Fourier Series/Absolute Value of x over Minus Pi to Pi/Proof 2

Proof
By Fourier Series for Absolute Value Function over Symmetric Range, the function $f: \openint {-\lambda} \lambda \to \R$ defined as:


 * $\forall x \in \openint {-\lambda} \lambda: \map f x = \size x$

has a Fourier series:


 * $\map f x \sim \dfrac \lambda 2 - \displaystyle \dfrac {4 \lambda} {\pi^2} \sum_{n \mathop = 0}^\infty \frac 1 {\paren {2 n + 1}^2} \cos \dfrac {\paren {2 n + 1} \pi x} \lambda$

Substituting for $\lambda = \pi$ gives:


 * $\size x = \dfrac \pi 2 - \displaystyle \dfrac 4 \pi \sum_{n \mathop = 1}^\infty \frac {\map \cos {2 n - 1} x} {\paren {2 n - 1}^2}$

as required.