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In number theory, a deficient number or defective number is a number n for which the sum of divisors of n is less than 2n. Equivalently, it is a number for which the sum of proper divisors is less than n. For example, the proper divisors of 8 are 1, 2, and 4, and their sum is less than 8, so 8 is deficient.
Denoting by σ the sum of divisors, the value 2n − σ is called the number's deficiency. In terms of the aliquot sum s, the deficiency is n − s.
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