Compute the following integrals using residues: $$\begin{align}\int_{|z|=1}z^3\sin(\frac{1}{z})dz\\\int_{|z|=\frac{2}{3}}\sin(\frac{1}{z^2}+e^{z^2}\cos(z))dz \end{align}$$
In the first integral I could not find any singular points, but removable ones. therefore to check out I was doing it right I derived the Laurent series:
$\sin(\frac{1}{z})=z^2-\frac{1}{3!}+\frac{1}{5!z^2}-….$
So there is no residue at infinity.
Regarding the second integral
$\int_{|z|=\frac{2}{3}}\sin(\frac{1}{z^2}+e^{z^2}\cos(z))dz$
I found no singularities but removable ones. However I was not able to compute the Laurent series to check that out.
So the value of both integrals would equal $0$.
Question:
Is my answer right?
Thanks in advance!
Best Answer
In the case of the first integral, you have a clear non-removable singularity at $0$. But the residue at $0$ is $0$ (it's an even function), and so the integral is equal to $0$. The same argument applies to the second integral.