OPEN
Let $a/b\in \mathbb{Q}_{>0}$ with $b$ squarefree. Are there integers $1<n_1<\cdots<n_k$, each the product of two distinct primes, such that
\[\frac{a}{b}=\frac{1}{n_1}+\cdots+\frac{1}{n_k}?\]
For $n_i$ the product of three distinct primes, this is true when $b=1$, as proved by Butler, Erdős and Graham
[BEG15] (this paper is perhaps Erdős' last paper, appearing 19 years after his death).
