\begin{definition}[Definition:Deficient Number/Definition 2]
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''' {{iff}}:
:$\dfrac {\map {\sigma_1} n} n < 2$
\end{definition}