From 70eba1d8d6400610c7fd42abaabecd2e452289f3 Mon Sep 17 00:00:00 2001 From: Alexander Hulpke Date: Fri, 1 Nov 2024 14:25:53 -0600 Subject: [PATCH] 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. --- lib/gpfpiso.gi | 2 +- tst/testbugfix/2024-Halloween-IsomorphismFp.tst | 8 ++++++++ 2 files changed, 9 insertions(+), 1 deletion(-) create mode 100644 tst/testbugfix/2024-Halloween-IsomorphismFp.tst diff --git a/lib/gpfpiso.gi b/lib/gpfpiso.gi index 2418ffd442..c386281719 100644 --- 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 new file mode 100644 index 0000000000..d004577d4b --- /dev/null +++ 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