Sufficient conditions for local tabularity of a polymodal logic

TL;DR

This paper identifies conditions under which certain operations on relational structures and logics preserve local tabularity in polymodal logics, expanding understanding of when such logics remain manageable.

Contribution

It introduces new semantic and axiomatic conditions for local tabularity, including the effects of reflexive closure and sums on frames, and provides a method to extend logics while preserving local tabularity.

Findings

Reflexive closure does not affect local tabularity.

Sum operations preserve local tabularity if component logics are locally tabular.

Extension with specific formulas maintains local tabularity in fused logics.

Abstract

On relational structures and on polymodal logics, we describe operations which preserve local tabularity. This provides new sufficient semantic and axiomatic conditions for local tabularity of a modal logic. The main results are the following. We show that local tabularity does not depend on reflexivity. Namely, given a class $\mathcal{F}$ of frames, consider the class $\mathcal{F}^\mathrm{r}$ of frames, where the reflexive closure operation was applied to each relation in every frame in $\mathcal{F}$. We show that if the logic of $\mathcal{F}^\mathrm{r}$ is locally tabular, then the logic of $\mathcal{F}$ is locally tabular as well. Then we consider the operation of sum on Kripke frames, where a family of frames-summands is indexed by elements of another frame. We show that if both the logic of indices and the logic of summands are locally tabular, then the logic of corresponding sums is also locally tabular. Finally, using the previous theorem, we describe an operation on logics that preserves local tabularity: we provide a set of formulas such that the extension of the fusion of two canonical locally tabular logics with these formulas is locally tabular.