TITLE: Closed form for $\sum_{n=1}^\infty\frac{1}{2^n\left(1+\sqrt[2^n]{2}\right)}$
QUESTION [34 upvotes]: Here is another infinite sum I need you help with:
$$\sum_{n=1}^\infty\frac{1}{2^n\left(1+\sqrt[2^n]{2}\right)}.$$
I was told it could be represented in terms of elementary functions and integers.

REPLY [13 votes]: Notice that
$$
\frac1{2^n(\sqrt[2^n]{2}-1)}
-\frac1{2^n(\sqrt[2^n]{2}+1)}
=\frac1{2^{n-1}(\sqrt[2^{n-1}]{2}-1)}
$$
We can rearrange this to
$$
\left(\frac1{2^n(\sqrt[2^n]{2}-1)}-1\right)
=\frac1{2^n(\sqrt[2^n]{2}+1)}
+\left(\frac1{2^{n-1}(\sqrt[2^{n-1}]{2}-1)}-1\right)
$$
and for $n=1$,
$$
\frac1{2^{n-1}(\sqrt[2^{n-1}]{2}-1)}-1=0
$$
therefore, the partial sum is
$$
\sum_{n=1}^m\frac1{2^n(\sqrt[2^n]{2}+1)}
=\frac1{2^m(\sqrt[2^m]{2}-1)}-1
$$
Taking the limit as $m\to\infty$, we get
$$
\sum_{n=1}^\infty\frac1{2^n(\sqrt[2^n]{2}+1)}
=\frac1{\log(2)}-1
$$