2 research outputs found
On finite complete presentations and exact decompositions of semigroups
We prove that given a finite (zero) exact right decomposition (M, T) of a semigroup S, if M is defined by a finite complete presentation, then S is also defined by a finite complete presentation. Exact right decompositions are natural generalizations to semigroups of coset decompositions in groups. As a consequence, we deduce that any Zappa–Szép extension of a monoid defined by a finite complete presentation, by a finite monoid, is also defined by such a presentation. It is also proved that a semigroup M 0[A; I, J; P], where A and P satisfy some very general conditions, is also defined by a finite complete presentation
Decomposition of semigroups into semidirect and Zappa-Sz\'{e}p products
This thesis focuses on semidirect and Zappa-Sz\'{e}p products in the context of semigroups and monoids. We present a survey of direct, semidirect and Zappa-Sz\'{e}p products and discuss correspondence between external and internal versions of these products for semigroups and monoids. Particular attention in this thesis is paid to a wide class of semigroups known as restriction semigroups. We consider Zappa-Sz\'{e}p product of a left restriction semigroup with semilattice of projections and determine algebraic properties of it. We prove that analogues of Green's lemmas and Green's theorem hold for certain semigroups where Green's relations \ar,\el,\eh and \dee are replaced by \art_E,\elt_E, \eht_E and \adt_E. We show that if \eht_E is a congruence on a certain semigroup , then any right congruence on the submonoid \wt H^e_E (the \eht_E-class of ), where , can be extended to a congruence on . We introduce the idea of an {\em inverse skeleton} of a semigroup and examine some conditions under which we obtain skeletons from monoids. We focus on a result of Kunze \cite{kunze:1983} for the Bruck-Reilly extension \mbox{BR}(M,\theta) of a monoid , showing that \mbox{BR}(M,\theta) is a Zappa-Sz\'{e}p product of under addition and a semidirect product . We put Kunze's result in more general framework and give an analogous result for certain restriction monoids. We consider the {\it -semidirect product} of two left restriction semigroups and prove that it is left restriction. In the two sided case using the notion of double action we prove that the -semidirect product of two restriction semigroups is restriction. introduce the notion of -propernessto prove the results analogous to McAlister's covering theorem and O'Carroll's embedding theorem for monoids and left restriction monoids under some conditions. We extend the notion of the -semidirect product of two restriction semigroups and to develop -Zappa-Sz\'{e}p products and construct a category. In the special case where is a semilattice and is a monoid we order our category to become inductive and thus obtain a restriction semigroup via the use of the standard pseudo-product