# solve for all integer solutions to the diophantine equation

$$1/x+1/y=1/14$$

Find all integer solutions for x and y.

I can solve linear diophantine equations without a problem normally but this has me stumped.

• In general, you count precisely $$\sum_{x | n}\sum_{y | n/x} 1 = \tau(n^2)$$ Positive integer solutions to $1/x + 1/y = 1/n$. Mar 28, 2014 at 13:29

The equation gives: $14y = xy - 14x$. So solving for $x$ and get: $x = 14 + \frac{196}{y-4}$.
So $x$ is an integer if $y - 4$ divides $196$. We can take it from here....
• Is moving from $14y = xy - 14x$ to $x = 14 + 196/(y-4)$ any easier than the original problem? Mar 28, 2014 at 13:10
• The $y-4$ in the denominator must be $y-14$ Mar 29, 2014 at 14:32
Rewrite as $xy-14x-14y=0$, and then, in an analogue of completing the square, as $(x-14)(y-14)=196$.
So $x-14$ ranges over the divisors of $196$. Since $196=2^2\cdot 7^2$, $196$ has $(2)(3)(3)$ integer divisors, including the negative divisors. That gives $18$ possible values of $x$.