2. Friendly Numbers
Friendly numbers are pairs of numbers where the sum of the proper divisors (positive divisors excluding the number itself) of each number equals the other number. For example, 220 and 284 are friendly numbers because the sum of the proper divisors of 220 (1, 2, 4, 5, 10, 11, 20, 22, 44, 55, 110) is 284, and vice versa.
