Evitable iterates of the consistency operator

Article type: Research Article

Authors: Walsh, James^{; *}

Affiliations: Sage School of Philosophy, Cornell University, Ithaca, NY 14853, USA

Correspondence: [*] Corresponding author. jameswalsh@cornell.edu.

Abstract: Why are natural theories pre-well-ordered by consistency strength? In previous work, an approach to this question was proposed. This approach was inspired by Martin’s Conjecture, one of the most prominent conjectures in recursion theory. Fixing a reasonable subsystem T of arithmetic, the goal was to classify the recursive functions that are monotone with respect to the Lindenbaum algebra of T. According to an optimistic conjecture, roughly, every such function must be equivalent to an iterate ConTα of the consistency operator “in the limit” within the ultrafilter of sentences that are true in the standard model. In previous work the author established the first case of this optimistic conjecture; roughly, every recursive monotone function is either as weak as the identity operator in the limit or as strong as ConT in the limit. Yet in this note we prove that this optimistic conjecture fails already at the next step; there are recursive monotone functions that are neither as weak as ConT in the limit nor as strong as ConT2 in the limit. In fact, for every α, we produce a function that is cofinally equivalent to ConTα yet cofinally equivalent to ConT.

Keywords: Proof theory, consistency, Martin’s Conjecture

DOI: 10.3233/COM-220400

Journal: Computability, vol. 12, no. 1, pp. 59-69, 2023