Abstract. The two main directions pursued in the present paper are the following.
The first direction was (perhaps) started by Pigozzi in 1969. Maksimova [M91, M79] proved that a normal modal logic (with a single unary modality) has the Craig interpolation property iff 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 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 [L66] 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 J\'onsson-Tarski~ [JT52] and Henkin-Monk-Tarski~ [HMT71], 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 define 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).
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