Ramanujan's sum

Noun

 * 1)  A function, usually denoted cq(n), of two positive integer variables q and n, defined by the formula $$c_q(n) = \sum_{1 \le a \leq q \atop (a,q)=1} e^{2 \pi i \tfrac{a}{q} n},$$, where (a, q) = 1 means that a only takes on values coprime to q. It is used in the proof of Vinogradov's theorem.