I got the following function: $f(x)=( \frac{x}{ \pi })^{2}$ With the fourier series: $\widetilde{f(x)} = \frac{1}{3} + \ \sum_{n=1}^{\infty} (-1)^{n}\frac{4}{ n^{2} \pi ^{2}} cos(nx)$
Now I must compute the series: $\ \sum_{n=1}^{\infty} \frac{1}{ n^{4}}$
Any ideas?