Definition:Almost Perfect Number/Definition 2

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

$n$ is almost perfect :
 * $\map {\sigma_1} n = 2 n - 1$

where $\map {\sigma_1} n$ denotes the divisor sum function of $n$.

Also see

 * Equivalence of Definitions of Almost Perfect Number