A Duality-Theoretic Perspective on Formal Languages and Recognition

A connection between recognisable languages and profinite identities is established through the composition of two famous theorems: Eilenberg’s theorem and Reiterman’s theorem. In this work, we present a detailed account of the duality-theoretic approach by Gehrke et al. that has been shown to bridge the gap and demonstrate that Eilenberg’s varieties and profinite theories are directly linked: they are at opposite ends of an extended Stone-type duality, instantiating a Galois correspondence between subobjects and quotients and resulting in an equational theory of recognisable languages. We give an in-depth overview of relevant components of algebraic language theory and the profinite equational theory of pseudovarieties in order to show how they are tied together by the duality-theoretic developments. Furthermore, we provide independent proofs of the key Galois connections at the heart of these bridging results.