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单词 ExampleOfDiracSequence
释义

example of Dirac sequence


We can construct a Dirac sequence {δn}n+ by choosing

δn(x)=nπ(1+n2x2).

To show that conditions 1 and 3 in the definition of a Dirac sequence are satisfied is trivial and condition 2 is also fulfilled since

-δn(x)dx=1π-n1+n2x2dx=[y=nxdy=ndx]=1π-11+y2dy=1πarctany|y=-=1ππ=1

for all n+, hence {δn}n+ is a Dirac sequence.

To prove that it actually converges in 𝒟() (the space of all distributionsDlmfPlanetmath on 𝒟()) to the Dirac delta distribution δ, we must show that

limnδn(x)φ(x)𝑑x=φ(0)

for any test function φ𝒟() (a topological vector spaceMathworldPlanetmath of smooth functions with compact support). Let us take an arbitrary test function φ𝒟() and assume that the closed and compact set supp(φ) is contained in some open intervalDlmfPlanetmath (a,b) (a<0 and b>0). Using the triangle inequalityMathworldMathworldPlanetmath and the fact that δn(x)𝑑x=1 for all n+ we can write

|-δn(x)φ(x)𝑑x-φ(0)|=|-δn(x)(φ(x)-φ(0))𝑑x|
φ(0)-a|δn(x)|𝑑xI1+ab|δn(x)(φ(x)-φ(0))|𝑑xI2+φ(0)b|δn(x)|𝑑xI3

It is easy to see that limnδn(x)=0, x(-,a][b,) and thereforelimnI1=0and limnI3=0.Finally we want to estimate I2 when n.

I2=ab|δn(x)||(φ(x)-φ(0))||x|sup|φ(x)|𝑑xsup|φ(x)|ab|δn(x)x|𝑑x=
=sup|φ(x)|1πab|nx1+(nx)2|𝑑x=sup|φ(x)|1π(-a0nx1+(nx)2𝑑x+0bnx1+(nx)2𝑑x)=
=sup|φ(x)|1π(-(12nln|1+(nx)2||x=a0)+(12nln|1+(nx)2||x=0b))=
=sup|φ(x)|1π(12nln|1+(na)2|+12nln|1+(nb)2|)

We now conclude that limnI2=0. This means that limnI1+I2+I3=0 which shows that {δn}n+ converges to the Dirac delta distribution δ.

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更新时间:2025/5/3 19:26:01