Divisor Sum of Non-Square Semiprime

Theorem
Let $n \in \Z_{>0}$ be a semiprime with distinct prime factors $p$ and $q$.

Then:
 * $\map {\sigma_1} n = \paren {p + 1} \paren {q + 1}$

where $\map {\sigma_1} n$ denotes the divisor sum function.