- #1
kq6up
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Homework Statement
Find the residue of ##\frac{1-cos2z}{z^3}## at ##z=0##
Homework Equations
##Res=\frac{1}{n!}\frac{d^n}{dz^n}[f(z)(z-z_0)^{n+1}]## Where the order of the pole is ##n+1##
The Attempt at a Solution
Differentiating ##(1-cos2z)z^3## twice, leaves me with zeros against every term giving me ##0## for a residue which is incorrect. Mathematica gives me 2 for a residue. Since ##z_0=0## I am only left multiplying the top function by z. I have no idea what I am missing here.
What am I doing incorrectly?
Thanks
Chris