One is not Prime/Proof 2

Proof
From Sigma Function of Prime Number, the sum $\map \sigma p$ of all the positive integer divisors of a prime number $p$ is $p + 1$.

But from Sigma Function of 1, $\map \sigma 1 = 1$.

If $1$ were to be classified as prime, then $\map \sigma 1$ would be an exception to the rule that $\map \sigma p = p + 1$.