Constructive Analysis of S1S and B\"uchi Automata

TL;DR

This paper explores the constructive analysis of S1S and B"uchi automata within the Coq proof assistant, establishing decidability under certain semantics and examining the implications of Ramseyan factorisations.

Contribution

It verifies B"uchi's translation for UP semantics constructively and analyzes the role of Ramseyan factorisations in the equivalence of semantics and classical principles.

Findings

B"uchi's translation is verified for UP semantics in Coq.

Decidability of S1S is established constructively for UP semantics.

RF assumption links UP and AS semantics and relates to classical logic principles.

Abstract

We study S1S and B\"uchi automata in the constructive type theory of the Coq proof assistant. For UP semantics (ultimately periodic sequences), we verify B\"uchi's translation of formulas to automata and thereby establish decidability of S1S constructively. For AS semantics (all sequences), we verify B\"uchi's translation assuming that sequences over finite semigroups have Ramseyan factorisations (RF). Assuming RF, UP semantics and AS semantics agree. RF is a consequence of Ramsey's theorem and implies the infinite pigeonhole principle, which is known to be unprovable constructively. We show that each of the following properties holds for UP semantics but is equivalent to RF for AS semantics: excluded middle of formula satisfaction, excluded middle of automaton acceptance, and existence of complement automata.