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Correct bug in DirectFactorsOfGroup #566

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Feb 1, 2016
Merged

Correct bug in DirectFactorsOfGroup #566

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4 changes: 2 additions & 2 deletions lib/grpnames.gi
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Original file line number Diff line number Diff line change
Expand Up @@ -310,7 +310,7 @@ InstallMethod(DirectFactorsOfGroup, "if normal subgroups are computed", true,
GGd := CommutatorFactorGroup(G);
# if GGd is not cyclic of prime power size then there are at least two
# maximal subgroups
if IsTrivial(GGd) or (IsCyclic(GGd) and IsPrimePowerInt(Size(GGd)))
if (IsTrivial(GGd) or (IsCyclic(GGd) and IsPrimePowerInt(Size(GGd))))
and Length(MaximalNormalSubgroups(G))= 1 then
# size of MaximalNormalSubgroups is an upper bound to the number of
# components
Expand Down Expand Up @@ -437,7 +437,7 @@ InstallMethod(DirectFactorsOfGroup, "generic method", true,
GGd := CommutatorFactorGroup(G);
# if GGd is not cyclic of prime power size then there are at least two
# maximal subgroups
if IsTrivial(GGd) or (IsCyclic(GGd) and IsPrimePowerInt(Size(GGd)))
if (IsTrivial(GGd) or (IsCyclic(GGd) and IsPrimePowerInt(Size(GGd))))
and Length(MaximalNormalSubgroups(G))= 1 then
# size of MaximalNormalSubgroups is an upper bound to the number of
# components
Expand Down
7 changes: 5 additions & 2 deletions tst/testinstall/direct_factors.tst
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Expand Up @@ -102,11 +102,14 @@ gap> DirectFactorsOfGroup(SymmetricGroup(4));
[ Sym( [ 1 .. 4 ] ) ]
gap> DirectFactorsOfGroup(SymmetricGroup(5));
[ Sym( [ 1 .. 5 ] ) ]
gap> G := Group([ (4,8)(6,10), (4,6,10,8,12), (2,4,12)(6,10,8), (3,9)(4,6,10,8,12)(7,11), (3,5)(4,6,10,8,12)(9,11), (1,3,11,9,5)(4,6,10,8,12) ]);;
gap> G := Group([ (6,7,8,9,10), (8,9,10), (1,2)(6,7), (1,2,3,4,5)(6,7,8,9,10) ]);;
gap> DirectFactorsOfGroup(G)=[G];
true
gap> G := Group([ (4,8)(6,10), (4,6,10,8,12), (2,4,12)(6,10,8), (3,9)(4,6,10,8,12)(7,11), (3,5)(4,6,10,8,12)(9,11), (1,3,11,9,5)(4,6,10,8,12) ]);;
gap> G := Group([ (6,7,8,9,10), (8,9,10), (1,2)(6,7), (1,2,3,4,5)(6,7,8,9,10) ]);;
gap> NormalSubgroups(G);;
gap> DirectFactorsOfGroup(G)=[G];
true
gap> G := Group([ (4,8)(6,10), (4,6,10,8,12), (2,4,12)(6,10,8), (3,9)(4,6,10,8,12)(7,11), (3,5)(4,6,10,8,12)(9,11), (1,3,11,9,5)(4,6,10,8,12) ]);;
gap> DirectFactorsOfGroup(G)=[ Group([ (4,8)(6,10), (4,6)(10,12), (2,12,8)(4,6,10) ]), Group([ (1,7,9)(3,5,11), (3,9)(7,11), (3,11)(5,7) ]) ];
true
gap> STOP_TEST("direct_factors.tst", 10000);