I don't know whether it's suitable to post this problem here, but I really need a help.
$f$ is a coutinuous function on $\mathbb{R}^+$, if the limit $\lim_{n\to\infty}f(nx)$ exists for all points $x$ of a nonempty closed set with no isolated point of $\mathbb{R}^+$, prove that the limit $\lim_{x\to\infty}f(x)$ exist.
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