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 2k 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.
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