Shifted convolution sums involving theta series

Abstract: Let $f$ be a cuspidal newform (holomorphic or Maass) of arbitrary level and nebentypus and denote by $\lambda_f(n)$ its $n$-th Hecke eigenvalue. Let $$ r(n)=#\left{(n_1,n_2)\in \mathbb{Z}^2:n_1^2+n_2^2=n\right}. $$ In this paper, we study the shifted convolution sum $$ \mathcal{S}h(X)=\sum{n\leq X}\lambda_f(n+h)r(n), \qquad 1\leq h\leq X, $$ and establish uniform bounds with respect to the shift $h$ for $\mathcal{S}_h(X)$.