Thanks for response.
1. Yes, I mean \lim_{k\rightarrow 0}(\omega+ik).
2. As pointed out by D H, it is x^l, x raised to the power of l.
3. Initially, I tried to do it by this formula, \frac{1}{\omega+i0_+-x^m} = \mathcal{P}\left(\frac{1}{\omega-x^m}\right)-i\pi \delta(\omega - x^m), with...