Just for fun and because I didn't believe roaminoz (sorry!), I googled:
An untouchable number is a positive integer that cannot be expressed as the sum of all the proper divisors of any positive integer.
Two integers are said to be amicable or friendly if each number is the sum of all the possible divisors of the other. One such example is 220 and 284. Since the possible divisiors of 220 are 1, 2, 4, 5, 10, 11, 20, 22, 44, 55, and 110 which sum to 284 and the possible divisors of 284 are 1, 2, 4, 71, and 142 which sum to 220, then 220 and 284 are said to be friendly.
How clever! I've got to look at all of them later.
