I have found the Fourier series of $\cot(ax)$ and i get:
$$\cot(ax) \sim \frac{ \sin(a \pi)}{a\pi}\left[ \left(\frac{1}{2a^2}\right)- \sum_1^\infty \frac{(-1)^n \cos(nx)}{n^2-a^2}\right]$$
How can I deduce the Fourier series of $\cot(x)$ where $x$ isn't multiple of $\pi$? Any help please...