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## <#GAPDoc Label="ExtExt">
## <Subsection Label="ExtExt">
## <Heading>ExtExt</Heading>
## This is Example B.2 in <Cite Key="BaSF"/>.
## <Example><![CDATA[
## gap> Qxyz := HomalgFieldOfRationalsInDefaultCAS( ) * "x,y,z";
## Q[x,y,z]
## gap> wmat := HomalgMatrix( "[ \
## > x*y, y*z, z, 0, 0, \
## > x^3*z,x^2*z^2,0, x*z^2, -z^2, \
## > x^4, x^3*z, 0, x^2*z, -x*z, \
## > 0, 0, x*y, -y^2, x^2-1,\
## > 0, 0, x^2*z, -x*y*z, y*z, \
## > 0, 0, x^2*y-x^2,-x*y^2+x*y,y^2-y \
## > ]", 6, 5, Qxyz );
## <A 6 x 5 matrix over an external ring>
## gap> W := LeftPresentation( wmat );
## <A left module presented by 6 relations for 5 generators>
## gap> Y := Hom( Qxyz, W );
## <A right module on 5 generators satisfying yet unknown relations>
## gap> SetInfoLevel( InfoWarning, 0 );
## gap> F := InsertObjectInMultiFunctor( Functor_Hom_for_fp_modules, 2, Y, "TensorY" );
## <The functor TensorY for f.p. modules and their maps over computable rings>
## gap> SetInfoLevel( InfoWarning, 1 );
## gap> G := LeftDualizingFunctor( Qxyz );;
## gap> II_E := GrothendieckSpectralSequence( F, G, W );
## <A stable homological spectral sequence with sheets at levels
## [ 0 .. 4 ] each consisting of left modules at bidegrees [ -3 .. 0 ]x
## [ 0 .. 3 ]>
## gap> Display( II_E );
## The associated transposed spectral sequence:
##
## a homological spectral sequence at bidegrees
## [ [ 0 .. 3 ], [ -3 .. 0 ] ]
## ---------
## Level 0:
##
## * * * *
## * * * *
## . * * *
## . . * *
## ---------
## Level 1:
##
## * * * *
## . . . .
## . . . .
## . . . .
## ---------
## Level 2:
##
## s s s s
## . . . .
## . . . .
## . . . .
##
## Now the spectral sequence of the bicomplex:
##
## a homological spectral sequence at bidegrees
## [ [ -3 .. 0 ], [ 0 .. 3 ] ]
## ---------
## Level 0:
##
## * * * *
## * * * *
## . * * *
## . . * *
## ---------
## Level 1:
##
## * * * *
## * * * *
## . * * *
## . . . *
## ---------
## Level 2:
##
## * * s s
## * * * *
## . * * *
## . . . *
## ---------
## Level 3:
##
## * s s s
## * s s s
## . . s *
## . . . *
## ---------
## Level 4:
##
## s s s s
## . s s s
## . . s s
## . . . s
## gap> filt := FiltrationBySpectralSequence( II_E, 0 );
## <An ascending filtration with degrees [ -3 .. 0 ] and graded parts:
##
## 0: <A non-zero left module presented by yet unknown relations for 23 generator\
## s>
## -1: <A non-zero left module presented by 37 relations for 22 generators>
## -2: <A non-zero left module presented by 31 relations for 10 generators>
## -3: <A non-zero left module presented by 33 relations for 5 generators>
## of
## <A non-zero left module presented by 102 relations for 37 generators>>
## gap> ByASmallerPresentation( filt );
## <An ascending filtration with degrees [ -3 .. 0 ] and graded parts:
## 0: <A non-zero left module presented by 26 relations for 16 generators>
## -1: <A non-zero left module presented by 30 relations for 14 generators>
## -2: <A non-zero left module presented by 18 relations for 7 generators>
## -3: <A non-zero left module presented by 12 relations for 4 generators>
## of
## <A non-zero left module presented by 48 relations for 20 generators>>
## gap> m := IsomorphismOfFiltration( filt );
## <A non-zero isomorphism of left modules>
## ]]></Example>
## </Subsection>
## <#/GAPDoc>
ReadPackage( "ExamplesForHomalg", "examples/ReducedBasisOfModule.g" );
SetInfoLevel( InfoWarning, 0 );
InsertObjectInMultiFunctor( Functor_Hom_for_fp_modules, 2, Y, "TensorY" );
SetInfoLevel( InfoWarning, 1 );
II_E := GrothendieckSpectralSequence( Functor_TensorY_for_fp_modules, LeftDualizingFunctor( Qxyz ), W );
filt := FiltrationBySpectralSequence( II_E );
ByASmallerPresentation( filt );
m := IsomorphismOfFiltration( filt );
#Display( StringTime( homalgTime( Qxyz ) ) );
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