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Fix AbsolutelyIrreducibleModules for a pc group and a finite prime field #5067

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Sep 26, 2022
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Fix AbsolutelyIrreducibleModules for a pc group and a finite prime field #5067

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8 changes: 4 additions & 4 deletions lib/grppcrep.gi
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Original file line number Diff line number Diff line change
Expand Up @@ -587,18 +587,18 @@ end );
##
#M AbsolutIrreducibleModules( <G>, <F>, <dim> ). . . . . . .up to equivalence
##
## <dim> is the limit of Dim( F ) * Dim( M ) for the modules M
##
InstallMethod( AbsolutIrreducibleModules,
"generic method for groups with pcgs",
"method for group with pcgs and finite prime field",
true,
[ IsGroup and CanEasilyComputePcgs, IsField and IsFinite and IsPrimeField, IsInt ],
0,

function( G, F, dim )
local modus;
modus := AbsAndIrredModules( G, F, dim );
return [modus[1],List( modus[2], x -> x.absirr )];
return [ modus[1],
Filtered( List( modus[2], x -> x.absirr ),
x -> IsPrimeField( x.field ) ) ];
end );

#############################################################################
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9 changes: 6 additions & 3 deletions lib/grpreps.gd
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Expand Up @@ -22,10 +22,13 @@
## <Oper Name="AbsolutIrreducibleModules" Arg='G, F, dim'/>
##
## <Description>
## returns a list of length 2. The first entry is a generating system of
## <A>G</A>. The second entry is a list of all absolute irreducible modules of
## <A>G</A> over the field <A>F</A> in dimension <A>dim</A>, given as MeatAxe modules
## <Ref Oper="AbsolutelyIrreducibleModules"/> returns a list of length 2.
## The first entry is a generating system of the group <A>G</A>.
## The second entry is a list of all those absolutely irreducible modules of
## <A>G</A> that can be realized over the finite field <A>F</A>
## and have dimension at most <A>dim</A>, given as MeatAxe modules
## (see&nbsp;<Ref Func="GModuleByMats" Label="for generators and a field"/>).
## <P/>
## The other two names are just synonyms.
## </Description>
## </ManSection>
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21 changes: 21 additions & 0 deletions tst/testinstall/grpreps.tst
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@@ -0,0 +1,21 @@
#@local G1, G2, F, res1, res2;
gap> START_TEST( "grpreps.tst" );

# Compare the two methods for 'AbsolutelyIrreducibleModules'.
gap> G1:= DihedralGroup( 10 );;
gap> G2:= DihedralGroup( IsPermGroup, 10 );;
gap> F:= GF(2);;
gap> # Make sure that different methods will be called.
gap> ApplicableMethod( AbsolutelyIrreducibleModules, [ G1, F, 10 ] )
> <> ApplicableMethod( AbsolutelyIrreducibleModules, [ G2, F, 10 ] );
true
gap> res1:= AbsolutelyIrreducibleModules( G1, F, 10 );;
gap> List( res1[2], r -> [ r.field, r.dimension ] );
[ [ GF(2), 1 ] ]
gap> res2:= AbsolutelyIrreducibleModules( G2, F, 10 );;
gap> List( res2[2], r -> [ r.field, r.dimension ] );
[ [ GF(2), 1 ] ]

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