Uniqueness of normal proofs in implicational intuitionistic logic

Abstract
A minimal theorem in a logic L is an L-theorem which is not a non-trivial substitution instance of another L-theorem. Komori (1987) raised the question whether every minimal implicational theorem in intuitionistic logic has a unique normal proof in the natural deduction system NJ. The answer has been known to be partially positive and generally negative. It is shown here that a minimal implicational theorem A in intuitionistic logic has a unique -normal proof in NJ whenever A is provable without non-prime contraction. The non-prime contraction rule in NJ is the implication introduction rule whose cancelled assumption differs from a propositional variable and appears more than once in the proof. Our result improves the known partial positive solutions to Komori's problem. Also, we present another simple example of a minimal implicational theorem in intuitionistic logic which does not have a unique -normal proof in NJ
Keywords Natural deduction  uniqueness of normal proofs  coherence  minimal formulas  Komori's  non-prime contraction
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Reprint years 2004
DOI 10.1023/A:1008254111992
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On the Membership Problem for Non-Linear Abstract Categorial Grammars.Sylvain Salvati - 2010 - Journal of Logic, Language and Information 19 (2):163-183.

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