How many positive integers less than 1 0 , 0 0 0 have exactly 1 5 divisors?
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@brian charlesworth Out of curiosity, Where did you get this question from?
FYI, we released the exact same problem in the past. As it turns out, many people guessed 15 because it was a number in the question. We then changed it to asking for integers up to 19,999, and the percentage of correct answers dropped tremendously.
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I answered this question on Yahoo! Answers a couple of years ago. It's a straightforward problem, but I found it amusing that the answer was, as you say, "in the question". I just thought that this coincidence would give solvers a little smile once they solved it, (even if they just guessed). :)
The question also had me wondering about the function N ( k ) , which returns the maximum integer such that there are precisely k positive integers less than N ( k ) that have exactly k divisors. I believe that N ( 1 5 ) = 2 9 2 ∗ 2 4 = 1 3 4 5 6 , as this would be the 1 6 th positive integer, (in ascending order), that has exactly 1 5 divisors. (I suppose this would have been a more interesting question to ask, but I was tired when I posted this question, so I wasn't feeling very creative.)
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The number n with exactly 15 divisor is either n = p 1 4 or n = p 2 q 4 , as the number of divisors is 1 4 + 1 = 1 5 ) or ( 2 + 1 ) × ( 4 + 1 ) = 1 5 ) , where p and q are primes But for n < 1 0 , 0 0 0 , n = p 1 4 is impossible, as 2 1 4 > 1 0 , 0 0 0 . The solutions for n = p 2 q 4 for n > 1 0 , 0 0 0 are as follows:
2 2 3 4 = 3 2 4 2 2 5 4 = 2 5 0 0 2 2 7 4 = 9 6 0 4
3 2 2 4 = 1 4 4 5 2 2 4 = 4 0 0 7 2 2 4 = 7 8 4
1 1 2 2 4 = 1 9 3 6 1 3 2 2 4 = 2 7 0 4 1 7 2 2 4 = 4 6 2 4
1 9 2 2 4 = 5 7 7 6 2 3 2 2 4 = 8 4 6 4 3 2 5 4 = 5 6 2 5
5 2 3 4 = 2 0 2 5 7 2 3 4 = 3 9 6 9 1 1 2 3 4 = 9 8 0 1
There are 1 5 such positive integers.