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Eliot Ness meets Paul Erdős (Posted on 2015-01-23) Difficulty: 2 of 5
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).

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.

No Solution Yet Submitted by Ady TZIDON    
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re: Solution | Comment 2 of 3 |
(In reply to Solution by Jer)

Not sure I understand.
Don't you have to prove the the number p+1 itself cannot be expressed as the sum of the divisors of any other positive integer?  Looks like you are proving that p+1 is a sum that validates p^2 to be not untouchable?

Or am I mistaken?

  Posted by Kenny M on 2015-01-25 17:24:57

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