# Showing e through summation

1. Apr 24, 2013

### Paul_G

Hi!

I'm currently taking a fairly early stats course, and I'm having a bit of a hangup learning exactly how to use "moments" properly. My general solution whenever I run into problems internalizing things is to do a bunch of easy problems, and to show it from the ground up.

This is my first post here, so I'm not certain how to use the math notationy stuff.

My issue here is fairly simple - I want to show that e = lim n-> inf (1+1/n)^n = 1/0!+1/1!+1/2!+...

So I start with my n-> inf (1+1/n)^n and do binomial expansion.

So e = lim n-> Infinity Sum[(n choose k)*1/n^k, {k, 0, Infinity}].

From here, we can go to Sum[n!/(n-k)!k! * 1/n^k, {k,0,Infinity}]

Next... Okay, next we can change the numerator to (n-1)! and the denominator of the right to n^(k-1), but I have no idea what I can do next, considering I'm trying to get to Sum[1/k!,{k,0,Infinity}]. How do I remove the n's?

Thank you very much, and apologies for the mess.

2. Apr 24, 2013

### Staff: Mentor

Do you know the Stirling formula? You can use it to expand some factorials.

3. Apr 24, 2013

### Paul_G

Nope, wikipedia doesn't seem to start at the ground for it, either. :( I'll youtube it!

4. Apr 24, 2013

### Staff: Mentor

What is wrong with the formulas here?
$$n! \sim \sqrt{2 \pi n} \left(\frac{n}{e}\right)^n$$
The ratio of both goes to 1 in the limit n->infinity

5. Apr 24, 2013

### Tobias Funke

6. Apr 24, 2013

### Paul_G

Perfect, that is exactly the sort of thing I was looking for.

Thanks! I think I need to learn more calculus.