Semigroups with idempotent powers, II: The lattices of equational subclasses of [(xy2 =xy] . J. A. Gerhard (1977) Semigroup forum
Semilattices of groups with distributive congruence lattice. J. B. Fountain, P. Lockley (1977) Semigroup forum
Semimodularity and weak modularity in subalgebra lattices of completely simple semigroups. K.G. Johnston (1986) Semigroup forum