Exceptional sets for compositions involving Euler's function,the divisor-sum function and Dedekind's function

Abstract

Let \(ψ(n)\), \(ϕ(n)\), and \(σ(n)\) denote Dedekind's arithmetic function, Euler's totient function, and the sum-of-divisors function, respectively. We study exceptional sets associated with the compositions \(ϕ(ψ(n))\), \(ϕ(σ(n))\), \(ψ(ψ(n))\), and \(ψ(σ(n))\). For every fixed \(c>0\), we obtain quantitative upper bounds for the numbers of integers \(n\le x\) satisfying \(ϕ(ψ(n))\ge cn\) and \(ϕ(σ(n))\ge cn\), thereby refining density results of Sándor and Dixit and Bhattacharjee, respectively. We further establish quantitative density-zero estimates for the sets of integers \(n\le x\) for which \(ψ(ψ(n))\le cn\) or \(ψ(σ(n))\le cn\). More generally, we allow the fixed thresholds in the first two problems to be replaced by thresholds involving non-decreasing functions subject to mild growth conditions.