A generalization of primitive sets and a conjecture of Erdős

Abstract: A set of integers greater than 1 is primitive if no element divides another. Erd\H{o}s proved in 1935 that the sum of $1/(n \log n)$ for $n$ running over a primitive set $A$ is universally bounded over all choices for $A$. In 1988 he asked if this universal bound is attained by the set of prime numbers. We answer the Erd\H{o}s question in the affirmative for 2-primitive sets. Here a set is 2-primitive if no element divides the product of 2 other elements.