The ancient Greeks noticed that there are natural numbers, each of which is equal to the sum of all its divisors different from this number! The least perfect number 6. It is divisible by itself and 1, 2, 3. 1+2+3=6. 28 is divided by itself and 1, 2, 4, 7 and 14. And 1+2+4+7+14=28.
In Ancient Rome the honor at banquets was considered the 6th place. And in various academies and scientific societies, until recently, was considered to have 28 members.
Perfect as well as 496, 8128. They found another 2 thousand years ago. But the fifth perfect number 33 550 336 revealed only 500 years ago, in 1460.
Today, 20 known perfect numbers.