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单词 CauchyPrincipalPartIntegral
释义

Cauchy principal part integral


Definition [1, 2, 3]Let C0() be the set of smooth functions with compact support on .Then the Cauchy principal part integral (or, more in line with the notation, the Cauchy principal value) p.v.(1x)is mapping p.v.(1x):C0() defined as

p.v.(1x)(u)=limε0+|x|>εu(x)x𝑑x

for uC0().

Theorem The mapping p.v.(1x)is a distribution of first order (http://planetmath.org/Distribution4).That is, p.v.(1x)𝒟1().

(proof. (http://planetmath.org/Operatornamepvfrac1xIsADistributionOfFirstOrder))

0.0.1 Properties

  1. 1.

    The distribution p.v.(1x) is obtained as the limit ([3], pp. 250)

    χn|x|xp.v.(1x).

    as n.Here, χ is the characteristic functionMathworldPlanetmathPlanetmathPlanetmathPlanetmath,the locally integrable functions on the left hand sideshould be interpreted as distributions(see this page (http://planetmath.org/EveryLocallyIntegrableFunctionIsADistribution)),and the limit should be taken in 𝒟(). It should also be noted that p.v.(1x) can be represented by a proper integral as

    p.v.(1x)(u)=0u(x)-u(-x)x,

    where we have used the fact that the integrand is continuousMathworldPlanetmathPlanetmath because of the differentiability at 0. In fact, this viewpoint can be used to somewhat vastly increase the set of functions for which this principal value is well-defined, such as functions that are integrable, satisfy a Lipschitz conditionMathworldPlanetmath at 0, and whose behavior for large x makes the integral converge at infinityMathworldPlanetmathPlanetmath.

  2. 2.

    Let ln|t| be the distribution induced by thelocally integrable function ln|t|:. Then,for the distributional derivativePlanetmathPlanetmath (http://planetmath.org/OperationsOnDistributions) D,we have ([2], pp. 149)

    D(ln|t|)=p.v.(1x).

References

  • 1 M. Reed, B. Simon,Methods of Modern Mathematical Physics: Functional AnalysisMathworldPlanetmath I,Revised and enlarged edition, Academic Press, 1980.
  • 2 S. Igari, Real analysis - With an introduction to Wavelet Theory, American Mathematical Society, 1998.
  • 3 J. Rauch,Partial Differential EquationsMathworldPlanetmath, Springer-Verlag, 1991.
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