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The undecidability of the disjunction property of propositional logics and other related problems. (English) Zbl 0799.03009
The paper proves undecidability for many general properties of intermediate and modal propositional logics (above S4), and also some other interesting results related to the disjunction property. The method of undecidability proofs is based on simulating Minsky machines in Kripke models [cf. L. A. Chagrova, ibid. 56, No. 4, 1261-1272 (1991; Zbl 0737.03018)] and on some earlier ideas of the first author [Algebra Logika 29, No. 3, 350-367 (1990; Zbl 0723.03005) and ibid., No. 5, 613-623 (1990; Zbl 0729.03009)]. It is proved that decidability and the finite model property are undecidable for normal modal calculi extending Grzegorczyk logic Grz (Theorems 1 and 2).
Then the authors prove sufficient conditions for the disjunction property (DP) of intermediate logics and Halldén-completeness (HC) of extensions of Grz; they are formulated in terms of “canonical formulas” introduced by the second author [Dokl. Akad. Nauk SSSR 269, 18-22 (1983; Zbl 0548.03009)]. They show that HC is not necessarily preserved for model companions (Theorem 5) and construct a modal logic with DP which lacks HC (Theorem 6) and a continuum of extensions of Grz with HC which lack DP (Theorem 6), and of those with both HC and DP. Also they prove undecidability of DP and of HC for calculi extending Grz. The axiomatizability of a given calculus by formulas of some special types (e.g. by implicative formulas in the intermediate case) turns out to be undecidable as well.
The last two sections study “Maksimova-completeness” and discuss some open problems.

##### MSC:
 03B45 Modal logic (including the logic of norms) 03B55 Intermediate logics 03D35 Undecidability and degrees of sets of sentences
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