Multi-Agent Subset Space Logic

Subset space logics have been introduced and studied as a framework for reasoning about a notion of effort in epistemic logic. The seminal Subset Space Logic (SSL) by Moss and Parikh modeled a single agent, and most work in this area has focused on different extensions of the language, or different model classes resulting from restrictions on subset spaces, while still keeping the single-agent assumption. In this paper we argue that the few existing attempts at multi-agent versions of SSL are unsatisfactory, and propose a new multi-agent subset space logic which is a natural extension of single-agent SSL. The main results are a sound and complete axiomatization of this logic, as well as an alternative and equivalent relational semantics.