Abstract :
Let a1, a2, a3,… be complex numbers. Then, for ε > 0, M ≥ 1 and T ≥ 1, [formula] where ζ(s) is Riemann′s zeta-function and Cε depends only on ε. The proof is based on a technical refinement within the circle of ideas to be found in Deshouillers and Iwaniec′s paper: Kloosterman sums and Fourier coefficients of cusp forms (Invent. Math.70 (1982), 219-288).