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Specifically, $a,b,c\in\mathbb{Q}(i)$ are complex numbers with rational parts whose squares sum to $a^2+b^2+c^2=3$. There's an answer to this question over $\mathbb{Q}$ already here but I couldn't find any more information.

Like, does the parametrization over $\mathbb{Q}$ still give you all solutions over $\mathbb{Q}(i)$ if you let the parameters vary over $\mathbb{Z}[i]$ instead of $\mathbb{Z}$?

Here's the substitutions for the parametrization from the linked post: $$a=\frac{r^2+s^2-t^2-2t(r+s)}{r^2+s^2+t^2}\quad b=\frac{r^2-s^2+t^2-2s(r+t)}{r^2+s^2+t^2}\quad c=\frac{-r^2+s^2+t^2-2r(s+t)}{r^2+s^2+t^2}$$ $$r=\frac{1}{b-1}\quad s=\frac{1}{c-1}\quad t=\frac{a-1}{(b-1)(c-1)}$$

My next bet is to try to find research papers on complex ternary quadratic forms. Either that or to try to do something with "pure" quaternions which I don't really understand.

I also spent some time studying the proof for the parametrization of Pythagorean triples over both $\mathbb{Z}$ and $\mathbb{Z}[i]$ and feel pretty comfortable with it. But I couldn't find anything there that was adaptable to this diophantine problem.

I did notice that you can scale the equation into $a^2+b^2+c^2=3d^2$ over $\mathbb{Z}[i]$ instead of $\mathbb{Q}(i)$ by clearing denominators. And that this rearranges to $$(a+bi)(a-bi)=(\sqrt{3}d+c)(\sqrt{3}d-c)$$ which gives a factorization of one number in two ways and that seems useful. But this would be a factorization in $\mathbb{Z}[i,\sqrt{3}]$ which might be useless as unique factorization might be messed up. Some other readings made it seem like $\mathbb{Z}[i,\sqrt{3}]$ is actually just the cyclotomic ring $\mathbb{Z}[\zeta_{12}]$ in disguise so I thought to spend some effort on that connection eventually.

Any guidance or help is appreciated, thank you.

  • yes, the stereographic projection also works over $\mathbb Q[i]$ although it is a bit messier to see when $r^2 + s^2 + t^2$ is nonzero. I will need to think about how well my answer generalizes; that is, my recipe gives some correct answers to $a^2 + b^2 + c^2 = 3 d^2,$ but I'm not sure it gives all answers in $\mathbb Z[i].$ Meanwhile, I put stuff at http://zakuski.math.utsa.edu/~kap/ – Will Jagy Jun 15 '23 at 20:34
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    I will leave in the zeros in the formulas for $a,b,c.$ They serve as spacers.

    $$ a = w^2 + x^2 - y^2 - z^2 + 0 , w x - 2 w y + 2 x y + 2 w z + 2 x z + 0 , y z $$ $$ b = w^2 - x^2 + y^2 - z^2 + 2 w x - 0 , w y + 2 x y - 2 w z + 0 , x z + 2 y z$$ $$ c = w^2 - x^2 - y^2 + z^2 - 2 w x + 2 w y + 0 , x y + 0 , w z + 2 x z + 2 y z$$ $$ d = w^2 + x^2 + y^2 + z^2 $$

    – Will Jagy Jun 15 '23 at 20:37
  • anyway, the point of my answer was finding all primitive integer solutions, meaning $\gcd(a,b,c,d) = 1.$ Extra work to deal with the desired condition in $\mathbb Z[i]$ and not something I've done ahead of time – Will Jagy Jun 15 '23 at 21:05
  • What is the problem with adapting the parametrization by stereographic projection to $\Bbb{Q}(i)$? – Servaes Jun 16 '23 at 16:40

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