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单词 AnotherProofOfDinisTheorem
释义

another proof of Dini’s theorem


This is the version of the Dini’s theoremMathworldPlanetmath I will prove:Let K be a compactPlanetmathPlanetmath metric space and (fn)nNC(K) which convergesPlanetmathPlanetmath pointwise to fC(K).

Besides, fn(x)fn+1(x)xK,n.

Then (fn)nN converges uniformly in K.

Proof

Suppose that the sequence does not converge uniformly. Then, by definition,

ε>0such that mNnm>m,xmKsuch that |fnm(xm)-f(xm)|ε.

So,

For m=1n1>1,x1Ksuch that |fn1(x1)-f(x1)|εn2>n1,x2Ksuch that |fn2(x2)-f(x2)|εnm>nm-1,xmKsuch that |fnm(xm)-f(xm)|ε

Then we have a sequence (xm)mK and (fnm)m(fn)nis a subsequence of the original sequence of functions. K is compact, so there is a subsequence of (xm)mwhich converges in K, that is, (xmj)j such that

xmjxK

I will prove that f is not continuousPlanetmathPlanetmath in x (A contradictionMathworldPlanetmathPlanetmath with one of the hypothesisMathworldPlanetmathPlanetmath).

To do this, I will show that f(xmj)j does not converge to f(x), using above’s ε.

Let j0such that jj0|fnmj(x)-f(x)|<ε/4,which exists due to the punctual convergence of the sequence. Then,particularly, |fnmjo(x)-f(x)|<ε/4.

Note that

|fnmj(xmj)-f(xmj)|=fnmj(xmj)-f(xmj)

because (using the hypothesis fn(y)fn+1(y)yK,n) it’s easy to see that

fn(y)f(y)yK,n

Then, fnmj(xmj)-f(xmj)εj. And also the hypothesis implies

fnmj(y)fnmj+1(y)yK,j

So, jj0fnmj0(xmj)fnmj(xmj), which implies

|fnmj0(xmj)-f(xmj)|ε

Now,

|fnmj0(xmj)-f(x)|+|f(xmj)-f(x)||fnmj0(xmj)-f(xmj)|εjj0

and so

|f(xmj)-f(x)|ε-|fnmj0(xmj)-f(x)|jj0.

On the other hand,

|fnmj0(xmj)-f(x)||fnmj0(xmj)-fnmj0(x)|+|fnmj0(x)-f(x)|

And as fnmj0 is continuous, there is a j1such that

jj1|fnmj0(xmj)-fnmj0(x)|<ε/4

Then,

jj1|fnmj0(xmj)-f(x)||fnmj0(xmj)-fnmj0(x)|+|fnmj0(x)-f(x)|<ε/2,

which implies

|f(xmj)-f(x)|ε-|fnmj0(xmj)-f(x)|ε/2jmax(j0,j1).

Then, particularly, f(xmj)j does not converge to f(x). QED.

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更新时间:2025/5/4 23:34:51