# Using Fourier series to calculate an infinite sum

Given the Fourier series of the $2\pi$-periodic function defined for $$-\pi\leqslant x \leqslant \pi$$ by $$f(x) = |x|$$ is $$\frac{\pi}{2} -\frac{4}{\pi} \sum_{k\geq 1, k\ odd}^{\infty} \frac{cos(kx)}{k^2}$$ use this to evaluate the sum of the series $$\sum_{k\geq 1,\, k\ odd}^{\infty} \frac{1}{k^4}$$

I don't know why I'm struggling with this, the answer is $s = \pi/96$ but I can't seem to get that. My approach is to let $x = \pi$ and this sets the given equation to $-1/k^2$, then I equate that with pi and get $\pi^2/8$. I've tried a number of things including Parsevals formula but I keep getting the wrong answer.

Thank you.

I think I've solved it with the help of the comments :)

$\frac{1}{2\pi} \int_{-\pi}^{\pi} x^2 = \frac{1}{4}(\pi)^2 + \frac{1}{2}(\frac{4}{\pi})^2 \sum_{k\geq 1, k odd}^{\infty } \frac{1}{k^4}$

As per Parsevals formula. My only remaining question is why can I just set $x = \pi$ like this, and get rid of the $cos(kx)$ term? The function is left and right differentiable at $x = pi$ but why does that help?

Anyway, solving the above equation yields $\frac{\pi^2}{6} = (\frac{4}{\pi})^2 \sum_{k\geq 1, k odd}^{\infty } \frac{1}{k^4}$.

This is equal to $\frac{\pi^4}{96}$ as required.

Thank you and I hope the solution saves somebody the trouble in the future.

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Seen this? –  Ｊ. Ｍ. Oct 11 '11 at 13:08
You are on the right track! Try Parseval once more! –  AD. Oct 11 '11 at 13:50
If you set $x=\pi$ you don't get rid of the cos term; you get $\cos(k\pi)=(-1)^k$. If you set $x=0$, on the other hand, then you get $\cos(k \cdot 0)=1$ for all $k$. –  Hans Lundmark Oct 11 '11 at 16:46

Parseval says $$\frac{1}{2\pi} \int_{-\pi}^{\pi} |x|^2 dx = \frac{1}{4}|A_0|^2 + \frac{1}{2} \sum_{n=1}^{\infty} |A_n|^2$$ (since all $B_n$ are zero). What's easy to forget is that the constant term, $\pi/2$ is not $A_0$ but $A_0/2$; hence $$\frac{\pi^2}{3} = \frac{1}{4} \left( 2 \frac{\pi}{2} \right)^2 + \dots$$ Maybe that's why you're not getting the right answer?