New New Modification of the Subformula Property for a Modal Logic

Abstract

A modified subformula property for the modal logic KD with the additional axiom $\Box\Diamond(A\vee B)\supset\Box\Diamond A\vee\Box\Diamond B$ is shown. A new modification of the notion of subformula is proposed for this purpose. This modification forms a natural extension of our former one on which modified subformula property for the modal logics K5, K5D and S4.2 has been shown (Bull Sect Logic 30:115--122, 2001 and 48:19--28, 2019). The finite model property as well as decidability for the logic follows from this.