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)}}