459. Sum of Positive Integer Parts

Given a non-negative integer $n$, calculate the sum of all positive integers $k$ such that $k$ is a divisor of $n$. This is also known as the sum of divisors function $\sigma_1(n)$, excluding the case where $n=0$.

If $n=0$, the function should return 0.

Example:
Input: n = 12
Output: 28
(Divisors of 12 are 1, 2, 3, 4, 6, 12. Sum = 1 + 2 + 3 + 4 + 6 + 12 = 28.)

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