chap2.txt

  
  2 Examples and Tests
  
  
  2.1 Basic Commands
  
    Example  
    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