Monday, 11 January 2016

sequences and series - limlimitsntoinftyfracnsqrt[n]n!=e




I don't know how to prove that

lim
Are there other different (nontrivial) nice limit that gives e apart from this and the following
\sum_{k = 0}^\infty \frac{1}{k!} = \lim_{n\to\infty}\left(1+\frac{1}{n}\right)^n = e\;?


Answer



In the series for e^n=\sum_{k=0}^\infty \frac{n^k}{k!},
the nth and biggest(!) of the (throughout positve) summands is \frac{n^n}{n!}.
On the other hand, all summands can be esimated as
\frac{n^k}{k!}\le \frac{n^n}{n!}
and especially those
with k\ge 2n can be estimated

\frac{n^k}{k!}<\frac{n^{k}}{(2n)^{k-2n}\cdot n^{n}\cdot n!}=\frac{n^{n}}{n!}\cdot \frac1{2^{k-2n}}
and thus we find
$$\begin{align}\frac{n^n}{n!}Taking nth roots we find
\frac n{\sqrt[n]{n!}}\le e\le \sqrt[n]{2n+3}\cdot\frac n{\sqrt[n]{n!}}.
Because \sqrt[n]{2n+3}\to 1 as n\to \infty, we obtain \lim_{n\to\infty}\frac n{\sqrt[n]{n!}}=e
from squeezing.


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