A natural number is called PERFECT when it is equal to the sum of all its divisors, excluding itself.

For example:

28 is divisible by 1, 2, 4, 7, 14 therefore it is a perfect number:

28 = 1 + 2 + 4 + 7 + 14

Also 6 is a perfect number as it is divisible by 1,2 and 3 and it turns out that:

6 = 1 + 2 + 3

On the contrary, 3 is not a perfect number as it is divisible just by 1 and 1 is less than 3.

PROPERTIES

When a number is less than the sum of its divisors, it is called abundant.
When a number is greater than the sum of its divisors, it is called deficient or defective.