Geodesics with One Self-intersection, and Other Stories

In this note we show that for any hyperbolic surface S, the number of geodesics of length bounded above by L in the mapping class group orbit of a fixed closed geodesic γ with a single double point is asymptotic to Ldim(Teichmuller space of S.). Since closed geodesics with one double point fall into a finite number of M≀⌈(S) orbits, we get the same asympotic estimate for the number of such geodesics of length bounded by L. We also use our (elementary) methods to do a more precise study of geodesics with a single double point on a punctured torus, including an extension of McShane’s identity to such geodesics. In the second part of the paper we study the question of when a covering of the boundary of an oriented surface S can be extended to a covering of the surface S itself, we obtain a complete answer to that question, and also to the question of when we can further require the extension to be a regular covering of S. Wealso analyze the question (first raised byK. Bou-Rabee) of theminimal index of a subgroup in a surface groupwhich does not contain a given element. We show that we have a linear bound for the index of an arbitrary subgroup, a cubic bound for the index of a normal subgroup, but also poly-log bounds for each fixed level in the lower central series (using elementary arithmetic considerations) – the results hold for free groups and fundamental groups of closed surfaces..