I am not sure that following posts obeys rules above or not(if not ignore it) but found interesting in encyclopedia about amicable numbers: Amicable numbers are two numbers so related that the sum of the proper divisors of the one is equal to the other, unity being considered as a proper divisor but not the number itself. Such a pair is (220, 284); for the proper divisors of 220 are 1, 2, 4, 5, 10, 11, 20, 22, 44, 55 and 110, of which the sum is 284; and the proper divisors of 284 are 1, 2, 4, 71, and 142, of which the sum is 220 Good nice thread.Better learning practice.Thanks to RajaHarischandra.