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单词 AnIntegrableFunctionWhichDoesNotTendToZero
释义

an integrable function which does not tend to zero


In this entry, we give an example of a function f such that f is LebesgueintegrableMathworldPlanetmath on but f(x) does not tend to zero asx.

Set

f(x)=k=1kk6(x-k)2+1.

Note that every term in this series is positive, hence we may integrateterm-by-term, then make a change of variable y=k3x-k4 andcompute the answer:

-+f(x)𝑑x=k=1-+kdxk6(x-k)2+1
=k=11k2-+dyy2+1
=π26π=π36

However, when k is an integer, f(k)>k, so not only does f(x) nottend to zero as x, it gets arbitrarily large.As we can see from the plot, we have a sequence of peaks which, as they gettaller, also get narrower in such a way that the total area underthe curve stays finite:

\\includegraphics

integrable-no-tend-zero.gif

By a variation of our procedure, we can produce a function which isdefined almost everywhere on the interval (0,1), is Lebesgue integrable,but is unboundedPlanetmathPlanetmath on any subinterval, no matter how small. For instance,define

f(x)=m=1n=11(m+n)6(x-m/(m+n))2+1.

Making a computation similar to the one above, we find that

-+f(x)𝑑x=m=1n=1-+dx(m+n)6(x-m/(m+n))2+1
=m=1n=11(m+n)3-+dyy2+1
=πk=1k-1k3

Hence the integral is finite.

Now, however, we find that f cannot be bounded in any interval,however small. For, in any interval, we can find rationalnumbers. Given a rational number r, there are an infiniteMathworldPlanetmathPlanetmathnumber of ways to express it as a fraction m/(m+n). Foreach of these ways, we have a term in the series which equals 1when x=r, hence f(r) diverges to infinityMathworldPlanetmath.

To help in understanding this function, we have made a slide showwhich shows partial sums of the series. As before, the successivepeaks become narrower in such a way that the arae under the curvestays finite but, this time, instead of marching off to infinity,they become dense in the interval.

\\includegraphics

integrable-everywhere-unbounded_3.gif

随便看

 

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更新时间:2025/5/4 15:25:36