The logic of the weak law of excluded middleKC p is obtained by adding the formula A A as an axiom scheme to Heyting's intuitionistic logicH p . A cut-free sequent calculus for this logic is given. As the consequences of the cut-elimination theorem, we get the decidability of the propositional part of this calculus, its separability, equality of the negationless fragments ofKC p andH p , interpolation theorems and so on. From the proof-theoretical point of view, the formulation presented (...) in this paper makes clearer the relations betweenKC p ,H p , and the classical logic. In the end, an interpretation of classical propositional logic in the propositional part ofKC p is given. (shrink)

A normalizable natural deduction formulation, with subformula property, of the implicative fragment of classical logic is presented. A traditional notion of normal deduction is adapted and the corresponding weak normalization theorem is proved. An embedding of the classical logic into the intuitionistic logic, restricted on propositional implicational language, is described as well. We believe that this multiple-conclusion approach places the classical logic in the same plane with the intuitionistic logic, from the proof-theoretical viewpoint.

The language of the propositional calculus is extended by two families of propositional probability operators, inductively applicable to the formulae, and the set of all formulae provable in an arbitrary superintuitionistic propositional logic is extended by the probability measure axioms concerning those probability operators. A logical system obtained in such a way, similar to a kind of polymodal logic, makes possible to express a probability measure of truthfulness of any formula. The paper contains a description of the Kripke-type possible worlds (...) semantics covering the considered logical systems, being followed by the corresponding completeness results. (shrink)

We prove that every finitely axiomatizable extension of Heyting's intuitionistic logic has a corresponding cut-free Gentzen-type formulation. It is shown how one can use this result to find the corresponding normalizable natural deduction system and to give a criterion for separability of considered logic. Obviously, the question how to obtain an effective definition of a sequent calculus which corresponds to a concrete logic remains a separate problem for every logic.

The propositional language extended by two families of unary propositional probability operators and the corresponding list of probability measure axioms concerning those operators is the basis of the system preseted here. We describe a Kripke-type possible worlds semantics covering such a kind of logical systems.1.

In distinction from the well-known double-negation embeddings of the classical logic we consider some variants of single-negation embeddings and describe some classes of superintuitionistic first-order predicate logics in which the classical first-order calculus is interpretable in such a way. Also we find the minimal extensions of Heyting's logic in which the classical predicate logic can be embedded by means of these translations.

In Ilić and Boričić, the right-handed cut-free sequent calculus $GRW$ for the contraction-less relevant logic $RW$ is defined. In this paper, we show that the enlargement of the system $GRW$ with the structural rule of intensional contraction presents the sequent system for the principal relevant logic $R$ but the rule of cut cannot be eliminated in $GRW+$.