Uniform Length Estimates for Trajectories on Flat Cone Surfaces

Abstract: In this paper, we study trajectories on flat cone surfaces. We prove that on any area-one flat cone surface, the length of any trajectory $\gamma$ is bounded below by $b_1\sqrt{\iota(\gamma,\gamma)} - b_2$, where $\iota(\gamma,\gamma)$ is the self-intersection number of $\gamma$, and the coefficients $b_1$ and $b_2$ depend solely on the flat metric of the surface. Moreover, these coefficients $b_1$ and $b_2$ are uniform in restriction to convex flat cone spheres with a curvature gap bounded below and a number of singularities bounded above. This follows from a careful study of degenerations of flat structures in the corresponding moduli spaces. We even construct explicit values. Combining this result with the upper bound from arXiv:2308.08940, we conclude that any regular closed geodesic $\gamma$ satisfies $$c_1\sqrt{\iota(\gamma,\gamma)} \leq |\gamma| \leq a_1\sqrt{\iota(\gamma,\gamma)},$$ where $a_1$ and $c_1$ are explicit and uniform for any such flat sphere. Finally, we use these bounds to address some counting problems on convex flat cone spheres and convex polygonal billiards, deducing uniform bounds on counting functions over the corresponding moduli spaces.