Saturday, December 23, 2017

real analysis - How to evaluate the following limit




How can I determine the following limit$$\lim_{n \to \infty} \frac{-\ln(n)}{n^x}$$ where $x \in [2, \infty)$



WolframAlpha tells me the limit is $0$, but I am not sure how to go about calculating it manually, I suspect L'Hospital plays a role?


Answer



From L'Hospital's Rule, we have



$$\lim_{n\to \infty}\frac{-\ln n}{n^x}=\lim_{n\to \infty}\frac{-1/n}{xn^{x-1}}=-\frac1x\lim_{n\to \infty}\frac{1}{n^x}=0$$



when $x>0$



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