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Fix a bug in the computation of a permutation group isomorphic to a group of automorphisms #1907

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Nov 16, 2017
Merged

Fix a bug in the computation of a permutation group isomorphic to a group of automorphisms #1907

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9f0cdd5
ENHANCE: Force abelian test before computing automorphism group.
hulpke Oct 12, 2017
c58a263
FIX: Automorphism group structure for type `C' only holds
hulpke Nov 14, 2017
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21 changes: 20 additions & 1 deletion lib/morpheus.gi
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Original file line number Diff line number Diff line change
Expand Up @@ -1568,7 +1568,7 @@ local d,id,H,iso,aut,auts,i,all,hom,field,dim,P,diag,mats,gens,gal;
all:=true;
fi;

elif id.series in ["L","2A","C"] then
elif id.series in ["L","2A"] or (id.series="C" and id.parameter[1]>2) then
hom:=EpimorphismFromClassical(G);
if hom=fail then return fail;fi;
field:=FieldOfMatrixGroup(Source(hom));
Expand Down Expand Up @@ -1978,6 +1978,25 @@ local A;
return A;
end);

#############################################################################
##
#M AutomorphismGroup(<G>) . . abelian case
##
InstallMethod(AutomorphismGroup,"test abelian",true,[IsGroup and IsFinite],
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I've tested this PR on Jenkins, and tests pass. A minor remark is that the method's description "test abelian" does not seem to be enough informative. "for a finite group to enforce check of IsAbelian" maybe?

RankFilter(IsSolvableGroup and IsFinite),
function(G)
local A;
if not IsAbelian(G) then
TryNextMethod();
fi;
A:=AutomorphismGroupAbelianGroup(G);
SetIsAutomorphismGroup(A,true);
SetIsGroupOfAutomorphismsFiniteGroup(A,true);
SetIsFinite(A,true);
SetAutomorphismDomain(A,G);
return A;
end);

# just in case it does not know to be finite
RedispatchOnCondition(AutomorphismGroup,true,[IsGroup],
[IsGroup and IsFinite],0);
Expand Down
21 changes: 21 additions & 0 deletions tst/teststandard/opers/AutomorphismGroup.tst
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@@ -0,0 +1,21 @@
gap> START_TEST("AutomorphismGroup.tst");
gap> SimpleAutTest:=function(from,to)
> local it,g,a,p;
> it:=SimpleGroupsIterator(from);
> g:=NextIterator(it);
> while Size(g)<=to do
> a:=AutomorphismGroup(g);
> Info(InfoWarning,2,g," ",Size(g)," ",Size(a)/Size(g));
> p:=Image(IsomorphismPermGroup(a));
> if Size(p)<>Size(a) then
> return g;
> fi;
> g:=NextIterator(it);
> od;
> return true;
> end;;
gap> SimpleAutTest(1,10^5);
true
gap> SimpleAutTest(10^5,3*10^6:nopsl2);
true
gap> STOP_TEST("AutomorphismGroup.tst",1);