[Math] Find the singular points, the pole type and compute the residue of: $\frac{\sinh(z)}{z^4(1-z^2)}$

complex integrationcomplex numberscomplex-analysis

I am trying to find the singular points, toe pole type and compute the residue of:
$$\frac{\sinh(z)}{z^4(1-z^2)}$$
With $z_0=0$ I am ok since it is easy to find the residue by Laurent expansion (it is a principal pole. Am i right? infinite times in the denominator).

But with $z=1$and $z=-1$ I have a problem. How can I compute the Laurent expansion around 1 and -1? Also are $z=1$, $z=-1$ simple poles?

Thanks!

Best Answer

In order to determine the Laurent expansion at $1$ and $-1$ see Claude Leibovici's answer.

To evaluate the residues, you may also note that $-1$ and $+1$ are simple poles. Therefore $$\mbox{Res}\left(\frac{\sinh(z)/z^4}{1-z^2},\pm 1\right)=\left.\frac{\sinh(z)/z^4}{(1-z^2)'}\right|_{z=\pm 1} =\left.\frac{\sinh(z)/z^4}{-2z}\right|_{z=\pm 1}=-\frac{\sinh(1)}{2}.$$ See the detailed explanation here.