The contraction rule and decision problems for logics without structural rules

This paper shows a role of the contraction rule in decision problems for the logics weaker than the intuitionistic logic that are obtained by deleting some or all of structural rules. It is well-known that for such a predicate logic L, if L does not have the contraction rule then it is decidable. In this paper, it will be shown first that the predicate logic FL_ec with the contraction and exchange rules, but without the weakening rule, is undecidable while the propositional fragment of FL_ec is decidable. On the other hand, it will be remarked that logics without the contraction rule are still decidable, if our language contains function symbols.