<h4>2.2 <span class="Heading">Intersection of Submodules</span></h4>
<div class="example"><pre>
<span class="GAPprompt">gap></span> <span class="GAPinput">Q := HomalgFieldOfRationalsInSingular();;</span>
<span class="GAPprompt">gap></span> <span class="GAPinput">R := Q * "x,y";</span>
Q[x,y]
<span class="GAPprompt">gap></span> <span class="GAPinput">fpres := LeftPresentations( R );;</span>
<span class="GAPprompt">gap></span> <span class="GAPinput">F := AsLeftPresentation( fpres, HomalgMatrix( [ [ 0 ] ], R ) );</span>
<An object in Category of left presentations of Q[x,y]>
<span class="GAPprompt">gap></span> <span class="GAPinput">I1 := AsLeftPresentation( fpres, HomalgMatrix( [ [ "x" ] ], R ) );;</span>
<span class="GAPprompt">gap></span> <span class="GAPinput">I2 := AsLeftPresentation( fpres, HomalgMatrix( [ [ "y" ] ], R ) );;</span>
<span class="GAPprompt">gap></span> <span class="GAPinput">Display( I1 );</span>
x
An object in Category of left presentations of Q[x,y]
<span class="GAPprompt">gap></span> <span class="GAPinput">Display( I2 );</span>
y
An object in Category of left presentations of Q[x,y]
<span class="GAPprompt">gap></span> <span class="GAPinput">eps1 := PresentationMorphism( F, HomalgMatrix( [ [ 1 ] ], R ), I1 );</span>
<A morphism in Category of left presentations of Q[x,y]>
<span class="GAPprompt">gap></span> <span class="GAPinput">eps2 := PresentationMorphism( F, HomalgMatrix( [ [ 1 ] ], R ), I2 );</span>
<A morphism in Category of left presentations of Q[x,y]>
<span class="GAPprompt">gap></span> <span class="GAPinput">kernelemb1 := KernelEmbedding( eps1 );</span>
<A monomorphism in Category of left presentations of Q[x,y]>
<span class="GAPprompt">gap></span> <span class="GAPinput">kernelemb2 := KernelEmbedding( eps2 );</span>
<A monomorphism in Category of left presentations of Q[x,y]>
<span class="GAPprompt">gap></span> <span class="GAPinput">P := FiberProduct( kernelemb1, kernelemb2 );;</span>
<span class="GAPprompt">gap></span> <span class="GAPinput">Display( P );</span>
(an empty 0 x 1 matrix)
An object in Category of left presentations of Q[x,y]
<span class="GAPprompt">gap></span> <span class="GAPinput">pi1 := ProjectionInFactorOfFiberProduct( [ kernelemb1, kernelemb2 ], 1 );</span>
<A monomorphism in Category of left presentations of Q[x,y]>
<span class="GAPprompt">gap></span> <span class="GAPinput">composite := PreCompose( pi1, kernelemb1 );</span>
<A monomorphism in Category of left presentations of Q[x,y]>
<span class="GAPprompt">gap></span> <span class="GAPinput">Display( composite );</span>
x*y
A monomorphism in Category of left presentations of Q[x,y]
</pre></div>
A monomorphism in Category of left presentations of Q[x,y,z]
<span class="GAPprompt">gap></span> <span class="GAPinput">Display( Source( iota1 ) );</span>
0, -z,y,
-z,0, x,
-y,x, 0
An object in Category of left presentations of Q[x,y,z]
<span class="GAPprompt">gap></span> <span class="GAPinput">pi1 := CoverByFreeModule( Source( iota1 ) );;</span>
<span class="GAPprompt">gap></span> <span class="GAPinput">d1 := PreCompose( pi1, iota1 );;</span>
<span class="GAPprompt">gap></span> <span class="GAPinput">Display( d1 );</span>
x,
y,
z
A morphism in Category of left presentations of Q[x,y,z]
<span class="GAPprompt">gap></span> <span class="GAPinput">iota2 := KernelEmbedding( d1 );;</span>
<span class="GAPprompt">gap></span> <span class="GAPinput">Display( iota2 );</span>
0, -z,y,
-z,0, x,
-y,x, 0
A monomorphism in Category of left presentations of Q[x,y,z]
<span class="GAPprompt">gap></span> <span class="GAPinput">Display( Source( iota2 ) );;</span>
x,-y,z
An object in Category of left presentations of Q[x,y,z]
<span class="GAPprompt">gap></span> <span class="GAPinput">pi2 := CoverByFreeModule( Source( iota2 ) );;</span>
<span class="GAPprompt">gap></span> <span class="GAPinput">d2 := PreCompose( pi2, iota2 );;</span>
<span class="GAPprompt">gap></span> <span class="GAPinput">Display( d2 );</span>
0, -z,y,
-z,0, x,
-y,x, 0
A morphism in Category of left presentations of Q[x,y,z]
<span class="GAPprompt">gap></span> <span class="GAPinput">iota3 := KernelEmbedding( d2 );;</span>
<span class="GAPprompt">gap></span> <span class="GAPinput">Display( iota3 );</span>
x,-y,z
A monomorphism in Category of left presentations of Q[x,y,z]
<span class="GAPprompt">gap></span> <span class="GAPinput">Display( Source( iota3 ) );</span>
(an empty 0 x 1 matrix)
An object in Category of left presentations of Q[x,y,z]
<span class="GAPprompt">gap></span> <span class="GAPinput">pi3 := CoverByFreeModule( Source( iota3 ) );;</span>
<span class="GAPprompt">gap></span> <span class="GAPinput">d3 := PreCompose( pi3, iota3 );;</span>
<span class="GAPprompt">gap></span> <span class="GAPinput">Display( d3 );</span>
x,-y,z
<div class="example"><pre>
<span class="GAPprompt">gap></span> <span class="GAPinput">R := HomalgRingOfIntegers( );;</span>
<span class="GAPprompt">gap></span> <span class="GAPinput">fpres := LeftPresentations( R );;</span>
<span class="GAPprompt">gap></span> <span class="GAPinput">M := AsLeftPresentation( fpres, HomalgMatrix( [ [ 2 ] ], 1, 1, R ) );</span>
<An object in Category of left presentations of Z>
<span class="GAPprompt">gap></span> <span class="GAPinput">N := AsLeftPresentation( fpres, HomalgMatrix( [ [ 3 ] ], 1, 1, R ) );</span>
<An object in Category of left presentations of Z>
<span class="GAPprompt">gap></span> <span class="GAPinput">T := TensorProductOnObjects( M, N );</span>
<An object in Category of left presentations of Z>
<span class="GAPprompt">gap></span> <span class="GAPinput">Display( T );</span>
[ [ 3 ],
[ 2 ] ]
An object in Category of left presentations of Z
<span class="GAPprompt">gap></span> <span class="GAPinput">IsZero( T );</span>
true
<span class="GAPprompt">gap></span> <span class="GAPinput">H := InternalHomOnObjects( DirectSum( M, M ), DirectSum( M, N ) );</span>
<An object in Category of left presentations of Z>
<span class="GAPprompt">gap></span> <span class="GAPinput">Display( H );</span>
[ [ 0, 0, 0, -2 ],
[ 1, 2, 0, 0 ],
[ 0, 2, 2, 0 ],
[ 2, 3, 0, 2 ] ]
An object in Category of left presentations of Z
<span class="GAPprompt">gap></span> <span class="GAPinput">alpha := StandardGeneratorMorphism( H, 3 );</span>
<A morphism in Category of left presentations of Z>
<span class="GAPprompt">gap></span> <span class="GAPinput">l := LambdaElimination( DirectSum( M, M ), DirectSum( M, N ), alpha );</span>
<A morphism in Category of left presentations of Z>
<span class="GAPprompt">gap></span> <span class="GAPinput">IsZero( l );</span>
false
<span class="GAPprompt">gap></span> <span class="GAPinput">Display( l );</span>
[ [ -2, 6 ],
[ -1, -3 ] ]
A morphism in Category of left presentations of Z
</pre></div>
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