1. Yuji Kobayashi,On the structure of exponent semigroups, J. Alg.78 (1982), 1, 1?19.
2. V.Koubek and J.Sichler,Universal varieties of semigroups, to appear in Austr. J. Math, in print.
3. L. Kucera,On the monoids of homomorphisms of semigroups with unity, Comment. Math. Univ. Carolinae23 (1982), 369?381.
4. A.Pultr and V.Trnkov�,Combinatorial, Algebraic and Topological Representations of Groups,Semigroups and Categories, North Holland 1980.