Published online by Cambridge University Press: 20 November 2018
An infinite family of perfect, non-extremal Riemann surfaces is constructed, the first examples of this type of surfaces. The examples are based on normal subgroups of the modular group $\text{PSL}\left( 2,\,\mathbb{Z} \right)$ of level 6. They provide non-Euclidean analogues to the existence of perfect, non-extremal positive definite quadratic forms. The analogy uses the function syst which associates to every Riemann surface $M$ the length of a systole, which is a shortest closed geodesic of $M$.