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单词 ProofOfRuffasFormulaForContinuousFunctions
释义

proof of Ruffa’s formula for continuous functions


Define sn to be the following sum:

sn=m=12n-12-nf(a+m(b-a)/2n)

Making the substitution m=2m and using the fact that 1+(-1)m=0 when m is odd to express the sum over even values of m as a sum over all values of m, this becomes

sn=m=12n+1-1(1+(-1)m)2-1-nf(a+m(b-a)/2n)

Subtracting this sum from sn+1 and simplifying gives

sn+1-sn=m=12n+1-1(-1)m+12-nf(a+m(b-a)/2n)

Using the telescoping sum trick, we may write

sk=n=1k(sn-sn-1)=n=1km=12n-1(-1)m+12-nf(a+m(b-a)/2n)

To completePlanetmathPlanetmathPlanetmathPlanetmathPlanetmathPlanetmath the proof, we must investigate the limit as k. Since f is assumed continuousMathworldPlanetmathPlanetmath and the interval [a,b] is compactPlanetmathPlanetmath, f is uniformly continuousPlanetmathPlanetmath. This means that, for every ϵ>0, there exists a δ>0 such that |x-y|<δ implies |f(x)-f(y)|<ϵ. By the Archimedean property, there exists an integer k>0 such that 2kδ>|a-b|. Hence, |f(x)-f(a+m(b-a)/2n)|ϵ when x lies in the interval [a+(m-1)(b-a)/2n,a+(m+1)(b-a)/2n]. Thus, (a-b)sk+|a-b|ϵ is a Darboux upper sum for the integral

abf(x)𝑑x

and (b-a)sk-|a-b|ϵ is a Darboux lower sum. (Darboux’s definition of the integral may be thought of as a modern incarnation of the ancient method of exhaustion.) Hence

|abf(x)𝑑x-sk||a-b|ϵ

Taking the limit ϵ0, we see that

abf(x)𝑑x=n=1An=(b-a)n=1m=12n-1(-1)m+12-nf(a+m(b-a)/2n)
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更新时间:2025/5/5 2:05:45