Definition:Deficient Number/Definition 2

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

Let $\sigma \left({n}\right)$ be the sigma function of $n$.

$n$ is deficient iff $\dfrac {\sigma \left({n}\right)} n < 2$.

Also see

 * Definition:Abundant/Definition 2
 * Definition:Abundancy