Find the sum of the first 12 smallest positive integers which have exactly 10 (positive) factors.
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If a positive integer n has 10 positive divisors, then it must be of the form n = p ⋅ q 4 or n = p 9 . where p and 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 .
The 12 smallest numbers of this form are 3 ⋅ 2 4 = 4 8 5 ⋅ 2 4 = 8 0 7 ⋅ 2 4 = 1 1 2 2 ⋅ 3 4 = 1 6 2 1 1 ⋅ 2 4 = 1 7 6 1 3 ⋅ 2 4 = 2 0 8 1 7 ⋅ 2 4 = 2 7 2 1 9 ⋅ 2 4 = 3 0 4 2 3 ⋅ 2 4 = 3 6 8 5 ⋅ 3 4 = 4 0 5 2 9 ⋅ 2 4 = 4 6 4 3 1 ⋅ 2 4 = 4 9 6 The sum is ( 3 + 5 + 7 + 1 1 + 1 3 + 1 7 + 1 9 + 2 3 + 2 9 + 3 1 ) ⋅ 2 4 + ( 2 + 5 ) ⋅ 3 4 = 3 0 9 5 .
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 = 5 1 2 .