Almost simple geodesics on the triply punctured sphere

Abstract

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