The alternation hierarchy for the theory of μ -lattices. Santocanale, Luigi (2001) Theory and Applications of Categories [electronic only]
The word problem in the tensor product of distributive semilattices. G.A. Fraser, A.M. Bell (1984) Semigroup forum
Two Congruence Lattices of Completely Simple Semigroups. Mario Petrich (1993) Monatshefte für Mathematik