Amicable number
= Amicable number
Two distinct <positive integers> $a,b$ are amicable when the <sum> of the <proper divisors> of $a$ is $b$ and the <sum> of the <proper divisors> of $b$ is $a$. The pair $220,284$ is an example. Iterating the proper-divisor <sum> can also produce longer <directed cycles>.