\newpage \section{Formula} From $(a+b)^2 = (a^2 + 2ab + b^2)$ the assertion follows. Function $f$ can be written as $$f(x) = \sum ^\infty_{k = - \infty} c_{k} \cdot e^{ikx} ~~~~{\rm with}~~~~ c_{k} = \frac {1} {\pi} \int f(x) \cdot e^{-ikx} dx$$