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Fix PreImagesRepresentative for IsToPcGroupHomomorphismByImages #5705

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Apr 24, 2024
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Fix PreImagesRepresentative for IsToPcGroupHomomorphismByImages #5705

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6 changes: 5 additions & 1 deletion lib/ghompcgs.gi
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Original file line number Diff line number Diff line change
Expand Up @@ -511,7 +511,11 @@
function( hom, elm )
local pcgsR, exp, imgs, pre, i;

# precompute pcgs
if not elm in ImagesSource( hom ) then
return fail;

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fi;

# Precompute a pcgs for the *image* of 'hom'.
InversePcgs( hom );

pcgsR := hom!.rangePcgs;
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21 changes: 21 additions & 0 deletions tst/testinstall/grppc.tst
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Expand Up @@ -183,5 +183,26 @@ true
gap> IsIdenticalObj( Range( iso ), ImagesSource( iso ) );
true

#
gap> G:= AbelianGroup( IsPcGroup, [ 3, 3, 3 ] );;
gap> S:= AbelianGroup( IsPcGroup, [ 3, 3 ] );;
gap> f:= GroupHomomorphismByImages( G, S, GeneratorsOfGroup( G ),
> [ GeneratorsOfGroup( S )[1], One( S ), One( S ) ] );;
gap> x:= GeneratorsOfGroup( S )[2];;
gap> x in Range( f ) and not x in Image( f );
true
gap> PreImagesRepresentative( f, x );
fail
gap> G:= AbelianGroup( IsPcGroup, [ 3, 3 ] );;
gap> S:= AbelianGroup( IsPcGroup, [ 3 ] );;
gap> f:= GroupHomomorphismByImages( G, S, GeneratorsOfGroup( G ),
> [ One( S ), One( S ) ] );;
gap> x:= GeneratorsOfGroup( S )[1];
f1
gap> x in Range( f ) and not x in Image( f );
true
gap> PreImagesRepresentative( f, x );
fail

# that's all, folks
gap> STOP_TEST( "grppc.tst" );
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