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Homework Statement
Find the Laurent expansions of
##f(z) = \frac{z+2}{z^2-z-2}## in ##1 < |z|<2## and then in ##2 < |z|< \infty##
in powers of ##z## and ##1/z##.
Homework Equations
Theorem:
Let ##f## be a rational function all of whose poles ##z_1,\dots , z_N## in the plane have order one and which has no pole at the origin and which is zero at ##\infty##. Suppose that no pole of ##f## lies in the annular region ##r<|z|<R##. Then
##f(z) = \sum_{-\infty}^\infty a_kz^k \; \; \; \text{ for } \; \; r < |z|<R,##
where
##a_k=\begin{cases}
\sum_{|z_j|< r} z_j^{k-1} \text{Res}(f;z_j), \; \; \; k \le -1\\
-\sum_{|z_j|>R} z_j^{-k-1} \text{Res}(f;z_j), \; \; \; k \ge 0.
\end{cases}
##
The Attempt at a Solution
We have the residues
##\text{Res}(f;-1) = -\frac{1}{3}##
##\text{Res}(f;2) = \frac{4}{3}##.
In the first case we have ##r=1## and ##R=2##. The coefficients are then
##a_k = (-1)^{k-1}\frac{-1}{3} = \frac{(-1)^k}{3}## for ##k=-1,-2,\dots##
##a_k = -2^{-k-1} \frac{4}{3} = \frac{-2}{3\cdot 2^k}## for ##k=0,1,2,\dots##.
The answer however says the first part is correct but the second one should be
##a_k = \frac{-4}{3^{k+2}}, \; \; \; k=0,1,2,\dots## which makes no sense to me.
For the second region we have ##r=2## and ##R=\infty##
Then
##a_k = (-1)^{k-1}\frac{-1}{3} + 2^{k-1}\frac{4}{3}## for ##k=-1,-2,\dots##
If we instead of summing over negative ##k## we write the series as
##\sum_{1}^\infty \frac{a_k}{z_k}## we have
##a_k = \frac{(-1)^k}{3} +\frac{2}{3\cdot 2^{k}}##.
The answer here however should be
##\sum_{1}^\infty \frac{a_k}{z_k}## with ##a_k=\frac{(-1)^k}{3}+\frac{4}{3^{k+2}}, \; \; \; k=1,2,\dots##.
I'm also wondering about the ##|z_j|<r## and ##|z_j|>R## parts in the theorem, why do I count singularities at ##r## and ##R## when it says strictly less/greater.
I managed to find an old thread covering the exact same question where they seem to get an answer similar to mine: https://www.physicsforums.com/threads/laurent-series-of-rational-functions.702294/
He seem to reach a answer similar to me so perhaps the answer is wrong. However he does it with partial fractions instead of the theorem (although how I understand it the theorem is essential partial fractions). So what I'm looking for is input if I solved this correctly or If I made a mistake? I also seem to get similar errors on other exercises on the method which makes me doubt.