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Elementary classes in basic modal logic. (English) Zbl 0951.03020

Summary: Dealing with topics of definability, this paper provides some interesting insights into the expressive power of basic modal logic. After some preliminary work it presents an abstract algebraic characterization of the elementary classes of basic modal logic, that is, of the classes of models that are definable by means of (sets of) basic modal formulas. Taking that for a start, the paper further contains characterization results for modal universal classes and modal positive classes.

MSC:

03B45 Modal logic (including the logic of norms)
03C40 Interpolation, preservation, definability
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