Definition:Deficient Number/Definition 2

Definition
Let $n \in \Z_{\ge 0}$ be a positive integer.

Let $\map {\sigma_1} n$ be the divisor sum function of $n$.

$n$ is deficient :
 * $\dfrac {\map {\sigma_1} n} n < 2$

Also see

 * Equivalence of Definitions of Deficient Number