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.
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Kanckos, A. A note on Gentzen’s ordinal assignment. Arch. Math. Logic 58, 347–352 (2019). https://doi.org/10.1007/s00153-018-0641-4
- Relative consistency proof (03F25)
- Normalization (03F05)
- Ordinal notations (03F15)
Mathematics Subject Classification
- 03F25 (Relative consistency and interpretations)
- 03F30 (First-order arithmetic and fragments)
- 03F05 (Cut-elimination and normal-form theorems)
- 03F15 (Recursive ordinals and ordinal notations)