For questions on exponential sums, such as $\sum \exp(2\pi ix_n)$.
In mathematics, an exponential sum may be a finite Fourier series (i.e. a trigonometric polynomial), or other finite sum formed using the exponential function, usually expressed by means of the function $ \exp(2\pi ix).\,$ Therefore a typical exponential sum may take the form $$ \sum \exp(2\pi ix_n), $$ summed over a finite sequence of real numbers $x_n$.
