Monday, 5 May 2014

sequences and series - Information on the sum $sum_{n=1}^infty frac{log n}{n!}$



In my personal study of interesting sums, I came up with the following sum that I could not evaluate:



$$\sum_{n=1}^\infty \frac{\log n}{n!} = 0.60378\dots$$




I would be very interested to see what can be done to this sum. Does a closed form of this fascinating sum exist?


Answer



Using Dobinski's formula for Bell numbers, we have
$$B(n)=\frac{1}{e}\sum_{k=0}^{\infty}\frac{k^n}{k!}$$
Hence,
$$\frac{d}{dn}B(n)=\frac{1}{e}\sum_{k=2}^{\infty}\frac{k^n\log k}{k!}$$
whence,
$$\sum_{k=1}^{\infty}\frac{\log k}{k!}=B'_0 e$$
Note that the first term ($k=1$) is $0$.


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