OPEN
Let $A\subseteq \mathbb{N}$ be an infinite set. Is
\[\sum_{n\in A}\frac{1}{2^n-1}\]
irrational?
If $A=\mathbb{N}$ then this series is $\sum_{n}\frac{d(n)}{2^n}$, where $d(n)$ is the number of divisors of $n$, which Erdős
[Er48] proved is irrational.
The case when $A$ is the set of primes is [69].
