Eliot Ness meets Paul Erdős (Posted on 2015-01-23)
An untouchable number is a positive integer that cannot be expressed as the sum of all the proper divisors of any positive integer (including the untouchable number itself).
Prove:
i. There is no untouchable number that is one more than a prime.
ii. Except number 5, there is no untouchable number that is three more than a prime.