Saturday, June 17, 2017

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




I don't know how to prove that
limnnnn!=e.


Are there other different (nontrivial) nice limit that gives e apart from this and the following
k=01k!=limn(1+1n)n=e?


Answer



In the series for en=k=0nkk!,


the nth and biggest(!) of the (throughout positve) summands is nnn!.

On the other hand, all summands can be esimated as
nkk!nnn!

and especially those
with k2n can be estimated
nkk!<nk(2n)k2nnnn!=nnn!12k2n

and thus we find
$$\begin{align}\frac{n^n}{n!}Taking nth roots we find
nnn!en2n+3nnn!.

Because n2n+31 as n, we obtain limnnnn!=e


from squeezing.


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