# At what rate do these common definitions converge to the constant $e=2.718…$?

$$e=2.718\ldots$$ is often defined to be $$\lim_{n\to\infty} (1+1/n)^n$$ (from continuously compounded interest) and $$\lim_{n\to\infty}\sum_{k=0}^n \frac{1}{k!}$$ (from Taylor series). My question is, what are the respective rates of convergence for each of these? For example, is the error from $$e$$ in $$O(\frac{1}{n})$$? Or is the number of digits of accuracy in $$O(n)$$? If it's not too involved, a basic overview of a proof would also be appreciated.

And just for bonus points, since I'm curious, what is the state-of-the-art algorithm for calculating digits of $$e$$?

• This is a cool question actually. I thought about this before where x is some irrational number. Of course you can “cheat” with the convergence algorithm, by making a single iteration of a new algorithm simply be two iterations if the previous algorithm. I remember doing some research online and finding some amazingly fast algorithms for computing pi. Like clash fast. – Spencer Kraisler Oct 10 '19 at 5:50

In the first case, \begin{align*} \left(1 + \frac{1}{n}\right)^n &= \exp \left[n \log \left(1 + \frac{1}{n}\right)\right] = \exp \left[1 - \frac{1}{2n} + O\left(n^{-2}\right)\right] = e - \frac{e}{2n} + O(n^{-2}) \end{align*} as $$n\to\infty$$. In the second case, \begin{align*} \sum_{k \leq n} \frac{1}{k!} - e = \sum_{k > n} \frac{1}{k!} &= \sum_{k > 0} \frac{1}{(k + n)!} \leq \sum_{k > 0} \frac{1}{n^k\, n!} = \frac{1}{n!(n-1)} = O\left(\frac{1}{n!}\right) \end{align*}

• So as a corollary, it's safe to say that, the limit expression has an error of $O(\frac{1}{n})$, and the number of digits of accuracy that the series gives you is $O(n \log n)$ (due to Stirling's approximation), yeah? – chausies Oct 10 '19 at 6:24
• @chausies: Sure. – anomaly Oct 10 '19 at 6:44

The Laurent series expansion of $$\left(1+\frac{1}{n}\right)^n$$ about the point $$n = \infty$$ gives you an idea of the error

$$\left(1+\frac{1}{n}\right)^n = e - \frac{e}{2n} + \frac{11e}{24n^2} - \frac{7e}{16n^3} + O\left( \frac{1}{n^4} \right)$$

On the other hand

$$\sum_{r = 1}^n \frac{1}{r!} = e + O\left(\frac{1}{n!}\right)$$

Hence clearly the summation converges faster than the limit.

Example: For $$n = 10$$, the limit is $$2.59374...,$$ whereas the sum is $$2.71828...$$.