Parseval's Theorem/Formulation 1

Theorem
Let $f$ be a real function which is square-integrable over the interval $\left[{-\pi \,.\,.\, \pi}\right]$.

Let $f$ be expressed by the Fourier series:


 * $\displaystyle f \left({x}\right) \cong \frac {a_0} 2 + \sum_{n \mathop = 1}^\infty \left({a_n \cos \left({n x}\right) + b_n \sin \left({n x}\right)}\right)$

Then:


 * $\displaystyle \frac 1 \pi \int_{-\pi}^\pi \left\lvert{f^2 \left({x}\right)}\right\rvert \, \mathrm d x = \frac { {a_0}^2} 2 + \sum_{n \mathop = 1}^\infty \left({ {a_n}^2 + {b_n}^2}\right)$