1. Burris, S., Nelson, E.: Embedding the dual of Π m in the lattice of equational classes of commutative semigroups. Proc. Am. Math. Soc. 30, 37–39 (1971)
2. Golubov, E.A., Sapir, M.V.: Residually small varieties of semigroups. Russian Math. (Iz. VUZ) 11, 21–29 (1982), in Russian; Engl. translation: Soviet Math. Izv. VUZ 26(11), 25–36 (1982)
3. Grätzer, G.: General Lattice Theory, 2nd edn. Birkhauser, Basel (1998)
4. Ježek, J.: The lattice of equational theories. Part I: Modular elements, Czechosl. Math. J. 31, 127–152 (1981)
5. Ježek, J., McKenzie, R.N.: Definability in the lattice of equational theories of semigroups. Semigroup Forum 46, 199–245 (1993)