FIX: Enforce that Natural(G/S)L test creates module with generators.

This fixes gap-system#2104
hulpke · Feb 1, 2018 · 8763634 · 8763634
1 parent 8691a62
commit 8763634
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Showing 2 changed files with 11 additions and 2 deletions.
diff --git a/lib/grpffmat.gi b/lib/grpffmat.gi
@@ -79,6 +79,13 @@ if Length(l)=0 then Error("list must be nonempty");fi;
   return GF(char^deg);
 end);
 
+BindGlobal("NonemptyGeneratorsOfGroup",function(grp)
+local l;
+   l:=GeneratorsOfGroup(grp);
+   if Length(l)=0 then l:=[One(grp)]; fi;
+   return l;
+end);
+
 #############################################################################
 ##
 #M  IsNaturalGL( <ffe-mat-grp> )
@@ -91,7 +98,7 @@ InstallMethod( IsNaturalGL,
 
 function( grp )
   return MTX.IsAbsolutelyIrreducible(
-    GModuleByMats(GeneratorsOfGroup(grp),DefaultFieldOfMatrixGroup(grp))) and
+    GModuleByMats(NonemptyGeneratorsOfGroup(grp),DefaultFieldOfMatrixGroup(grp))) and
    Size( grp ) = Size( GL( DimensionOfMatrixGroup( grp ),
 		  Size( FieldOfMatrixGroup( grp ) ) ) );
 end );
@@ -108,7 +115,7 @@ local gen, d, f;
   d := DimensionOfMatrixGroup( grp );
   gen := GeneratorsOfGroup( grp );
   return MTX.IsAbsolutelyIrreducible(
-    GModuleByMats(GeneratorsOfGroup(grp),DefaultFieldOfMatrixGroup(grp))) and
+    GModuleByMats(NonemptyGeneratorsOfGroup(grp),DefaultFieldOfMatrixGroup(grp))) and
     ForAll(gen, x-> DeterminantMat(x) = One(f)) 
 	    and Size(grp) = Size(SL(d, Size(f)));
 end );

diff --git a/tst/testinstall/grpmat.tst b/tst/testinstall/grpmat.tst
@@ -50,6 +50,8 @@ gap> iso:= IsomorphismPermGroup( g );;
 gap> img:=Image( iso );;
 gap> Size(img);
 67010895544320000
+gap> IsNaturalGL( TrivialSubgroup( GL(2,2) ) );
+false
 
 # Unbind variables so we can GC memory
 gap> Unbind(img); Unbind(iso); Unbind(g); Unbind(hom); Unbind(u);