Sums of Proper Divisors with Missing Digits

Let \(s(n)\) denote the sum of proper divisors of an integer n. In 1992, Erdős, Granville, Pomerance, and Spiro (EGPS) conjectured that if \(\mathcal {A}\) is a set of integers with asymptotic density zero, then \(s^{-1}(\mathcal {A})\) also has asymptotic density zero. In this chapter we show that the EGPS conjecture holds when \(\mathcal {A}\) is taken to be a set of integers with missing digits. In particular, we give a sharp upper bound for the size of this preimage set. We also provide an overview of progress toward the EGPS conjecture and survey recent work on sets of integers with missing digits.