Integers which are Divisor Sum for 3 Integers

Theorem
The sequence of integers which are the divisor sum of $3$ different integers begins:


 * $24, 42, 48, 60, 84, 90, \ldots$

Proof
For a given $n$, to determine every $m$ such that $\map {\sigma_1} m = n$ can be determined by evaluating the divisor sum of all integers up to $n - 1$.

It is hence noted: