Non-congruence of homology Veech groups in genus two

Abstract

We study the action of the Veech group of square-tiled surfaces of genus two on homology. This action defines the homology Veech group which is a subgroup of $${\mathrm{SL}}_2(\mathcal {O}_D)$$ where $$\mathcal {O}_D$$ is a quadratic order of square discriminant. Extending a result of Weitze-Schmithusen we show that also the homology Veech group is a totally non-congruence subgroup with exceptions stemming only from the prime ideals lying above 2. While Weitze-Schmithusen’s result for Veech groups is asymmetric with respect to the spin structure our use of the homology Veech group yields a completely symmetric picture.