TITLE: Closed form to an interesting series: $\sum_{n=1}^\infty \frac{1}{1+n^3}$
QUESTION [10 upvotes]: Intutitively, I feel that there is a closed form to 
$$\sum_{n=1}^\infty \frac{1}{1+n^3}$$
I don't know why but this sum has really proved difficult. Attempted manipulating a Mellin Transform on the integral solution:
$$\int_0^\infty \frac{\text{d}x}{1+x^3}=\frac{\pi}{3}\csc \frac{\pi}{3}$$ But to little avail.
Checking W|A gives the austere solution: $$\frac{1}{3}\sum_{\{x|x^3+1=0\}} x \space\text{digamma}(1-x) $$
Which I completely don't understand. Thank you for any help.

REPLY [9 votes]: Hint. You may use the following series representation of the digamma function
$$
\psi(z+1) + \gamma = \sum_{n=1}^{\infty}\left( \frac{1}n - \frac{1}{n+z}\right).\tag1
$$ Then your goal is to rewrite the general term of your series in a form allowing to use $(1)$. You may start with 
$$
\begin{align}
\frac{1}{1+n^3}=\frac{1}{(n+1)(n-z_0)(n-\bar{z}_0)}
\end{align}
$$ where $\displaystyle z_0=\frac{1+i\sqrt{3}}2$, then make a partial fraction decomposition giving

$$
\frac{1}{1+n^3}=a_1\left(\frac{1}n - \frac{1}{n+1}\right)+a_2\left(\frac{1}n - \frac{1}{n-z_0}\right)+a_3\left(\frac{1}n - \frac{1}{n-\bar{z}_0}\right). \tag2
$$ 

By summing $(2)$ you get a closed form of your initial series.