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jmc

number theory senior

Problem

A positive integer is nice if there is a positive integer with exactly four positive divisors (including and ) such that the sum of the four divisors is equal to Find the sum of all the nice numbers in the set
Solution
The positive integers with exactly four positive divisors are the integers of the form , where is a prime, or , where and are distinct primes. We consider each case:

Suppose that for some prime . Then the sum of the divisors of is For this value of is too low, and for the value of is too high; therefore, no prime gives a value of in the given set.

Therefore, we must have , for some distinct primes and Then the sum of the divisors of is , which we can factor as . First suppose that one of and equals ; without loss of generality, let . Then Since , we see that is odd, and so is even. Thus is divisible by so it must be either or Trying both cases, we see that both and give a non-prime value of

If neither nor equals , then both are odd primes, so is the product of two even numbers, which must be divisible by The only multiples of in the given range are and . We have so the only way to write as the product of two even positive integers is But we cannot have or , since is not prime. Note that Since both 3 and 503 are prime, 2016 is nice.

Thus, is the only nice number in the given set.
Final answer
2016