 The sum of divisors of a number, excluding the number itself, is called its aliquot sum, so a perfect number is one that is equal to its aliquot sum.Equivalently, a perfect number is a number that is half the sum of all of its positive divisors including itself; in symbols, "σ"1("n") = 2"n" where "σ"1 is the sum-of-divisors function.For instance, 28 is perfect as 1 + 2 + 4 + 7 + 14 + 28 = 56 = 2 × 28.