New Modification of the Subformula Property for a Modal Logic
Keywords:subformula property, modal logic, sequent calculus, scope of □
A modified subformula property for the modal logic KD with the additional
axiom □ ◊(A ∨ B) ⊃ □ ◊ A ∨ □ ◊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 ( and ). The finite model property as well as decidability for the logic follows from this.
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 M. Takano, A semantical analysis of cut-free calculi for modal logics, Reports on Mathematical Logic, vol. 53 (2018), pp. 43–65, DOI: http://dx.doi.org/10.4467/20842589RM.18.003.8836
 M. Takano, A modified subformula property for the modal logic S4.2, Bulletin of the Section of Logic, vol. 48(1) (2019), pp. 19–28, DOI: http://dx.doi.org/10.18778/0138-0680.48.1.02
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