Permanence vs. termination: a logical analysis

The present article is devoted to a logical inquiry on the notions of permanence and termination, which play a central role in many areas of temporal reasoning. In the first part, we introduce a bimodal framework to represent these notions and provide a syntactic and semantic comparison with a monomodal framework representing the notion of future necessity. In the second part, we focus on the problem of defining synonymous logical systems over the two frameworks; as an example, we provide an extended analysis of two systems, the monomodal K4 and the bimodal S4X. The third part of the article indicates possible developments of the proposed line of inquiry, such as finding a simple representation in the bimodal language of some interesting properties of time and identifying further pairs of synonymous systems.