Revisiting the correspondence between cut-elimination and normalisation

Cut-free proofs in Herbelin's sequent calculus are in 1-1 correspondence with normal natural deduction proofs. For this reason Herbelin's sequent calculus has been considered a privileged middle-point between L-systems and natural deduction. However, this bijection does not extend to proofs containing cuts and Herbelin observed that his cut-elimination procedure is not isomorphic to $\beta$-reduction. In this paper we equip Herbelin's system with rewrite rules which, at the same time: (1) complete in a sense the cut elimination procedure firstly proposed by Herbelin; and (2) perform the intuitionistic "fragment'' of the tq-protocol - a cut-elimination procedure for classical logic defined by Danos, Joinet and Schellinx. Moreover we identify the subcalculus of our system which is isomorphic to natural deduction, the isomorphism being with respect not only to proofs but also to normalisation. Our results show, for the implicational fragment of intuitionistic logic, how to embed natural deduction in the much wider world of sequent calculus and what a particular cut-elimination procedure normalisation is.Fundação para a Ciência e a Tecnologia (FCT)

Sobre disjunções, confluências e o centro de gravidade da lógica filosófica

No dizer de Burgess [B2009], o centro de gravidade da Lógica Filosófica é, hoje em dia, a área científica que, em Inglês, se designa por Theoretical Computer Science. Não sei bem como traduzir esta designação, mas, se assim é, a área científica assim designada parece ser relevante num painel sobre Matemática e Ciências da Computação, integrado num colóquio sobre as disjunções e confluências entre Humanidades e Ciências. O pequeno trabalho técnico sobre o qual vou aqui escrever pertence à Teoria da Demonstração – pertence, portanto, a um dos compartimentos principais da Lógica Matemática, dos mais próximos da Theoretical Computer Science; e é relativo às distinções entre lógicas clássicas e não-clássicas – ora o estudo das lógicas não-clássicas é outra definição da Lógica Filosófica. O referido trabalho técnico ocupa a segunda parte deste artigo e debruça-se (curiosamente) sobre a operação lógica de disjunção, no contexto da lógica clássica. O objectivo do exercício não é tanto o de publicitar o resultado final, mas antes o de ilustrar como, através de exercícios técnicos deste género, que constituem o dia-a-dia de alguns lógicos, se tocam indirectamente questões que talvez possam ser do interesse de alguns colegas das Humanidades. Esta tentativa de confluência ocupará a terceira parte deste artigo. Depois de explicitado o objectivo do artigo, é evidente qual será o seu tom: o cientista tentará, numa linguagem que não é a sua, falar ao humanista em problemas estranhos a este, sem perder a esperança de ser transparente. A ver vamos.Fundação para a Ciência e a Tecnologia (FCT

Delayed substitutions

This paper investigates an approach to substitution alternative to the implicit treatment of the $\lambda$-calculus and the explicit treatment of explicit substitution calculi. In this approach, substitutions are delayed (but not executed) explicitly. We implement this idea with two calculi, one where substitution is a primitive construction of the calculus, the other where substitutions is represented by a $\beta$-redex. For both calculi, confluence and (preservation of) strong normalisation are proved (the latter fails for a related system due to Revesz, as we show). Applications of delayed substitutions are of theoretical nature. The strong normalisation result implies strong normalisation for other calculi, like the computational lambda-calculus, lambda-calculi with generalised applications, or calculi of cut-elimination for sequent calculus. We give an investigation of the computational interpretation of cut-elimination in terms of generation, execution, and delaying of substitutions, paying particular attention to how generalised applications improve such interpretation

The polarized λ-calculus

A natural deduction system isomorphic to the focused sequent calculus for polarized intuitionistic logic is proposed. The system comes with a language of proof-terms, named polarized λ-calculus, whose reduction rules express simultaneously a normalization procedure and the isomorphic copy of the cut-elimination procedure pertaining to the focused sequent calculus. Noteworthy features of this natural deduction system are: how the polarity of a connective determines the style of its elimination rule; the existence of a proof-search strategy which is equivalent to focusing in the sequent calculus; the highlydisciplined organization of the syntax - even atoms have introduction, elimination and normalization rules. The polarized λ-calculus is a programming formalism close to call-by-push-value, but justified by its proof-theoretical pedigree.This research was financed by Portuguese Funds through FCT Fundac¸ao para a Ci ˜ encia ˆ e a Tecnologia, within the Project UID/MAT/00013/2013.info:eu-repo/semantics/publishedVersio

Turing e a normalização

Em publicaçãoAlgumas contribuições de Turing em Lógica dizem respeito à Teoria de Tipos e foram desenvolvidas durante a Segunda Guerra Mundial. Um dos resultados desse trabalho é uma demonstração do teorema da normalização para tipos simples, que permaneceu ignorada durante 40 anos. O resultado de Turing é, portanto, um dos episódios da complicada história da (re)descoberta da normalização.The part of Turing's work in Logic concerning Type Theory was developed during Second World War. He obtained a proof of the normalization theorem for simple types, but his result was ignored for 40 years. Turing's result is therefore one of the episodes in the complex history of (re)discovery of normalization

Spirantization occurences in German and Portuguese

Este artigo visa trazer à discussão, principalmente, traços fonológicos comuns ao Alemão e ao Português, os quais carecem de clarificação quanto à sua origem, como p. ex. a fricatização das oclusivas sonoras /b, d, g/, bem como, em certo grau, da vibrante uvular /R/. Contudo, outras semelhanças também serão abordadas.The purpose of this paper is to bring up for discussion mainly phonological traits common to German and Portuguese requiring clarification as to their origin, like the spirantization of the voiced plosives /b,d,g/, and, to some extent, of the uvular vibrant /R/. However, further similarities will be approached as well

Curry-Howard for sequent calculus at last!

This paper tries to remove what seems to be the remaining stumbling blocks in the way to a full understanding of the Curry-Howard isomorphism for sequent calculus, namely the questions: What do variables in proof terms stand for? What is co-control and a co-continuation? How to define the dual of Parigot's mu-operator so that it is a co-control operator? Answering these questions leads to the interpretation that sequent calculus is a formal vector notation with first-class co-control. But this is just the "internal" interpretation, which has to be developed simultaneously with, and is justified by, an "external" one, offered by natural deduction: the sequent calculus corresponds to a bi-directional, agnostic (w.r.t. the call strategy), computational lambda-calculus. Next, the duality between control and co-control is studied and proved in the context of classical logic, where one discovers that the classical sequent calculus has a distortion towards control, and that sequent calculus is the de Morgan dual of natural deduction.(undefined