How to transform genus zero curve to conic section?

156 Views Asked by At

I have a curve of genus zero:

$$C: 2 x^5-4 x^3 y+x^2 y+2 x y^3+2 x y^2+y^5$$

or in homogenized form:

$$C': 2 x^5-4 x^3 y z+x^2 y z^2+2 x y^3 z+2 x y^2 z^2+y^5$$

How to transform it to some conic section curve with rational substitutions $x\to f_1(u, v), y\to f_2(u, v)$?

(or do you know Sagemath code that would do that?)

I computed singular points of the curve and also rational parametrization of the curve if that can be useful to find the transformation.

Singular points: $$(-1:1:1), (0:0:1), (\frac{1}{2}+\frac{i \sqrt{3}}{2}:-\frac{1}{2}+\frac{i \sqrt{3}}{2}:1), (\frac{1}{2}-\frac{i \sqrt{3}}{2}:-\frac{1}{2}-\frac{i \sqrt{3}}{2}:1)$$

Rational parametrization (in one parameter or in two parameters): $$x\to -\frac{p (2 p+1)}{4 p^5+1}, y\to \frac{2 p^3 (2 p+1)}{4 p^5+1}$$ $$x\to -\frac{p q^3 (2 p+q)}{4 p^5+q^5}, y\to \frac{2 p^3 q (2 p+q)}{4 p^5+q^5}$$

Also some small rational points on the curve:

$$(0,0), (-1,1), (\frac{1}{3},-\frac{2}{3}), (-\frac{3}{5},\frac{6}{5}), (-\frac{8}{9},\frac{4}{9}), (-\frac{54}{53},\frac{48}{53}), (-\frac{96}{13},\frac{108}{13})$$

UPDATE:

Here is an example of a curve of degree 3 and genus 0 that I was able to transform to conic - specifically parabola (with forward and backward transformations):

$$y^2=x^3-x^2,\left(x\to \frac{v}{u^2},y\to \frac{v}{u^3}\right)\\ v=u^2+1,\left(u\to \frac{x}{y},v\to \frac{x^3}{y^2}\right)$$

2

There are 2 best solutions below

11
On BEST ANSWER

The accepted answer does not give any details of how the transformation was found, so let me give an alternative answer. First, you have a parametrisation so you have a morphism $C \to \mathbb{P}^1$. Embedding $\mathbb{P}^1$ as a conic can be done in many ways (choose three generic homogeneous degree 2 polynomials in $k[x,y]$).

Let me then deal with the more interesting case: We have a genus $0$ curve $C \subset \mathbb{P}^n$ and we don't know whether or not it has a smooth rational point. The key point to note is that if $C/k$ is a genus $0$ curve, then (by Riemann-Roch) the complete linear system of the anti-canonical divisor $|-K_C|$ defines a degree $1$ rational map $C \dashrightarrow \mathbb{P}^2$.

I don't know how to make this algorithmic in SageMath, but Magma has very good canonical divisor functionality for curves:

P2<x,y,z> := ProjectiveSpace(Rationals(), 2);
f := 2*x^5 - 4*x^3*y*z + x^2*y*z^2 + 2*x*y^3*z + 2*x*y^2*z^2+ y^5;
C := Curve(P2, f);
K := CanonicalDivisor(C);
lin_sys := Basis(-K);
phi := map<C -> P2 | lin_sys>;
X := Image(phi);
phi := map<C -> X | lin_sys>;
assert IsInvertible(phi);

// print Inverse(phi); // access the inverse of phi

// In response to the comment below, here's how to make the 
// map nicer because we know a point
P1 := ProjectiveSpace(Rationals(), 1);
lin_sys_P := Basis(Divisor(C![1,-2,3]));
phi := map<C -> P1 | lin_sys_P>;
assert IsInvertible(phi);

// print Inverse(phi); // access the inverse of phi
```
0
On

$$2 x^5-4 x^3 y+x^2 y+2 x y^3+2 x y^2+y^5=0,\left(x\to -\frac{8 u v^3-u+4 v^3-2 v}{16 v^5-1},y\to \frac{2 v \left(8 u v^3-u+4 v^3-2 v\right)}{16 v^5-1}\right)\\v-u^2=0,\left(u\to \frac{-2 x^5+2 x^3 y-x y^3-y^5}{2 x \left(x^3+y^3\right)},v\to -\frac{y}{2 x}\right)$$