I don't know how to prove that
limn→∞nn√n!=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 k≥2n can be estimated
nkk!<nk(2n)k−2n⋅nn⋅n!=nnn!⋅12k−2n
and thus we find
$$\begin{align}\frac{n^n}{n!}
nn√n!≤e≤n√2n+3⋅nn√n!.
Because n√2n+3→1 as n→∞, we obtain limn→∞nn√n!=e
from squeezing.
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