Numbers that are the sum of their proper divisors (i.e. all of their divisors except for themselves). For example, 6 is a perfect number, since its proper divisors are 1, 2, and 3, and 1 + 2 + 3 = 6.
Numbers that are the sum of their proper divisors (i.e. all of their divisors except for themselves). For example, 6 is a perfect number, since its proper divisors are 1, 2, and 3, and 1 + 2 + 3 = 6.