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fingolfin merged 2 commits into from
Jan 23, 2026
+25 −4
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Avoid errors due to too large range bounds #6201

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9b0e757
avoid errors due to too large range bounds
ThomasBreuer Jan 20, 2026
978c0a6
simplify the test
ThomasBreuer Jan 21, 2026
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14 changes: 10 additions & 4 deletions lib/morpheus.gi
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Original file line number Diff line number Diff line change
Expand Up @@ -2627,7 +2627,13 @@ local d,iso,a,b,c,o,s,two,rt,r,z,e,y,re,m,gens,cnt,lim,p,
a:=PseudoRandom(gp);
fi;
e:=Order(a);
if e in r then
# `r` is either a list or a record,
# the latter describing the range
# `[ r.first, r.first + r.offset .. r.last ]`,
# also if this range cannot be created in GAP.
if ( IsList( r ) and e in r ) or
( IsRecord( r ) and r.first <= e and e <= r.last
and ( e - r.first ) mod r.offset = 0 ) then
a:=a^QuoInt(e,o);
if z=fail or Size(Centralizer(gp,a))=z then
return a;
Expand Down Expand Up @@ -2731,7 +2737,7 @@ local d,iso,a,b,c,o,s,two,rt,r,z,e,y,re,m,gens,cnt,lim,p,
if gens=fail then
Info(InfoMorph,1,"Isomorphism simple: ad-hoc");
# not found by table or other -- try a 2/something ad-hoc
rt:=[2,4..Size(g)];
rt:= rec( first:= 2, offset:= 2, last:= Size(g) );
gens:=[findElm(g,2,fail,rt)];
z:=Size(Centralizer(g,gens[1]));

Expand All @@ -2740,7 +2746,7 @@ local d,iso,a,b,c,o,s,two,rt,r,z,e,y,re,m,gens,cnt,lim,p,
m:=Maximum(Filtered(Factors(Size(g)),x->x<100));
cnt:=0;
repeat
gens[2]:=findElm(g,m,fail,[m,2*m..Size(g)]);
gens[2]:=findElm(g,m,fail, rec( first:= m, offset:= m, last:= Size(g) ));
if isFull(SubgroupNC(g,gens)) then
b:=gens;
y:=Size(Centralizer(g,gens[2]));
Expand All @@ -2765,7 +2771,7 @@ local d,iso,a,b,c,o,s,two,rt,r,z,e,y,re,m,gens,cnt,lim,p,
od;
gens:=b;
e:=Order(gens[2]);
re:=[e,2*e..Size(g)];
re:= rec( first:= e, offset:= e, last:= Size(g) );
y:=Size(Centralizer(g,gens[2]));
fi;
Info(InfoMorph,1,"generators ",List(gens,Order));
Expand Down
15 changes: 15 additions & 0 deletions tst/testbugfix/2026-01-20-IsomorphismSimpleGroups.tst
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@@ -0,0 +1,15 @@
# Fix #6200 IsomorphismSimpleGroups
#@local G, x, H
gap> START_TEST("IsomorphismSimpleGroups.tst");

# The runtime depends very much on random choices in the code,
# it can vary from a few milliseconds (rare) to several minutes.
gap> Reset( GlobalMersenneTwister, 2^19 );;
gap> G:= AlternatingGroup( 20 );;
gap> x:= ();;
gap> H:= ConjugateGroup( G, x );;
gap> IsomorphismSimpleGroups( G, H : cheap:= true ) <> fail;
true

#
gap> STOP_TEST("IsomorphismSimpleGroups.tst");
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