Maths Olympiad Prep

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Number theory Difficulty 5.3 AIME, harder Prove it JBMO

Problem:
Find all ordered triples (x,y,z)(x, y, z) of positive integers satisfying the equation
1x2+yxz+1z2=12013 \frac{1}{x^{2}}+\frac{y}{x z}+\frac{1}{z^{2}}=\frac{1}{2013}

Solution

Solution:
We have x2z2=2013(x2+xyz+z2)x^{2} z^{2}=2013\left(x^{2}+x y z+z^{2}\right). Let d=gcd(x,z)d=\operatorname{gcd}(x, z) and x=da,z=dbx=d a, z=d b. Then a2b2d2=2013(a2+aby+b2)a^{2} b^{2} d^{2}=2013\left(a^{2}+a b y+b^{2}\right).
As gcd(a,b)=1\operatorname{gcd}(a, b)=1, we also have gcd(a2,a2+aby+b2)=1\operatorname{gcd}\left(a^{2}, a^{2}+a b y+b^{2}\right)=1 and gcd(b2,a2+aby+b2)=1\operatorname{gcd}\left(b^{2}, a^{2}+a b y+b^{2}\right)=1. Therefore a22013a^{2} \mid 2013 and b22013b^{2} \mid 2013. But 2013=311612013=3 \cdot 11 \cdot 61 is squarefree and therefore a=1=ba=1=b.
Now we have x=z=dx=z=d and d2=2013(y+2)d^{2}=2013(y+2). Once again as 2013 is squarefree, we must have y+2=2013n2y+2=2013 n^{2} where nn is a positive integer.
Hence (x,y,z)=(2013n,2013n22,2013n)(x, y, z)=\left(2013 n, 2013 n^{2}-2,2013 n\right) where nn is a positive integer.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.