What is a perfect number?
The thing is that there are plenty of perfect numbers. Perfect numbers are positive integers. It's unknown if there are any odd perfect numbers; and nobody also knows if perfect numbers have end or not. The perfect numbers are known from the ancient times: they appeared nearly at the same time with Euclid's elements. However, the first "perfect one" is 6, because 1, 2, and 3 are its proper positive divisors, and 1 + 2 + 3 = 6. The next perfect number is 28 = 1 + 2 + 4 + 7 + 14.