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单词 CoefficientsOfLaurentSeries
释义

coefficients of Laurent series


Suppose that f is analytic in the annulus  {zR1<|z-a|<R2},  where R1 may be 0 and R2 may be .  Then the coefficients of the Laurent seriesMathworldPlanetmath (http://planetmath.org/LaurentSeries)

n=-cn(z-a)n

of f can be obtained from

cn=12πiγf(t)(t-a)n+1dt(n=0,±1,±2,),(1)

where the path (http://planetmath.org/ContourIntegral) γ goes anticlockwise once around the point  z=a  within the annulus.  Especially, the residueDlmfPlanetmath of f in the point a is

c-1=12πiγf(t)𝑑t.(2)

Remark.  Usually, the Laurent series of a functionMathworldPlanetmath, i.e. the coefficients cn, are not determined by using the integral formula (1), but directly from known series .  Often it is sufficient to know the value of c-1 or the residue, which is used to compute integrals (see the Cauchy residue theorem —  cf. (2)).  There is also the usable

Rule.  In the case that the limit  limza(z-a)f(z)  exists and has a non-zero value r, the point  z=a  is a pole of the 1 for the function f and

Res(f;a)=r.

Examples

  1. 1.

    Let  f(z):=1sinz,  and  a=0.  Using the Taylor seriesMathworldPlanetmath of the complex sine we obtain

    limz0z1sinz=limz011-z23!+-= 1,

    whence  Res(1sinz; 0)=1.  Thus we can write

    γdzsinz= 2πi,

    where the must be chosen such that it encloses only the pole 0 of1sinz.

  2. 2.

    The Taylor series of the complex exponential function gives the Laurent series

    e1z 1+1z+12!z2+13!z3+

    which shows that  Res(e1z; 0)=1.

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更新时间:2025/5/4 19:09:08