Numbers friendly are pairs of numbers where one is the sum of the divisors of the other.
For example, the dividers of 220 are 1, 2, 4, 5, 10, 11, 20, 22, 44, 55 and 110, whose sum is 284.
On the other hand, the divisors of 284 are 1, 2, 4, 71, and 142, and their sum is 220. Fermat also discovered par. 17,296 and 18,416. Descartes discovered pair 9,363,584 and 9,437,056.
