Homework Help: How to prove the convergence

1. Jan 17, 2005

twoflower

Hi all,

suppose the following sum:

$$\sum_{n = 1}^{\infty} \frac{n!}{n^{n}}$$

I had a feeling it should converge, but I can't find a way how to prove that. I tried d'Alembert's criterion:

$$\lim_{n \rightarrow \infty} \frac{a_{n+1}}{a_n} < 1 \Rightarrow \sum a_{n} converges$$

But the limit is 1 so it doesn't give anything. In fact, if I were able to prove that

$$\lim_{n \rightarrow \infty} \sqrt[n]{n!} = 1$$

I would have it. But I don't know how to make a prove of that...Which criterion or rule should I use here?

Thank you.

2. Jan 17, 2005

twoflower

Whoops, the limit isn't 1, it is infinity....

3. Jan 17, 2005

Galileo

Check d'Alembert again.
$\lim_{n \rightarrow \infty} |\frac{a_{n+1}}{a_n}|$ is smaller than 1.

Hint: Make use of: $\left(1+\frac{1}{n}\right)^n \to e$ as $n \to \infty$

4. Jan 18, 2005

twoflower

Thank you Galileo, now I can see it goes to 1/e.

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