S. Barré

On the kissing number of hyperbolic surfaces with a small systole

Parlier proved that there exists a constant $U>0$ such that for any hyperbolic surface $S$ of genus $g$ with systole $\ell$ the following holds: \[ Kiss(S)\leq U \frac {e^\ell}\ell g \] where $Kiss(S)$ is the maximum number of homotopically distinct systoles in $S$ (Kissing number). We show that there exists a constant $V>0$ and a sequence $S_n$ of hyperbolic homogeneous surfaces of genus $g_n\to \infty$ with a bounded systole such that \[ Kiss(S_n)\geq Vg_n \] for all $n\geq 1$. This shows that Parlier's theorem is sharp for a fixed systole length. Our proof uses Bruhat--Tits buildings of type $\tilde A_2$ and Hamiltonian surfaces.