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An application of a Theorem of Ash to finite covers

ISSN:0039-3215
2004年第78卷第1-2期


The technique of covers is now well established in semigroup theory. The idea is, given a semigroup S, to find a semigroup Scirc having a better understood structure than that of S, and an onto morphism theta of a specific kind from Scirc to S. With the right conditions on theta, the behaviour of S is closely linked to that of Scirc. If S is finite one aims to choose a finite Scirc. The celebrated results for inverse semigroups of McAlister in the 1970rsquos form the flagship of this theory.Weakly left quasi-ample semigroups form a quasivariety (of algebras of type(2, 1)), properly containing the classes of groups, and of inverse, left ample, and weakly left ample semigroups. We show how the existence of finite proper covers for semigroups in this quasivariety is a consequence of Ashrsquos powerful theorem for pointlike sets. Our approach is to obtain a cover Scirc of a weakly left quasi-ample semigroup S as a subalgebra of S × G, where G is a group. It follows immediately from the fact that weakly left quasi-ample semigroups form a quasivariety, that Scirc is weakly left quasi-ample. We can then specialise our covering results to the quasivarieties of weakly left ample, and left ample semigroups. The latter have natural representations as (2, 1)-subalgebras of partial (one-one) transformations, where the unary operation takes a transformation agr to the identity map in the domain of agr. In the final part of this paper we consider representations of weakly left quasi-ample semigroups.This work was supported by the London Mathematical Society, Fundação para a Ciência e a Tecnologia, Centro de Álgebra da Universidade de Lisboa, Centro de Matemática da Universidade do Porto and projects POCTI/32440/MAT/2000 and POCTI/32817/Mat/2000 of FEDER.Special issue of Studia Logica: ldquoAlgebraic Theory of Quasivarietiesrdquo Presented byM. E. Adams, K. V. Adaricheva, W. Dziobiak, and A. V. Kravchenko

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ISSN:0039-3215
2004年第78卷第1-2期

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