Dilator-based analysis of KP

Abstract

Proof theorists developed various frameworks to analyze impredicative systems like $Π^1_1\text{-}\mathsf{CA}_0$ or $\mathsf{KP}$; One is an operator-controlled derivation system, and the other is Girard's dilator-based $β$-logic. In this paper, we provide a functorial formulation of operator-controlled analysis of $\mathsf{KP}$, thereby unifying the two approaches into a single framework. As an application, a new proof of Girard's boundedness theorem is also provided, which states that every $Σ_1$-over-$L_{ω_1^{\mathsf{CK}}}$-definable function $ω_1^\mathsf{CK}\to ω_1^\mathsf{CK}$ is bounded by a recursive dilator.