A Divisor-Sum Analogue of the Collatz Map

Abstract

Let $σ$ denote the sum-of-divisors function and let $\mathcal{R}$ send an odd integer $n$ to $σ(n)$ and an even integer $n$ to $n/2$. We conjecture that every orbit of $\mathcal{R}$ reaches $1$; this implies that there is no odd $2^{k}$-perfect number for any $k \ge 1$. We prove a pointwise descent estimate for the map $T$ induced by $\mathcal{R}$ on the odd integers, and show that a single application of $T$ divides almost every odd $n$ by $(\log n)^{2\log 2-\varepsilon}$, for every fixed $\varepsilon>0$. The estimate does not iterate, and we determine what is missing.