021.c


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#include <math.h>

int sum_divisors(int number)
{
	int i;
	int sum = 1;
	int limit = (int) ceil(sqrt(number));
	for(i=2; i<limit; i++)
	{
		if(number % i == 0)
		{
			sum += i;
			if(number / i != i)
				sum += number / i;
		}
	}
	return sum;
}

int main()
{
	int sum = 0;
	int a;
	for(a=1; a<10000; a++)
	{
		int b = sum_divisors(a);
		if(a == sum_divisors(b) && a != b)
			sum += a;
	}

	printf("%d\n", sum);

	return 0;
}