least deficient number | slightly defective number

An almost perfect number, also known as a least deficient or slightly defective number, is a positive integer n for which the divisor function satisfies σ(n) = 2n - 1. The only known almost perfect numbers are the powers of 2, namely 1, 2, 4, 8, 16, 32, ... (OEIS A000079). It seems to be an open problem to show that a number is almost perfect only if it is of the form 2^n.

deficient number | perfect number | quasiperfect number