Given a positive integer $n$ the sum of the integers $0 < d \le n$ such that $d|n$ is the value of the sum of divisors function for $n$ often symbolized by a Greek lowercase $\sigma$ Thus, $$\sigma(n) = \sum_{d|n} d.$$ Sometimes this function is referred to as $\sigma_1(n)$ highlighting its relation to the divisor function.

Given coprime integers $m$ and $n$ (that is, $\gcd(m, n) = 1$ then $\sigma(mn) = \sigma(m)\sigma(n)$ meaning that the sum of divisors function is a multiplicative function.