Fix for IsomorphismFpGroup for perm groups (#5831)

This fixes a bug reported to me by Andries Brouwer on 10/31/24 A homomorphism is created in a way that does not enforce it to be onto.
gap-system · Nov 1, 2024 · 70eba1d · 70eba1d
1 parent 6d93f61
commit 70eba1d
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diff --git a/lib/gpfpiso.gi b/lib/gpfpiso.gi
@@ -392,7 +392,7 @@ function(g,str,N)
       fi;
 
       # the range is elementary. Use this for the fp group isomorphism
-      f:=Range(hom);
+      f:=Image(hom);
       # calculate automorphisms of f induced by G
       fgens:=GeneratorsOfGroup(f);
       auts:=List(GeneratorsOfGroup(g),i->

diff --git a/tst/testbugfix/2024-Halloween-IsomorphismFp.tst b/tst/testbugfix/2024-Halloween-IsomorphismFp.tst
@@ -0,0 +1,8 @@
+# Fix for bug in IsomorphismFp
+# reported by Andries Brouwer, 10/31/24
+
+gap> G:=PSL(3,7);;
+gap> maxes := MaximalSubgroupClassReps(G);;
+gap> maxes1 := MaximalSubgroupClassReps(maxes[1]);;
+gap> maxes12:= maxes1[2];;
+gap> StructureDescription(maxes12:short);;  # used to trigger an error