Regular integers modulo n

Abstract

Let $n=p_1^{ν_1}... p_r^{ν_r} >1$ be an integer. An integer $a$ is called regular (mod $n$) if there is an integer $x$ such that $a^2x\equiv a$ (mod $n$). Let $\varrho(n)$ denote the number of regular integers $a$ (mod $n$) such that $1\le a\le n$. Here $\varrho(n)=(ϕ(p_1^{ν_1})+1)... (ϕ(p_r^{ν_r})+1)$, where $ϕ(n)$ is the Euler function. In this paper we first summarize some basic properties of regular integers (mod $n$). Then in order to compare the rates of growth of the functions $\varrho(n)$ and $ϕ(n)$ we investigate the average orders and the extremal orders of the functions $\varrho(n)/ϕ(n)$, $ϕ(n)/\varrho(n)$ and $1/\varrho(n)$.