Abstract
We prove that a C 2+α-smooth orientation-preserving circle diffeomorphism with rotation number in Diophantine class D δ , 0≤δ<α≤1, α−δ≠1, is C 1+α−δ-smoothly conjugate to a rigid rotation. This is the first sharp result on the smoothness of the conjugacy. We also derive the most precise version of Denjoy’s inequality for such diffeomorphisms.
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Khanin, K., Teplinsky, A. Herman’s theory revisited. Invent. math. 178, 333–344 (2009). https://doi.org/10.1007/s00222-009-0200-z
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DOI: https://doi.org/10.1007/s00222-009-0200-z