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ConjugacyClasses fails to compute or is very slow #45

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stertooy opened this issue Jul 25, 2020 · 1 comment · May be fixed by #63
Open

ConjugacyClasses fails to compute or is very slow #45

stertooy opened this issue Jul 25, 2020 · 1 comment · May be fixed by #63

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@stertooy
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It seems something can go wrong when computing conjugacy classes (I swear I'm not doing this on purpose...):

gap> G := ExamplesOfSomePcpGroups( 8 );;
gap> N := NormalClosure( G, Subgroup( G, List( Pcp( G ), g -> g^6 ) ) );;
gap> Q := FactorGroup( G, N );
Pcp-group with orders [ 6, 6, 3, 3, 6 ]
gap> ConjugacyClasses( Q );
Error, Variable: 'c' must have an assigned value in
  C := LiftBlockToPointNormalizer( CR, cc, C, H, L, c )
 ; at /usr/lib/gap/pkg/polycyclic-master/gap/pcpgrp/normcon.gi:250 called from
NormalizerOfComplement( C, H, N, I ) at /usr/lib/gap/pkg/polycyclic-master/gap/pcpgrp/normcon.gi:287 called from
NormalizerBySeries( GG, UU, EfaSeries( GG ) ) at /usr/lib/gap/pkg/polycyclic-master/gap/pcpgrp/normcon.gi:315 called from
NormalizerPcpGroup( G, U ) at /usr/lib/gap/pkg/polycyclic-master/gap/pcpgrp/normcon.gi:328 called from
oper( Parent( D ), D ) at /usr/lib/gap/lib/domain.gd:462 called from
attr( sub ) at /usr/lib/gap/lib/domain.gd:450 called from
...  at *stdin*:1025
type 'quit;' to quit to outer loop

This bug was introduced in 5988322, although the code does not work in earlier versions either - presumably due to the bug mentioned by this commit.

If we transform the group Q to a pc-group, calculating the conjugacy classes works fine:

gap> iso := IsomorphismPcGroup( Q );;
gap> ConjugacyClasses( Image( iso ) );
[ <identity> of ...^G, f7*f8^G, f8^G, f7*f8^2^G, f8^2^G, f7^G, f6*f8^G, f6*f7*f8^2^G, f6*f8^2^G, f6*f7^G,
  ...
  f1*f2^2*f3^G, f1*f2^2*f3*f4^G, f1*f2^2*f3*f4^2^G ]

I'm not sure how related this is, so perhaps it should be its own separate issue (if an issue at all), but when ConjugacyClasses does work for a pcp-group, it can be much slower than for the isomorphic pc-group.

gap> N := NormalClosure( G, Subgroup( G, List( Pcp( G ), g -> g^11 ) ) );;
gap> Q := FactorGroup( G, N );;
gap> ConjugacyClasses( Q );;time;
11944
gap> iso := IsomorphismPcGroup( Q );;
gap> ConjugacyClasses( Image( iso ) );;time;
28
@fingolfin
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Thank you once more for your report, we'll look into it (but it might take some time, as now it's the holiday season).

@stertooy stertooy linked a pull request May 15, 2021 that will close this issue
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Fix bug in NormalizerOfComplement stertooy/polycyclic
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@fingolfin @stertooy