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Fix TwoCohomologyGeneric when there are no conditions on the tails, leading to either an unexpected error or even a wrong output #6477

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Merged
fingolfin merged 1 commit into from
Aug 3, 2026
+19 −1
Merged

Fix TwoCohomologyGeneric when there are no conditions on the tails, leading to either an unexpected error or even a wrong output #6477

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3 changes: 2 additions & 1 deletion lib/twocohom.gi
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Original file line number Diff line number Diff line change
Expand Up @@ -987,7 +987,8 @@ local field,fp,fpg,gens,hom,mats,fm,mon,tzrules,dim,rules,eqs,i,j,k,l,o,l1,
#eqs:=Filtered(TriangulizedMat(eqs),x->not IsZero(x));
eqs:=ShallowCopy(BasisVectors(eqs));
if Length(eqs)=0 then
eqs:=IdentityMat(Length(rules),field);
# no conditions, so the whole space of tail vectors consists of cocycles
eqs:=IdentityMat(nvars,field);
else
eqs:=ImmutableMatrix(field,eqs);
eqs:=NullspaceMat(TransposedMat(eqs)); # basis of cocycles
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17 changes: 17 additions & 0 deletions tst/testbugfix/2026-08-02-TwoCohomologyGeneric.tst
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@@ -0,0 +1,17 @@
# TwoCohomologyGeneric used the wrong dimension for the space of cocycles
# in case no conditions on the tails were found, which made it error out
# (or return a wrong answer) for modules of dimension > 1.
gap> triv:=function(G,d,F)
> local mo;
> mo:=GModuleByMats(List(GeneratorsOfGroup(G),x->IdentityMat(d,F)),d,F);
> return Length(TwoCohomologyGeneric(G,mo).cohomology);
> end;;
gap> List([1..3],d->triv(CyclicGroup(2),d,GF(2)));
[ 1, 2, 3 ]
gap> List([1..3],d->triv(ElementaryAbelianGroup(8),d,GF(2)));
[ 6, 12, 18 ]
gap> List([1..3],d->triv(DihedralGroup(8),d,GF(2)));
[ 3, 6, 9 ]
gap> List([1..3],d->triv(CyclicGroup(3),d,GF(3)));
[ 1, 2, 3 ]
gap> Unbind(triv);
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