Almost perfect number

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Science and technology

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What Wikipedia says

In mathematics, an almost perfect number (sometimes also called slightly defective or least deficient number) is a natural number n such that the sum of all divisors of n (the sum-of-divisors function σ(n)) is equal to 2n − 1, the sum of all proper divisors of n, s(n) = σ(n) − n, then being equal to n − 1.

Excerpt from the Wikipedia article “Almost perfect number”, under the CC BY-SA 4.0 license. Read the article on Wikipedia

Image: Hyacinth, CC BY-SA 4.0, via Wikimedia Commons.