Classification conjecture for monotone proof-theoretic operators

A monotone proof-theoretic operator is a monotone function sending sentences to sentences; a function is recursive when it is computable, and it is non-constant when it is not constant. Let TT be the fixed base theory, let βα\beta\preceq\alpha denote the ordering between ordinal notations, and let [θ][\theta] denote the equivalence class of a sentence under the paper's proof-theoretic equivalence. For a nice elementary presentation \prec of a recursive well-ordering, define the iterated consistency statements by

TConTα(φ)βαConT(φConTβ(φ)).T \vdash \mathsf{Con}^\alpha_T(\varphi) \leftrightarrow \forall \beta \prec \alpha\,\mathsf{Con}_T\bigl(\varphi\wedge\mathsf{Con}^\beta_T(\varphi)\big).

Classification conjecture for monotone proof-theoretic operators. Suppose g\mathfrak{g} is monotone, non-constant, and recursive, with g(φ)Π1\mathfrak{g}(\varphi)\in\Pi_1 for every sentence φ\varphi. Let \prec be a nice elementary presentation of a well-ordering and let α\alpha be an ordinal notation. If, for every φ\varphi,

T+φ+ConTα(φ)g(φ),T+\varphi+\mathsf{Con}^\alpha_T(\varphi)\vdash\mathfrak{g}(\varphi),

then there is some βα\beta\preceq\alpha such that, for all φ\varphi in a true cone,

[φg(φ)]=[φConTβ(φ)].[\varphi\wedge\mathfrak{g}(\varphi)]=[\varphi\wedge\mathsf{Con}^\beta_T(\varphi)].

The conjecture seeks a classification of monotone proof-theoretic operators analogous to the Slaman–Steel classification of increasing Borel order-preserving functions. It refines the preceding dichotomy for recursive monotone operators bounded by the consistency operator; the source provides no resolution, so the conjecture remains open.

Sources & referencesView supporting material

Primary source

James Walsh, “A note on the consistency operator”, arXiv:1905.00998 (2019).

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