A k ( n ) = ∑ 1 < h ≤ k ( h , k ) = 1 e π i f ( h , k ) − ( 2 π i n h / k ) A_{k}(n) = \sum_{1 < h \leq k(h,k) = 1} {\mathrm{e}^{\pi\mathrm{i}f(h,k)-(2\pi\mathrm{i}nh/k)}}