In number theory, a perfect number is a positive integer that is equal to the sum of its positive divisors, excluding the number itself. For instance, 6 has divisors 1, 2 and 3 (excluding itself), and 1 + 2 + 3 = 6, so 6 is a perfect number.
The sum of divisors of a number, excluding the number itself, is called its aliquot sum, so a perfect number is one that is equal to its aliquot sum. Equivalently, a perfect number is a number that is half the sum of all of its positive divisors including itself; in symbols,




{\displaystyle \sigma _{1}(n)=2n}



{\displaystyle \sigma _{1}}
is the sum-of-divisors function. For instance, 28 is perfect as 1 + 2 + 4 + 7 + 14 = 28.
This definition is ancient, appearing as early as Euclid's Elements (VII.22) where it is called τέλειος ἀριθμός (perfect, ideal, or complete number). Euclid also proved a formation rule (IX.36) whereby




{\displaystyle q(q+1)/2}
is an even perfect number whenever


{\displaystyle q}
is a prime of the form




{\displaystyle 2^{p}-1}
for positive integer


{\displaystyle p}
—what is now called a Mersenne prime. Two millennia later, Leonhard Euler proved that all even perfect numbers are of this form. This is known as the Euclid–Euler theorem.
It is not known whether there are any odd perfect numbers, nor whether infinitely many perfect numbers exist. The first few perfect numbers are 6, 28, 496 and 8128 (sequence A000396 in the OEIS).

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Did you know?

Credit is the trust which allows one party to provide money or resources to another party wherein the second party does not reimburse the first party immediately, but promises either to repay or return those resources at a later date.

Lenders want to make sure all their bases are covered before they extend you credit. That means they may look at factors other than your credit score to determine whether to lend you money. Your employment status also can play a role: If your income is too low or you haven’t been at your current workplace long, those factors could weigh against you.

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