Sunday, 11 January 2015

limits - Is limntoinftyfracn(n!)frac1n=e any easier than Stirling?










Stirling's approximation says that
lim

Since \lim_{n \to \infty} x^\frac{1}{n} \to 1 uniformly on a neighbourhood of \frac{1}{\sqrt{ 2 \pi}}, it follows that
\lim_{n \to \infty} \left( \frac{n^n \sqrt{n}}{n! e^n } \right)^\frac{1}{n} = \lim_{n \to \infty} \frac{n}{(n!)^\frac{1}{n}} \cdot \frac{n^\frac{1}{2n}}{e} = 1.
Since \lim_{n \to \infty} n^\frac{1}{2n} = 1, we get the limit in the title
\lim_{n \to \infty} \frac{n}{(n!)^\frac{1}{n}} = e.




Question: Is Stirling's approximation is really needed to derive the above limit? Or is there an easier way to reach the same conclusion?




Motivation: The radius of convergence R of a power series \sum_{n=0}^\infty a_nx^n is given by Hadamard's formula

\frac{1}{R} = \limsup_{n \to \infty} |a_n|^\frac{1}{n}.
If we know ahead of time that R > 0 then the coefficients are given by
a_n = \frac{f^{(n)}(0)}{n!}
where f is the function defined by the power series. Then we get
\frac{1}{R} = \limsup_{n \to \infty} |a_n|^\frac{1}{n} = \limsup_{n \to \infty} \frac{|f^{(n)}(0)|^\frac{1}{n}}{n} \cdot \frac{n}{(n!)^\frac{1}{n}} = e \cdot \limsup_{n \to \infty} \frac{|f^{(n)}(0)|^\frac{1}{n}}{n}.
So one use of the limit is to clean up the formula for the radius of convergence of a power series in terms of the derivatives of the corresponding function.


Answer



HINT: write \lim_{n \to \infty} \displaystyle\frac{n}{(n!)^\frac{1}{n}} as \lim_{n \to \infty} \left(\displaystyle\frac{n^n}{(n!)}\right)^\frac{1}{n} and then compute \lim_{n \to \infty} \frac{a_{n+1}}{a_{n}} where a_n=\displaystyle\frac{n^n}{(n!)}


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