Perfect number

This article defines a property that can be evaluated for a natural number, i.e., every natural number either satisfies the property or does not satisfy the property.
View a complete list of properties of natural numbers

Definition

A natural number $n$ is termed a perfect number if $\sigma (n)=2n$, where $\sigma$ denotes the divisor sum function: the sum of all the positive divisors of $n$. In particular, $n$ equals the sum of all its proper positive divisors.

Facts

• If $M_{n}=2^{n}-1$ (the $n^{th}$ Mersenne number) is a prime number (and hence, a Mersenne prime), then $2^{n-1}(2^{n}-1)$ is a perfect number.
• Every even perfect number arises in the above fashion.
• The existence of odd perfect numbers is an open problem.