If N is a natural number, we write σ(N ) := ),d|N d for the sum of the divisors
of N . We call N perfect if σ(N ) = 2N , i.e., if N is equal to the sum of its proper
divisors. The even perfect numbers were completely classified by Euclid and Euler, but the odd perfect numbers remain utterly mysterious: despite millennia of effort, we don’t know of a single example, but we possess no argument ruling out their existence.