Permutative conversions transform a proof with a disjunction or existential quantification elimination rule followed by an elimination rule into a proof with the second rule in the minor deduction of the first rule [14,21]. Permutative conversions are indispensable for normalizing a proof in a natural deduction system with disjunction or existential quantification. Without permutative conversions, a normal proof fails to have the subformula property, because there may exist an introduction rule of a logical symbol in a minor deduction of a disjunction or existential quantification elimination rule followed by the elimination rule of the same logical symbol, which may break the subformula property.Strong normalization property is important. First it implies weak normalization property, which proves the subformula property and consistency [14,21]. In particular, weak normalization of a second order system has been often proved through showing its strong normalization [5,6]. Secondly, when we consider proof normalization as computation by the proofs-as-programs paradigm [8], strong normalization guarantees termination of programs. Thirdly, it is itself an interesting combinatorial problem in mathematics.Several papers have studied strong normalization of systems with or without permutative conversions. We summarize them in Table 1. Strong normalization of second order natural deduction with disjunction, existential quantification, and their commutative conversions is proved in [15,12]. Strong normalization of second order natural deduction with disjunction, first-order existential quantification, and their commutative conversions is proved in [19]. Strong normalization of second order natural deduction with disjunction and existential quantification without their commutative conversions is proved in [5,6]. Strong normalization of the negative fragment of second order natural deduction is discussed in [4,7,11,20,21]. Strong normalization of first order natural deduction with disjunction, existential quantification, and their commutative conversions is proved in [10,20]. Strong normalization of a type theory with P types, S types, and their weak permutative conversions is proved in [18]. Strong normalization of propositional natural deduction with disjunction and commutative conversions is proved in [3,2,9].We will prove strong normalization of second order intuitionistic natural deduction with permutative conversions by using Prawitz's strong validity. This paper completes Prawitz's original proof given in [15].By Curry-Howard isomorphism [8], a second order logical system corresponds to a functional programming language with polymorphic types and abstract data types. Second order universal quantification gives polymorphic Second order permutative conversions with Prawitz's strong validity
Makoto TATSUTA
National Institute of InformaticsThe Graduate University for Advanced Studies ABSTRACT A clear and complete proof of strong normalization of second order natural deduction with permutative conversions is given by us...