In mathematics, an almost perfect number (sometimes also called slightly defective number) is a natural number n such that the sum of all divisors of n (the divisor function σ(n)) is equal to 2n - 1. The only almost perfect numbers known are those of the form 2^k for some natural number k; however, it has not been shown yet that all almost perfect numbers are of this form. Almost perfect numbers are deficient numbers.

See also

• Perfect number
• Quasiperfect number
• Abundant number

External links

• MathWorld: Almost perfect number