Rational Parametrizations of Rational Plane Curves
In this section, will denote a complex projective plane curve, defined by an absolutely irreducible, homogeneous polynomial in three variables, with coefficients in . Moreover, we will write .
Recall that the curve is rational if it is birationally equivalent to the projective line . In other words, there exists a rational parametrization of , that is, a birational map . Note that such a parametrization is given by three homogeneous polynomials of the same degree in the homogeneous coordinates on .
The curve is rational iff its geometric genus is zero.
Based on work of Max Noether on adjoint curves, Hilbert und Hurwitz showed that if is rational, then there is a birational map defined over such that if is odd, and is a conic if is even.
If a conic contains a rational point, then there exists a parametrization of defined over ; otherwise, there exists a parametrization of defined over a quadratic field extension of .
The approach of Hilbert und Hurwitz is constructive and allows one, in principle, to find rational parametrizations. The resulting algorithm is not very practical, however, as the approach asks to compute adjoint curves repeatedly, at each of a number of reduction steps.
The algorithm implemented in OSCAR relies on reduction steps of a different type and requires the computation of adjoint curves only once. Its individual steps are interesting in their own right:
- Assure that the curve is rational by checking that its geometric genus is zero;
- compute a basis of the adjoint curves of of degree ; each such basis defines a birational map where is a rational normal curve in ;
- the anticanonical linear system on defines a birational map , where is a rational normal curve in in ;
- iterate the previous step to obtain a birational map , where if is odd, and is a conic if is even;
- invert the birational map ;
- if is even, compute a parametrization of the conic and compose it with the inverted map above.
The defining property of an adjoint curve is that it passes with “sufficiently high” multiplicity through the singularities of . There are several concepts of making this precise. For each such concept, there is a corresponding adjoint ideal of , namely the homogeneous ideal formed by the defining polynomials of the adjoint curves. In OSCAR, we follow the concept of Gorenstein which leads to the largest possible adjoint ideal.
See [Bhm99] and [BDLP17] for details and further references.
Adjoint Ideals of Plane Curves
adjoint_ideal
— Methodadjoint_ideal(C::ProjectivePlaneCurve{QQField})
Return the Gorenstein adjoint ideal of C
.
Examples
julia> R, (x,y,z) = graded_polynomial_ring(QQ, [:x, :y, :z]);
julia> C = ProjectivePlaneCurve(y^4-2*x^3*z+3*x^2*z^2-2*y^2*z^2)
Projective plane curve
defined by 0 = 2*x^3*z - 3*x^2*z^2 - y^4 + 2*y^2*z^2
julia> I = adjoint_ideal(C)
Ideal generated by
-x*z + y^2
x*y - y*z
x^2 - x*z
Rational Points on Conics
rational_point_conic
— Methodrational_point_conic(D::ProjectivePlaneCurve{QQField})
If the plane conic D
contains a rational point, return the homogeneous coordinates of such a point. If no such point exists, return a point on D
defined over a quadratic field extension of .
Examples
julia> R, (x,y,z) = graded_polynomial_ring(QQ, [:x, :y, :z]);
julia> C = ProjectivePlaneCurve(y^4-2*x^3*z+3*x^2*z^2-2*y^2*z^2)
Projective plane curve
defined by 0 = 2*x^3*z - 3*x^2*z^2 - y^4 + 2*y^2*z^2
julia> I = adjoint_ideal(C)
Ideal generated by
-x*z + y^2
x*y - y*z
x^2 - x*z
julia> R, (x,y,z) = graded_polynomial_ring(QQ, [:x, :y, :z]);
julia> D = ProjectivePlaneCurve(x^2 + 2*y^2 + 5*z^2 - 4*x*y + 3*x*z + 17*y*z);
julia> P = rational_point_conic(D)
3-element Vector{AbstractAlgebra.Generic.MPoly{AbsSimpleNumFieldElem}}:
-1//4*a
-1//4*a + 1//4
0
julia> S = parent(P[1])
Multivariate polynomial ring in 3 variables x, y, z
over number field of degree 2 over QQ
julia> NF = base_ring(S)
Number field with defining polynomial t^2 - 2
over rational field
julia> a = gen(NF)
a
julia> minpoly(a)
t^2 - 2
Parametrizing Rational Plane Curves
parametrization
— Methodparametrization(C::ProjectivePlaneCurve{QQField})
Return a rational parametrization of C
.
Examples
julia> R, (x,y,z) = graded_polynomial_ring(QQ, [:x, :y, :z]);
julia> C = ProjectivePlaneCurve(y^4-2*x^3*z+3*x^2*z^2-2*y^2*z^2)
Projective plane curve
defined by 0 = 2*x^3*z - 3*x^2*z^2 - y^4 + 2*y^2*z^2
julia> parametrization(C)
3-element Vector{QQMPolyRingElem}:
12*s^4 - 8*s^2*t^2 + t^4
-12*s^3*t + 2*s*t^3
8*s^4
Contact
Please direct questions about this part of OSCAR to the following people:
You can ask questions in the OSCAR Slack.
Alternatively, you can raise an issue on github.