Inventiones mathematicae

, Volume 183, Issue 2, pp 337–383

Dichotomy for the Hausdorff dimension of the set of nonergodic directions


DOI: 10.1007/s00222-010-0279-2

Cite this article as:
Cheung, Y., Hubert, P. & Masur, H. Invent. math. (2011) 183: 337. doi:10.1007/s00222-010-0279-2


Given an irrational 0<λ<1, we consider billiards in the table Pλ formed by a \(\tfrac{1}{2}\times1\) rectangle with a horizontal barrier of length \(\frac{1-\lambda}{2}\) with one end touching at the midpoint of a vertical side. Let NE (Pλ) be the set of θ such that the flow on Pλ in direction θ is not ergodic. We show that the Hausdorff dimension of NE (Pλ) can only take on the values 0 and \(\tfrac{1}{2}\), depending on the summability of the series \(\sum_{k}\frac{\log\log q_{k+1}}{q_{k}}\) where {qk} is the sequence of denominators of the continued fraction expansion of λ. More specifically, we prove that the Hausdorff dimension is \(\frac{1}{2}\) if this series converges, and 0 otherwise. This extends earlier results of Boshernitzan and Cheung.

Copyright information

© Springer-Verlag 2010

Authors and Affiliations

  1. 1.Department of MathematicsSan Francisco State UniversitySan FranciscoUSA
  2. 2.LATP, case cour AFaculté de Saint JérômeMarseille Cedex 20France
  3. 3.Department of MathematicsUniversity of ChicagoChicagoUSA