On Berger's Isoperimetric Problem

Berger’s isoperimetric problem asks if the flat equilateral torus is ${\lambda }_1$-maximal. In 1996, Nadirashvili first gave a positive answer. In this paper, we use El Soufi-Ilias-Ros’s method and Bryant’s result (arXiv:1507.01485) to give a new proof.