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Make RestrictedMapping of a group homomorphism use the same Range as the original morphism #6230

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Merged
fingolfin merged 1 commit into from
Mar 3, 2026
Merged

Make RestrictedMapping of a group homomorphism use the same Range as the original morphism #6230

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12 changes: 2 additions & 10 deletions lib/ghom.gi
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Original file line number Diff line number Diff line change
Expand Up @@ -95,23 +95,15 @@ end);
InstallMethod(RestrictedMapping,"create new GHBI",
CollFamSourceEqFamElms,[IsGroupHomomorphism,IsGroup],0,
function(hom,U)
local rest,gens,imgs,imgp;
local rest,gens,imgs;

gens:=GeneratorsOfGroup(U);
imgs:=List(gens,i->ImageElm(hom,i));

if HasImagesSource(hom) then
imgp:=ImagesSource(hom);
else
imgp:=Subgroup(Range(hom),imgs);
fi;
rest:=GroupHomomorphismByImagesNC(U,imgp,gens,imgs);
rest:=GroupHomomorphismByImagesNC(U,Range(hom),gens,imgs);
if HasIsInjective(hom) and IsInjective(hom) then
SetIsInjective(rest,true);
fi;
if HasIsTotal(hom) and IsTotal(hom) then
SetIsTotal(rest,true);
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@fingolfin fingolfin Feb 21, 2026

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Why is this removed? If the map was total (= not partial) then surely its restriction is also total? And this is kinda important to know about a map

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@stertooy stertooy Feb 21, 2026

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IsGroupHomomorphism implies IsTotal, so both hom and rest should have IsTotal set to true already.

fi;

return rest;
end);
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