Perfect number

An integer is called a perfect number if it is equal to the sum of its positive factors, excluding the number itself.

e.g. 6, 28, 496, ...

The factors of 6 are 1, 2 and 3.
1 + 2 + 3 = 6
6 is a perfect number.

The factors of 28 are 1, 2, 4, 7 and 14.
1 + 2 + 4 + 7 + 14 = 28
28 is a perfect number.

The factors of 496 are 1, 2, 4, 8, 16, 31, 62, 124 and 248.
1 + 2 + 4 + 8 + 16 + 31 + 62 + 124 + 248 = 496
496 is a perfect number.