Modal Definability in Enriched Languages
The paper deals with polymodal languages combined with stan­ dard semantics defined by means of some conditions on the frames. So a notion of "polymodal base" arises which provides various enrichments of the classical modal language. One of these enrichments, viz. the base £(R,-R), with modalities over a relation and over its complement, is the paper's main paradigm. The modal definability (in the spirit of van Benthem's correspon­ dence theory) of arbitrary and ~-elementary classes of frames in this base and in some of its extensions, e.g., £(R,-R,R-1 ,_R-1), £(R,-R,=I=) etc., is described, and numerous examples of conditions definable there, as well as undefinable ones, are adduced. 81