Iām working on the question below.
Find the value of $\sum_{k=0}^\infty \frac{1}{(2k + 1)^2}$ by adopting Parseval's identity for the function
$$f(x) = \begin{cases} -1 & \text{if } -\pi < x < 0 \\ 1 & \text{if }0 < x < \pi \\ 0 & \text{if }x = 0.\end{cases}$$
I've already got the Fourier series: $$f(x) = \sum_{n=-\infty}^\infty \frac{1 -(-1)^n}{in\pi} e^{inx}.$$
So, I think equation of Parseval's identity is $$\sum_{n=-\infty}^\infty \left(\frac{1-(-1)^n}{in\pi}\right)^2 = \frac{1}{2\pi}\int_{-\pi}^\pi |f(x)|^2 \; dx.$$ Is this ok?
But, I'm not sure how to conclude. (Where does (2k+1) appear from this equation ?)