The example of a perfect number six, because it factors into 1, 2, 3
which sum exactly to itself, leads to the question of an
imperfect number. There are two kinds
deficient (defective) (factors sum to less than the
number) or abundant (excessive) (they sum to more).
If a number is almost perfect the sum of all its factors could be
close to the number. If the sum is one less
than the number this is called slightly deficient. A sum that's one more
is called slightly abundant. No one knows if there is a number
with factors that are slightly abundant.[Singh, p. 13]