Arithmetic properties of the sum of divisors

Tewodros Amdeberhan, Victor H. Moll, Vaishavi Sharma, Diego Villamizar

Published 2020-07-06Version 1

The divisor function $\sigma(n)$ denotes the sum of the divisors of the positive integer $n$. For a prime $p$ and $m \in \mathbb{N}$, the $p$-adic valuation of $m$ is the highest power of $p$ which divides $m$. Formulas for $\nu_{p}(\sigma(n))$ are established. For $p=2$, these involve only the odd primes dividing $n$. These expressions are used to establish the bound $\nu_{2}(\sigma(n)) \leq \lceil\log_{2}(n) \rceil$, with equality if and only if $n$ is the product of distinct Mersenne primes, and for an odd prime $p$, the bound is $\nu_{p}(\sigma(n)) \leq \lceil \log_{p}(n) \rceil$, with equality related to solutions of the Ljunggren-Nagell diophantine equation.