A perfect number is a number whose sum of factors is equal to
A perfect number is a number whose sum of factors is equal to twice the number.
For example, 6 is a perfect number since the sum of its factors i.e. 1, 2, 3 and 6 is 12 i.e. twice of 6.
A perfect number is also defined as a positive integer that is equal to the sum of its positive divisors including 1 but excluding the number itself.
Taking the same example of number 6: Its factors other than 6 are: 1, 2, 3 and 1+2 +3 = 6.