Definition:Almost Perfect Number/Definition 2

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

$n$ is almost perfect :
 * $\sigma \left({n}\right) = 2 n - 1$

where $\sigma \left({n}\right)$ denotes the $\sigma$ function of $n$.

Also see

 * Equivalence of Definitions of Almost Perfect