Divisor Sum of 1

Example of Sigma Function of Integer

 * $\sigma \left({1}\right) = 1$

where $\sigma$ denotes the $\sigma$ function.

Proof
By definition, the $\sigma$ function of an integer $n$ is the sum of the positive integer divisors of $n$.

There is only one positive integer which is a divisor of $1$, and that is $1$ itself.

Hence the result.