# Almost perfect number

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 2

*n* - 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.

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