[Math] Number of zeros that $z^4-5z+1$ has on the annulus $\{z | 1 < |z| <2 \}$

complex-analysis

This is a trivial question, but I'm very inexperienced with Rouche's theorem.

I need the number of zeros that $z^4-5z+1$ has in the annulus $\{z | 1 < |z| <2 \}$

I applied Rouche's theorem for functions $g(z)=z^4-5z+1$ and $f(z)=-5z$, and I got that $g$ has one zero in the unit disk and one zero in the disk of radius 2 and therefore no zeros in the annulus. However WolframAlpha says otherwise. What am I doing wrong?

Best Answer

This might help: On $|z|=2,$

$$|(z^4-5z+1)-z^4| < |z^4|.$$ On $|z|=1,$

$$|(z^4-5z+1)-(-5z+1)|< |-5z+1|$$