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Fix PreImagesSet for input sets not contained in image #48

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Fix PreImagesSet for input sets not contained in image #48

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5 changes: 5 additions & 0 deletions CHANGES.md
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Original file line number Diff line number Diff line change
@@ -1,5 +1,10 @@
This file describes changes in the GAP package 'polycyclic'.

X.YY (YYYY-MM-DD)
- Fix a bug in `PreImagesSet` which lead to a "method not found" error
if the given input set was not contained in the image of the given
homomorphism.

2.16 (2020-07-25)
- Fix a bug in `NormalIntersection` which could lead to wrong results;
this also affected other operations, such `Core`, `Intersection`
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9 changes: 3 additions & 6 deletions gap/basic/grphoms.gi
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Expand Up @@ -296,14 +296,11 @@ InstallMethod( PreImagesSet,
CollFamRangeEqFamElms,
[ IsFromPcpGHBI and IsToPcpGHBI, IsPcpGroup ],
function( hom, U )
local prei, kern;
local prei, gens, kern;
prei := List( Igs(U), x -> PreImagesRepresentative(hom,x) );
if fail in prei then
TryNextMethod();
# Potential solution: Intersect U with ImagesSource(hom)
# and then compute the preimage of that.
#gens := GeneratorsOfGroup( Intersection( ImagesSource(hom), U ) );
#prei := List( gens, x -> PreImagesRepresentative(hom,x) );
gens := GeneratorsOfGroup( Intersection( ImagesSource(hom), U ) );
prei := List( gens, x -> PreImagesRepresentative(hom,x) );
fi;
kern := Igs( KernelOfMultiplicativeGeneralMapping( hom ) );
return SubgroupByIgs( Source(hom), kern, prei );
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13 changes: 13 additions & 0 deletions tst/bugfix.tst
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Expand Up @@ -479,5 +479,18 @@ true
gap> HirschLength( Ker );
5

#
# PreImages resp. PreImagesSet used to run into a "method not found"
# error when the input set is not contained in the image of the map.
#
gap> G := AbelianPcpGroup([0]);
Pcp-group with orders [ 0 ]
gap> phi := GroupHomomorphismByImages(G,G,[G.1],[One(G)]);
[ g1 ] -> [ id ]
gap> H := PreImagesSet(phi, G);
Pcp-group with orders [ 0 ]
gap> G = H;
true

#
gap> STOP_TEST( "bugfix.tst", 10000000);