It is generally accepted that H. Friedman’s gap condition is closely related to iterated collapsing functions from ordinal analysis. But what precisely is the connection? We offer the following answer: In a previous paper we have shown that the gap condition arises from an iterative construction on transformations of partial orders. Here we show that the parallel construction for linear orders yields familiar collapsing functions. The iteration step in the linear case is an instance of a general construction that we (...) call ‘Bachmann–Howard derivative’. In the present paper, we focus on the unary case, i.e., on the gap condition for sequences rather than trees and, correspondingly, on addition-free ordinal notation systems. This is partly for convenience, but it also allows us to clarify a phenomenon that is specific to the unary setting: As shown by van der Meeren, Rathjen and Weiermann, the gap condition on sequences admits two linearizations with rather different properties. We will see that these correspond to different recursive constructions of sequences. (shrink)

We introduce and analyze two theories for typed inductive definitions and establish their proof-theoretic ordinal to be the small Veblen ordinal \. We investigate on the one hand the applicative theory \ of functions, inductive definitions, and types. It includes a simple type structure and is a natural generalization of S. Feferman’s system \\). On the other hand, we investigate the arithmetical theory \ of typed inductive definitions, a natural subsystem of \, and carry out a wellordering proof within \ (...) that makes use of fundamental sequences for ordinal notations in an ordinal notation system based on the finitary Veblen functions. The essential properties for describing the ordinal notation system are worked out. (shrink)