Definition:Divisor Sum Function

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

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