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Number theory Difficulty 6.5 National Olympiad Prove it Estonia

Find all pairs (x,y)(x, y) of positive integers such that
1x2+249xy+1y2=12012. \frac{1}{x^2} + \frac{249}{xy} + \frac{1}{y^2} = \frac{1}{2012}.

Solution

Let gcd(x,y)=d\gcd(x, y) = d and x=adx = ad, y=bdy = bd. Then the equation can be written as a2+249ab+b2a2b2d2=12012\frac{a^2+249ab+b^2}{a^2b^2d^2} = \frac{1}{2012} or
a2b2d2=2012(a2+249ab+b2). a^2 b^2 d^2 = 2012(a^2 + 249ab + b^2).
As aa and bb are relatively prime, a2a^2 and b2b^2 are both relatively prime to a2+249ab+b2a^2 + 249ab + b^2 and therefore both they must be divisors of 20122012. As 2012=225032012 = 2^2 \cdot 503 and 503503 is a prime, the possible cases are (a,b)=(1,1)(a, b) = (1, 1), (a,b)=(1,2)(a, b) = (1, 2), (a,b)=(2,1)(a, b) = (2, 1). If we substitute (a,b)=(1,1)(a, b) = (1, 1) into the last equation, we get d2=2012251d^2 = 2012 \cdot 251, which is not solvable in integers. The other two cases give 4d2=20125034d^2 = 2012 \cdot 503, from which d=503d = 503. This leads to the solutions (x,y)=(503,1006)(x, y) = (503, 1006) and (x,y)=(1006,503)(x, y) = (1006, 503).

2012x2+2492012xy+2012y2=x2y2. 2012x^2 + 249 \cdot 2012xy + 2012y^2 = x^2y^2.
From the left-hand side we see that both sides of the equation must be divisible by 503503. As 503503 is a prime, one of the numbers xx and yy must be divisible by 503503. So x2x^2 or y2y^2 is divisible by 5032503^2, giving that both sides of the equation are divisible by 5032503^2. If xx is divisible by 503503, the summands 2012x22012x^2 and 2492012xy249 \cdot 2012xy on the left-hand side are divisible by 5032503^2, meaning that 2012y22012y^2 is divisible by 5032503^2. Therefore yy is divisible by 503503. Analogously, we get that if yy is divisible by 503503, then xx is also divisible by 503503. Consequently, both xx and yy are divisible by 503503. Denote x=503ax = 503a, y=503by = 503b. Then the equation, after dividing both sides by 5033503^3, simplifies to 4a2+996ab+4b2=503a2b24a^2 + 996ab + 4b^2 = 503a^2b^2. Assume aba \ge b. If b2b \ge 2, then 503a2b2503a22b=1006a2b=4a2b+4a2b+998a2b>4a2+4b2+996ab503a^2b^2 \ge 503a^2 \cdot 2b = 1006a^2b = 4a^2b + 4a^2b + 998a^2b > 4a^2 + 4b^2 + 996ab, so the last equation cannot hold. Therefore b=1b = 1. Now we get a quadratic equation 499a2996a4=0499a^2 - 996a - 4 = 0 with respect to aa, whose only positive solution is a=2a = 2. From here we obtain the solution (1006,503)(1006, 503) to our original equation. The case bab \ge a is symmetrical and gives the solution (503,1006)(503, 1006).

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