What are perfact numbers?

If the sum of all a number's factors (factors that are smaller than the number itself) is equal to the number itself, the number is said to be "perfect".

For example, the factors of 6 (excluding 6 itself) are 1, 2, and 3; and the sum of these numbers is exactly 6.

The smallest perfect numbers are 6, 28, 496, 8128.

It isn't known whether the set of perfect numbers is finite or infinite. Also, it isn't known whether there are any odd perfect number; all known perfect numbers are even.