We assume that basic notions and facts of the theory of semigroups are known to the reader. Necessary information may be found, for instance, in the books [C1 Pr], [Howie], [La], [Lya], [Petri 1]; Chapter IV in [GA] is devoted to semigroups and contains a lot of basic information concerning semigroups. Some relevant information will be recalled in the course of our account. It is also assumed that the reader has a rudimentary knowledge of the theory of lattices and the theory of groups. Facts that we need can be found in many books; we mention, for instance, [Hall], [Kar Mer], [Kur], [Bir 2], [Gr], [Cr Dil], see also [GA], Chapters V and II.
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