Behaviour of Hankel function $H_s^{(1)}(x)$ near $x=0$

Question

I am looking for a reference to the fact $H_s^{(1)}(x) \approx i (\frac{2}{x})^s \frac{\Gamma(s)}{\pi}$ for small $x$,and $s\in \mathbb{C}$. I think it is obtained from some integral representation of $H_s^{(1)}(x)$.

Edit: By reference I mean a book that also explains this estimation.

Thank you.

Answer 1

For $\Re s > 0$ you have from http://dlmf.nist.gov/10.7.E7 $$H_s^{(1)}(x) \sim - H_s^{(2)}(x) \sim - \frac{i}{\pi}\Gamma(s)\left(\frac{1}{2}x\right)^{-s}$$ The Abramowitz/Stegun equivalent is 9.1.9 (there $Y_s$ is considered, you have to divide by $\pm i$)