Tuesday, 19 November 2013

calculus - Show that limntoinftyfracln(n!)n=+infty





Show that
lim




The only way i've been able to show that is using Stirling's approximation:
n! \sim\sqrt{2\pi n}\left(\frac{n}{e}\right)^n




Let:
\begin{cases} x_n = \frac{\ln(n!)}{n}\\ n \in \Bbb N \end{cases}



So we may rewrite x_n as:

x_n \sim \frac{\ln(2\pi n)}{2n} + \frac{n\ln(\frac{n}{e})}{n}



Now using the fact that \lim(x_n + y_n) = \lim x_n + \lim y_n :
\lim_{n\to\infty}x_n = \lim_{n\to\infty}\frac{\ln(2\pi n)}{2n} + \lim_{n\to\infty}\frac{n\ln(\frac{n}{e})}{n} = 0 + \infty



I'm looking for another way to show this, since Stirling's approximation has not been introduced at the point where i took the exercise from yet.



Answer



Another way to show \lim_{n\to\infty}\frac{\ln(n!)}{n}=\infty is to consider the following property of logarithms: \log(n!)=\log(n)+\log(n-1)+\cdots+\log(2)>\frac{n}{2}\log\left(\frac n2\right). Now \frac{\log (n!)}{n}>\frac{\log(n/2)}{2}. As n\to\infty, this clearly diverges to +\infty.


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