# Does the following Diophantine equation have nontrivial rational solutions?

Are there any solutions to the equation $s^{2}(1+t^{2})^{2}+t^{2}(1+s^{2})^{2}=u^2$ where $s,t,u\in \mathbb{Q}$ and $0 < s,t<1$? If so, is there a simple way to parametrize them all?

If I am understanding the geometry behind this problem, even if we pick a specific value for $t$ we are left with an elliptic curve, and it is possible there could be infinitely many solutions.

This question arises from some related (and somewhat esoteric) questions I have about rational points on unit circles. But this question looked "pretty" enough, I thought I'd ask the experts here.

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I don't know about parametrising them all, but if I got my calculations right then setting t=1/2 gives an elliptic curve of rank 1 and I see plenty of points. For example (all with t=1/2) s=15/16 or 14911/369120. –  Kevin Buzzard May 16 '11 at 23:11
Random specialisations of $t$ all seem to have rank at least 1, so probably the curve in $s$ and $u$ has rank 1 over $\mathbf{Q}(t)$ which means there will be, amongst other solutions, infinitely many parametric solutions in $s$ and $t$. –  Kevin Buzzard May 16 '11 at 23:24
As Kevin Buzzard indicated, now the $(s,u)$ curve is isomorphic to $$y^2=x^3+(-2t^4-8t^2-2)x^2+(t^8+8t^6+14t^4+8t^2+1)x,$$ and a point on this is $(4t^2,2t^5-2t)$, not being torsion. Mapping back to your $(s,u)$, this is $$s={t^4-1\over 4t^2},u={t^8+8t^6-2t^4+8t^2+1\over 16t^3}.$$ There is also $$s={t-1\over t+1},u={(t^2+1)^2\over(t+1)^2},$$ deriving from $(x,y)=(t^4+2t^3+2t^2+2t+1,2t^5-2t)$. Further, Modulo primes like 11 and 19, along with the torsion results $x=0,(t^2+1)^2,t^4+6t^2+1$, these are the only $F_p$ solutions with degree bounded by 4, and so are the only rational ones there. –  Junkie May 17 '11 at 4:28
There is a short GP package (based on a paper done by Mike Artin, Fernando Rodriguez-Villegas and John Tate) to pass from y^2=quartic to y^2=cubic (although some of this was done in an old paper by Hermite much before, I can post the reference if needed). Check the link ma.utexas.edu/cnt/cnt-frames.html (the file jacobians) –  A. Pacetti May 17 '11 at 23:53
If $u^2=quartic(s)$ and we know a point, here $(0,t)$, you can follow Cassels 8(iii) in Lectures on Elliptic Curves, or computer algebra systems will do it, to get the rational isomorphism. I suspect we can show that the Mordell-Weil group over $Q(t)$ is only $(Z/2)^2\oplus Z$ by descent or cohomology, but the details maybe tricky, especially if by hand. Another approach, is noting the rank 1 specializations and use Silverman's theorem, III.11 in his Advanced Topics in the Arithmetic of Elliptic Curves, effectively. Finally, Rank increase upon specialization is difficult, generally. –  Junkie May 18 '11 at 2:07