Cut-elimination for the modal Grzegorczyk logic via non-well-founded proofs

We present a sequent calculus for the modal Grzegorczyk logic Grz allowing non-well-founded proofs and obtain the cut-elimination theorem for it by constructing a continuous cut-elimination mapping acting on these proofs.Comment: WOLLIC'17, 12 pages, 1 appendi

Cyclic proof systems for modal fixpoint logics

This thesis is about cyclic and ill-founded proof systems for modal fixpoint logics, with and without explicit fixpoint quantifiers.Cyclic and ill-founded proof-theory allow proofs with infinite branches or paths, as long as they satisfy some correctness conditions ensuring the validity of the conclusion. In this dissertation we design a few cyclic and ill-founded systems: a cyclic one for the weak Grzegorczyk modal logic K4Grz, based on our explanation of the phenomenon of cyclic companionship; and ill-founded and cyclic ones for the full computation tree logic CTL* and the intuitionistic linear-time temporal logic iLTL. All systems are cut-free, and the cyclic ones for K4Grz and iLTL have fully finitary correctness conditions.Lastly, we use a cyclic system for the modal mu-calculus to obtain a proof of the uniform interpolation property for the logic which differs from the original, automata-based one

Current research on G\"odel's incompleteness theorems

We give a survey of current research on G\"{o}del's incompleteness theorems from the following three aspects: classifications of different proofs of G\"{o}del's incompleteness theorems, the limit of the applicability of G\"{o}del's first incompleteness theorem, and the limit of the applicability of G\"{o}del's second incompleteness theorem.Comment: 54 pages, final accepted version, to appear in The Bulletin of Symbolic Logi

Constructivisation through Induction and Conservation

The topic of this thesis lies in the intersection between proof theory and alge- braic logic. The main object of discussion, constructive reasoning, was intro- duced at the beginning of the 20th century by Brouwer, who followed Kant’s explanation of human intuition of spacial forms and time points: these are constructed step by step in a finite process by certain rules, mimicking con- structions with straightedge and compass and the construction of natural numbers, respectively. The aim of the present thesis is to show how classical reasoning, which admits some forms of indirect reasoning, can be made more constructive. The central tool that we are using are induction principles, methods that cap- ture infinite collections of objects by considering their process of generation instead of the whole class. We start by studying the interplay between cer- tain structures that satisfy induction and the calculi for some non-classical logics. We then use inductive methods to prove a few conservation theorems, which contribute to answering the question of which parts of classical logic and mathematics can be made constructive.Tämän opinnäytetyön aiheena on todistusteorian ja algebrallisen logiikan leikkauspiste. Keskustelun pääaiheen, rakentavan päättelyn, esitteli 1900-luvun alussa Brouwer, joka seurasi Kantin selitystä ihmisen intuitiosta tilamuodoista ja aikapisteistä: nämä rakennetaan askel askeleelta äärellisessä prosessissa tiettyjen sääntöjen mukaan, jotka jäljittelevät suoran ja kompassin konstruktioita ja luonnollisten lukujen konstruktiota. Tämän opinnäytetyön tavoitteena on osoittaa, kuinka klassista päättelyä, joka mahdollistaa tietyt epäsuoran päättelyn muodot, voidaan tehdä rakentavammaksi. Keskeinen työkalu, jota käytämme, ovat induktioperiaatteet, menetelmät, jotka keräävät äärettömiä objektikokoelmia ottamalla huomioon niiden luomisprosessin koko luokan sijaan. Aloitamme tutkimalla vuorovaikutusta tiettyjen induktiota tyydyttävien rakenteiden ja joidenkin ei-klassisten logiikan laskelmien välillä. Todistamme sitten induktiivisten menetelmien avulla muutamia säilymislauseita, jotka auttavat vastaamaan kysymykseen siitä, mitkä klassisen logiikan ja matematiikan osat voidaan tehdä rakentaviksi

Uniform Interpolation in provability logics

We prove the uniform interpolation theorem in modal provability logics GL and Grz by a proof-theoretical method, using analytical and terminating sequent calculi for the logics. The calculus for G\"odel-L\"ob's logic GL is a variant of the standard sequent calculus, in the case of Grzegorczyk's logic Grz, the calculus implements an explicit loop-preventing mechanism inspired by work of Heuerding

Through and beyond classicality: analyticity, embeddings, infinity

Structural proof theory deals with formal representation of proofs and with the investigation of their properties. This thesis provides an analysis of various non-classical logical systems using proof-theoretic methods. The approach consists in the formulation of analytic calculi for these logics which are then used in order to study their metalogical properties. A specific attention is devoted to studying the connections between classical and non-classical reasoning. In particular, the use of analytic sequent calculi allows one to regain desirable structural properties which are lost in non-classical contexts. In this sense, proof-theoretic versions of embeddings between non-classical logics - both finitary and infinitary - prove to be a useful tool insofar as they build a bridge between different logical regions

Philosophical logics - a survey and a bibliography

Intensional logics attract the attention of researchers from differing academic backgrounds and various scientific interests. My aim is to sketch the philosophical background of alethic, doxastic, and deontic logics, their formal and metaphysical presumptions and their various problems and paradoxes, without attempting formal rigor. A bibliography, concise on philosophical writings, is meant to allow the reader\u27s access to the maze of literature in the field