I give a class forcing that adds a real which is Pi-1-2 and for which no forcing extension (by a set of conditions) can destroy this definability.

We give a direct, purely arithmetical and elementary proof of the strong normalization of the cut-elimination procedure for full (i.e., in presence of all the usual connectives) classical natural deduction.

In [gallier], general results (due to Coppo, Dezani and Veneri) relating properties of pure lambda terms and their typability in some systems with conjunctive types are proved in a uniform way by using the reducibility method.This paper gives a very short proof of the same results (actually, one of them is a bit stronger) using purely arithmetical methods.

The λμ-calculus is an extension of the λ-calculus that has been introduced by M Parigot to give an algorithmic content to classical proofs. We show that Bohm's theorem fails in this calculus.

We give a procedure for counting the number of different proofs of a formula in various sorts of propositional logic. This number is either an integer (that may be 0 if the formula is not provable) or infinite.

The $\lambda\mu$-calculus is an extension of the $\lambda$-calculus that has been introduced by M Parigot to give an algorithmic content to classical proofs. We show that Bohm's theorem fails in this calculus.

We prove the strong normalization of full classical natural deduction by using a translation into the simply typed λμ-calculus. We also extend Mendler’s result on recursive equations to this system.

Storage operators have been introduced by J. L. Krivine in [5] they are closed λ-terms which, for a data type, allow one to simulate a "call by value" while using the "call by name" strategy. In this paper, we introduce the directed λ-calculus and show that it has the usual properties of the ordinary λ-calculus. With this calculus we get an equivalent--and simple--definition of the storage operators that allows to show some of their properties: $\bullet$ the stability of the set (...) of storage operators under the β-equivalence (Theorem 5.1.1); $\bullet$ the undecidability (and semidecidability) of the problem "is a closed λ-term t a storage operator for a finite set of closed normal λ-terms?" (Theorems 5.2.2 and 5.2.3); $\bullet$ the existence of storage operators for every finite set of closed normal λ-terms (Theorem 5.4.3); $\bullet$ the computation time of the "storage operation" (Theorem 5.5.2). (shrink)