Characterization of $\PSL(2,q)$ by the number of singular elements

Abstract

Given a finite group $G$, let $π(G)$ denote the set of all primes that divide the order of $G$. For a prime $r \in π(G)$, we define $r$-singular elements as those elements of $G$ whose order is divisible by $r$. Denote by $S_r(G)$ the number of $r$-singluar elements of $G$. We denote the proportion $S_r(G)/|G|$ of $r$-singular elements in $G$ by ${μ_r}(G)$. Let $μ(G) := {\{μ_r}(G) | r\in π(G)\}$ be the set of all proportions of $r$-singular elements for each prime $r$ in $π(G)$. In this paper, we prove that if a finite group $G$ has the same set $μ(G)$ as the simple group $\PSL(2,q)$, then $G$ is isomorphic to $\PSL(2,q)$.