Divisor sums representable as the sum of two squares

Abstract

Let $s(n)$ denote the sum of the proper divisors of the natural number $n$. We show that the number of $n\le x$ such that $s(n)$ is a sum of two squares has order of magnitude $x/\sqrt{\log x}$, which agrees with the count of $n\le x$ which are a sum of two squares. Our result confirms a special case of a conjecture of Erd{\H o}s, Granville, Pomerance and Spiro, who in a 1990 paper asserted that if $\mathcal{A}\subset \mathbb{N}$ has asymptotic density zero (e.g. if $\mathcal{A}$ is the set of $n\le x$ which are a sum of two squares), then $s^{-1}(\mathcal{A})$ also has asymptotic density zero.

14 pages