Decidability of Krohn-Rhodes complexity $c = 1$ of finite semigroups and automata

Abstract

When decomposing a finite semigroup into a wreath product of groups and aperiodic semigroups, complexity measures the minimal number of groups that are needed. Determining an algorithm to compute complexity has been an open problem for almost 60 years. The main result of this paper proves decidability of Krohn-Rhodes complexity $c = 1$ of finite semigroups and automata. This is achieved by showing the lower bounds in work by Henckell, Rhodes and Steinberg from 2012 is sharp using profinite methods and results of McCammond from 1991 and 2001.