Olympiad Maths Prep

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Problem 1042

National olympiad, first round
Number theory Difficulty 6.0 Prove it

3. Prove: the equation
x2+y22017xy2017=0 x^{2}+y^{2}-2017 x y-2017=0

has no integer solutions.

This one wants a proof. Work it on paper, read the official solution, then mark yourself honestly — the ladder only means something if the record is true.

Official solution

Let (x0,y0)\left(x_{0}, y_{0}\right) be a set of integer solutions to equation (1). Clearly, x00,y00x_{0} \neq 0, y_{0} \neq 0, and
2017(x0y0+1)=x02+y02>0 2017\left(x_{0} y_{0}+1\right)=x_{0}^{2}+y_{0}^{2}>0 \text {. }

From this, we know that x0y0>0x_{0} y_{0}>0, hence x0x_{0} and y0y_{0} have the same sign.
Without loss of generality, assume x0x_{0} and y0y_{0} are both positive integers (otherwise, replace x0x_{0} and y0y_{0} with x0-x_{0} and y0-y_{0} respectively), and x0y0x_{0} \geqslant y_{0}.
Consider equation (1) as a quadratic equation in xx
x22017y0x+y022017=0 x^{2}-2017 y_{0} x+y_{0}^{2}-2017=0 \text {. }

Thus, from a set of positive integer solutions (x0,y0)\left(x_{0}, y_{0}\right) (x0y0)\left(x_{0} \geqslant y_{0}\right) of equation (1), we can derive another set of positive integer solutions (y0,x)\left(y_{0}, x^{\prime}\right)
(y0x), and y0+x<x0+y0 \left(y_{0} \geqslant x^{\prime}\right) \text {, and } y_{0}+x^{\prime}<x_{0}+y_{0} \text {. }

The above process can be carried out infinitely, each time yielding a new set of solutions where the sum of the two numbers is strictly less than the sum of the two numbers in the previous set, and all these solutions are positive integer solutions, which is clearly impossible. Therefore, the original indeterminate equation has no integer solutions.

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic, difficulty and ordering added by this site.