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Let $F$ be a continuous function on the real set $\mathbb R$ such that the function $x \mapsto xF(x)$ is uniformly continuous on $\mathbb R$ . Prove that $F$ is also uniformly continuous on $\mathbb R$ .

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    What did you try? Where is this failing?2012-10-27

1 Answers 1

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Some hints:

  • Prove that there exists $A,B>0$ such that for all real number $x$, $|xF(x)|\leq A|x|+B$. In particular, $F$ is bounded, say by $M$.

  • We write for $x\geq 0$, $|F(x)-F(y)|\leq \frac 1x|xF(x)-yF(y)|+\frac 1x|F(y)|\cdot |x-y|.$ So if $|x|\geq 1$, we have $|F(x)-F(y)|\leq |xF(x)-yF(y)|+M\cdot |x-y|.$

  • Conclude, using uniform continuity of $F$ on $[-2,2]$.

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    @CityOfGod I'm pretty sure it has been asked recently in the site. But I don't find the link.2013-02-17