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 $\textrm{SL}_2(\mathcal{O}_D)$ where $\mathcal{O}_D$ is a quadratic order of square discriminant. Extending a result of Weitze-Schmith\"usen 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-Schmith\"usen'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.