## Products of ‘transitive’ modal logics.(English)Zbl 1103.03020

Products of modal logics, introduced in the 1970s [see K. Segerberg, “Two-dimensional modal logic”, J. Philos. Log. 2, 77–96 (1973; Zbl 0259.02013), V. Shekhtman, “Two-dimensional modal logics”, Math. Notes 23, 417–424 (1978; Zbl 0403.03015), translation from Mat. Zametki 23, 759–772 (1978; Zbl 0384.03010)], have been intensively studied over the last decade [see D. Gabbay, A. Kurucz, F. Wolter, and M. Zakharyaschev, Many-dimensional modal logics: theory and applications. Amsterdam: Elsevier (2003; Zbl 1051.03001)]. In this paper, the authors introduce a novel technique for dealing with products of logics with transitive branching frames and solve a major open problem concerning algorithmic properties of products of ‘transitive’ modal logics by showing that products and commutators of standard logics such as K4, S4, S4.1, K4.3, GL and Grz are undecidable and do not have the finite model property. More generally, they prove that all products of two Kripke-complete modal logics with transitive frames of arbitrary finite or infinite depth are undecidable, in many cases these products are not axiomatizable and do not enjoy the abstract finite model property, and, sometimes they are even $$\Pi^1_1$$-hard. As a byproduct, the first known examples of Kripke-incomplete commutators of Kripke-complete logics are obtained.

### MSC:

 03B45 Modal logic (including the logic of norms) 03B25 Decidability of theories and sets of sentences

### Citations:

Zbl 0259.02013; Zbl 0384.03010; Zbl 0403.03015; Zbl 1051.03001
Full Text:

### References:

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