Saturday, July 20, 2019

Limit of a sequence involving root of a factorial: $lim_{n to infty} frac{n}{ sqrt [n]{n!}}$




I need to check if
$$\lim_{n \to \infty} \frac{n}{ \sqrt [n]{n!}}$$ converges or not. Additionally, I wanted to show that the sequence is monotonically increasing in n and so limit exists. Any help is appreciated. I had tried taking log and manipulating the sequence but I could not prove monotonicity this way.



Answer



Use Stirling's approximation:
$ n! \sim \sqrt{2 \pi n} \left(\frac{n}{e}\right)^n $
and you'll get
$$
\lim_{n \rightarrow \infty} \frac{n}{(n!)^{1/n}}
=\lim_{n \rightarrow \infty} \frac{n}{(\sqrt{2 \pi n} \left(\frac{n}{e}\right)^n)^{1/n}}
=\lim_{n \rightarrow \infty} \frac{n}{({2 \pi n})^{1/2n} \left(\frac{n}{e}\right)}
=\lim_{n \rightarrow \infty} \frac{e}{({2 \pi n})^{1/2n} }=e,
$$

because $\lim_{n\to \infty} ({2 \pi n})^{1/2n}= \lim_{n\to \infty} n^{1/n}=1$.


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