Divisor Sum of Square-Free Integer/Proof 1

Proof
We have that the Sigma Function is Multiplicative.

From the definition of prime number, each of the prime factors of $n$ is coprime to all other divisors of $n$.

From Sigma of Prime Number, we have:
 * $\displaystyle \sigma \left({p_i}\right) = p_i + 1$

Hence the result.