Path-filtration for modal logics applied to revisiting quasi-dense logics

Abstract: In https://arxiv.org/pdf/2405.10094 (also published at LICS'24 conference), Lyon and Ostropolski-Nalewaja answer the question of the decidability of quasi-dense modallogics, and give an upper bound in EXPSPACE. Unfortunately, their intricate proof contains a major flaw that cannot be fixed, leaving the question wide open. In this paper we provide a correct and rather simple and direct proof of it by introducing a new variant of the well-know filtration method based on paths in a canonical model and improve the hypothetical membership to membership NEXPTIME.