Nonvanishing and central critical values of twisted $L$-functions of cusp forms on average

Abstract

Let $f$ be a cusp form of integral weight $k \geq 4$ for $\Gamma_{0}(N)$ with nebentypus $\psi$. Generalising work of Kohnen we construct a kernel function for the $L$-function $L(f,\chi,s)$ of $f$ twisted by a primitive Dirichlet character $\chi$ and use it to show that the average $\sum_{f \in S_{k}(N,\psi)}\frac{L(f,\chi,s)}{\langle f,f\rangle}\overline{a_{f}(1)}$ over an orthogonal basis of $S_{k}(N,\psi)$ does not vanish on certain rectangles inside the critical strip if the weight $k$ or the level $N$ is big enough. As another application of the kernel function we prove an averaged version of Waldspurger's Theorem.