Saturday, February 6, 2016

Limit as $x to infty$ of $frac{log(x)^{log(log(x))}}{x}$



I want to compute $\lim\limits_{x \to \infty}\frac{\log(x)^{\log(\log(x))}}{x}$



By graphing it, clearly $x$ grows larger than $\log(x)^{\log(\log(x))}$, so the limit will go to $0$.



I tried iterating L'Hopital's rule, but after three derivations, the sequence of limits gets successively more complicated.




How can you prove that the limit is indeed $0$?


Answer



HINT:



Let $x=\exp(e^u)$. Then your limit is equal to $$\lim_{u\to\infty}\frac{(e^u)^{\log(e^u)}}{\exp(e^u)}=\lim_{u\to\infty}\frac{e^{u^2}}{e^{e^u}}=\lim_{u\to\infty}e^{u^2-e^u}=\cdots$$


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