Almost simple geodesics on the triply punctured sphere

Abstract

Every closed hyperbolic geodesic $γ$ on the triply--punctured sphere $M =\widehat{\mathbb C} - \{0,1,\infty\}$ has a self--intersection number $I(γ) \ge 1$ and a combinatorial length $L(γ) \ge 2$, the latter defined by the number of times $γ$ passes through the upper halfplane. In this paper we show that $δ(γ) = I(γ) - L(γ) \ge -1$ for all closed geodesics; and that for each fixed $δ$, the number of geodesics with invariants $(δ,L)$ is given exactly by a quadratic polynomial $p_δ(L)$ for all $L \ge 4 + δ$.