Large Aperiodic Semigroups

We search for the largest syntactic semigroups of star-free languages having n left quotients; equivalently, we look for the largest transition semigroups of aperiodic finite automata with n states. We first introduce unitary semigroups generated by transformations that change only one state. In particular, we study unitary-complete semigroups which have a special structure, and show that each maximal unitary semigroup is unitary-complete. For [Formula: see text] we exhibit a unitary-complete semigroup that is larger than any aperiodic semigroup known to date. We then present even larger aperiodic semigroups, generated by transformations that map a non-empty subset of states to a single state; we call such transformations and semigroups semiconstant. We examine semiconstant tree semigroups which have a structure based on full binary trees. The semiconstant tree semigroups are at present the best candidates for largest aperiodic semigroups.

ON THE TRANSITION SEMIGROUPS OF CENTRALLY LABELED RAUZY GRAPHS

Rauzy graphs of subshifts are endowed with an automaton structure. For Sturmian subshifts, it is shown that its transition semigroup is the syntactic semigroup of the language recognized by the automaton. An inverse limit of the partial semigroups of nonzero regular elements of their transition semigroups is described. If the subshift is minimal, then this inverse limit is isomorphic, as a partial semigroup, to the [Formula: see text]-class associated to it in the free pro-aperiodic semigroup.

ON THE CAYLEY SEMIGROUP OF A FINITE APERIODIC SEMIGROUP

Let S be a finite semigroup. In this paper, we introduce the functions φs:S* → S*, first defined by Rhodes, given by φs([a1,a2,…,an]) = [sa1,sa1a2,…,sa1a2 ⋯ an]. We show that if S is a finite aperiodic semigroup, then the semigroup generated by the functions {φs}s ∈ S is finite and aperiodic.

NORMAL FORMS FOR FREE APERIODIC SEMIGROUPS

The implicit operation ω is the unary operation which sends each element of a finite semigroup to the unique idempotent contained in the subsemigroup it generates. Using ω there is a well-defined algebra which is known as the free aperiodic semigroup. In this article we introduce a specific and rather elementary list of pseudoidentitites, we show that for each n, the n-generated free aperiodic semigroup is defined by this list of pseudoidentities, and then we use this identification to show that it has a decidable word problem. In the language of implicit operations, this shows that the pseudovariety of finite aperiodic semigroups is κ-recursive. This completes a crucial step towards showing that the Krohn–Rhodes complexity of every finite semigroup is decidable.