Definition:Divisor Sum Function

Definition
Let $n$ be an integer such that $n \ge 2$.

The sigma function $\sigma \left({n}\right)$ is defined on $n$ as being the sum of all the positive integer divisors of $n$.

That is:
 * $\displaystyle \sigma \left({n}\right) = \sum_{d \mathop \backslash n} d$

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