Determine all pairs of positive integers ( x , y ) such that the expression below is an integer. x y 2 + y + 1 1 x 2 y + x + y
Note: If the solutions are of the form ( x 1 , y 1 ) , ( x 2 , y 2 ) , ( x 3 , y 3 ) , ( a k 2 , b k ) , where k is a variable integer, then find x 1 + y 1 + x 2 + y 2 + x 3 + y 3 + a + b .
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Multiply by y and subtract x to get x y 2 + y + 1 1 x 2 y 2 + x y + y 2 − x = x y 2 + y + 1 1 y 2 − 1 1 x . This must also be an integer. If y 2 − 1 1 x is positive, this is impossible, because the numerator will be less than the denominator. If y 2 = 1 1 x , then y = 1 1 k for some integer k and x = 1 1 k 2 . The tuple ( 1 1 k 2 , 1 1 k ) gives a solution for any positive integer k .
Finally, if y 2 < 1 1 x , then we need 1 1 x − y 2 to be at least as large as the denominator: 1 1 x − y 2 ≥ x y 2 + y + 1 1 ⇒ x ( y 2 − 1 1 ) + ( y 2 + y + 1 1 ) ≤ 0 . Note that if y ≥ 4 , the right side equals m + n x where m and n are positive, which would be a contradiction.
So this leaves the cases y = 1 , 2 , 3 .
If y = 1 we get x + 1 2 x 2 + x + 1 is an integer, but this equals x − 1 1 + x + 1 2 1 3 3 , and the only divisors of 1 3 3 larger than 1 2 are 1 9 and 1 3 3 , so the two solutions are x = 7 , 1 2 1 .
If y = 2 we get 4 x + 1 3 2 x 2 + x + 2 is an integer, but this equals 2 1 ( x − 4 x + 1 3 1 1 x − 4 ) . So 4 x + 1 3 1 1 x − 4 is an integer. Clearly this integer is less than 3 , so it's 1 or 2 . If it's 1 then 7 x = 1 7 , which is no good. If it's 2 then x = 1 0 , and that does in fact work (the expression is 5 3 2 1 2 = 4 ), so that's the one solution in this case.
If y = 3 then we get 9 x + 1 4 3 x 2 + x + 3 is an integer, but this equals 3 1 ( x − 9 x + 1 4 1 1 x − 9 ) . So 9 x + 1 4 1 1 x − 9 is an integer. Clearly this integer is less than 2 , so it's 1 . But this leads to 2 x = 2 3 , which is no good.
Putting it all together, our solutions are ( 7 , 1 ) , ( 1 2 1 , 1 ) , ( 1 0 , 2 ) , and ( 1 1 k 2 , 1 1 k ) for any k . The sum is 7 + 1 + 1 2 1 + 1 + 1 0 + 2 + 1 1 + 1 1 = 1 6 4 .