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\section{Formula}


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

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