An Isomorphism between Scalar-Valued Modular Forms and Modular Forms for Weil Representations

In this note, we consider discriminant forms that are given by the norm form of real quadratic fields and their induced Weil representations. We prove that there exists an isomorphism between the space of vector-valued modular forms for the Weil representations that are invariant under the action of the automorphism group and the space of scalar-valued modular forms that satisfy some epsilon-condition, with which we translate Borcherds’s theorem of obstructions to scalar-valued modular forms. In the end, we consider an example in the case of level 12.