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Fix regression in IsomorphismPermGroupOrFailFpGroup that caused it to fail in situations that worked in GAP 4.12.2 #5709

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May 6, 2024
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Fix regression in IsomorphismPermGroupOrFailFpGroup that caused it to fail in situations that worked in GAP 4.12.2 #5709

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3 changes: 3 additions & 0 deletions lib/grpfp.gi
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Original file line number Diff line number Diff line change
Expand Up @@ -4051,6 +4051,7 @@ local mappow, G, max, p, gens, rels, comb, i, l, m, H, t, gen, sz,
RelatorsOfFpGroup(G),[gen],true,false:
cyclic:=true,limit:=1+max,quiet:=true );
fi;

if t=fail then
# we cannot get the size within the permitted limits -- give up
return fail;
Expand Down Expand Up @@ -4098,6 +4099,8 @@ local mappow, G, max, p, gens, rels, comb, i, l, m, H, t, gen, sz,
max:=10^3*sz;
fi;

amax:=Maximum(amax,max+1);

useind:=false;
t1:=timerFunc();
while max<amax do
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12 changes: 12 additions & 0 deletions tst/testbugfix/2024-05-06-IsomorphismPermGroupOrFailFpGroup.tst
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@@ -0,0 +1,12 @@
# Verify regression in IsomorphismPermGroupOrFailFpGroup is
# resolved, see <https://github.com/gap-system/gap/issues/5697>
gap> F:= FreeGroup(2);
<free group on the generators [ f1, f2 ]>
gap> gens:= GeneratorsOfGroup( F );
[ f1, f2 ]
gap> x:= gens[1];; y:= gens[2];;
gap> rels:= [ y*x^-1*y^-1*x*y^-1*x^-1, x^-1*y*x*y*x^-1*y^-2 ];;
gap> G:= F / rels;
<fp group on the generators [ f1, f2 ]>
gap> IsomorphismPermGroupOrFailFpGroup(G, 100000) <> fail;
true
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