I need help finding the limit of

e^(-n) * (1 + 1/n)^(n^2)

Printable View

- Oct 28th 2006, 08:41 AMguntHelp with limit
I need help finding the limit of

e^(-n) * (1 + 1/n)^(n^2) - Oct 28th 2006, 09:47 AMgunt
n is natural, and n->+inf.

The problem is you can't separate the expression and calculate the product of both limits... - Oct 28th 2006, 01:29 PMSoroban
Hello, gunt!

I assume that

Quote:

We have: .

Then: .

- Oct 28th 2006, 02:01 PMTD!
- Oct 28th 2006, 03:55 PMtopsquark
Hmmmmm....

That's the same "error" that TD pointed out (if error it is). And it appears I lied... The expression with n = 1000 (as far as my calculator will go) appears to be decreasing but fairly flat (first derivative at n = 1000 is -2 x 10^(-7)) and only at 0.6 or so.

I can't say why TPH and Soroban's methods would be wrong, but I don't think the answer is 1.

-Dan - Oct 28th 2006, 04:17 PMAfterShock
According to Maple,

lim(as x approaches infinity) [((1+(1/x))^x)^x]/(e^x) = 1/sqrt(e)

Not 1. - Oct 28th 2006, 04:52 PMThePerfectHacker
You seem to be right, I graphed it too.

However, I am confused why my results produced the incorrect answer. It is certainly that the limit composition rule failed, for some reason. Perhaps, when I said it works for was wrong, I know that it works for numbers, I just always assumed it works for infinite limits also. Apparently it seems by assumption was wrong.

Though I can prepared to be wrong, and think I am. What you shown was not a proof :). It needs to be formal.

EDIT. I realized my mistake.

What I said about composition functions holds true. But I have not properly expressed the outer function, I assumed it was raised to the , but that does not succesfully express the outer function. - Oct 28th 2006, 05:44 PMgunt
I tried a few algebric manipulations and got stuck on each one of them.

It's giving me more trouble than I expected. It looked so innocent... - Oct 28th 2006, 06:47 PMPlato
- Oct 29th 2006, 03:32 AMTD!
- Oct 29th 2006, 05:04 AMtopsquark
- Oct 29th 2006, 05:21 AMgunt
Great! Thanks for your help.

- Oct 29th 2006, 05:26 AMThePerfectHacker
I was also thinking about using a series expansion on this one, but I think you made it as simple as possible. Good job.

It looks correct to me. Again as I said the limit composition rule is used here.

The only think I am concerned about is whether a distribution of the limit to each individual term is allowed in an infinite series. I think yes, I believe you can always do it. Or whether the series need to be absolutely convergent, but I think you are correct about this one. - Oct 29th 2006, 06:06 AMtopsquark
- Oct 29th 2006, 06:12 AMThePerfectHacker
Not to offend you but I realized that in my engineering class. The professor just manipulates that stuff and differencials in an way he wants to.

It some ways I agree with that approach. I think I am the only mathemation in my differencial equations class. I have made some remarks about the professors' solution after class, he told me he would be concered with it if he was in a roomful of mathemations but engineers do not need to know that stuff. In fact, If I want to for example, differencial equations on a serious level I would first learn how to solve them informally and incorrectly. Then I would learn the theory, so that I not have to memorize techniques to how to solve them, that stuff I would already know and only be concerned with about the theory.