Inspired by IMO shortlist question

Find the number of ordered pairs of positive integers ( x , y ) (x,y) such that the following equation is satisfied,

x 3 + y 2 + z = x y z x^{3}+y^{2}+z=xyz

where z = gcd ( x , y ) z=\gcd(x,y) .


The answer is 0.

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1 solution

Daniel Liu
Dec 30, 2015

Let x = z x x=zx' and y = z y y=zy' . Then the equation turns into z 3 x 3 + z 2 y 2 + z = z 3 x y z^3x'^3+z^2y'^2+z=z^3x'y' or z 2 x 3 + z y 2 + 1 = z 2 x y z^2x'^3+zy'^2+1=z^2x'y' Since the RHS is divisible by z z it follows that the LHS is also divisible by z z , so z 1 z = 1 z\mid 1\implies z=1 and our original equation reduces to x 3 + y 2 + 1 = x y y 2 x y + ( x 3 + 1 ) = 0 x^3+y^2+1=xy\implies y^2-xy+(x^3+1)=0

Since y y is a real number, the discriminant of the quadratic w.r.t y y must be positive, i.e x 2 4 ( x 3 + 1 ) 0 x 2 4 x 3 + 4 x^2-4(x^3+1)\ge 0\implies x^2\ge 4x^3+4 which clearly cannot be satisfied if x x is a positive integer; thus, there are no solutions.

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