Compatibility between modal operators in distributive modal logic
Unlike in classical modal logic, in non-classical modal logics the box and diamond operators frequently fail to be interdefinable. Instead, these logics impose some compatibility conditions which tie the box and diamond operators together and ensure that in terms of Kripke semantics they arise from the same accessibility relation. This is, for instance, the case in the intuitionistic modal logic of Fischer Servi and the positive modal logic of Dunn. In these logics, however, such compatibility conditions also impose further conditions on the accessibility relation. In this paper, we identify the basic compatibility conditions which ensure that modal operators in distributive modal logics arise from a single accessibility relation without imposing any restrictions on the relation. As in the distributive logic of Gehrke, Nagahashi, and Venema, we allow for negative box and diamond operators here in addition to the usual positive ones. Intuitionistic modal logic and positive modal logic, or more precisely the corresponding classes of algebras, are then obtained in a modular way by adding certain canonical axioms which we call locality conditions on top of these basic compatibility conditions.