Perfect Number — from Wolfram MathWorld:
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,
I don’t know any more than when I started. But these examples:
6 = 1+2+3
28 = 1+2+4+7+14
496 = 1+2+4+8+16+31+62+124+248
suggest that
Perfect numbers are positive integers such that the largest factor is the sum of all the other factors.
might also define them. I’m sure that sucks the fun out of it for some people.
More fun numeric facts here.