Zachary Stence

Let d(n) be defined as the sum of proper divisors of n (numbers less than n which divide evenly into n).
If d(a) = b and d(b) = a, where a ≠ b, then a and b are an amicable pair and each of a and b are called amicable numbers.

For example, the proper divisors of 220 are 1, 2, 4, 5, 10, 11, 20, 22, 44, 55 and 110; therefore d(220) = 284. The proper divisors of 284 are 1, 2, 4, 71 and 142; so d(284) = 220.

Evaluate the sum of all the amicable numbers under 10000.

#include <iostream>
#include <cmath>
using namespace std;

int d(int num) {
  int sum = 0;
  for (int i = 1; i <= num/2; i++) {
    if (num % i == 0)
      sum += i;
  }
  return sum;
}

int main () {
  int sum = 0;
  for (int i = 1; i < 10000; i++) {
    if (i == d(d(i)) && i != d(i))
      sum += i;
  }
  cout << sum << endl;
}

Answer: 31626