Let's mine for factors!

Find the sum of the first 12 smallest positive integers which have exactly 10 (positive) factors.


The answer is 3095.

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1 solution

Arjen Vreugdenhil
Apr 15, 2016

If a positive integer n n has 10 positive divisors, then it must be of the form n = p q 4 or n = p 9 . n = p\cdot q^4\ \ \ \ \text{or}\ \ \ \ n = p^9. where p p and q q are distinct prime numbers. The factors will be 1 , q , q 2 , q 3 , q 4 , p , p q , p q 2 , p q 3 , p q 4 ; or 1 , p , p 2 , , p 9 . 1, q, q^2, q^3, q^4, p, pq, pq^2, pq^3, pq^4;\ \ \ \ \ \text{or}\ \ \ \ \ 1, p, p^2, \dots, p^9.

The 12 smallest numbers of this form are 3 2 4 = 48 5 2 4 = 80 7 2 4 = 112 2 3 4 = 162 11 2 4 = 176 13 2 4 = 208 17 2 4 = 272 19 2 4 = 304 23 2 4 = 368 5 3 4 = 405 29 2 4 = 464 31 2 4 = 496 3\cdot 2^4 = 48 \\ 5\cdot 2^4 = 80 \\ 7\cdot 2^4 = 112 \\ \color{#3D99F6}{2\cdot 3^4 = 162} \\ 11\cdot 2^4 = 176 \\ 13\cdot 2^4 = 208 \\ 17\cdot 2^4 = 272 \\ 19\cdot 2^4 = 304 \\ 23\cdot 2^4 = 368 \\ \color{#3D99F6}{5\cdot 3^4 = 405} \\ 29\cdot 2^4 = 464 \\ 31\cdot 2^4 = 496 The sum is ( 3 + 5 + 7 + 11 + 13 + 17 + 19 + 23 + 29 + 31 ) 2 4 + ( 2 + 5 ) 3 4 = 3095 . (3+5+7+11+13+17+19+23+29+31)\cdot 2^4 \\ + (2 + 5)\cdot 3^4 = \boxed{3095}.

The interesting thing is (and I believe that is what Joel Yip was really after) that the next number with 10 positive divisors is 2 9 = 512 2^9 = 512 .

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