By Koebe's retrosection theorem, every closed Riemann surface of genus $g \geq 2$ is uniformized by a Schottky group. Marden observed that there are Schottky groups that are not classical ones, that is, they cannot be defined by a suitable collection of circles. This opened the question of whether every closed Riemann surface can be uniformized by a classical Schottky group. In this paper, we observe that every Belyi curve can be uniformized by a classical Schottky group. Since Belyi curves form a dense locus in the moduli space ${ \mathcal M}_{ g}$ and the locus ${ \mathcal M}_{ g}^{ cs} \subset { \mathcal M}_{ g}$ of those Riemann surfaces uniformized by classical Schottky groups is a non-empty open set, this ensures that ${ \mathcal M}_{ g}^{ cs}$ is open and dense in ${ \mathcal M}_{ g}$.
Categories: math.CV