Let λ(n) be the n-th normalized Fourier coefficient of a holomorphic Hecke eigenform f (z) ∈ S k (Γ) for the full modular group. In one of his papers, Sankaranarayanan mentioned that it is an open problem to give a non-trivial estimate for the sum of λ(n) over cubes, i.e. n x λ(n 3 ). In this paper, we are able to use the analytic properties of symmetric power L-functions to solve his problem. More precisely, we prove that n x λ(n 3 ) x 3 4 +ε , n x λ(n 4 ) x 7 9 +ε .