An almost perfect number, also known as a least deficient or slightly defective (Singh 1997) number, is a positive integer for which the divisor function
satisfies .
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 .
for which the divisor function
satisfies 
.
The only known almost perfect numbers are the powers of
2, namely 1, 2, 4, 8, 16, 32, ... (OEIS A000079).
.
