In one of his papers, Dragalin proposed a very general approach to the construction of models for nonstandard logics of uniform-algebra type and, in particular, for intuitionistic logic (see [1]). The presentation (clear enough, but without the obvious details) is accompanied by a number of arithmetical examples (see also [1]). In the last of these examples, Kleene’s slash realizability is considered (see [2, column C]), and the author gives a “ . . . model corresponding to Kleene’s slash realizability . . . ” [1, p. 194]. However, the connection between this model and Kleene’s slash realizability is as follows: “ . . . ‖φ‖ = T ⇔ ((|φ)∧HA φ)”; (see also [1, p. 195]; cf. [3]). Of course, by means of the above-mentioned model (which corresponds exactly to the formula realizability from [3]) one can prove the properties of disjunctiveness and existentiality for arithmetic (this is just the result the author seeks to obtain, using a suitable uniform algebra). However, Kleene’s slash realizability does not coincide with deducibility in the intuitionistic HA arithmetic. As mentioned above, the paper [1] contains a number of examples of functional pseudoboolean algebras corresponding to various HA models (including, in particular, realizability models). Exact formulation of functional pseudoboolean algebra is given in [1], and here we shall only briefly mention facts needed in what follows. If F is the set of forms of a functional pseudoboolean algebra, then for any f and g from F (where f and g are forms of the same arity), there exist the forms f ∧ g , f ∨ g , f ⊃ g in F of equal arity. After that, a functional algebraic model for logic-mathematical language is defined, and for a given functional algebraic model, we define the value of any formula of our language in it. The main distinction from usual algebraic models is in the fact that the value ‖φ‖ of the formula φ is a certain form fφ from a functional pseudoboolean algebra. Now we can fix the language of arithmetic, the object area, and the function Ĉnst . After that, each model is defined by specifying the following set: B (a pseudoboolean algebra), F , and Pr-valuation of predicates. Let a given functional pseudoboolean algebra be a model for Kleene’s slash realizability, i.e., suppose formulas of the language can be mapped to the set of forms so that any formula φ is |realizable if and only if fφ ∈ 1 (the unit of algebra B). Consider two distinct statements, φ and ψ , undecidable in HA, i.e., HA φ, HA ψ, HA ¬φ, HA ¬ψ. Since φ and ψ cannot be deduced in HA, the formulas ¬φ and ¬ψ are nondeducible |-realizable formulas of the language of arithmetic. If the forms corresponding to them in the functional algebraic model are F¬φ and F¬ψ , respectively, then these forms belong to algebra’s 1 , and then the form F¬φ ∨F¬ψ = F¬φ∨¬ψ (which corresponds in the functional algebraic model to the formula ¬φ∨¬ψ , see [1, p. 187]), also belongs to our algebra’s unit, and hence, the formula ¬φ∨¬ψ is |realizable. But this implies that HA φ or HA ψ , which is impossible by the choice of φ and ψ . Thus, we have proved the following theorem:
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