A note on Gentzen’s ordinal assignment
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Gentzen’s height measure of the 1938 consistency proof is a cumulative complexity measure for sequents that is measured bottom-up in a derivation. By a factorisation of the ordinal assignment a top-down ordinal assignment can be given that does not depend on information occurring below the sequent to which the ordinal is assigned. Furthermore, an ordinal collapsing function is defined in order to collapse the top-down ordinal to the one assigned by Gentzen’s own ordinal assignment. A direct definition of the factorised assignment follows as a corollary. This extraction of an ordinal collapsing function hopes to provide a formal or conceptual clarification of Gentzen’s ordinal assignment and its height-line argument.
KeywordsRelative consistency proof (03F25) Normalization (03F05) Ordinal notations (03F15)
Mathematics Subject Classification03F25 (Relative consistency and interpretations) 03F30 (First-order arithmetic and fragments) 03F05 (Cut-elimination and normal-form theorems) 03F15 (Recursive ordinals and ordinal notations)
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- 4.Takeuti, G.: Proof Theory: Second Edition. Dover Books on Mathematics. Dover Publications, New York (2013). First ed. 1987Google Scholar