Fri, 03.11.2017, 13:30
From Cut Elimination to Pi^1_1-Comprehension

Speaker: Anton Freund, University of Leeds
Room: S2|15-201

I will begin with an informal explanation of cut elimination and its use in the ordinal analysis of Peano arithmetic. The latter shows that Pi^0_2-soundness is equivalent to the primitive recursive well-foundedness of an ordinal notation system. On a conceptual level, we observe that the focus is on low logical complexity. Next, I explain how an ordinal notation system can be relativized to a given well-order. This brings us to the level of Pi^1_2-statements. Many well-known axioms of that complexity have been characterized in terms of relativized ordinal notation systems: from arithmetical comprehension (Girard, Hirst) up to the existence of omega-models of bar induction (Rathjen and Vizcaíno). Finally, I present a new result at the next type-level: Following a suggestion by Montalbán and Rathjen, I relativize an ordinal notation system to a relativized ordinal notation system. In this way one can get a characterization of Pi^1_1-comprehension, which is a Pi^1_3-statement (preprint available as arXiv:1704.01662).


Technische Universität Darmstadt
Department of Mathematics
Logic Group

Schloßgartenstraße 7
64289 Darmstadt

Phone: +49-(0)6151-1622863
Fax:     +49-(0)6151-1622840

Secretary's office
Betina Schubotz

Office hours:
Mo.-Fr.  10:00-15:00


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