uniqueness of Laurent expansion
The Laurent series expansion of a function in an annulus is unique.
Proof. Suppose that has in the annulus two Laurent expansions:
It follows that
where is an integer. Let now be an arbitrary closed contour in the annulus, going once around . Since is a compact set of points, those two Laurent series converge uniformly (http://planetmath.org/UniformConvergence) on it and therefore they can be integrated termwise (http://planetmath.org/SumFunctionOfSeries) along , i.e.
(1) |
But
when integrated anticlockwise (see calculation of contour integral). Thus (1) reads
i.e. , for any integer , whence both expansions are identical.