [   1,   0,   0,   0 ],
[   0,   1,   0,  -1 ],
[  -1,   0,   2,   1 ] ]

A morphism in Category of matrices over Q
gap> homalg_matrix := HomalgMatrix( [ [ 1, 1, 0, 0 ],
>                                   [ 0, 1, 0, -1 ],
>                                   [ -1, 0, 2, 1 ] ], 3, 4, Q );
<A 3 x 4 matrix over an internal ring>
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( c );
[ [     0 ],
[     1 ],
[  -1/2 ],
[     1 ] ]

A split epimorphism in Category of matrices over Q
gap> gamma := UniversalMorphismIntoDirectSum( [ c, c ] );;
gap> Display( gamma );
[ [     0,     0 ],
[     1,     1 ],
[  -1/2,  -1/2 ],
[     1,     1 ] ]

A morphism in Category of matrices over Q
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( PreCompose( p1, alpha ) );
[ [   0,   1,   0,  -1 ],
[  -1,   0,   2,   1 ] ]

A morphism in Category of matrices over Q
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>
gap> Display( 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 ] ]

A morphism in Category of matrices over Q
gap> KernelObjectFunctorial( u, IdentityMorphism( Source( u ) ), u ) = IdentityMorphism( VectorSpaceObject( 3, Q ) );
true
gap> IsZero( CokernelObjectFunctorial( u, IdentityMorphism( Range( u ) ), u ) );
true
gap> DirectProductFunctorial( [ u, u ] ) = DirectSumFunctorial( [ u, u ] );
true
gap> CoproductFunctorial( [ u, u ] ) = DirectSumFunctorial( [ u, u ] );
true
gap> IsOne( FiberProductFunctorial( [ [ u, IdentityMorphism( Source( u ) ), u ], [ u, IdentityMorphism( Source( u ) ) , u ] ] ) );
true
gap> IsOne( PushoutFunctorial( [ [ u, IdentityMorphism( Range( u ) ), u ], [ u, IdentityMorphism( Range( u ) ) , u ] ] ) );
true

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