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user58955
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growth of derivative

Suppose that $f:[a,\infty)\to \mathbb{R}$ is smooth at infinity. I would like to have some result like if $f$ does not grow too fast, then something like the following holds

(1) $\liminf_{x\to\infty} |f'(x)|/|x f(x)| = 0$.

(2) $\liminf_{x\to\infty} |f'(x)|/|f(x)| = 0$.

I managed to show that (1) holds when $|f(x)|\lesssim e^{x^{1-\delta}}$ for some $\delta > 0$. I think it should hold for $|f(x)|\lesssim e^x$ but I do not have a proof. It actually holds even for $f(x)$ growing faster than $e^x$, say $f(x) = e^{x^{1.1}}$. Does anyone see how to obtain a bigger range of $f$ than $|f(x)|\lesssim e^{x^{1-\delta}}$?

For (2), I can only show that if there exists $\alpha,\beta$ with $\alpha\leq \beta<\alpha+1$ such that $x^\alpha \leq |f(x)| \leq x^\beta$ then (2) holds. Can this be relaxed? I don't feel that a lower bound for $f$ essential?

user58955
  • 640
  • 5
  • 17