A modal logic internalizing normal proofs

  • Authors:
  • Sungwoo Park;Hyeonseung Im

  • Affiliations:
  • Pohang University of Science and Technology, San 31 Hyojadong Namgu, Pohang Gyungbuk 790-784, Republic of Korea;Pohang University of Science and Technology, San 31 Hyojadong Namgu, Pohang Gyungbuk 790-784, Republic of Korea

  • Venue:
  • Information and Computation
  • Year:
  • 2011

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Abstract

In the proof-theoretic study of logic, the notion of normal proof has been understood and investigated as a metalogical property. Usually we formulate a system of logic, identify a class of proofs as normal proofs, and show that every proof in the system reduces to a corresponding normal proof. This paper develops a system of modal logic that is capable of expressing the notion of normal proof within the system itself, thereby making normal proofs an inherent property of the logic. Using a modality @? to express the existence of a normal proof, the system provides a means for both recognizing and manipulating its own normal proofs. We develop the system as a sequent calculus with the implication connective @? and the modality @?, and prove the cut elimination theorem. From the sequent calculus, we derive two equivalent natural deduction systems.