Seqfans, The second Chebyshev function can be seen to be related to the first by writing it as: Note: p_n is the n-th prime. psi(x) = sum(with p_n <= x, k*log(p_n)), where k is the unique integer such that p_n^k ≤ x but p_n^(k+1) > x. Which sequence, if any, holds the k value for each n? John W. Nicholson