Definition:Divisor Sum Function

Definition
Let $n$ be an integer such that $n \ge 1$.

The sigma function $\map \sigma n$ is defined on $n$ as being the sum of all the positive integer divisors of $n$.

That is:
 * $\ds \map \sigma n = \sum_{d \mathop \divides n} d$

where $\ds \sum_{d \mathop \divides n}$ is the sum over all divisors of $n$.

Also see

 * Definition:Divisor Function