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单词 AbsoluteConvergenceOfIntegralAndBoundednessOfDerivative
释义

absolute convergence of integral and boundedness of derivative


Theorem.  Assume that we have an http://planetmath.org/node/11865absolutely converging integralDlmfPlanetmath

af(x)𝑑x

where the real function f and its derivative f are continuousMathworldPlanetmath and f additionally bounded on the interval [a,).  Then

limxf(x)= 0.(1)

Proof.  If  c>a,  we obtain

acf(x)f(x)dx=12/ac(f(x))2=(f(c))2-(f(a))22,

from which

(f(c))2=(f(a))2+2acf(x)f(x)𝑑x.(2)

Using the boundedness of f and the absolute convergenceMathworldPlanetmath, we can estimate upwards the integral

ac|f(x)f(x)|𝑑x=ac|f(x)||f(x)|𝑑xMac|f(x)|𝑑xMa|f(x)|𝑑xc[a,)

whence a|f(x)f(x)|𝑑x is finite and thus af(x)f(x)𝑑x converges absolutely.  Hence (2) implies

limc(f(c))2=(f(a))2+2af(x)f(x)𝑑x,

i.e. limx(f(x))2 exists as finite, therefore also

limx|f(x)|:=A.

Antithesis:  A>0.  It implies that there is an x0(a) such that

|f(x)|A2xx0.

If now  b>x0, then we had

x0b|f(x)|𝑑xA2(b-x0)asb.

This means that x0|f(x)|𝑑x and consequently also a|f(x)|𝑑x would be divergent.  Since it is not true, we infer that  A=0,  i.e. that the assertion (1) is true.

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