A 'perfect number' is one in which the factors of a number (apart from the number itself) add up to the number. Thus, 6 is a perfect number because its factors are 1, 2, 3, and 1 + 2 + 3 = 6.
It is an unsolved problem in mathematics if there are an infinite number of perfect numbers.
See Wikipedia: Perfect Numbers
The Perfect numbers less than 1,000,000 (plus one):