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Number theory Difficulty 4.9 AIME Prove it Mongolia

Find all positive integer solutions to the equation
4z2(2z2+1)=4xy2xy. 4z^2(2z^2 + 1) = 4xy - 2x - y.

Solution

Answer: There is no solution.
Our equation is equivalent to
(4z2+1)2=(4x1)(2y1). (4z^2 + 1)^2 = (4x - 1)(2y - 1).
Since 4x13(mod4)4x - 1 \equiv 3 \pmod{4}, there exists a prime divisor p3p \ge 3 of 4x14x - 1 such that p3(mod4)p \equiv 3 \pmod{4}. Then p4z2+1p \mid 4z^2 + 1, thus 4z21(modp)4z^2 \equiv -1 \pmod{p}. By Fermat's theorem, we have
1(2z)p1(1)p121(modp). 1 \equiv (2z)^{p-1} \equiv (-1)^{\frac{p-1}{2}} \equiv -1 \pmod{p}.
This is a contradiction. So there is no solution.

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