Non-finitely axiomatisable two-dimensional modal logics. (English) Zbl 1259.03032

In this very clearly structured paper, the authors consider the problem of axiomatizing products of two finitely axiomatizable unimodal propositional logics. The paper settles some open problems from the literature on product logics in the negative. It is shown that certain recursively enumerable product logics characterized by classes of product frames with a linearly ordered first component, including the decidable logic K4.3 \(\times\) K, cannot be axiomatized with only a finite number of propositional variables. Moreover, the notion of vertical depth of a bimodal formula is defined, and it is shown that for K4.3 \(\times\) K and certain two-dimensional modal logics extending K4.3 \(\times\) K, every axiomatization must contain formulas of arbitrarily large vertical depth. The paper ends with a list of open problems.


03B45 Modal logic (including the logic of norms)
03B62 Combined logics
Full Text: DOI Euclid


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