# Projective curves¶

EXAMPLES:

We can construct curves in either a projective plane:

sage: P.<x,y,z> = ProjectiveSpace(QQ, 2)
sage: C = Curve([y*z^2 - x^3], P); C
Projective Plane Curve over Rational Field defined by -x^3 + y*z^2


or in higher dimensional projective spaces:

sage: P.<x,y,z,w> = ProjectiveSpace(QQ, 3)
sage: C = Curve([y*w^3 - x^4, z*w^3 - x^4], P); C
Projective Curve over Rational Field defined by -x^4 + y*w^3, -x^4 + z*w^3


AUTHORS:

• William Stein (2005-11-13)
• David Joyner (2005-11-13)
• David Kohel (2006-01)
• Moritz Minzlaff (2010-11)
• Grayson Jorgenson (2016-8)
sage.schemes.curves.projective_curve.Hasse_bounds(q, genus=1)

Return the Hasse-Weil bounds for the cardinality of a nonsingular curve defined over $$\GF{q}$$ of given genus.

INPUT:

• q (int) – a prime power
• genus (int, default 1) – a non-negative integer,

OUTPUT:

(tuple) The Hasse bounds (lb,ub) for the cardinality of a curve of genus genus defined over $$\GF{q}$$.

EXAMPLES:

sage: Hasse_bounds(2)
(1, 5)
sage: Hasse_bounds(next_prime(10^30))
(999999999999998000000000000058, 1000000000000002000000000000058)

class sage.schemes.curves.projective_curve.ProjectiveCurve(A, X)

Initialization function.

EXAMPLES:

sage: P.<x,y,z,w,u> = ProjectiveSpace(GF(7), 4)
sage: C = Curve([y*u^2 - x^3, z*u^2 - x^3, w*u^2 - x^3, y^3 - x^3], P); C
Projective Curve over Finite Field of size 7 defined by -x^3 + y*u^2,
-x^3 + z*u^2, -x^3 + w*u^2, -x^3 + y^3

sage: K.<u> = CyclotomicField(11)
sage: P.<x,y,z,w> = ProjectiveSpace(K, 3)
sage: C = Curve([y*w - u*z^2 - x^2, x*w - 3*u^2*z*w], P); C
Projective Curve over Cyclotomic Field of order 11 and degree 10 defined
by -x^2 + (-u)*z^2 + y*w, x*w + (-3*u^2)*z*w

affine_patch(i, AA=None)

Return the i-th affine patch of this projective curve.

INPUT:

• i – affine coordinate chart of the projective ambient space of this curve to compute affine patch with respect to.
• AA – (default: None) ambient affine space, this is constructed if it is not given.

OUTPUT:

• a curve in affine space.

EXAMPLES:

sage: P.<x,y,z,w> = ProjectiveSpace(CC, 3)
sage: C = Curve([y*z - x^2, w^2 - x*y], P)
sage: C.affine_patch(0)
Affine Curve over Complex Field with 53 bits of precision defined by
y*z - 1.00000000000000, w^2 - y

sage: P.<x,y,z> = ProjectiveSpace(QQ, 2)
sage: C = Curve(x^3 - x^2*y + y^3 - x^2*z, P)
sage: C.affine_patch(1)
Affine Plane Curve over Rational Field defined by x^3 - x^2*z - x^2 + 1

sage: A.<x,y> = AffineSpace(QQ, 2)
sage: P.<u,v,w> = ProjectiveSpace(QQ, 2)
sage: C = Curve([u^2 - v^2], P)
sage: C.affine_patch(1, A).ambient_space() == A
True

arithmetic_genus()

Return the arithmetic genus of this projective curve.

This is the arithmetic genus $$g_a(C)$$ as defined in [Har1977]. If $$P$$ is the Hilbert polynomial of the defining ideal of this curve, then the arithmetic genus of this curve is $$1 - P(0)$$. This curve must be irreducible.

OUTPUT: Integer.

EXAMPLES:

sage: P.<x,y,z,w> = ProjectiveSpace(QQ, 3)
sage: C = P.curve([w*z - x^2, w^2 + y^2 + z^2])
sage: C.arithmetic_genus()
1

sage: P.<x,y,z,w,t> = ProjectiveSpace(GF(7), 4)
sage: C = P.curve([t^3 - x*y*w, x^3 + y^3 + z^3, z - w])
sage: C.arithmetic_genus()
10

is_complete_intersection()

Return whether this projective curve is a complete intersection.

OUTPUT: Boolean.

EXAMPLES:

sage: P.<x,y,z,w> = ProjectiveSpace(QQ, 3)
sage: C = Curve([x*y - z*w, x^2 - y*w, y^2*w - x*z*w], P)
sage: C.is_complete_intersection()
False

sage: P.<x,y,z,w> = ProjectiveSpace(QQ, 3)
sage: C = Curve([y*w - x^2, z*w^2 - x^3], P)
sage: C.is_complete_intersection()
True

sage: P.<x,y,z,w> = ProjectiveSpace(QQ, 3)
sage: C = Curve([z^2 - y*w, y*z - x*w, y^2 - x*z], P)
sage: C.is_complete_intersection()
False

plane_projection(PP=None)

Return a projection of this curve into a projective plane.

INPUT:

• PP – (default: None) the projective plane the projected curve will be defined in. This space must be defined over the same base field as this curve, and must have dimension two. This space is constructed if not specified.

OUTPUT:

• a tuple consisting of two elements: a scheme morphism from this curve into a projective plane, and the projective curve that is the image of that morphism.

EXAMPLES:

sage: P.<x,y,z,w,u,v> = ProjectiveSpace(QQ, 5)
sage: C = P.curve([x*u - z*v, w - y, w*y - x^2, y^3*u*2*z - w^4*w])
sage: L.<a,b,c> = ProjectiveSpace(QQ, 2)
sage: proj1 = C.plane_projection(PP=L)
sage: proj1
(Scheme morphism:
From: Projective Curve over Rational Field defined by x*u - z*v, -y +
w, -x^2 + y*w, -w^5 + 2*y^3*z*u
To:   Projective Space of dimension 2 over Rational Field
Defn: Defined on coordinates by sending (x : y : z : w : u : v) to
(x : -z + u : -z + v),
Projective Plane Curve over Rational Field defined by a^8 + 6*a^7*b +
4*a^5*b^3 - 4*a^7*c - 2*a^6*b*c - 4*a^5*b^2*c + 2*a^6*c^2)
sage: proj1[1].ambient_space() is L
True
sage: proj2 = C.projection()
sage: proj2[1].ambient_space() is L
False

sage: P.<x,y,z,w,u> = ProjectiveSpace(GF(7), 4)
sage: C = P.curve([x^2 - 6*y^2, w*z*u - y^3 + 4*y^2*z, u^2 - x^2])
sage: C.plane_projection()
(Scheme morphism:
From: Projective Curve over Finite Field of size 7 defined by x^2 +
y^2, -y^3 - 3*y^2*z + z*w*u, -x^2 + u^2
To:   Projective Space of dimension 2 over Finite Field of size 7
Defn: Defined on coordinates by sending (x : y : z : w : u) to
(y : z : -x + w),
Projective Plane Curve over Finite Field of size 7 defined by x0^10 -
2*x0^9*x1 + 3*x0^8*x1^2 - 2*x0^7*x1^3 + x0^6*x1^4 + 2*x0^6*x1^2*x2^2 -
2*x0^5*x1^3*x2^2 - x0^4*x1^4*x2^2 + x0^2*x1^4*x2^4)

sage: P.<x,y,z> = ProjectiveSpace(GF(17), 2)
sage: C = P.curve(x^2 - y*z - z^2)
sage: C.plane_projection()
Traceback (most recent call last):
...
TypeError: this curve is already a plane curve

projection(P=None, PS=None)

Return a projection of this curve into projective space of dimension one less than the dimension of the ambient space of this curve.

This curve must not already be a plane curve. Over finite fields, if this curve contains all points in its ambient space, then an error will be returned.

INPUT:

• P – (default: None) a point not on this curve that will be used to define the projection map; this is constructed if not specified.
• PS – (default: None) the projective space the projected curve will be defined in. This space must be defined over the same base ring as this curve, and must have dimension one less than that of the ambient space of this curve. This space will be constructed if not specified.

OUTPUT:

• a tuple consisting of two elements: a scheme morphism from this curve into a projective space of dimension one less than that of the ambient space of this curve, and the projective curve that is the image of that morphism.

EXAMPLES:

sage: K.<a> = CyclotomicField(3)
sage: P.<x,y,z,w> = ProjectiveSpace(K, 3)
sage: C = Curve([y*w - x^2, z*w^2 - a*x^3], P)
sage: L.<a,b,c> = ProjectiveSpace(K, 2)
sage: proj1 = C.projection(PS=L)
sage: proj1
(Scheme morphism:
From: Projective Curve over Cyclotomic Field of order 3 and degree 2
defined by -x^2 + y*w, (-a)*x^3 + z*w^2
To:   Projective Space of dimension 2 over Cyclotomic Field of order
3 and degree 2
Defn: Defined on coordinates by sending (x : y : z : w) to
(x : y : -z + w),
Projective Plane Curve over Cyclotomic Field of order 3 and degree 2
defined by a^6 + (-a)*a^3*b^3 - a^4*b*c)
sage: proj1[1].ambient_space() is L
True
sage: proj2 = C.projection()
sage: proj2[1].ambient_space() is L
False

sage: P.<x,y,z,w,a,b,c> = ProjectiveSpace(QQ, 6)
sage: C = Curve([y - x, z - a - b, w^2 - c^2, z - x - a, x^2 - w*z], P)
sage: C.projection()
(Scheme morphism:
From: Projective Curve over Rational Field defined by -x + y, z - a -
b, w^2 - c^2, -x + z - a, x^2 - z*w
To:   Projective Space of dimension 5 over Rational Field
Defn: Defined on coordinates by sending (x : y : z : w : a : b : c)
to
(x : y : -z + w : a : b : c),
Projective Curve over Rational Field defined by x1 - x4, x0 - x4, x2*x3
+ x3^2 + x2*x4 + 2*x3*x4, x2^2 - x3^2 - 2*x3*x4 + x4^2 - x5^2, x2*x4^2 +
x3*x4^2 + x4^3 - x3*x5^2 - x4*x5^2, x4^4 - x3^2*x5^2 - 2*x3*x4*x5^2 -
x4^2*x5^2)

sage: P.<x,y,z,w> = ProjectiveSpace(GF(2), 3)
sage: C = P.curve([(x - y)*(x - z)*(x - w)*(y - z)*(y - w), x*y*z*w*(x+y+z+w)])
sage: C.projection()
Traceback (most recent call last):
...
NotImplementedError: this curve contains all points of its ambient space

sage: P.<x,y,z,w,u> = ProjectiveSpace(GF(7), 4)
sage: C = P.curve([x^3 - y*z*u, w^2 - u^2 + 2*x*z, 3*x*w - y^2])
sage: L.<a,b,c,d> = ProjectiveSpace(GF(7), 3)
sage: C.projection(PS=L)
(Scheme morphism:
From: Projective Curve over Finite Field of size 7 defined by x^3 -
y*z*u, 2*x*z + w^2 - u^2, -y^2 + 3*x*w
To:   Projective Space of dimension 3 over Finite Field of size 7
Defn: Defined on coordinates by sending (x : y : z : w : u) to
(x : y : z : w),
Projective Curve over Finite Field of size 7 defined by b^2 - 3*a*d,
a^5*b + a*b*c^3*d - 3*b*c^2*d^3, a^6 + a^2*c^3*d - 3*a*c^2*d^3)
sage: Q.<a,b,c> = ProjectiveSpace(GF(7), 2)
sage: C.projection(PS=Q)
Traceback (most recent call last):
...
TypeError: (=Projective Space of dimension 2 over Finite Field of size
7) must have dimension (=3)

sage: PP.<x,y,z,w> = ProjectiveSpace(QQ, 3)
sage: C = PP.curve([x^3 - z^2*y, w^2 - z*x])
sage: Q = PP([1,0,1,1])
sage: C.projection(P=Q)
(Scheme morphism:
From: Projective Curve over Rational Field defined by x^3 - y*z^2,
-x*z + w^2
To:   Projective Space of dimension 2 over Rational Field
Defn: Defined on coordinates by sending (x : y : z : w) to
(y : -x + z : -x + w),
Projective Plane Curve over Rational Field defined by x0*x1^5 -
6*x0*x1^4*x2 + 14*x0*x1^3*x2^2 - 16*x0*x1^2*x2^3 + 9*x0*x1*x2^4 -
2*x0*x2^5 - x2^6)
sage: LL.<a,b,c> = ProjectiveSpace(QQ, 2)
sage: Q = PP([0,0,0,1])
sage: C.projection(PS=LL, P=Q)
(Scheme morphism:
From: Projective Curve over Rational Field defined by x^3 - y*z^2,
-x*z + w^2
To:   Projective Space of dimension 2 over Rational Field
Defn: Defined on coordinates by sending (x : y : z : w) to
(x : y : z),
Projective Plane Curve over Rational Field defined by a^3 - b*c^2)
sage: Q = PP([0,0,1,0])
sage: C.projection(P=Q)
Traceback (most recent call last):
...
TypeError: (=(0 : 0 : 1 : 0)) must be a point not on this curve

sage: P.<x,y,z> = ProjectiveSpace(QQ, 2)
sage: C = P.curve(y^2 - x^2 + z^2)
sage: C.projection()
Traceback (most recent call last):
...
TypeError: this curve is already a plane curve

class sage.schemes.curves.projective_curve.ProjectivePlaneCurve(A, f)

Initialization function.

EXAMPLES:

sage: P.<x,y,z> = ProjectiveSpace(QQbar, 2)
sage: C = Curve([y*z - x^2 - QQbar.gen()*z^2], P); C
Projective Plane Curve over Algebraic Field defined by
-x^2 + y*z + (-I)*z^2

sage: P.<x,y,z> = ProjectiveSpace(GF(5^2, 'v'), 2)
sage: C = Curve([y^2*z - x*z^2 - z^3], P); C
Projective Plane Curve over Finite Field in v of size 5^2 defined by y^2*z - x*z^2 - z^3

arithmetic_genus()

Return the arithmetic genus of this projective curve.

This is the arithmetic genus $$g_a(C)$$ as defined in [Har1977]. For a projective plane curve of degree $$d$$, this is simply $$(d-1)(d-2)/2$$. It need not equal the geometric genus (the genus of the normalization of the curve). This curve must be irreducible.

OUTPUT: Integer.

EXAMPLES:

sage: x,y,z = PolynomialRing(GF(5), 3, 'xyz').gens()
sage: C = Curve(y^2*z^7 - x^9 - x*z^8); C
Projective Plane Curve over Finite Field of size 5 defined by -x^9 + y^2*z^7 - x*z^8
sage: C.arithmetic_genus()
28
sage: C.genus()
4

sage: P.<x,y,z> = ProjectiveSpace(QQ, 2)
sage: C = Curve([y^3*x - x^2*y*z - 7*z^4])
sage: C.arithmetic_genus()
3

degree()

Return the degree of this projective curve.

For a plane curve, this is just the degree of its defining polynomial.

OUTPUT: integer.

EXAMPLES:

sage: P.<x,y,z> = ProjectiveSpace(QQ, 2)
sage: C = P.curve([y^7 - x^2*z^5 + 7*z^7])
sage: C.degree()
7

divisor_of_function(r)

Return the divisor of a function on a curve.

INPUT: r is a rational function on X

OUTPUT:

• list - The divisor of r represented as a list of coefficients and points. (TODO: This will change to a more structural output in the future.)

EXAMPLES:

sage: FF = FiniteField(5)
sage: P2 = ProjectiveSpace(2, FF, names = ['x','y','z'])
sage: R = P2.coordinate_ring()
sage: x, y, z = R.gens()
sage: f = y^2*z^7 - x^9 - x*z^8
sage: C = Curve(f)
sage: K = FractionField(R)
sage: r = 1/x
sage: C.divisor_of_function(r)     # todo: not implemented  !!!!
[[-1, (0, 0, 1)]]
sage: r = 1/x^3
sage: C.divisor_of_function(r)     # todo: not implemented  !!!!
[[-3, (0, 0, 1)]]

excellent_position(Q)

Return a transformation of this curve into one in excellent position with respect to the point Q.

Here excellent position is defined as in [Ful1989]. A curve $$C$$ of degree $$d$$ containing the point $$(0 : 0 : 1)$$ with multiplicity $$r$$ is said to be in excellent position if none of the coordinate lines are tangent to $$C$$ at any of the fundamental points $$(1 : 0 : 0)$$, $$(0 : 1 : 0)$$, and $$(0 : 0 : 1)$$, and if the two coordinate lines containing $$(0 : 0 : 1)$$ intersect $$C$$ transversally in $$d - r$$ distinct non-fundamental points, and if the other coordinate line intersects $$C$$ transversally at $$d$$ distinct, non-fundamental points.

INPUT:

• Q – a point on this curve.

OUTPUT:

• a scheme morphism from this curve to a curve in excellent position that is a restriction of a change of coordinates map of the projective plane.

EXAMPLES:

sage: P.<x,y,z> = ProjectiveSpace(QQ, 2)
sage: C = Curve([x*y - z^2], P)
sage: Q = P([1,1,1])
sage: C.excellent_position(Q)
Scheme morphism:
From: Projective Plane Curve over Rational Field defined by x*y - z^2
To:   Projective Plane Curve over Rational Field defined by -x^2 -
3*x*y - 4*y^2 - x*z - 3*y*z
Defn: Defined on coordinates by sending (x : y : z) to
(-x + 1/2*y + 1/2*z : -1/2*y + 1/2*z : x + 1/2*y - 1/2*z)

sage: R.<a> = QQ[]
sage: K.<b> = NumberField(a^2 - 3)
sage: P.<x,y,z> = ProjectiveSpace(K, 2)
sage: C = P.curve([z^2*y^3*x^4 - y^6*x^3 - 4*z^2*y^4*x^3 - 4*z^4*y^2*x^3 + 3*y^7*x^2 + 10*z^2*y^5*x^2\
+ 9*z^4*y^3*x^2 + 5*z^6*y*x^2 - 3*y^8*x - 9*z^2*y^6*x - 11*z^4*y^4*x - 7*z^6*y^2*x - 2*z^8*x + y^9 +\
2*z^2*y^7 + 3*z^4*y^5 + 4*z^6*y^3 + 2*z^8*y])
sage: Q = P([1,0,0])
sage: C.excellent_position(Q)
Scheme morphism:
From: Projective Plane Curve over Number Field in b with defining
polynomial a^2 - 3 defined by -x^3*y^6 + 3*x^2*y^7 - 3*x*y^8 + y^9 +
x^4*y^3*z^2 - 4*x^3*y^4*z^2 + 10*x^2*y^5*z^2 - 9*x*y^6*z^2 + 2*y^7*z^2 -
4*x^3*y^2*z^4 + 9*x^2*y^3*z^4 - 11*x*y^4*z^4 + 3*y^5*z^4 + 5*x^2*y*z^6 -
7*x*y^2*z^6 + 4*y^3*z^6 - 2*x*z^8 + 2*y*z^8
To:   Projective Plane Curve over Number Field in b with defining
polynomial a^2 - 3 defined by 900*x^9 - 7410*x^8*y + 29282*x^7*y^2 -
69710*x^6*y^3 + 110818*x^5*y^4 - 123178*x^4*y^5 + 96550*x^3*y^6 -
52570*x^2*y^7 + 18194*x*y^8 - 3388*y^9 - 1550*x^8*z + 9892*x^7*y*z -
30756*x^6*y^2*z + 58692*x^5*y^3*z - 75600*x^4*y^4*z + 67916*x^3*y^5*z -
42364*x^2*y^6*z + 16844*x*y^7*z - 3586*y^8*z + 786*x^7*z^2 -
3958*x^6*y*z^2 + 9746*x^5*y^2*z^2 - 14694*x^4*y^3*z^2 +
15174*x^3*y^4*z^2 - 10802*x^2*y^5*z^2 + 5014*x*y^6*z^2 - 1266*y^7*z^2 -
144*x^6*z^3 + 512*x^5*y*z^3 - 912*x^4*y^2*z^3 + 1024*x^3*y^3*z^3 -
816*x^2*y^4*z^3 + 512*x*y^5*z^3 - 176*y^6*z^3 + 8*x^5*z^4 - 8*x^4*y*z^4
- 16*x^3*y^2*z^4 + 16*x^2*y^3*z^4 + 8*x*y^4*z^4 - 8*y^5*z^4
Defn: Defined on coordinates by sending (x : y : z) to
(1/4*y + 1/2*z : -1/4*y + 1/2*z : x + 1/4*y - 1/2*z)

sage: set_verbose(-1)
sage: a = QQbar(sqrt(2))
sage: P.<x,y,z> = ProjectiveSpace(QQbar, 2)
sage: C = Curve([(-1/4*a)*x^3 + (-3/4*a)*x^2*y + (-3/4*a)*x*y^2 + (-1/4*a)*y^3 - 2*x*y*z], P)
sage: Q = P([0,0,1])
sage: C.excellent_position(Q)
Scheme morphism:
From: Projective Plane Curve over Algebraic Field defined by
(-0.3535533905932738?)*x^3 + (-1.060660171779822?)*x^2*y +
(-1.060660171779822?)*x*y^2 + (-0.3535533905932738?)*y^3 + (-2)*x*y*z
To:   Projective Plane Curve over Algebraic Field defined by
(-2.828427124746190?)*x^3 + (-2)*x^2*y + 2*y^3 + (-2)*x^2*z + 2*y^2*z
Defn: Defined on coordinates by sending (x : y : z) to
(1/2*x + 1/2*y : (-1/2)*x + 1/2*y : 1/2*x + (-1/2)*y + z)

fundamental_group()

Return a presentation of the fundamental group of the complement of self.

Note

The curve must be defined over the rationals or a number field with an embedding over $$\QQbar$$.

EXAMPLES:

sage: P.<x,y,z> = ProjectiveSpace(QQ,2)
sage: C = P.curve(x^2*z-y^3)
sage: C.fundamental_group() # optional - sirocco
Finitely presented group < x0 | x0^3 >


In the case of number fields, they need to have an embedding into the algebraic field:

sage: a = QQ[x](x^2+5).roots(QQbar)[0][0]
sage: a
-2.236067977499790?*I
sage: F = NumberField(a.minpoly(), 'a', embedding=a)
sage: P.<x,y,z> = ProjectiveSpace(F, 2)
sage: F.inject_variables()
Defining a
sage: C = P.curve(x^2 + a * y^2)
sage: C.fundamental_group() # optional - sirocco
Finitely presented group < x0 |  >


Warning

This functionality requires the sirocco package to be installed.

is_ordinary_singularity(P)

Return whether the singular point P of this projective plane curve is an ordinary singularity.

The point P is an ordinary singularity of this curve if it is a singular point, and if the tangents of this curve at P are distinct.

INPUT:

• P – a point on this curve.

OUTPUT:

• Boolean. True or False depending on whether P is or is not an ordinary singularity of this curve, respectively. An error is raised if P is not a singular point of this curve.

EXAMPLES:

sage: P.<x,y,z> = ProjectiveSpace(QQ, 2)
sage: C = Curve([y^2*z^3 - x^5], P)
sage: Q = P([0,0,1])
sage: C.is_ordinary_singularity(Q)
False

sage: R.<a> = QQ[]
sage: K.<b> = NumberField(a^2 - 3)
sage: P.<x,y,z> = ProjectiveSpace(K, 2)
sage: C = P.curve([x^2*y^3*z^4 - y^6*z^3 - 4*x^2*y^4*z^3 - 4*x^4*y^2*z^3 + 3*y^7*z^2 + 10*x^2*y^5*z^2\
+ 9*x^4*y^3*z^2 + 5*x^6*y*z^2 - 3*y^8*z - 9*x^2*y^6*z - 11*x^4*y^4*z - 7*x^6*y^2*z - 2*x^8*z + y^9 +\
2*x^2*y^7 + 3*x^4*y^5 + 4*x^6*y^3 + 2*x^8*y])
sage: Q = P([0,1,1])
sage: C.is_ordinary_singularity(Q)
True

sage: P.<x,y,z> = ProjectiveSpace(QQ, 2)
sage: C = P.curve([z^5 - y^5 + x^5 + x*y^2*z^2])
sage: Q = P([0,1,1])
sage: C.is_ordinary_singularity(Q)
Traceback (most recent call last):
...
TypeError: (=(0 : 1 : 1)) is not a singular point of (=Projective Plane
Curve over Rational Field defined by x^5 - y^5 + x*y^2*z^2 + z^5)

is_singular(P=None)

Return whether this curve is singular or not, or if a point P is provided, whether P is a singular point of this curve.

INPUT:

• P – (default: None) a point on this curve.

OUTPUT:

• Boolean. If no point P is provided, returns True of False depending on whether this curve is singular or not. If a point P is provided, returns True or False depending on whether P is or is not a singular point of this curve.

EXAMPLES:

Over $$\QQ$$:

sage: F = QQ
sage: P2.<X,Y,Z> = ProjectiveSpace(F,2)
sage: C = Curve(X^3-Y^2*Z)
sage: C.is_singular()
True


Over a finite field:

sage: F = GF(19)
sage: P2.<X,Y,Z> = ProjectiveSpace(F,2)
sage: C = Curve(X^3+Y^3+Z^3)
sage: C.is_singular()
False
sage: D = Curve(X^4-X*Z^3)
sage: D.is_singular()
True
sage: E = Curve(X^5+19*Y^5+Z^5)
sage: E.is_singular()
True
sage: E = Curve(X^5+9*Y^5+Z^5)
sage: E.is_singular()
False


Over $$\CC$$:

sage: F = CC
sage: P2.<X,Y,Z> = ProjectiveSpace(F,2)
sage: C = Curve(X)
sage: C.is_singular()
False
sage: D = Curve(Y^2*Z-X^3)
sage: D.is_singular()
True
sage: E = Curve(Y^2*Z-X^3+Z^3)
sage: E.is_singular()
False


Showing that trac ticket #12187 is fixed:

sage: F.<X,Y,Z> = GF(2)[]
sage: G = Curve(X^2+Y*Z)
sage: G.is_singular()
False

sage: P.<x,y,z> = ProjectiveSpace(CC, 2)
sage: C = Curve([y^4 - x^3*z], P)
sage: Q = P([0,0,1])
sage: C.is_singular()
True

is_transverse(C, P)

Return whether the intersection of this curve with the curve C at the point P is transverse.

The intersection at P is transverse if P is a nonsingular point of both curves, and if the tangents of the curves at P are distinct.

INPUT:

• C – a curve in the ambient space of this curve.
• P – a point in the intersection of both curves.

OUTPUT: Boolean.

EXAMPLES:

sage: P.<x,y,z> = ProjectiveSpace(QQ, 2)
sage: C = Curve([x^2 - y^2], P)
sage: D = Curve([x - y], P)
sage: Q = P([1,1,0])
sage: C.is_transverse(D, Q)
False

sage: K = QuadraticField(-1)
sage: P.<x,y,z> = ProjectiveSpace(K, 2)
sage: C = Curve([y^2*z - K.0*x^3], P)
sage: D = Curve([z*x + y^2], P)
sage: Q = P([0,0,1])
sage: C.is_transverse(D, Q)
False

sage: P.<x,y,z> = ProjectiveSpace(QQ, 2)
sage: C = Curve([x^2 - 2*y^2 - 2*z^2], P)
sage: D = Curve([y - z], P)
sage: Q = P([2,1,1])
sage: C.is_transverse(D, Q)
True

local_coordinates(pt, n)

Return local coordinates to precision n at the given point.

Behaviour is flaky - some choices of $$n$$ are worse than others.

INPUT:

• pt - an F-rational point on X which is not a point of ramification for the projection (x,y) - x.
• n - the number of terms desired

OUTPUT: x = x0 + t y = y0 + power series in t

EXAMPLES:

sage: FF = FiniteField(5)
sage: P2 = ProjectiveSpace(2, FF, names = ['x','y','z'])
sage: x, y, z = P2.coordinate_ring().gens()
sage: C = Curve(y^2*z^7-x^9-x*z^8)
sage: pt = C([2,3,1])
sage: C.local_coordinates(pt,9)     # todo: not implemented  !!!!
[2 + t, 3 + 3*t^2 + t^3 + 3*t^4 + 3*t^6 + 3*t^7 + t^8 + 2*t^9 + 3*t^11 + 3*t^12]

ordinary_model()

Return a birational map from this curve to a plane curve with only ordinary singularities.

Currently only implemented over number fields. If not all of the coordinates of the non-ordinary singularities of this curve are contained in its base field, then the domain and codomain of the map returned will be defined over an extension. This curve must be irreducible.

OUTPUT:

• a scheme morphism from this curve to a curve with only ordinary singularities that defines a birational map between the two curves.

EXAMPLES:

sage: set_verbose(-1)
sage: P.<x,y,z> = ProjectiveSpace(K, 2)
sage: C = Curve([x^5 - K.0*y*z^4], P)
sage: C.ordinary_model()
Scheme morphism:
From: Projective Plane Curve over Number Field in a with defining polynomial x^2 - 3 with a = 1.732050807568878? defined by x^5 + (-a)*y*z^4
To:   Projective Plane Curve over Number Field in a with defining polynomial x^2 - 3 with a = 1.732050807568878? defined by (-a)*x^5*y + (-4*a)*x^4*y^2 + (-6*a)*x^3*y^3 + (-4*a)*x^2*y^4 + (-a)*x*y^5 + (-a - 1)*x^5*z + (-4*a + 5)*x^4*y*z + (-6*a - 10)*x^3*y^2*z + (-4*a + 10)*x^2*y^3*z + (-a - 5)*x*y^4*z + y^5*z
Defn: Defined on coordinates by sending (x : y : z) to
(-1/4*x^2 - 1/2*x*y + 1/2*x*z + 1/2*y*z - 1/4*z^2 : 1/4*x^2 + 1/2*x*y + 1/2*y*z - 1/4*z^2 : -1/4*x^2 + 1/4*z^2)

sage: set_verbose(-1)
sage: P.<x,y,z> = ProjectiveSpace(QQ, 2)
sage: C = Curve([y^2*z^2 - x^4 - x^3*z], P)
sage: D = C.ordinary_model(); D # long time (2 seconds)
Scheme morphism:
From: Projective Plane Curve over Rational Field defined by -x^4 -
x^3*z + y^2*z^2
To:   Projective Plane Curve over Rational Field defined by 4*x^6*y^3
- 24*x^5*y^4 + 36*x^4*y^5 + 8*x^6*y^2*z - 40*x^5*y^3*z + 24*x^4*y^4*z +
72*x^3*y^5*z - 4*x^6*y*z^2 + 8*x^5*y^2*z^2 - 56*x^4*y^3*z^2 +
104*x^3*y^4*z^2 + 44*x^2*y^5*z^2 + 8*x^6*z^3 - 16*x^5*y*z^3 -
24*x^4*y^2*z^3 + 40*x^3*y^3*z^3 + 48*x^2*y^4*z^3 + 8*x*y^5*z^3 -
8*x^5*z^4 + 36*x^4*y*z^4 - 56*x^3*y^2*z^4 + 20*x^2*y^3*z^4 +
40*x*y^4*z^4 - 16*y^5*z^4
Defn: Defined on coordinates by sending (x : y : z) to
(-3/64*x^4 + 9/64*x^2*y^2 - 3/32*x*y^3 - 1/16*x^3*z -
1/8*x^2*y*z + 1/4*x*y^2*z - 1/16*y^3*z - 1/8*x*y*z^2 + 1/16*y^2*z^2 :
-1/64*x^4 + 3/64*x^2*y^2 - 1/32*x*y^3 + 1/16*x*y^2*z - 1/16*y^3*z +
1/16*y^2*z^2 : 3/64*x^4 - 3/32*x^3*y + 3/64*x^2*y^2 + 1/16*x^3*z -
3/16*x^2*y*z + 1/8*x*y^2*z - 1/8*x*y*z^2 + 1/16*y^2*z^2)
sage: all(D.codomain().is_ordinary_singularity(Q) for Q in D.codomain().singular_points()) # long time
True

sage: set_verbose(-1)
sage: P.<x,y,z> = ProjectiveSpace(QQ, 2)
sage: C = Curve([(x^2 + y^2 - y*z - 2*z^2)*(y*z - x^2 + 2*z^2)*z + y^5], P)
sage: C.ordinary_model() # long time (5 seconds)
Scheme morphism:
From: Projective Plane Curve over Number Field in a with defining
polynomial y^2 - 2 defined by y^5 - x^4*z - x^2*y^2*z + 2*x^2*y*z^2 +
y^3*z^2 + 4*x^2*z^3 + y^2*z^3 - 4*y*z^4 - 4*z^5
To:   Projective Plane Curve over Number Field in a with defining
polynomial y^2 - 2 defined by (-29*a + 1)*x^8*y^6 + (10*a + 158)*x^7*y^7
+ (-109*a - 31)*x^6*y^8 + (-80*a - 198)*x^8*y^5*z + (531*a +
272)*x^7*y^6*z + (170*a - 718)*x^6*y^7*z + (19*a - 636)*x^5*y^8*z +
(-200*a - 628)*x^8*y^4*z^2 + (1557*a - 114)*x^7*y^5*z^2 + (2197*a -
2449)*x^6*y^6*z^2 + (1223*a - 3800)*x^5*y^7*z^2 + (343*a -
1329)*x^4*y^8*z^2 + (-323*a - 809)*x^8*y^3*z^3 + (1630*a -
631)*x^7*y^4*z^3 + (4190*a - 3126)*x^6*y^5*z^3 + (3904*a -
7110)*x^5*y^6*z^3 + (1789*a - 5161)*x^4*y^7*z^3 + (330*a -
1083)*x^3*y^8*z^3 + (-259*a - 524)*x^8*y^2*z^4 + (720*a -
605)*x^7*y^3*z^4 + (3082*a - 2011)*x^6*y^4*z^4 + (4548*a -
5462)*x^5*y^5*z^4 + (2958*a - 6611)*x^4*y^6*z^4 + (994*a -
2931)*x^3*y^7*z^4 + (117*a - 416)*x^2*y^8*z^4 + (-108*a - 184)*x^8*y*z^5
+ (169*a - 168)*x^7*y^2*z^5 + (831*a - 835)*x^6*y^3*z^5 + (2225*a -
1725)*x^5*y^4*z^5 + (1970*a - 3316)*x^4*y^5*z^5 + (952*a -
2442)*x^3*y^6*z^5 + (217*a - 725)*x^2*y^7*z^5 + (16*a - 77)*x*y^8*z^5 +
(-23*a - 35)*x^8*z^6 + (43*a + 24)*x^7*y*z^6 + (21*a - 198)*x^6*y^2*z^6
+ (377*a - 179)*x^5*y^3*z^6 + (458*a - 537)*x^4*y^4*z^6 + (288*a -
624)*x^3*y^5*z^6 + (100*a - 299)*x^2*y^6*z^6 + (16*a - 67)*x*y^7*z^6 -
5*y^8*z^6
Defn: Defined on coordinates by sending (x : y : z) to
((-5/128*a - 5/128)*x^4 + (-5/32*a + 5/32)*x^3*y + (-1/16*a +
3/32)*x^2*y^2 + (1/16*a - 1/16)*x*y^3 + (1/32*a - 1/32)*y^4 - 1/32*x^3*z
+ (3/16*a - 5/8)*x^2*y*z + (1/8*a - 5/16)*x*y^2*z + (1/8*a +
5/32)*x^2*z^2 + (-3/16*a + 5/16)*x*y*z^2 + (-3/16*a - 1/16)*y^2*z^2 +
1/16*x*z^3 + (1/4*a + 1/4)*y*z^3 + (-3/32*a - 5/32)*z^4 : (-5/128*a -
5/128)*x^4 + (5/32*a)*x^3*y + (3/32*a + 3/32)*x^2*y^2 + (-1/16*a)*x*y^3
+ (-1/32*a - 1/32)*y^4 - 1/32*x^3*z + (-11/32*a)*x^2*y*z + (1/8*a +
5/16)*x*y^2*z + (3/16*a + 1/4)*y^3*z + (1/8*a + 5/32)*x^2*z^2 + (-1/16*a
- 3/8)*x*y*z^2 + (-3/8*a - 9/16)*y^2*z^2 + 1/16*x*z^3 + (5/16*a +
1/2)*y*z^3 + (-3/32*a - 5/32)*z^4 : (1/64*a + 3/128)*x^4 + (-1/32*a -
1/32)*x^3*y + (3/32*a - 9/32)*x^2*y^2 + (1/16*a - 3/16)*x*y^3 - 1/32*y^4
+ (3/32*a + 1/8)*x^2*y*z + (-1/8*a + 1/8)*x*y^2*z + (-1/16*a)*y^3*z +
(-1/16*a - 3/32)*x^2*z^2 + (1/16*a + 1/16)*x*y*z^2 + (3/16*a +
3/16)*y^2*z^2 + (-3/16*a - 1/4)*y*z^3 + (1/16*a + 3/32)*z^4)

plot(*args, **kwds)

Plot the real points of an affine patch of this projective plane curve.

INPUT:

• self - an affine plane curve
• patch - (optional) the affine patch to be plotted; if not specified, the patch corresponding to the last projective coordinate being nonzero
• *args - optional tuples (variable, minimum, maximum) for plotting dimensions
• **kwds - optional keyword arguments passed on to implicit_plot

EXAMPLES:

A cuspidal curve:

sage: R.<x, y, z> = QQ[]
sage: C = Curve(x^3 - y^2*z)
sage: C.plot()
Graphics object consisting of 1 graphics primitive


The other affine patches of the same curve:

sage: C.plot(patch=0)
Graphics object consisting of 1 graphics primitive
sage: C.plot(patch=1)
Graphics object consisting of 1 graphics primitive


An elliptic curve:

sage: E = EllipticCurve('101a')
sage: C = Curve(E)
sage: C.plot()
Graphics object consisting of 1 graphics primitive
sage: C.plot(patch=0)
Graphics object consisting of 1 graphics primitive
sage: C.plot(patch=1)
Graphics object consisting of 1 graphics primitive


A hyperelliptic curve:

sage: P.<x> = QQ[]
sage: f = 4*x^5 - 30*x^3 + 45*x - 22
sage: C = HyperellipticCurve(f)
sage: C.plot()
Graphics object consisting of 1 graphics primitive
sage: C.plot(patch=0)
Graphics object consisting of 1 graphics primitive
sage: C.plot(patch=1)
Graphics object consisting of 1 graphics primitive

quadratic_transform()

Return a birational map from this curve to the proper transform of this curve with respect to the standard Cremona transformation.

The standard Cremona transformation is the birational automorphism of $$\mathbb{P}^{2}$$ defined $$(x : y : z)\mapsto (yz : xz : xy)$$.

OUTPUT:

• a scheme morphism representing the restriction of the standard Cremona transformation from this curve to the proper transform.

EXAMPLES:

sage: P.<x,y,z> = ProjectiveSpace(QQ, 2)
sage: C = Curve(x^3*y - z^4 - z^2*x^2, P)
Scheme morphism:
From: Projective Plane Curve over Rational Field defined by x^3*y -
x^2*z^2 - z^4
To:   Projective Plane Curve over Rational Field defined by -x^3*y -
x*y*z^2 + z^4
Defn: Defined on coordinates by sending (x : y : z) to
(y*z : x*z : x*y)

sage: P.<x,y,z> = ProjectiveSpace(GF(17), 2)
sage: C = P.curve([y^7*z^2 - 16*x^9 + x*y*z^7 + 2*z^9])
Scheme morphism:
From: Projective Plane Curve over Finite Field of size 17 defined by
x^9 + y^7*z^2 + x*y*z^7 + 2*z^9
To:   Projective Plane Curve over Finite Field of size 17 defined by
2*x^9*y^7 + x^8*y^6*z^2 + x^9*z^7 + y^7*z^9
Defn: Defined on coordinates by sending (x : y : z) to
(y*z : x*z : x*y)

rational_parameterization()

Return a rational parameterization of this curve.

This curve must have rational coefficients and be absolutely irreducible (i.e. irreducible over the algebraic closure of the rational field). The curve must also be rational (have geometric genus zero).

The rational parameterization may have coefficients in a quadratic extension of the rational field.

OUTPUT:

• a birational map between $$\mathbb{P}^{1}$$ and this curve, given as a scheme morphism.

EXAMPLES:

sage: P.<x,y,z> = ProjectiveSpace(QQ, 2)
sage: C = Curve([y^2*z - x^3], P)
sage: C.rational_parameterization()
Scheme morphism:
From: Projective Space of dimension 1 over Rational Field
To:   Projective Plane Curve over Rational Field defined by -x^3 + y^2*z
Defn: Defined on coordinates by sending (s : t) to
(s^2*t : s^3 : t^3)

sage: P.<x,y,z> = ProjectiveSpace(QQ, 2)
sage: C = Curve([x^3 - 4*y*z^2 + x*z^2 - x*y*z], P)
sage: C.rational_parameterization()
Scheme morphism:
From: Projective Space of dimension 1 over Rational Field
To:   Projective Plane Curve over Rational Field defined by x^3 - x*y*z + x*z^2 - 4*y*z^2
Defn: Defined on coordinates by sending (s : t) to
(4*s^2*t + s*t^2 : s^2*t + t^3 : 4*s^3 + s^2*t)

sage: P.<x,y,z> = ProjectiveSpace(QQ, 2)
sage: C = Curve([x^2 + y^2 + z^2], P)
sage: C.rational_parameterization()
Scheme morphism:
From: Projective Space of dimension 1 over Number Field in a with defining polynomial a^2 + 1
To:   Projective Plane Curve over Number Field in a with defining
polynomial a^2 + 1 defined by x^2 + y^2 + z^2
Defn: Defined on coordinates by sending (s : t) to
((-a)*s^2 + (-a)*t^2 : s^2 - t^2 : 2*s*t)

riemann_surface(**kwargs)

Return the complex Riemann surface determined by this curve

OUTPUT:

• RiemannSurface object

EXAMPLES:

sage: R.<x,y,z>=QQ[]
sage: C=Curve(x^3+3*y^3+5*z^3)
sage: C.riemann_surface()
Riemann surface defined by polynomial f = x^3 + 3*y^3 + 5 = 0, with 53 bits of precision

tangents(P, factor=True)

Return the tangents of this projective plane curve at the point P.

These are found by homogenizing the tangents of an affine patch of this curve containing P. The point P must be a point on this curve.

INPUT:

• P – a point on this curve.
• factor – (default: True) whether to attempt computing the polynomials of the individual tangent lines over the base field of this curve, or to just return the polynomial corresponding to the union of the tangent lines (which requires fewer computations).

OUTPUT:

• a list of polynomials in the coordinate ring of the ambient space of this curve.

EXAMPLES:

sage: set_verbose(-1)
sage: P.<x,y,z> = ProjectiveSpace(QQbar, 2)
sage: C = Curve([x^3*y + 2*x^2*y^2 + x*y^3 + x^3*z + 7*x^2*y*z + 14*x*y^2*z + 9*y^3*z], P)
sage: Q = P([0,0,1])
sage: C.tangents(Q)
[x + 4.147899035704788?*y, x + (1.426050482147607? + 0.3689894074818041?*I)*y,
x + (1.426050482147607? - 0.3689894074818041?*I)*y]
sage: C.tangents(Q, factor=False)
[6*x^3 + 42*x^2*y + 84*x*y^2 + 54*y^3]

sage: P.<x,y,z> = ProjectiveSpace(QQ,2)
sage: C = P.curve([x^2*y^3*z^4 - y^6*z^3 - 4*x^2*y^4*z^3 - 4*x^4*y^2*z^3 + 3*y^7*z^2 +\
10*x^2*y^5*z^2 + 9*x^4*y^3*z^2 + 5*x^6*y*z^2 - 3*y^8*z - 9*x^2*y^6*z - 11*x^4*y^4*z -\
7*x^6*y^2*z - 2*x^8*z + y^9 + 2*x^2*y^7 + 3*x^4*y^5 + 4*x^6*y^3 + 2*x^8*y])
sage: Q = P([0,1,1])
sage: C.tangents(Q)
[-y + z, 3*x^2 - y^2 + 2*y*z - z^2]

sage: P.<x,y,z> = ProjectiveSpace(QQ, 2)
sage: C = P.curve([z^3*x + y^4 - x^2*z^2])
sage: Q = P([1,1,1])
sage: C.tangents(Q)
Traceback (most recent call last):
...
TypeError: (=(1 : 1 : 1)) is not a point on (=Projective Plane Curve
over Rational Field defined by y^4 - x^2*z^2 + x*z^3)

class sage.schemes.curves.projective_curve.ProjectivePlaneCurve_finite_field(A, f)
rational_points(algorithm='enum', sort=True)

Return the rational points on this curve computed via enumeration.

INPUT:

• algorithm (string, default: ‘enum’) – the algorithm to use. Currently this is ignored.
• sort (boolean, default True) – whether the output points should be sorted. If False, the order of the output is non-deterministic.

OUTPUT:

A list of all the rational points on the curve defined over its base field, possibly sorted.

Note

This is a slow Python-level implementation.

EXAMPLES:

sage: F = GF(7)
sage: P2.<X,Y,Z> = ProjectiveSpace(F,2)
sage: C = Curve(X^3+Y^3-Z^3)
sage: C.rational_points()
[(0 : 1 : 1), (0 : 2 : 1), (0 : 4 : 1), (1 : 0 : 1), (2 : 0 : 1), (3 : 1 : 0), (4 : 0 : 1), (5 : 1 : 0), (6 : 1 : 0)]

sage: F = GF(1237)
sage: P2.<X,Y,Z> = ProjectiveSpace(F,2)
sage: C = Curve(X^7+7*Y^6*Z+Z^4*X^2*Y*89)
sage: len(C.rational_points())
1237

sage: F = GF(2^6,'a')
sage: P2.<X,Y,Z> = ProjectiveSpace(F,2)
sage: C = Curve(X^5+11*X*Y*Z^3 + X^2*Y^3 - 13*Y^2*Z^3)
sage: len(C.rational_points())
104

sage: R.<x,y,z> = GF(2)[]
sage: f = x^3*y + y^3*z + x*z^3
sage: C = Curve(f); pts = C.rational_points()
sage: pts
[(0 : 0 : 1), (0 : 1 : 0), (1 : 0 : 0)]

rational_points_iterator()

Return a generator object for the rational points on this curve.

INPUT:

• self – a projective curve

OUTPUT:

A generator of all the rational points on the curve defined over its base field.

EXAMPLES:

sage: F = GF(37)
sage: P2.<X,Y,Z> = ProjectiveSpace(F,2)
sage: C = Curve(X^7+Y*X*Z^5*55+Y^7*12)
sage: len(list(C.rational_points_iterator()))
37

sage: F = GF(2)
sage: P2.<X,Y,Z> = ProjectiveSpace(F,2)
sage: C = Curve(X*Y*Z)
sage: a = C.rational_points_iterator()
sage: next(a)
(1 : 0 : 0)
sage: next(a)
(0 : 1 : 0)
sage: next(a)
(1 : 1 : 0)
sage: next(a)
(0 : 0 : 1)
sage: next(a)
(1 : 0 : 1)
sage: next(a)
(0 : 1 : 1)
sage: next(a)
Traceback (most recent call last):
...
StopIteration

sage: F = GF(3^2,'a')
sage: P2.<X,Y,Z> = ProjectiveSpace(F,2)
sage: C = Curve(X^3+5*Y^2*Z-33*X*Y*X)
sage: b = C.rational_points_iterator()
sage: next(b)
(0 : 1 : 0)
sage: next(b)
(0 : 0 : 1)
sage: next(b)
(2*a + 2 : a : 1)
sage: next(b)
(2 : a + 1 : 1)
sage: next(b)
(a + 1 : 2*a + 1 : 1)
sage: next(b)
(1 : 2 : 1)
sage: next(b)
(2*a + 2 : 2*a : 1)
sage: next(b)
(2 : 2*a + 2 : 1)
sage: next(b)
(a + 1 : a + 2 : 1)
sage: next(b)
(1 : 1 : 1)
sage: next(b)
Traceback (most recent call last):
...
StopIteration

class sage.schemes.curves.projective_curve.ProjectivePlaneCurve_prime_finite_field(A, f)
rational_points(algorithm='enum', sort=True)

INPUT:

• algorithm - string:
• 'enum' - straightforward enumeration
• 'bn' - via Singular’s brnoeth package.

EXAMPLES:

sage: x, y, z = PolynomialRing(GF(5), 3, 'xyz').gens()
sage: f = y^2*z^7 - x^9 - x*z^8
sage: C = Curve(f); C
Projective Plane Curve over Finite Field of size 5 defined by
-x^9 + y^2*z^7 - x*z^8
sage: C.rational_points()
[(0 : 0 : 1), (0 : 1 : 0), (2 : 2 : 1), (2 : 3 : 1),
(3 : 1 : 1), (3 : 4 : 1)]
sage: C = Curve(x - y + z)
sage: C.rational_points()
[(0 : 1 : 1), (1 : 1 : 0), (1 : 2 : 1), (2 : 3 : 1),
(3 : 4 : 1), (4 : 0 : 1)]
sage: C = Curve(x*z+z^2)
sage: C.rational_points('all')
[(0 : 1 : 0), (1 : 0 : 0), (1 : 1 : 0), (2 : 1 : 0),
(3 : 1 : 0), (4 : 0 : 1), (4 : 1 : 0), (4 : 1 : 1),
(4 : 2 : 1), (4 : 3 : 1), (4 : 4 : 1)]


Note

The Brill-Noether package does not always work (i.e., the ‘bn’ algorithm. When it fails a RuntimeError exception is raised.

riemann_roch_basis(D)

Return a basis for the Riemann-Roch space corresponding to $$D$$.

This uses Singular’s Brill-Noether implementation.

INPUT:

• D - a divisor

OUTPUT:

A list of function field elements that form a basis of the Riemann-Roch space

EXAMPLES:

sage: R.<x,y,z> = GF(2)[]
sage: f = x^3*y + y^3*z + x*z^3
sage: C = Curve(f); pts = C.rational_points()
sage: D = C.divisor([ (4, pts[0]), (4, pts[2]) ])
sage: C.riemann_roch_basis(D)
[x/y, 1, z/y, z^2/y^2, z/x, z^2/(x*y)]

sage: R.<x,y,z> = GF(5)[]
sage: f = x^7 + y^7 + z^7
sage: C = Curve(f); pts = C.rational_points()
sage: D = C.divisor([ (3, pts[0]), (-1,pts[1]), (10, pts[5]) ])
sage: C.riemann_roch_basis(D)
[(-x - 2*y)/(-2*x - 2*y), (-x + z)/(x + y)]


Note

Currently this only works over prime field and divisors supported on rational points.