Friday, 3 January 2014

calculus - How to find the limit $limlimits_{ntoinfty}1/sqrt[n]{n}$ which is indeterminate on evaluation but is convergent?



When I evaluate the limit in the title above I get the following:



\begin{align}
\lim\limits_{n\to\infty}\dfrac{1}{\sqrt[n]{n}} &= \lim\limits_{n\to\infty} \dfrac{1}{n^{\frac{1}{n}}} = \dfrac{1}{\infty^0} \quad\Rightarrow\quad Indeterminate\\
&= \lim\limits_{n\to\infty}\left(\dfrac{1}{n}\right)^\frac{1}{n} = 0^0 \quad\Rightarrow\quad Indeterminate
\end{align}



But when I use a computer software (mathematica) to evaluate the same limit it says the limit is 1. What am I doing wrong?



Answer



Indeterminate forms can have values.



Note from L'Hospital's Rule that $\lim_{n\to \infty}\frac{\log(n)}{n}=\lim_{n\to \infty}\frac{1/n}{1}=0$. Hence, we have



$$\begin{align}
\lim_{n\to \infty}\frac{1}{n^{1/n}}&=\lim_{n\to \infty}e^{-\frac1n \log(n)}\\\\
&e^{-\lim_{n\to \infty}\left(\frac1n \log(n)\right)}\\\\
&=e^0\\\\
&=1

\end{align}$$



as expected!


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