It is well known that for monoids and completely simple semigroups the property of being automatic does not depend on the choice of the semigroup generating set. In this paper we extend these results to semigroups satisfying the equality S=SS. As an immediate corollary of this result we obtain that for regular semigroups being automatic is invariant under the change of generators

HNN extensions of inverse semigroups, where the associated inverse subsemigroups are order ideals of the base, are defined by means of a construction based upon the isomorphism between the categories of inverse semigroups and inductive groupoids. The resulting HNN extension may conveniently be described by an inverse semigroup presentation, and we determine when an HNN extension with finitely generated or finitely presented base is again finitely generated or finitely presented. Our main results depend upon properties of the [Formula: see text]-preorder in the associated subsemigroups. Let S be a finitely generated inverse semigroup and let U, V be inverse subsemigroups of S, isomorphic via φ: U → V, that are order ideals in S. We prove that the HNN extension S*U,φ is finitely generated if and only if U is finitely [Formula: see text]-dominated. If S is finitely presented, we give a necessary and suffcient condition for S*U,φ to be finitely presented. Here, in contrast to the theory of HNN extensions of groups, it is not necessary that U be finitely generated.

In this paper we prove two main results. The first is a necessary and sufficient condition for a semidirect product of a semilattice by a group to be finitely generated. The second result is a necessary and sufficient condition for such a semidirect product to be finitely presented.2000 Mathematics subject classification: primary 20M05; secondary 20M18, 20M30.

We introduce the notion of path extensions of tiling semigroups and investigate their properties. We show that the path extension of a tiling semigroup yields a strongly F*-inverse cover of the tiling semigroup and that it is isomorphic to an HNN* extension of its semilattice of idempotents

To give a general framework for the theory of automatic groups and semigroups, we introduce the notion of automaticity for semigroup acts. We investigate their basic properties and discuss how the property of being automatic behaves under changing the generators of the acting semigroup and under changing the generators of the semigroup act. In particular, we prove that under some conditions on the acting semigroup, the automaticity of the act is invariant under changing the generators. Since automatic semigroups can be seen as a special case of automatic semigroup acts, our result generalizes and extends the corresponding result on automatic semigroups, where the semigroup S satisfies S=SSS=SS. We also give a geometric approach in terms of the fellow traveller property and discuss the solvability of the equality problem in automatic semigroup acts. Our notion gives rise to a variety of definitions of automaticity depending on the set chosen as a semigroup act and we discuss future research directions