We introduce an implication-free fragment image of ω-arithmetic, having Exchange rule for sequents dropped. Exchange rule for formulas is, instead, an admissible rule in image. Our main result is that cut-free proofs of image are isomorphic with recursive winning strategies of a set of games called “1-backtracking games” in [S. Berardi, Th. Coquand, S. Hayashi, Games with 1-backtracking, Games for Logic and Programming Languages, Edinburgh, April 2005].We also show that image is a sound and complete formal system for the implication-free (...) fragment of LCM , Seventh International Conference on Typed Lambda Calculi and Applications, in: Lecture Notes in Computer Science, vol. 3461, 2005, pp. 11–22. Invited paper]). A simple syntactical description of this fragment was still missing. No sound and complete formal system is known for LCM itself. (shrink)

This paper presents proof that Buss's S22 can prove the consistency of a fragment of Cook and Urquhart's PV from which induction has been removed but substitution has been retained. This result improves Beckmann's result, which proves the consistency of such a system without substitution in bounded arithmetic S12. Our proof relies on the notion of "computation" of the terms of PV. In our work, we first prove that, in the system under consideration, if an equation is proved and either (...) its left- or right-hand side is computed, then there is a corresponding computation for its right- or left-hand side, respectively. By carefully computing the bound of the size of the computation, the proof of this theorem inside a bounded arithmetic is obtained, from which the consistency of the system is readily proven. This result apparently implies the separation of bounded arithmetic because Buss and Ignjatović stated that it is not possible to prove the consistency of a fragment of PV without induction but with substitution in Buss's S12. However, their proof actually shows that it is not possible to prove the consistency of the system, which is obtained by the addition of propositional logic and other axioms to a system such as ours. On the other hand, the system that we have considered is strictly equational, which is a property on which our proof relies. (shrink)

We extend Barbanera and Berardi's symmetric lambda calculus [2] to second-order classical propositional logic and prove its strong normalization.