Perfect Number : If the sum of all the factors of a number is equal to twice the number, then the number is called a perfect number.

Examples :

Perfect Number : The sum of all factors of those numbers which is two times the given number are called perfect numbers.

Example - 6 and 28 are perfect numbers because 1+ 2 + 3 + 6 = 2×6 and 1 + 2 + 4 + 7 + 14 + 28 = 2× 28.