Denjoy's theorem on rotation number
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In mathematics, a Denjoy diffeomorphism is a diffeomorphism of a circle that has an irrational rotation number, but is not topologically conjugate to an irrational rotation. It is a theorem of Arnaud Denjoy[1] that a Denjoy diffeomorphism cannot be twice differentiable.
Notes
- ^ See this Postscript file.