Fix a bug in JenningsLieAlgebra and PCentralLieAlgebra

Fix a bug reported by Laurent Bartholdi.

Author: willemdegraaf

lib/alglie.gi

@@ -5671,8 +5671,7 @@ InstallMethod( JenningsLieAlgebra,
                 co:=[];
                 for k in [1,3..Length(e)-1] do
                     pp:= Position( enum_gens[pos[i]+pos[j]], e[k] );
-                    f:= GeneratorsOfGroup( pcgps[pos[i]+pos[j]] )[pp];
-                    t:= Position( gens, f );
+                    t:= Sum( enum_gens{[1..pos[i]+pos[j]-1]}, Length )+pp;
                     Add( co, One( F )*e[k+1] );
                     Add( co, t );
                 od;
@@ -5725,8 +5724,7 @@ InstallMethod( JenningsLieAlgebra,
             x:= Zero( L );
             for k in [1,3..Length(e)-1] do
                 pp:= Position( enum_gens[pos[i]*p], e[k] );
-                f:= GeneratorsOfGroup( pcgps[pos[i]*p] )[pp];
-                t:= Position( gens, f );
+                t:= Sum( enum_gens{[1..pos[i]*p-1]}, Length )+pp;
                 x:= x+ One( F )*e[k+1]*vv[t];
             od;
             pimgs[i]:= x;
@@ -5870,8 +5868,7 @@ InstallMethod( PCentralLieAlgebra,
                 co:=[];
                 for k in [1,3..Length(e)-1] do
                     pp:= Position( enum_gens[pos[i]+pos[j]], e[k] );
-                    f:= GeneratorsOfGroup( pcgps[pos[i]+pos[j]] )[pp];
-                    t:= Position( gens, f );
+                    t:= Sum( enum_gens{[1..pos[i]+pos[j]-1]}, Length )+pp;
                     Add( co, One( F )*e[k+1] );
                     Add( co, t );
                 od;
@@ -5926,8 +5923,7 @@ InstallMethod( PCentralLieAlgebra,
             x:= Zero( L );
             for k in [1,3..Length(e)-1] do
                 pp:= Position( enum_gens[pos[i]+1], e[k] );
-                f:= GeneratorsOfGroup( pcgps[pos[i]+1] )[pp];
-                t:= Position( gens, f );
+                t:= Sum( enum_gens{[1..pos[i]]}, Length )+pp;
                 x:= x+ One( F )*e[k+1]*vv[t];
             od;
             pimgs[i]:= x;

tst/testbugfix/jennings.tst

@@ -0,0 +1,31 @@
+# test for JenningsLieAlgebra (fixing a bug reported by Laurent Bartholdi):
+
+gap> g := Group((1,10)(2,9)(3,11)(4,12)(5,15)(6,16)(7,13)(8,14)(17,21)(18,22)*
+> (19,24)(20,23)(25,27)(26,28)(29,30)(33,51)(34,52)(35,49)(36,50)(37,54)(38,53)*
+> (39,55)(40,56)(41,61)(42,62)(43,64)(44,63)(45,57)(46,58)(47,60)(48,59), 
+> (1,19)(2,20)(3,17)(4,18)(5,22)(6,21)(7,23)(8,24)(9,29)(10,30)(11,32)(12,31)*
+> (13,25)(14,26)(15,28)(16,27)(33,42)(34,41)(35,43)(36,44)(37,47)(38,48)(39,45)*
+> (40,46)(49,53)(50,54)(51,56)(52,55)(57,59)(58,60)(61,62), 
+> (1,37)(2,38)(3,40)(4,39)(5,33)(6,34)(7,36)(8,35)(9,43)(10,44)(11,41)(12,42)*
+> (13,46)(14,45)(15,47)(16,48)(17,58)(18,57)(19,59)(20,60)(21,63)(22,64)(23,61)*
+> (24,62)(25,50)(26,49)(27,51)(28,52)(29,55)(30,56)(31,53)(32,54));
+<permutation group with 3 generators>
+gap> L:= JenningsLieAlgebra(g);
+<Lie algebra of dimension 13 over GF(2)>
+gap> List(Basis(L),PthPowerImage);
+[ 0*v.1, v.6, 0*v.1, v.7, v.8, v.9, 0*v.1, v.10, v.11, v.12, 0*v.1, v.13, 
+  0*v.1 ]
+gap> LieLowerCentralSeries(L);
+[ <Lie algebra of dimension 13 over GF(2)>, 
+  <Lie algebra of dimension 3 over GF(2)>, 
+  <Lie algebra of dimension 0 over GF(2)> ]
+gap> L:= PCentralLieAlgebra(g);
+<Lie algebra of dimension 13 over GF(2)>
+gap> LieLowerCentralSeries(L);
+[ <Lie algebra of dimension 13 over GF(2)>, 
+  <Lie algebra of dimension 10 over GF(2)>, 
+  <Lie algebra of dimension 7 over GF(2)>, 
+  <Lie algebra of dimension 4 over GF(2)>, 
+  <Lie algebra of dimension 2 over GF(2)>, 
+  <Lie algebra of dimension 1 over GF(2)>, 
+  <Lie algebra of dimension 0 over GF(2)> ]