Equivalence of Definitions of Almost Perfect Number

Proof
By definition of abundance:


 * $\map A n = \map {\sigma_1} n - 2 n$

By definition of divisor sum function:
 * $\map {\sigma_1} n$ is the sum of all the divisors of $n$.

Thus $\map {\sigma_1} - n$ is the sum of the aliquot parts of $n$.

Hence the result.