Interpolation in Algebraizable Logics; Semantics for Non-normal Multi-modal Logic

The two main directions pursued in the present paper are the following. The rst direction was (perhaps) started by Pigozzi in 1969. In Mak 91] and Mak 79] Maksimova proved that a normal modal logic (with a single u-nary modality) has the Craig interpolation property ii the corresponding class of algebras has the superamalgamation property. In this paper we extend Maksimova's theorem to normal multi-modal logics with arbitrarily many, not necessarily unary modalities, and to not necessarily normal multi-modal logics with modalities of ranks smaller than 2. To extend the characterization beyond multi-modal logics, we look at arbitrary algebraizable logics. We will introduce an algebraic property equivalent with the Craig interpolation property in algebraizable (and in strongly nice) logics, and prove that the superamalgamation property implies the Craig interpolation property. The problem of extending the characterization result to non-normal non-unary modal logics will be discussed, too. On the second direction pursued herein: For non-normal modal logic with one unary modality Lemmon Lem 66] gave a possible worlds semantics. Here we give a more general possible worlds semantics for not necessarily normal multi-modal logics with arbitrarily many not necessarily unary modalities. Strongly related to the above is the theorem, proved e.g. in JJ onsson-Tarski JT 52] and Henkin-Monk-Tarski HMT 71], that every normal Boolean algebra with operators (BAO) can be represented as a subalgebra of the complex algebra of some relational structure. We extend this result to not necessarily normal BAO's as follows. We deene partial relational structures and show that every not necessarily normal BAO is embeddable into the complex algebra of a partial relational structure. This gives a possible worlds semantics for not necessarily normal multi-modal logics (with arbitrarily many, not necessarily unary modalities).