Deficient number

In number theory, a deficient number or defective number is a positive integer $n$ for which the sum of divisors of $n$ is less than $2 n$ . Equivalently, it is a number for which the sum of proper divisors (or aliquot sum) 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 $σ (n)$ the sum of divisors, the value $2 n - σ (n)$ is called the number's deficiency. In terms of the aliquot sum $s (n)$ , the deficiency is $n - s (n)$ .