# Definition:Almost Perfect Number/Definition 2

## Definition

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

$n$ is almost perfect if and only if:

$\sigma \left({n}\right) = 2 n - 1$

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

## Sequence

The sequence of almost perfect numbers begins:

$1, 2, 4, 8, 16, 32, 64, 128, \ldots$