Topoi of automata I: Four topoi of automata and regular languages

TL;DR

This paper explores the topos-theoretic foundations of automata and regular languages, showing how different automata types form Grothendieck topoi and how regular languages are characterized within this framework.

Contribution

It introduces four topos-theoretic models of automata and unifies regular language characterizations as Morita-equivalent definitions of a Boolean-ringed topos.

Findings

Four automata notions form four Grothendieck topoi

Regular languages characterized by a single Boolean-ringed topos

Automata theory details are described via topos theory

Abstract

Both topos theory and automata theory are known for their multi-faceted nature and relationship with topology, algebra, logic, and category theory. This paper aims to clarify the topos-theoretic aspects of automata theory, particularly demonstrating through two main theorems how regular (and non-regular) languages arise in topos-theoretic calculation. First, it is shown that the four different notions of automata form four types of Grothendieck topoi, illustrating how the technical details of automata theory are described by topos theory. Second, we observe that the four characterizations of regular languages (DFA, Myhill-Nerode theorem, finite monoids, profinite words) provide Morita-equivalent definitions of a single Boolean-ringed topos, situating this within the context of Olivia Caramello's 'Toposes as Bridges.' This paper also serves as a preparation for follow-up papers, which deal with the relationship between hyperconnected geometric morphisms and algebraic/geometric aspects of formal language theory.