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Fix an unexpected error in GQuotients (by fixing a bug in the cyclic modified Todd-Coxeter) #5532

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Nov 17, 2023
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Fix an unexpected error in GQuotients (by fixing a bug in the cyclic modified Todd-Coxeter) #5532

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7 changes: 6 additions & 1 deletion lib/sgpres.gi
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Original file line number Diff line number Diff line change
Expand Up @@ -2847,7 +2847,12 @@
od;
if i>r then
if f<>alpha then
NEWTC_ModifiedCoincidence(DATA,f,alpha,WordProductLetterRep(-Reversed(fp),y));
if DATA.useAddition then
NEWTC_ModifiedCoincidence(DATA,f,alpha,-fp+y);

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else
NEWTC_ModifiedCoincidence(DATA,f,alpha,
WordProductLetterRep(-Reversed(fp),y));
fi;

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fi;
return;
fi;
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7 changes: 7 additions & 0 deletions tst/testbugfix/2023-10-28-GQuotients.tst
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@@ -0,0 +1,7 @@
# Fix unexpected error in GQuotients.
# See https://github.com/gap-system/gap/issues/5525
#
gap> G := FreeGroup(2);;
gap> Q := G / [G.1*G.2^-2*G.1, G.1^-1*G.2^-3];;
gap> GQuotients(Q, SmallGroup(12, 1));
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