Intuitionistic Systems Research Articles

Non-null implication

An implication A ⊃ B is said to be null in case A is false in all instances. This note describes several formal systems for first-order predicate logic and arithmetic in which null implications do not occur. In fact, the most restrictive systems avoid the occurrences of any formulas which represent null predicates. The precise sense of this avoidance is explained below.An intuitionistic negation is a statement of the form A ⊃ F where F is a contradiction. The absence of null implications among true statements entails the absence of intuitionistic negations, and hence systems described here might be called negationless intuitionistic systems.

Modalities in linear logic weaker than the exponential ?of course?: Algebraic and relational semantics

We present a semantic study of a family of modal intuitionistic linear systems, providing various logics with both an algebraic semantics and a relational semantics, to obtain completeness results. We call modality a unary operator □ on formulas which satisfies only one rale (regularity), and we consider any subsetW of a list of axioms which defines the exponential “of course” of linear logic. We define an algebraic semantics by interpreting the modality □ as a unary operationμ on an IL-algebra. Then we introduce a relational semantics based on pretopologies with an additional binary relationr between information states. The interpretation of □ is defined in a suitable way, which differs from the traditional one in classical modal logic. We prove that such models provide a complete semantics for our minimal modal system, as well as, by requiring the suitable conditions onr (in the spirit of correspondence theory), for any of its extensions axiomatized by any subsetW as above. We also prove an embedding theorem for modal IL-algebras into complete ones and, after introducing the notion of general frame, we apply it to obtain a duality between general frames and modal IL-algebras.

The intuitionistic alternative set theory

Lano, K., The intuitionistic alternative set theory, Annals of Pure and Applied Logic 59 (1993) 141–156 The Alternative Set Theory, as defined in Vopenka and Sochor, demonstrates how a set theory which avoids actually infinite sets can serve as a framework for much of classical mathematics. This paper defines a theory which can serve as an intuitionistic analogue of AST, and examines motivations for alternative formulations of classical AST from an intuitionistic and finitistic viewpoint. The intuitionistic AST uses appropriate modifications of the concepts of AST, with the notion of feasibility replacing finiteness, and with new distinctions between alternative definitions of Countable Class and Revealment. Results of classical AST which are still valid in this new system are given, and an interpretation of the corresponding classical system in the intuitionistic system is defined. This shows that the adoption of an intuitionistic logic does not essentially deprive us of classical methods or results for AST.

Some properties of the syntactic p-recursion categories generated by consistent, recursively enumerable extensions of Peano arithmetic.

The dominical categories were introduced by Di Paola and Heller, as a first step toward a category-theoretic treatment of the generalized first Godel incompleteness theorem [1]. In his Ph.D. dissertation [7], Rosolini subsequently defined the closely related p-categories, which should prove pertinent to category-theoretic representations of incompleteness for intuitionistic systems. The precise relationship between these two concepts is as follows: every dominical category is a pointed p-category, but there are p-categories, indeed pointed p-isotypes (all pairs of objects being isomorphic) with a Turing morphism that are not dominical. The first of these assertions is an easy consequence of the fact that in a dominical category C by definition the near product functor when restricted to the subcategory Ct, of total morphisms of C (as “total” is defined in [1]) constitutes a bona fide product such that the derived associativity and commutativity isomorphisms are natural on C × C × C and C × C, respectively, as noted in [7]. As to the second, p-recursion categories (that is, pointed p-isotypes having a Turing morphism) that are not dominical were defined and studied by Montagna in [6], the syntactic p-categories ST and S′T associated with consistent, recursively enumerable extensions of Peano arithmetic, PA. These merit detailed investigation on several counts.

Recursion over realizability structures

Realizability structures play a major role in the metamathematics of intuitionistic systems and they are a basic tool in the extraction of the computational content of constructive proofs. Besides their rich categorical structure and effectiveness properties provide a privileged mathematical setting for the semantics of data types of programming languages. In this paper we emphasize the modelling of recursive definitions of programs and types. A realizability model for a language including Girard's system F and an operator of recursion on types is given and some of its local properties are studied.

Completeness proofs for propositional logic with polynomial-time connectives

We introduce a conservative extension of propositional calculus which allows conjunctions and disjunctions of variable length. Consistency, completeness and decidability for both classical and intuitionistic systems are given provided some simple conditions on the formation of propositional letters are satisfied. We describe an application to PROLOG programming in the context of databases.

Large sets in intuitionistic set theory

We consider properties of sets in an intuitionistic setting corresponding to large cardinals in classical set theory. Adding such ‘large set axioms’ to intuitionistic ZF set theory does not violate well-know metamathematical properties of intuitionistic systems. Moreover, we consider statements in constructive analysis equivalent to the consistency of such ‘large set axioms’.

Forcing in intuitionistic systems without power-set

AbstractIt is shown how to define forcing semantics within metatheories not containing the power-set construction, in particular, how to construct exponents assuming only (a slightly strengthened form of) exponents in the metatheory. Some straightforward applications (consistency and independence results, and derived rules) are obtained for such systems.

Note on the fan theorem

The principal aim of this paper is to establish a theorem stating, roughly, that the addition of the fan theorem and the. continuity schema to an intuitionistic system of elementary analysis results in a conservative extension with respect to arithmetical statements.The result implies that the consistency of first order arithmetic cannot be proved by use of the fan theorem, in addition to standard elementary methods—although it was the opposite assumption which led Gentzen to withdraw the first version of his consistency proof for arithmetic (see [B]).We must presuppose acquaintance with notation and principal results of [K, T], and with §1.6, Chapter II, and Chapter III, §4-6 of [T1]. In one respect we shall deviate from the notation in [K, T]: We shall use (n)x (instead of g(n, x)) to indicate the xth component of the sequence coded by n, if x < lth(n), 0 otherwise.We also introduce abbreviations n ≤* m, a ≤ b which will be used frequently below:

Disjunctive properties of intuitionistic systems

Disjunctive properties of intuitionistic systems

A sequence of decidable finitely axiomatizable intermediate logics with the disjunction property

The intuitionistic propositional logic I has the following (disjunction) property.We are interested in extensions of the intuitionistic logic which are both decidable and have the disjunction property. Systems with the disjunction property are known, for example the Kreisel-Putnam system [1] which is I + (∼ϕ → (ψ ∨ α))→ ((∼ϕ→ψ) ∨ (∼ϕ→α)) and Scott's system I + ((∼ ∼ϕ→ϕ)→(ϕ ∨ ∼ϕ))→ (∼∼ϕ ∨ ∼ϕ). It was shown in [3c] that the first system has the finite-model property.In this note we shall construct a sequence of intermediate logics Dn with the following properties:These systems are presented both semantically and syntactically, using the remarkable correspondence between properties of partially ordered sets and axiom schemata of intuitionistic logic. This correspondence, apart from being interesting in itself (for giving geometric meaning to intuitionistic axioms), is also useful in giving independence proofs and obtaining proof theoretic results for intuitionistic systems (see for example, C. Smorynski, Thesis, University of Illinois, 1972, for independence and proof theoretic results in Heyting arithmetic).

The consistency of classical set theory relative to a set theory with intu1tionistic logic

Let ZF be the usual Zermelo-Fraenkel set theory formulated without identity, and with the collection axiom scheme. Let ZF−-extensionality be obtained from ZF by using intuitionistic logic instead of classical logic, and dropping the axiom of extensionality. We give a syntactic transformation of ZF into ZF−-extensionality.Let CPC, HPC respectively be classical, intuitionistic predicate calculus without identity, whose only homological symbol is ∈. We use the ~ ~-translation, a basic tool in the metatheory of intuitionistic systems, which is defined byThe two fundamental lemmas about this ~ ~ -translation we will use areFor proofs, see Kleene [3, Lemma 43a, Theorem 60d].This - would provide the desired syntactic transformation at least for ZF into ZF− with extensionality, if A− were provable in ZF− for each axiom A of ZF. Unfortunately, the ~ ~-translations of extensionality and power set appear not to be provable in ZF−. We therefore form an auxiliary classical theory S which has no extensionality and has a weakened power set axiom, and show in §2 that the ~ ~-translation of each axiom of Sis provable in ZF−-extensionality. §1 is devoted to the translation of ZF in S.

Induction and transfinite induction in intuitionistic systems

The present paper has to be considered as a kind of supplement to the authors work Proof theory and intuitionistic systems; we will henceforth refer to this volume as [LN]. The paper consists of two parts: in part one we present the content of chapters VII and VIII of [LN] in a more simplified and coherent fashion, in part two we prove some results on lhe relalionsliip between ordinary induction and transfinite induction, which were not included in [LN]. First a few remarks concerning part one. In chapters VII, VIII of [LN] one considers a certain theory, denoted by T tbr the moment, based on the language of second order ~ri+.hmetic, which is essentially intuition~stic numberthcory plus a certain schema of transfinite induction. The theot)' T has proot'theoretil~:ally the same strength as intuitionistic analysis such as described eg. in [KV]. In connection with T we consider in VIII of [LN] a certain t~cpe of fonnulas, called Harrop formulas, whose definition will be given below. The main result proved in VII and VIII of [LN] says that ifV is a set of Harrop formulas such that r u v is consistent then T u V has the usual disjunctive properties of intuitionistic systems such as: if A, B are closed and if T u V 1- A v B then T u V I-- A or r tJ v t- B, etc. An important role in the proof of this statement is played by a certain notion 'valuation of a proof. The presentation in [LN ] has two drawbacks. First, the most general case is treated, that is the case where the schema of transfinite induction contains function parameters; as a consequence

Interpretations of Kleene's metamathematical predicate Γ ∣ a in intuitionistic arithmetic

Let Pp, Pd, and N be the intuitionistic systems of prepositional calculus, predicate calculus, and elementary arithmetic, respectively, described in [3].Kleene [4] introduces a metamathematical predicate Γ ∣ A for each of the systems Pp, Pd, and N, where Γ ranges over finite sequences of wffs, and A ranges over wffs, of that system. In the case of N, if Γ is consistent, then ‘ Γ ∣ A’ is essentially the result of deleting all references to recursive functions from the metamathematical predicate ‘A is realizable-(Γ ⊦)’ described in [3], pp. 502–503.Through use of this predicate, Kleene [4] obtains elegant constructive proofs of the following results for N:Metatheorem 0.1. If B ∨ C is a closed theorem of N, then ⊦ B or ⊦ C.Metatheorem 0.2. If (∃a)D(a) is a closed theorem of N, then there is a numeral n such that ⊦D(n).Metatheorem 0.3. If A is a closed wff of N, then A ∣ A is a necessary and sufficient condition that, for all closed B, C, (∃a)D(a) inN:(0.3.1) ⊦ A ⊃ B ∨ C implies ⊦ A ⊃ B or ⊦A ⊃ Cand(0.3.2) ⊦A ⊃ (∃)D(a) implies there is a numeral n such that ⊦ A ⊃ D(n).

Ronald Harrop. Concerning formulas of the types A → B v C, A → (Ex)B(x) in intuitionistic formal systems. The journal of symbolic logic, vol. 25 no. 1 (for 1960, pub. 1961), pp. 27–32.

Ronald Harrop. Concerning formulas of the types A → B v C, A → (Ex)B(x) in intuitionistic formal systems. The journal of symbolic logic, vol. 25 no. 1 (for 1960, pub. 1961), pp. 27–32.

On disjunctions and existential statements in intuitionistic systems of logic

On disjunctions and existential statements in intuitionistic systems of logic