A set $A\subset \mathbb{N}$ is primitive if no member of $A$ divides another. Is the sum
\[\sum_{n\in A}\frac{1}{n\log n}\]
maximised over all primitive sets when $A$ is the set of primes?

Erdős

[Er35] proved that this sum always converges for a primitive set. Solved by Lichtman

[Li23].