\[\frac{1}{\pi}\int^{\pi}_{-\pi}{\left|f(x)\right|}^{2}\,\mathrm{d}x=\tfrac{1}{2%
}{\left|a_{0}\right|}^{2}+\sum^{\infty}_{n=1}({\left|a_{n}\right|}^{2}+{\left|%
b_{n}\right|}^{2}),\]