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单词 UniquenessOfLaurentExpansion
释义

uniqueness of Laurent expansion


The Laurent seriesMathworldPlanetmath expansion of a function f(z) in an annulus   r<|z-z0|<R  is unique.

Proof.  Suppose that f(z) has in the annulus two Laurent expansions:

f(z)=n=-an(z-z0)n=n=-bn(z-z0)n

It follows that

f(z)(z-z0)-ν-1=n=-an(z-z0)n-ν-1=n=-bn(z-z0)n-ν-1

where ν is an integer.  Let now γ be an arbitrary closed contour in the annulus, going once around z0.  Since γ 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 γ, i.e.

n=-anγ(z-z0)n-ν-1𝑑z=n=-bnγ(z-z0)n-ν-1𝑑z.(1)

But

γ(z-z0)n-ν-1𝑑z={2iπifn=ν,0  ifnν,

when integrated anticlockwise (see calculation of contour integral).  Thus (1) reads

2iπaν= 2iπbν,

i.e.  aν=bν,  for any integer ν, whence both expansions are identical.

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更新时间:2025/5/4 8:16:20