Proof Society 2025

Tutorial

Impredicative ordinal analysis

Henry Towsner
University of Pennsylvania

on  Mon, 10:00in  Class A (Zaal 1.05)for  60min and on  Mon, 14:30in  Class A (Zaal 1.05)for  60min and on  Tue, 10:00in  Class A (Zaal 1.05)for  60min

Ordinal analysis is the branch of proof theory concerned with measuring the strength of a mathematical theory using a transfinite ordinal. We’ll introduce some of the main ideas of modern ordinal analysis, especially the use of infinitary sequent calculi and how ordinals arise from these proof systems. Along the way, we’ll see Buchholz’s classic (but neglected) ordinal analysis of one of the basic impredicative theories, Π11CA0\Pi^1_1-\mathrm{CA}_0^-.

Slides and proof rules.

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