New Foundations for the Proof Theory of Bi-Intuitionistic and Provability Logics Mechanized in Coq
Creators: name: Shillito, Ian
Publisher: name: The Australian National University
Year: 2023

description: We contribute to the shift towards logic as a fully formalised science by formalising in the interactive theorem prover Coq both new and known results. Our contribution is threefold. First, we define pen-and-paper general notions required in our dissertation: propo- sitional and first-order syntax, axiomatic calculus, sequent calculus, Kripke semantics, soundness and completeness results. Furthermore, we instantiate these notions in modal logic, both in pen-and-paper and in Coq. Consequently, we constitute a readily available and usable Coq library containing foundational elements of the study of modal logic, which can thus serve as a basis for the formalisation of further works in non-classical logics. Second, we exhibit and rectify a mistake that has gone unnoticed for almost fifty years in the foundations of propositional and first-order bi-intuitionistic logic. We show that both in the propositional and first-order case, what was conceived of as a unique bi-intuitionistic logic is in fact two distinct logics. We trace these confusions back to a fundamental problem in the axiomatic proof theory of propositional and first-order bi-intuitionistic logic: traditional Hilbert calculi are not designed to treat logics as conse- quence relations. They lead to an ambiguous notion of deduction from assumptions that can cause us to conflate distinct rules and thus distinct logics. More precisely, we show that the bi-intuitionistic rule (DN) is ambiguous in a traditional context and splits into two distinct rules in the context of generalized Hilbert calculi. As a consequence, we obtain two generalized Hilbert calculi in the propositional (wBIH and sBIH) and first-order (FOwBIH and FOsBIH) case, for bi-intuitionistic logic that differ only in the disambiguated version of the (DN) rule they incorporate. Unsurprisingly, these systems capture two distinct propositional logics (wBIL and sBIL) and first-order logics (FOwBIL and FOsBIL), which have been conflated in the literature. Third, we obtain cut-elimination results for the sequent calculi GLS and GL4ip, which are sequent calculi for, respectively, the classical modal provability logic GLL and the in- tuitionistic modal provability logic iGLL. These results involve a novel proof technique for the admissibility of additive-cut called the "mhd proof technique", which relies on the fact that terminating backward proof-search allows us to attribute a "maximal height of derivations" to each sequent. So, for each of the calculi GLS and GL4ip, we define a back- ward proof-search procedure for it and develop a thorough termination argument using a local measure on sequents and a well-founded relations along which this measure decreases upwards in the proof-search. Then, we directly prove the admissibility of additive-cut for both calculi using the mhd proof technique. We use this number, the mhd of a sequent, as an induction measure in arguments involving local and syntactic transformations, allowing us to exhibit and hence extract from our Coq formalisation Haskell programs constituting cut-elimination procedures.