The basis problem for modular forms for the Weil representation

The vector valued theta series of a positive-definite even lattice is a modular form for the Weil representation of ${\mathrm{S}\mathrm{L}}_2(\mathbb{Z})$. We show that the space of cusp forms for the Weil representation is generated by such functions. This gives a positive answer to Eichler’s basis problem in this case. As applications we derive Waldspurger’s result on the basis problem for scalar valued modular forms and give a new proof of the surjectivity of the Borcherds lift based on the analysis of local Picard groups.