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- #1

For example, given

$$

f = \frac{4}{\pi}\sum_{n=1}^{\infty}\frac{\sin(2n-1)\theta}{2n-1} = \begin{cases} 1, & 0 < \theta < \pi\\

0, & \theta = 0, \pm\pi\\

-1, & -\pi < \theta < 0

\end{cases}

$$

Can this be differentiating term wise? If so, what conditions must be met?