Model theoretic methods for fragments of FO and special classes of (finite) structures

Abstract

Some prominent fragments of first-order logic are discussed from a game-oriented and modal point of view, with an emphasis on model theoretic techniques for the non-classical context of finite model theory or of other natural non-elementary classes of structures. We stress the modularity and compositionality of the games as a key ingredient in the exploration of the expressive power of logics over specific classes of structures. The leading model theoretic theme is expressive completeness – or the characterisation of fragments of first-order logic as expressively complete over some class of (finite) structures for first-order properties with some prescribed semantic preservation behaviour. In contrast with classical expressive completeness arguments, the emphasis here is on explicit model constructions and transformations, which are guided by the game analysis of both first-order logic and of the imposed semantic constraints. keywords: finite model theory, model theoretic games, bisimulation, modal and guarded logic, expressive completeness, preservation and characterisation theorems