Fix forming trivial quotients of algebra modules

gap-system · Jan 4, 2018 · dfb8827 · dfb8827
1 parent 869d720
commit dfb8827
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Showing 2 changed files with 27 additions and 1 deletion.
diff --git a/lib/algrep.gi b/lib/algrep.gi
@@ -1194,6 +1194,15 @@ InstallMethod( NaturalHomomorphismBySubAlgebraModule,
  f:= NaturalHomomorphismBySubspace( V, W );
  quot:= ImagesSource( f );
 
+ if IsTrivial(quot) then
+ quot := SubAlgebraModule( V, [], "basis" );
+ nathom := ZeroMapping( V, quot );
+ SetIsSurjective( nathom, true );
+ SetImagesSource(nathom, quot);
+ SetKernelOfAdditiveGeneralMapping( nathom, W );
+ return nathom;
+ fi;
+
  imgs:= List( Basis( V ), x -> Coefficients( Basis( quot ),
  ImagesRepresentative(f,x)));
 
@@ -1231,7 +1240,9 @@ InstallMethod( NaturalHomomorphismBySubAlgebraModule,
 
  # Enter the preimages info.
  nathom!.basisimage:= Basis( qmod );
- nathom!.preimagesbasisimage:= f!.preimagesbasisimage;
+ if IsBound(f!.preimagesbasisimage) then
+ nathom!.preimagesbasisimage:= f!.preimagesbasisimage;
+ fi;
  SetKernelOfAdditiveGeneralMapping( nathom, W );
 
  # Run the implications for the factor.

diff --git a/tst/testinstall/algrep.tst b/tst/testinstall/algrep.tst
@@ -55,6 +55,21 @@ gap> S1:=SubAlgebraModule(S,bas,"basis");
 <16-dimensional left-module over <Lie algebra of dimension 4 over GF(3)>>
 
 #
+# factors
+#
+gap> S0/S0;
+<0-dimensional left-module over <Lie algebra of dimension 4 over GF(3)>>
+gap> S1/S0;
+<16-dimensional left-module over <Lie algebra of dimension 4 over GF(3)>>
+gap> S1/S1;
+<0-dimensional left-module over <Lie algebra of dimension 4 over GF(3)>>
+gap> S/S0;
+<35-dimensional left-module over <Lie algebra of dimension 4 over GF(3)>>
+gap> S/S1;
+<19-dimensional left-module over <Lie algebra of dimension 4 over GF(3)>>
+gap> S/S;
+<0-dimensional left-module over <Lie algebra of dimension 4 over GF(3)>>
+
 #
 # cocycles etc.
 #