Sum of Digits - Recursive Formula

The recursive formula is: $\text{sum}(n) = (n \bmod 10) + \text{sum}(\lfloor n / 10 \rfloor)$ This says: the sum of digits equals the last digit plus the sum of the remaining digits.

For example, $\text{sum}(1234) = 4 + \text{sum}(123)$, and $\text{sum}(123) = 3 + \text{sum}(12)$, continuing until you reach a single digit. Each recursive call shrinks $n$ by removing its last digit, moving you toward the base case.