In number theory, Ramanujan's sum, usually denoted cq, is a function of two positive integer variables q and n defined by the formula c q = ∑ 1 ≤ a ≤ q = 1 e 2 π i a q n, {\displaystyle c_{q}=\sum _{1\leq a\leq q \atop =1}e^{2\pi i{\tfrac {a}{q}}n},} where = 1 means that a only takes on values coprime to q. Wikipedia