[1] ŠULKA R.: On three lattices that belong to every semigroup. Math. Slovaca 34, 1984, 217-228. MR 0744956 | Zbl 0597.20053
[2] ŠULKA R.: Nilpotency in semigroups and sublattices of their Booleans. Math. Slovaca (to appear). MR 0899430 | Zbl 0616.20033
[3] CLIFFORD A. H., PRESTON G. B.: The algebraic theory of Semigroups. Amer. Math. Soc., Providence, R. I. Vol. I (1961), Vol. II (1967). Zbl 0111.03403