1. Crvenković, S. andRuškuc, N.,On semigroups having n 2 essentially n-ary polynomials, Algebra Universal30 (1993), 269–271.
2. Grätzer, G.,Composition of functions, inProceedings of the Conference on Universal Algebra (Kingston, 1969), Queen's Univ., Kingston, Ont., 1970, pp. 1–106.
3. Grätzer, G. andKisielewicz, A.,A survey of some open problems on p n -sequences and free spectra of algebras and varieties, inUniversal Algebra and Quasigroup Theory (eds. A. Romanowska and J. D. H. Smith), Haldermann Verlag, Berlin, 1992, pp. 57–88.
4. Perkins, P.,Bases of equational theories in semigroups, J. Algebra11 (1969), 298–314.
5. Pollák, G.,A class of hereditarily finitely based varieties of semigroups, inAlgebraic Theory of Semigroups (Proc. Sixth Algebraic Conf., Szeged, 1976), Coll. Math. Soc. János Bolyai20 (1979), 433–445.