Wednesday, 6 March 2019

real analysis - Intuitive explanation for why left(1frac1nright)ntofrac1e



I am aware that e, the base of natural logarithms, can be defined as:




e=lim



Recently, I found out that



\lim_{n\to\infty}\left(1-\frac{1}{n}\right)^n = e^{-1}



How does that work? Surely the minus sign makes no difference, as when n is large, \frac{1}{n} is very small?



I'm not asking for just any rigorous method of proving this. I've been told one: as n goes to infinity, \left(1+\frac{1}{n}\right)^n\left(1-\frac{1}{n}\right)^n = 1, so the latter limit must be the reciprocal of e. However, I still don't understand why changing such a tiny component of the limit changes the output so drastically. Does anyone have a remotely intuitive explanation of this concept?


Answer




Perhaps think about the binomial expansions of \left(1 + \frac{1}{n}\right)^n and \left(1 - \frac{1}{n}\right)^n. The first two terms are 1 + n \frac{1}{n} and 1 - n \frac{1}{n} respectively. And after that the terms in \left(1 + \frac{1}{n}\right)^n are all positive, whereas the terms in \left(1 - \frac{1}{n}\right)^n alternate. So the difference between the two limits is going to be at least 2.


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