# Evaluating the sum $\sum_{n=1}^{\infty} \frac{e - \left(1 + \frac{1}{n}\right)^n}{\sqrt{n}}$

I want to evaluate $$\sum_{n=1}^{\infty} \frac{e - \left(1 + \frac{1}{n}\right)^n}{\sqrt{n}}$$ But I'm not sure how to approach it. Mathematica suggests that it converges pretty slowly and gives something like 2.57... after around 20,000 terms, but then starts to choke.

To see that it converges, I think I can write the following: $$\left(1 + \frac{1}{n}\right)^n = e^{n \ln\left(1 + \frac{1}{n}\right)} = e^{n\left(\frac{1}{n} - \frac{1}{2n^2} + O(n^{-3})\right)} = ee^{-\frac{1}{2n}}e^{O(n^{-2})}$$ and note that $e^{O(n^{-2})} \to 1$ as $n \to \infty$, so that $$e - \left(1 + \frac{1}{n}\right)^n \approx e - ee^{-\frac{1}{2n}} = e(1 - e^{-\frac{1}{2n}}) = e\left(\frac{1}{2n} + O(n^{-2})\right) = O(n^{-1})$$

and we have $$\frac{e - \left(1 + \frac{1}{n}\right)^n}{\sqrt{n}} = O(n^{-3/2})$$ which says this is asymptotically just a p-series.

Any ideas?

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There might be no known closed form –  Amr Feb 16 '13 at 18:29

First, excellent job on estimating the summand - few people are skilled with power-series manipulations like that.

If you expand the summand into a power series (at $n=\infty$), you'll get something of the form $$\frac{e-(1+\frac1n)^n}{\sqrt n} = \frac e2n^{-3/2} - \frac{11e}{24}n^{-5/2} + \frac{7e}{16}n^{-7/2} + \cdots = \sum_{k=0}^\infty c_k n^{-k-3/2}$$ for certain easily computed constants $c_k$. Choosing whatever truncation points $K$ and $N$ you like, you can then write \begin{align*} \sum_{n=1}^\infty \frac{e-(1+\frac1n)^n}{\sqrt n} &= \sum_{n=1}^\infty \bigg( \sum_{k=0}^K c_k n^{-k-3/2} + \bigg( \frac{e-(1+\frac1n)^n}{\sqrt n} - \sum_{k=0}^K c_k n^{-k-3/2} \bigg) \bigg) \\ &= \sum_{k=0}^K c_k \zeta(k+3/2) + \sum_{n=1}^N \bigg( \frac{e-(1+\frac1n)^n}{\sqrt n} - \sum_{k=0}^K c_k n^{-k-3/2} \bigg) + \sum_{n>N} O(n^{-K-5/2}). \end{align*} Everything other than the error term can be calculated to as many decimal places as you want (and the implicit constant in the $O$-notation could also be calculated, if you want to be completely rigorous). With this accelerated convergence, I find the value of your original sum to be $2.5912775703968337633$, probably accurate to that many decimal places.

(Then, if you want, you can try an inverse constant lookup to see if that value matches any closed form - although I doubt it will in this case.)

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I've added a bounty in the (probably vain) hope that there's a closed form, but if no one finds one, I'll just give it to you instead. –  AndrewG Feb 18 '13 at 22:06
We have $$\begin{eqnarray*} e - \left(1 + \frac{1}{n}\right)^n &=& \sum_{k=0}^\infty \frac{1}{k!} - \sum_{k=0}^n {n\choose k} \frac{1}{n^k} \\ &=& \sum_{k=0}^\infty \frac{1}{k!} \left(1-\frac{n!}{(n-k)!}\frac{1}{n^k}\right) \\ &=& \sum_{k=1}^\infty \frac{1}{k!} \left(1-\frac{n^{\underline k}}{n^k}\right), \end{eqnarray*}$$ where $n^{\underline k}$ is the falling factorial. But $$n^{\underline k} = \sum_{j=1}^k (-1)^{k-j} \left[ k\atop j\right] n^j,$$ where $\left[ k\atop j\right]$ is the unsigned Stirling number of the first kind. Thus, $$\begin{eqnarray*} e - \left(1 + \frac{1}{n}\right)^n &=& -\sum_{k=2}^\infty \frac{1}{k!} \sum_{j=1}^{k-1} (-1)^{k-j} \left[ k\atop j\right] \frac{1}{n^{k-j}}, \end{eqnarray*}$$ and so $$\begin{eqnarray*} \sum_{n=1}^{\infty} \frac{e - \left(1 + \frac{1}{n}\right)^n}{\sqrt{n}} &=& \sum_{k=2}^\infty \underbrace{\frac{1}{k!} \sum_{j=1}^{k-1} (-1)^{k-j+1} \left[ k\atop j\right] \zeta(k-j+\textstyle \frac{1}{2})}_{a_k}. \end{eqnarray*}$$
Below we give the partial sums to 50 digits. $$\begin{array}{cl} N & \sum_{k=2}^N a_k \\ \hline 10 & 2.5912768743966448667041943446771083714079106923575 \\ 20 & 2.5912775703968337622590116959919424146275651059115 \\ 30 & 2.5912775703968337632934326247597530975932248904119 \\ 40 & 2.5912775703968337632934326247597625731591505820979 \\ 50 & 2.5912775703968337632934326247597625731591505821009 \end{array}$$