Formula

From $(a+b)^2 = (a^2 + 2ab + b^2)$ the assertion follows.

Function $f$ can be written as

$$f(x) = \sum ^\infty_{k = - \infty} c_{k} \cdot e^{ikx}     {\rm with}     c_{k} = \frac {1} {\pi} \int f(x) \cdot e^{-ikx} dx$$

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