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Published online by Cambridge University Press: 29 June 2020
In this paper we explore the following question: how weak can a logic be for Rosser’s essential undecidability result to be provable for a weak arithmetical theory? It is well known that Robinson’s Q is essentially undecidable in intuitionistic logic, and P. Hájek proved it in the fuzzy logic BL for Grzegorczyk’s variant of Q which interprets the arithmetic operations as nontotal nonfunctional relations. We present a proof of essential undecidability in a much weaker substructural logic and for a much weaker arithmetic theory, a version of Robinson’s R (with arithmetic operations also interpreted as mere relations). Our result is based on a structural version of the undecidability argument introduced by Kleene and we show that it goes well beyond the scope of the Boolean, intuitionistic, or fuzzy logic.
This paper is an extension and generalisation of work started by Cintula and Hájek before the unfortunate death of the latter in 2016. Hájek is included among the authors in recognition of his work on this topic, and with the blessing of his family, but it should be noted that he was not able to contribute directly to the final version of this paper.