A perfect number is a positive integer that equals the sum of its divisors.

Perfect numbers are positive integers n such that n = s(n), where s(n) is the restricted divisor function (i.e., the sum of proper divisors of n), or equivalently σ(n) = 2n, where σ(n) is the divisor function (i.e., the sum of divisors of n including n itself).

abundant number | aliquot sequence | amicable pair | amicable quadruple | aspiring number | deficient number | divisor function | e-perfect number | even perfect number | harmonic number | hyperperfect number | infinitary perfect number | Mersenne number | Mersenne prime | multiperfect number | multiplicative perfect number | odd perfect number | pseudoperfect number | quasiperfect number | Smith number | sociable numbers | sublime number | superperfect number | unitary perfect number | weird number

college level