18 What are Perfect Numbers?

A000396 – OEIS

A natural number is called perfect if it is equal to the sum of its divisors except itself.

The first perfect numbers are
6
28
496
8128
33550336
8589869056
137438691328
2305843008139952128
2658455991569831744654692615953842176
191561942608236107294793378084303638130997321548169216