Thursday, 5 September 2013

Limit $lim_{xtoinfty} frac {(ln(x))^k}x$




The limit is $$\lim_{x\to\infty} \frac {(\ln(x))^k}x$$ $\forall k \in N, k \ge 1$



For $k = 1$ or $2$, the limit is fairly easy to solve using the Stolz–Cesàro theorem. Intuitively, because $ln(x)$ grows so much slower than $x$, the answer should be $0$. But how can we mathematically solve the limit for any positive integer k?


Answer



$$
\lim_{x\to\infty}\frac{\ln^kx}{x}= \left(\lim_{x\to\infty}\frac{k\ln x^{\frac1{k}}}{x^{\frac1{k}}} \right)^k = k^k \left( \lim_{y\to\infty} \frac{\ln y}{y} \right)^k= 0
$$


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