Tuesday, January 20, 2009


JSH: Pell's Equation, circle, ellipses, nifty little result

Oh yeah, a while back I found I could get Pythagorean triples from x^2 - Dy^2 = 1, which I thought was neat, but I ended up moving to other things. Recently, however, a poster brought the subject up in a somewhat different context, which got me to thinking about it again, so here's the result:

Given x^2 - Dy^2 = 1

you have solutions for an ellipse or Pythagorean triples with

(D-1)j^2 + (j+/-1)^2 = (x+y)^2

where j = ((x+Dy) -/+1)/D.

Notice that an integer j will always exist as x = +/-1 mod D, from Pell's Equation.

And that's it. I think it's nifty. Tiny. Concise. Does the job.

You can also at times use it to go BACKWARDS and get a solution to Pell's Equation from Pythagorean triples.

And it gives one other result which is that in general, with the Diophantine equation:

ax^2 + y^2 = z^2

where 'a' is a natural number there are always integer solutions and always an infinity of them, driven, I think intriguingly, by solutions to Pell's Equation.

So it's like, discrete ellipses and circles connected to discrete hyperbolas. You could kind of like call it, discrete conic love.

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