Aperiodic flows on finite semigroups: Foundations and first examples

The theory of flows is used as a crucial tool in the recent proof by Margolis, Rhodes and Schilling that Krohn–Rhodes complexity is decidable. In this paper, we begin a systematic study of aperiodic flows. We give the foundations of the theory of flows and give a unified approach to the Presentation Lemma and its relations to flows and the Slice Theorem. We completely describe semigroups having a flow over the trivial semigroup and connect this to classical results in inverse semigroup theory. We reinterpret Tilson’s Theorem on the complexity of small monoids in terms of flows. We conclude with examples of semigroups built from the character table of Abelian Groups that have aperiodic flows.