Equivalence of Definitions of Perfect Number

Theorem
The following definitions of a perfect number are equivalent:

Proof
Consider a strictly positive integer $n$.

By Definition 1, $n$ is a perfect number $n$ equals the sum of its proper divisors.

By definition of sigma function, $\sigma \left({n}\right)$ equals the sum of all the divisors of $n$.

Thus $\sigma \left({n}\right) - n$ equals the sum of all the proper divisors of $n$.

So by Definition 2, $n$ is a perfect number $\sigma \left({n}\right) - n = n$.

Hence the definitions are equivalent.