Ensure subgroup is tested at least once, (#5709)

in finding induced permutation representation of FpGroup, even if max parameter was increased in between. This resolves #5697 Co-authored-by: Alexander Hulpke <[email protected]>
gap-system · May 6, 2024 · fcd5de4 · fcd5de4
1 parent 98a4b90
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diff --git a/lib/grpfp.gi b/lib/grpfp.gi
@@ -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;
@@ -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

diff --git a/tst/testbugfix/2024-05-06-IsomorphismPermGroupOrFailFpGroup.tst b/tst/testbugfix/2024-05-06-IsomorphismPermGroupOrFailFpGroup.tst
@@ -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