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Ordered orbits of the shift, square roots, and the devil's staircase

Published online by Cambridge University Press:  24 October 2008

Shaun Bullett
Affiliation:
School of Mathematical Sciences, Queen Mary and Westfield College, Mile End Road, LondonE1 4NS
Pierrette Sentenac
Affiliation:
Mathématique, Bâtiment 425, Université de Paris-Sud, 91405 Orsay, France

Abstract

An orbit of the shift σ: t ↦ 2t on the circle = ℝ/ℤ is ordered if and only if it is contained in a semi-circle Cμ = [μ, μ+½]. We investigate the ‘devil's staircase’ associating to each μ ε the rotation number ν of the unique minimal closed σ-invariant set contained in Cμ; we present algorithms for μ in terms of ν, and we prove (after Douady) that if ν is irrational then μ is transcendental. We apply some of this analysis to questions concerning the square root map, and mode-locking for families of circle maps, we generalize our algorithms to orbits of the shift having ‘sequences of rotation numbers’, and we conclude with a characterization of all orders of points around realizable by orbits of σ.

Type
Research Article
Copyright
Copyright © Cambridge Philosophical Society 1994

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