Catch trivial group LatticeViaRadical and IsomorphismFpGroup

Reported-By: Madeleine Whybrow <mlw10@ic.ac.uk>
gap-system · Jan 18, 2019 · 485b8e0 · 485b8e0
1 parent e0ac2e5
commit 485b8e0
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diff --git a/lib/gpprmsya.gi b/lib/gpprmsya.gi
@@ -1739,6 +1739,11 @@ local   F,      # free group
 	relators,
 	i, k;       # loop variables
 
+    if IsTrivial(G) then
+      return GroupHomomorphismByFunction(G, TRIVIAL_FP_GROUP,
+                                         x->One(TRIVIAL_FP_GROUP),
+                                         x->One(G):noassert);
+    fi;
     # test for internal rep
     if HasGeneratorsOfGroup(G) and 
       not ForAll(GeneratorsOfGroup(G),IsInternalRep) then

diff --git a/lib/grplatt.gi b/lib/grplatt.gi
@@ -866,6 +866,9 @@ InstallGlobalFunction(LatticeViaRadical,function(arg)
   end;
 
   G:=arg[1];
+  if IsTrivial(G) then
+    return LatticeFromClasses(G,[G^G]);
+  fi;
   H:=fail;
   select:=fail;
   if Length(arg)>1 then
@@ -1242,6 +1245,11 @@ InstallMethod(LatticeSubgroups,"via radical",true,[IsGroup and
 InstallMethod(LatticeSubgroups,"cyclic extension",true,[IsGroup and
   IsFinite],0, LatticeByCyclicExtension );
 
+InstallMethod(LatticeSubgroups, "for the trivial group", true,
+  [IsGroup and IsTrivial],
+  0,
+  G -> LatticeFromClasses(G,[G^G]));
+
 RedispatchOnCondition( LatticeSubgroups, true,
     [ IsGroup ], [ IsFinite ], 0 );
 

diff --git a/tst/testbugfix/2019-01-18-grplatt.tst b/tst/testbugfix/2019-01-18-grplatt.tst
@@ -0,0 +1,8 @@
+gap> LatticeSubgroups(Group(()));
+<subgroup lattice of Group(()), 1 classes, 1 subgroups>
+gap> IsomorphismFpGroup(SymmetricGroup(1));
+MappingByFunction( Group(()), <fp group on the generators 
+[  ]>, function( x ) ... end, function( x ) ... end )
+gap> IsomorphismFpGroup(SymmetricGroup(1), "F");
+MappingByFunction( Group(()), <fp group on the generators 
+[  ]>, function( x ) ... end, function( x ) ... end )