Finiteness results for flat surfaces: large cusps and short geodesics

Abstract

For fixed $g$ and $T$ we show the finiteness of the set of affine equivalence classes of flat surfaces of genus $g$ whose Veech group contains a cusp of hyperbolic co-area less than $T$. We obtain new restrictions on Veech groups: any non-elementary Fuchsian group can appear only finitely many times in a fixed stratum; any non-elementary Veech group is of finite index in its normalizer; and the quotient of $H$ by a non-lattice Veech group admits arbitrarily large embedded disks. A key ingredient of the proof is the finiteness of the set of affine equivalence classes of flat surfaces of genus $g$ whose Veech group contains a hyperbolic element with eigenvalue less than $T$.