Modal Logics with a Linear Hierarchy of Local Propositional Quantifiers

Local propositions arise in the context of the semantics for logics of knowledge in multi-agent systems. A proposition is local to an agent when it depends only on that agent's local state. We consider a logic, LLP, that extends S5, the modal logic of necessity (in which the modality refers to truth at all worlds) by adding a quantifier ranging over the set of all propositions and, for each agent, a propositional quantifier ranging over the agent's local propositions. LLP is able to express a large variety of epistemic modalities, including knowledge, common knowledge and distributed knowledge. However, this expressiveness comes at a cost: the logic is equivalent to second order predicate logic when two independent agents are present [5], hence undecidable and not axiomatizable. This paper identifies a class of multi-agent S5 structures, hierarchical structures, in which the agents' information has the structure of a linear hierarchy. All systems with just a single agent are hierarchical. It is shown that LLP becomes decidable with respect to hierarchical systems. The main result of the paper is the completeness of an axiomatization for the hierarchical case.