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Use NC variants of PreImages operations #58

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16 changes: 16 additions & 0 deletions init.g
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Original file line number Diff line number Diff line change
Expand Up @@ -28,6 +28,22 @@ if not IsBound(MultVector) then
DeclareSynonym( "MultVector", MultRowVector );
fi;

#
# GAP >= 4.17 has unchecked NC variants of the PreImages operations
#
if not IsBound( PreImagesNC ) then
BindGlobal( "PreImagesNC", PreImages );
fi;
if not IsBound( PreImagesElmNC ) then
BindGlobal( "PreImagesElmNC", PreImagesElm );
fi;
if not IsBound( PreImagesSetNC ) then
BindGlobal( "PreImagesSetNC", PreImagesSet );
fi;
if not IsBound( PreImagesRepresentativeNC ) then
BindGlobal( "PreImagesRepresentativeNC", PreImagesRepresentative );
fi;

ReadPackage("fining","lib/geometry.gd");

ReadPackage("fining","lib/liegeometry.gd");
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4 changes: 2 additions & 2 deletions lib/gpolygons.gi
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Expand Up @@ -4020,7 +4020,7 @@ InstallMethod( EGQByBLTSet,
[0,0,0,1,0,0],[0,0,0,0,1,0]]*One(f));
q4qcanonical := Range(duality)!.geometry;
iso := IsomorphismPolarSpaces(q4q, q4qcanonical);
bltdual := PreImagesSet(duality, ImagesSet(iso, blt));
bltdual := PreImagesSetNC(duality, ImagesSet(iso, blt));

Info(InfoFinInG, 1, "Now embedding dual BLT-set into W(5,q)...");

Expand Down Expand Up @@ -4301,7 +4301,7 @@ InstallMethod( FlockGQByqClan, [ IsqClanObj ],
imgs := Filtered([1..Size(omega)], x -> omega[x] in blt);;
stabblt := Stabilizer(Image(hom), imgs, OnSets);
gens := GeneratorsOfGroup(stabblt);
gens := List(gens, x -> PreImagesRepresentative(hom, x));
gens := List(gens, x -> PreImagesRepresentativeNC(hom, x));
stabblt := GroupWithGenerators(gens);

Info(InfoFinInG, 1, "..computed stabiliser of BLT set");
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14 changes: 12 additions & 2 deletions lib/morphisms.gi
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Expand Up @@ -204,15 +204,25 @@ InstallOtherMethod( PreImageElm,

# CHECKED 27/09/11 jdb
#############################################################################
#O PreImagesSet( <em>, <x> )
#O PreImagesSetNC( <em>, <x> )
##
InstallOtherMethod( PreImagesSet,
InstallOtherMethod( PreImagesSetNC,
"for a geometry morphism and an element of an incidence structure",
[IsGeometryMorphism, IsElementOfIncidenceStructureCollection],
function(em, x)
return List(x, t -> em!.prefun(t));
end );

# The generic PreImagesSet fails its IsSubset( Range( em ), x ) check for
# geometry morphisms. Without PreImagesSetNC in GAP, both names denote the
# same operation, see init.g.
if not IsIdenticalObj( PreImagesSet, PreImagesSetNC ) then
InstallOtherMethod( PreImagesSet,
"for a geometry morphism and an element of an incidence structure",
[IsGeometryMorphism, IsElementOfIncidenceStructureCollection],
PreImagesSetNC );
fi;

##########################################################
## User methods for the "natural geometry morphisms"
##########################################################
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4 changes: 2 additions & 2 deletions lib/orbits-stabilisers.gi
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Expand Up @@ -411,7 +411,7 @@ InstallMethod( FiningStabiliserPerm, [IsProjectiveGroupWithFrob, IsElementOfInci
nr:=Position(enum,el);
stab:=Stabilizer(Image(hom),nr);
gens:=GeneratorsOfGroup(stab);;
gens:=List(gens,x->PreImagesRepresentative(hom,x));
gens:=List(gens,x->PreImagesRepresentativeNC(hom,x));
stab:=GroupWithGenerators(gens);
return stab;
end );
Expand All @@ -433,7 +433,7 @@ InstallMethod( FiningStabiliserPerm2,
stab:=Stab(im,nr,OnPoints,rec( DoEstimate := size )).stab;
#stab:=Stabiliser(Image(hom),nr);
gens:=GeneratorsOfGroup(stab);;
gens:=List(gens,x->PreImagesRepresentative(hom,x));
gens:=List(gens,x->PreImagesRepresentativeNC(hom,x));
stab:=GroupWithGenerators(gens);
return stab;
end );
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8 changes: 8 additions & 0 deletions tst/bugfix.tst
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Expand Up @@ -20,5 +20,13 @@ gap> ElationOfProjectiveSpace(l, p, q);
gap> SingerCycleCollineation(2, 2^6);
< a collineation: <cmat 3x3 over GF(2,6)>, F^0>

# GAP >= 4.17 calls PreImagesSetNC internally
gap> d := NaturalDuality(SymplecticSpace(3, 3));;
gap> pts := AsList(Points(Range(d)!.geometry)){[1..3]};;
gap> PreImagesSetNC(d, pts) = List(pts, x -> PreImageElm(d, x));
true
gap> PreImagesSet(d, pts) = PreImagesSetNC(d, pts);
true

#
gap> STOP_TEST("bugfix.tst", 1 );
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