Axiomatization of modal logic with counting

Abstract Modal logic with counting is obtained from basic modal logic by adding cardinality comparison formulas of the form $ \#\varphi \succsim \#\psi $, stating that the cardinality of successors satisfying $ \varphi $ is larger than or equal to the cardinality of successors satisfying $ \psi $. It is different from graded modal logic where basic modal logic is extended with formulas of the form $ \Diamond _{k}\varphi $ stating that there are at least $ k$-many different successors satisfying $ \varphi $. In this paper, we investigate the axiomatization of ML(#) with respect to different frame classes, such as image-finite frames and arbitrary frames. Drawing inspiration from existing works, we employ a similar proof strategy that uses the characterization of binary relations on finite Boolean algebras capable of representing generalized probability measures or finite (respectively arbitrary) cardinality measures. Our main result shows that any formula not provable in the Hilbert system can be refuted within a finite (respectively arbitrary) cardinality measure Kripke frame with a finite domain. We then transform this finite (respectively arbitrary) cardinality measure Kripke frame into a Kripke frame in the corresponding class, refuting the unprovable formula.