Two numbers are said to be amicable when each one is the sum of the divisors of the other.
For instance 220 and 284
Sum of divisors of 220=2²×5×11 is 1+2+4+5+10+11+20+22+44+55+110=284
Sum of divisors of 284=4×71 is 1+2+4+71+142=220
Other related problems and relations about divisors :
perfect numbers, multiperfect numbrs, aliquot sequence...