FLAP Journal 2025 Journal Article
Alternative Cut-free Sequent Calculi for S4 that are Compatible with Paradefinite Four-valued Logic
- Norihiro Kamide
Three new Gentzen-style sequent calculi GS41, GS42, and GS43 for normal modal logic S4 are introduced. These calculi are obtained from a Gentzen-style sequent calculus GMA4 for a modal extension MA4 of Avron’s self-extensional paradefinite four-valued logic by adding some special inference rules and/or some special initial sequents. The cut-elimination theorems for GS41, GS42, GS43, and GMA4 are proved. An alternative Kripke semantics for S4 is intro- duced and the completeness theorems with respect to this semantics are proved for GS41 and GS42. An extended Kripke semantics is introduced for an ex- tended S4 with an auxiliary negation connective and the completeness theorem with respect to this semantics is proved for GS43. Finally, a Gödel–McKinsey– Tarski theorem for embedding Gurevich logic into S4 is proved.