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单词 SincIsNotL1
释义

sinc is not L1


The main results used in the proof will be that fL1(A)|f|L1(A) and the dominated convergence theorem.

Proof by contradictionMathworldPlanetmathPlanetmath:

Let f(x)=|sinc(x)| and suppose it’s Lebesgue integrableMathworldPlanetmath in +.

Consider the intervals Ik=[kπ,(k+1)π] and Uk=i=0kIk=[0,(k+1)π].

and the succession of functions fn(x)=f(x)χUn(x), where χUn is the characteristic functionMathworldPlanetmathPlanetmathPlanetmath of the set Un.

Each fn is a continuous functionMathworldPlanetmathPlanetmath of compact support and will thus be integrable in +. Furthermore fn(x)f(x) (pointwise)

in each Ik, f(x)|sin(x)|(k+1)π.

So

+fn=k=0nkπ(k+1)π|sin(x)|x𝑑xk=0nkπ(k+1)π|sin(x)|(k+1)π=k=0n2(k+1)π.

Suppose f is integrable in +. Then by the dominated convergence theorem limn+fn=+f.

But limn+fnlimnk=0n2(k+1)π=+ and we get the contradictionMathworldPlanetmathPlanetmath +f+.

So f cannot be integrable in +.This implies that f cannot be integrable in and since a function is integrable in a set iff its absolute valueMathworldPlanetmathPlanetmath is

sinc(x)L1()

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更新时间:2025/5/4 20:14:33