2013 AMC 10B Problem 24

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Concepts:sum of factorsprimecasework

Difficulty rating: 2180

24.

A positive integer nn is "nice" if there is a positive integer mm with exactly four positive divisors (including 11 and mm) such that the sum of the four divisors is equal to n.n. How many numbers in the set {2010,2011,2012,,2019}\{ 2010,2011,2012,\dotsc,2019 \} are nice?

1 1

2 2

3 3

4 4

5 5

Solution:

An integer with exactly four positive divisors is either p3p^3 or pqpq, where pp and qq are distinct primes.

If m=p3m=p^3, the divisor sum is 1+p+p2+p31+p+p^2+p^3. The values for p=11p=11 and p=13p=13 fall below and above the interval 20102010 to 20192019, so this case gives none.

In the m=pqm=pq case, the divisor sum is 1+p+q+pq=(p+1)(q+1)1+p+q+pq=(p+1)(q+1). If one prime is 22, the sum is divisible by 33; only 20102010 and 20162016 qualify, but 2010/31=6692010/3-1=669 and 2016/31=6712016/3-1=671 are not prime.

If both primes are odd, then the sum is divisible by 44, leaving 20122012 and 20162016. The factorization 2012=45032012=4\cdot503 would give primes 33 and 502502, impossible, while 2016=4504=(3+1)(503+1)2016=4\cdot504=(3+1)(503+1) works.

Thus exactly one number is nice, and the correct answer is A .

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