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Suppose that $f_n : [a,b] \to \mathbb{R}$ is a sequence of differentiable functions that is pointwise bounded. Assume in addition that $|f_n'(x)| \leq 1$ for all $n \geq 1$ and all $x \in [a,b]$. Prove that some subsequence of $(f_n)$ is uniformly convergent.

I'm not really sure where to start

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Do you know the Ascoli-Arzela Teorem? –  Julián Aguirre Feb 8 '13 at 11:40
    
A related problem. –  Mhenni Benghorbal Feb 8 '13 at 16:58
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1 Answer 1

Hint: By mean Value theorem We have $|\frac{f_n(x)-f_n(y)}{x-y}|=|f_n'(c_n)|<1$,try to show by the given data $f_n$s are uniformly bounded equicontinous, Now apply Arzela-Ascolli Theorem.

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What about the condition that $(f_n)$ is uniformly bounded? –  icaruss Feb 8 '13 at 12:38
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@carl It also follows from the MVT that $|f_n(x)| \le |f_n'(c)||x| + |f_n(0)|$ which combines with the two conditions you are given to yield uniform boundedness. –  brom Feb 8 '13 at 15:14
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