Erd\H{o}s inequality for primitive sets

Abstract

A set of natural numbers $A$ is called primitive if no element of $A$ divides any other. Let $\Omega(n)$ be the number of prime divisors of $n$ counted with multiplicity. Let $f_z(A) = \sum_{a \in A}\frac{z^{\Omega(a)}}{a (\log a)^z}$, where $z \in \mathbb{R}_{> 0}$. Erd\H{o}s proved in 1935 that $f_1(A) = \sum_{a \in A}\frac{1}{a \log a}$ is uniformly bounded over all choices of primitive sets $A$. We prove the same fact for $f_z(A)$, when $z \in (0, 2)$. Also we discuss the $\lim_{z \to 0} f_z(A)$. Some other results about primitive sets are generalized. In particular we study the asymptotic of $f_z(\mathbb{P}_k)$, where $\mathbb{P}_k = \{ n : \Omega(n) = k \}$. In case of $z = 1$ we find the next term in asymptotic expansion of $f_1(\mathbb{P}_k)$ compared to the recent result of Gorodetsky, Lichtman, Wong.