The logical system P-W is an implicational non-commutative intuitionistic logic defined by axiom schemes B = (b → c) → (a → b) → a → c, B' = (a → b) → (b → c) → a → c, I = a → a with the rules of modus ponens and substitution. The P-W problem is a problem asking whether α = β holds if α → β and β → α are both provable in P-W. The answer is (...) affirmative. The first to prove this was E. P. Martin by a semantical method. In this paper, we give the first proof of Martin's theorem based on the theory of simply typed λ-calculus. This proof is obtained as a corollary to the main theorem of this paper, shown without using Martin's Theorem, that any closed hereditary right-maximal linear (HRML) λ-term of type α → α is βη-reducible to λ x.x. Here the HRML λ-terms correspond, via the Curry-Howard isomorphism, to the P-W proofs in natural deduction style. (shrink)

A reduction rule is introduced as a transformation of proof figures in implicational classical logic. Proof figures are represented as typed terms in a -calculus with a new constant P (()). It is shown that all terms with the same type are equivalent with respect to -reduction augmented by this P-reduction rule. Hence all the proofs of the same implicational formula are equivalent. It is also shown that strong normalization fails for P-reduction. Weak normalization is shown for P-reduction with another (...) reduction rule which simplifies of (( ) ) into an atomic type. (shrink)

A principal type-scheme of a -term is the most general type-scheme for the term. The converse principal type-scheme theorem (J.R. Hindley, The principal typescheme of an object in combinatory logic, Trans. Amer. Math. Soc. 146 (1969) 29–60) states that every type-scheme of a combinatory term is a principal type-scheme of some combinatory term.This paper shows a simple proof for the theorem in -calculus, by constructing an algorithm which transforms a type assignment to a -term into a principal type assignment to (...) another -term that has the type as its principal type-scheme. The clearness of the algorithm is due to the characterization theorem of principal type-assignment figures. The algorithm is applicable to BCIW--terms as well. Thus a uniform proof is presented for the converse principal type-scheme theorem for general -terms and BCIW--terms. (shrink)

The syntactic structure of the system of pure implicational relevant logic P - W is investigated. This system is defined by the axioms B = (b → c) → (a → b) → a → c, B' = (a → b) → (b → c) → a → c, I = a → a, and the rules of substitution and modus ponens. A class of λ-terms, the closed hereditary right-maximal linear λ-terms, and a translation of such λ-terms M to BB'I-combinators (...) M + is introduced. It is shown that a formula α is provable in P - W if and only if α is a type of some λ-term in this class. Hence these λ-terms represent proof figures in the Natural Deduction version of P - W. Errol Martin (1982) proved that no formula with form α → α is provable in P - W without using the axiom I. We show that a β-normal form λ-term M in the class is η reducible to λ x.x if the translated BB'I-combinator M + contains I. Using this theorem and Martin's result, we prove that a λ-term in the class is βη-reducible to λ x.x if the λ-term has a type α → α. Hence the structure of proofs of α → α in P - W is determined. (shrink)

The syntactic structure of the system of pure implicational relevant logic $P - W$ is investigated. This system is defined by the axioms $B = (b \rightarrow c) \rightarrow (a \rightarrow b) \rightarrow a \rightarrow c, B' = (a \rightarrow b) \rightarrow (b \rightarrow c) \rightarrow a \rightarrow c, I = a \rightarrow a$, and the rules of substitution and modus ponens. A class of $\lambda$-terms, the closed hereditary right-maximal linear $\lambda$-terms, and a translation of such $\lambda$-terms $M$ to $BB'I$-combinators (...) $M^+$ is introduced. It is shown that a formula $\alpha$ is provable in $P - W$ if and only if $\alpha$ is a type of some $\lambda$-term in this class. Hence these $\lambda$-terms represent proof figures in the Natural Deduction version of $P - W$. Errol Martin (1982) proved that no formula with form $\alpha \rightarrow \alpha$ is provable in $P - W$ without using the axiom $I$. We show that a $\beta$-normal form $\lambda$-term $M$ in the class is $\eta$ reducible to $\lambda x.x$ if the translated $BB'I$-combinator $M^+$ contains $I$. Using this theorem and Martin's result, we prove that a $\lambda$-term in the class is $\beta\eta$-reducible to $\lambda x.x$ if the $\lambda$-term has a type $\alpha \rightarrow \alpha$. Hence the structure of proofs of $\alpha \rightarrow \alpha$ in $P - W$ is determined. (shrink)