properties: the empty semigroup is not monogenic

semigroups · Nov 26, 2018 · feb7cf2 · feb7cf2
1 parent da98a22
commit feb7cf2
Show file tree

Hide file tree

Showing 2 changed files with 6 additions and 1 deletion.
diff --git a/gap/attributes/properties.gi b/gap/attributes/properties.gi
@@ -777,7 +777,7 @@ function(S)
  gens := GeneratorsOfSemigroup(S);
  if Length(gens) <= 1 then
  SetMinimalSemigroupGeneratingSet(S, gens);
- return true;
+ return Length(gens) = 1;
  elif CanEasilyCompareElements(gens)
  and ForAll([2 .. Length(gens)], i -> gens[1] = gens[i]) then
  SetMinimalSemigroupGeneratingSet(S, [gens[1]]);

diff --git a/tst/standard/properties.tst b/tst/standard/properties.tst
@@ -945,6 +945,11 @@ gap> x := MinimalSemigroupGeneratingSet(S)[1];;
 gap> S := Semigroup(x, x, x);;
 gap> IsMonogenicSemigroup(S);
 true
+gap> S := Subsemigroup(FullTransformationMonoid(2), []);;
+gap> IsEmpty(S);
+true
+gap> IsMonogenicSemigroup(S);
+false
 
 # properties: IsMonogenicInverseSemigroup, 1
 gap> IsMonogenicInverseSemigroup(AsSemigroup(IsBooleanMatSemigroup,