Filtering unification and most general unifiers in modal logic

We characterize (both from a syntactic and an algebraic point of view) the normal K 4-logics for which unification is filtering. We also give a sufficient semantic criterion for existence of most general unifiers, covering natural extensions of K 4.2 + (i.e., of the modal system obtained from K 4 by adding to it, as a further axiom schemata, the modal translation of the weak excluded middle principle).

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