attr: resolve Issue #868

semigroups · Sep 17, 2022 · fd61d2b · fd61d2b
1 parent 733b2d1
commit fd61d2b
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diff --git a/gap/attributes/attr.gi b/gap/attributes/attr.gi
@@ -526,8 +526,9 @@ function(S)
   return DClasses(S){DigraphSources(D)};
 end);
 
-InstallMethod(MaximalDClasses, "for a finite monoid as semigroup",
-[IsFinite and IsMonoidAsSemigroup],
+InstallMethod(MaximalDClasses, 
+"for a finite monoid as semigroup with mult. neutral elt",
+[IsFinite and IsMonoidAsSemigroup and HasMultiplicativeNeutralElement],
 S -> [DClass(S, MultiplicativeNeutralElement(S))]);
 
 InstallMethod(MaximalLClasses,

diff --git a/tst/standard/attributes/attr.tst b/tst/standard/attributes/attr.tst
@@ -2113,6 +2113,17 @@ gap> MaximalLClasses(S);
 gap> MaximalRClasses(S);
 [ <Green's R-class: PBR([ [ -1 ] ], [ [ 1 ] ])> ]
 
+# Issue 868 - IsMonoidAsSemigroup assumed HasMultiplicativeNeutralElement 
+gap> S := SemigroupByMultiplicationTable(
+> [[1, 1, 1, 1, 5, 6], 
+>  [1, 1, 1, 2, 5, 6], 
+>  [3, 3, 3, 3, 5, 6], 
+>  [1, 2, 3, 4, 5, 6], 
+>  [5, 5, 5, 5, 5, 5],
+>  [6, 6, 6, 6, 6, 6]]);;
+gap> M := AsMonoid(IsFpMonoid, M);
+<fp monoid with 5 generators and 25 relations of length 80>
+
 # SEMIGROUPS_UnbindVariables
 gap> Unbind(D);
 gap> Unbind(G);