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Does $ \frac{z}{x+y}+\frac{y}{x+z} +\frac{z}{x-y}+\frac{y}{x-z}=9$ have a nonzero integer solution?

数论 Math StackExchange -3 票 1 回答 129 浏览 提问者: Evan2013 2026-08-14 13:09
number-theory diophantine-equations

问题内容

I came up with the following Diophantine equation:

$$ \frac{z}{x+y}+\frac{y}{x+z} +\frac{z}{x-y}+\frac{y}{x-z}=9, $$

where $x,y,z\in\mathbb Z,$ $xyz\ne0,$ and $x+y\ne0,$ $x-y\ne0,$ $x+z\ne0,$ $x-z\ne0.$

Does this equation have a nonzero integer solution?

If a solution exists, I would be interested in finding one, preferably a primitive solution satisfying $\gcd(x,y,z)=1$.

If no such solution exists, is there a proof of nonexistence?

I came up with this equation and became curious whether it has any nonzero integer solutions. I am particularly interested in whether the structure of the denominators (x\pm y) and (x\pm z) can be used to prove the existence or nonexistence of integer solutions.

I have not yet found a solution or a proof of nonexistence.

Any ideas or approaches would be appreciated.

回答 (1)

Dietrich Burde 3 票 2026-08-14 14:04 原文

Solution idea: Writing $a=\frac{y}{x}$ and $b=\frac{z}{x}$ the equation is equivalent to $$ \frac{2a}{1-b^2}+\frac{2b}{1-a^2}=9. $$ This leads to an elliptic curve having no rational points so that the only rational solutions are $$ (a,b)=(0,9/2),(9/2,0), $$ which contradicts $xyz\neq 0$.