An Ehrenfeucht-Fraïssé Game for Inquisitive First-Order Logic

Inquisitive first-order logic, InqBQ, is an extension of classi- cal first-order logic with questions. From a mathematical point of view, formulas in this logic express properties of sets of relational structures. In this paper we describe an Ehrenfeucht-Fraïssé game for InqBQ and show that it characterizes the distinguishing power of the logic. We exploit this result to show a number of undefinability results: in particular, several variants of the question how many individuals have property P are not expressible in InqBQ, even in restriction to finite models