, Volume 38, Issue 3, pp 247-262

On two problems of Harvey Friedman

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The paper considers certain properties of intermediate and moda propositional logics.

The first part contains a proof of the theorem stating that each intermediate logic is closed under the Kreisel-Putnam rule ∼x→y∨z/(∼x→y)∨(∼x→z).

The second part includes a proof of the theorem ensuring existence of a greatest structurally complete intermediate logic having the disjunction property. This theorem confirms H. Friedman's conjecture 41 (cf. [2], problem 41).

In the third part the reader will find a criterion which allows us to obtain sets satisfying the conditions of Friedman's problem 42, on the basis of intermediate logics satisfying the conditions of problem 41.

Finally, the fourth part contains a proof of a criterion which allows us to obtain modal logics endowed with Hallden's property on the basis of structurally complete intermediate logics having the disjunction property.

Dedicated to Professor Roman Suszko
The author would like to thank professors J. Perzanowski and A. Wroński for valuable suggestions.