Using only elementary arguments, Cassels solved the Diophantine equation (x − 1) 3 + x 3 + (x + 1) 3 = z 2 (with x, z ∈ Z). The generalization (x − 1) k + x k + (x + 1) k = z n (with x, z, n ∈ Z and n ≥ 2) was considered by Zhongfeng Zhang who solved it for k ∈ {2, 3, 4} using Frey-Hellegouarch curves and their corresponding Galois representations. In this paper, by employing some sophisticated refinements of this approach, we show that the only solution for k = 5 is x = z = 0, and that there are no solutions for k = 6. The chief innovation we employ is a computational one, which enables us to avoid the full computation of data about cuspidal newforms of high level.