A Poincaré section for horocycle flow on the space of lattices

Abstract

We construct a Poincaré section for the horocycle flow on the modular surface $SL(2,\mathbb{R})/SL(2,\mathbb{Z})$, and study the associated first return map, which coincides with a transformation (the {\it BCZ map}) defined by Boca-Cobeli-Zaharescu. We classify ergodic invariant measures for this map and prove equidistribution of periodic orbits. As corollaries, we obtain results on the average depth of cusp excursions and on the distribution of gaps for Farey sequences and slopes of lattice vectors.

arXiv comment39 pages, 12 figures, some updates and added references. The large figure on page 3 has been changed to be more manageable. Submitted to Inventiones Mathematicae