Let f(x)=∑n=1∞sin(nx)n4f(x) = \sum_{n=1}^{\infty} \frac{\sin(nx)}{n^4}f(x)=∑n=1∞n4sin(nx). Which of the following is true regarding its differentiability?
It is nowhere differentiable.
f′(x)f'(x)f′(x) is continuous everywhere.
f′(x)f'(x)f′(x) is undefined at x=πx=\pix=π.
It is continuous but not differentiable at x=0x=0x=0.