A perfect number can be expressed by the sum of its positive divisors except itself. For example, the specified divisors of 6 are 1, 2, and 3; then, we have 1 + 2 + 3 = 6. Thus, 6 is a perfect number. Other perfect numbers are:
28
496
8128
33550336
8589869056
137438691328
2305843008139952128
2658455991569831744654692615953842176
191561942608236107294793378084303638130997321548169216

Interestingly, in superstring theory, the dimension of the gauge group of type I is 496, which is a perfect number.