A graph related to the sum of element orders of a finite group

A finite group is called $\psi$-divisible iff $\psi (H)\mathrm{\mid }\psi (G)$ for any subgroup $H$ of a finite group $G$. Here, $\psi (G)$ is the sum of element orders of $G$. For now, the only known examples of such groups are the cyclic ones of square-free order. The existence of non-abelian $\psi$-divisible groups still constitutes an open question. The aim of this paper is to make a connection between the $\psi$-divisibility property and graph theory. Hence, for a finite group $G$, we introduce a simple undirected graph called the $\psi$-divisibility graph of $G$. We denote it by ${\psi }_G$. Its vertices are the non-trivial subgroups of $G$, while two distinct vertices $H$ and $K$ are adjacent iff $H\subset K$ and $\psi (H)\mathrm{\mid }\psi (K)$ or $K\subset H$ and $\psi (K)\mathrm{\mid }\psi (H)$. We prove that $G$ is $\psi$-divisible iff ${\psi }_G$ has a universal (dominating) vertex. Also, we study various properties of ${\psi }_G$, when $G$ is a finite cyclic group. The choice of restricting our study to this specific class of groups is motivated in the paper.