# Base class of curves¶

This module defines the base class of curves in Sage.

Curves in Sage are reduced subschemes of dimension 1 of an ambient space. The ambient space is either an affine space or a projective space.

EXAMPLES:

sage: A.<x,y,z> = AffineSpace(QQ, 3)
sage: C = Curve([x - y, z - 2])
sage: C
Affine Curve over Rational Field defined by x - y, z - 2
sage: C.dimension()
1


AUTHORS:

• William Stein (2005)

class sage.schemes.curves.curve.Curve_generic(A, polynomials)

Generic curve class.

EXAMPLES:

sage: A.<x,y,z> = AffineSpace(QQ,3)
sage: C = Curve([x-y,z-2])
True

change_ring(R)

Return a new curve which is this curve coerced to R.

INPUT:

• R – ring or embedding

OUTPUT: a new curve which is this curve coerced to R

EXAMPLES:

sage: P.<x,y,z,w> = ProjectiveSpace(QQ, 3)
sage: C = Curve([x^2 - y^2, z*y - 4/5*w^2], P)
Projective Curve over Number Field in a with defining polynomial x^2 + 1 with a = 1*I defined by x^2 - y^2, y*z - 4/5*w^2

sage: R.<a> = QQ[]
sage: K.<b> = NumberField(a^3 + a^2 - 1)
sage: A.<x,y> = AffineSpace(K, 2)
sage: C = Curve([K.0*x^2 - x + y^3 - 11], A)
sage: L = K.embeddings(QQbar)
sage: set_verbose(-1)  # suppress warnings for slow computation
sage: C.change_ring(L)
Affine Plane Curve over Algebraic Field defined by y^3 +
(-0.8774388331233464? - 0.744861766619745?*I)*x^2 - x - 11

sage: P.<x,y,z> = ProjectiveSpace(QQ, 2)
sage: C = P.curve([y*x - 18*x^2 + 17*z^2])
sage: C.change_ring(GF(17))
Projective Plane Curve over Finite Field of size 17 defined by -x^2 + x*y

defining_polynomial()

Return the defining polynomial of the curve.

EXAMPLES:

sage: x,y,z = PolynomialRing(QQ, 3, names='x,y,z').gens()
sage: C = Curve(y^2*z - x^3 - 17*x*z^2 + y*z^2)
sage: C.defining_polynomial()
-x^3 + y^2*z - 17*x*z^2 + y*z^2

divisor(v, base_ring=None, check=True, reduce=True)

Return the divisor specified by v.

Warning

The coefficients of the divisor must be in the base ring and the terms must be reduced. If you set check=False and/or reduce=False it is your responsibility to pass a valid object v.

EXAMPLES:

sage: x,y,z = PolynomialRing(QQ, 3, names='x,y,z').gens()
sage: C = Curve(y^2*z - x^3 - 17*x*z^2 + y*z^2)

divisor_group(base_ring=None)

Return the divisor group of the curve.

INPUT:

• base_ring – the base ring of the divisor group. Usually, this is $$\ZZ$$ (default) or $$\QQ$$.

OUTPUT: the divisor group of the curve

EXAMPLES:

sage: x,y,z = PolynomialRing(QQ, 3, names='x,y,z').gens()
sage: C  = Curve(y^2*z - x^3 - 17*x*z^2 + y*z^2)
sage: Cp = Curve(y^2*z - x^3 - 17*x*z^2 + y*z^2)
sage: C.divisor_group() is Cp.divisor_group()
True

genus()

Return the geometric genus of the curve.

EXAMPLES:

sage: x,y,z = PolynomialRing(QQ, 3, names='x,y,z').gens()
sage: C = Curve(y^2*z - x^3 - 17*x*z^2 + y*z^2)
sage: C.genus()
1

geometric_genus()

Return the geometric genus of the curve.

This is by definition the genus of the normalization of the projective closure of the curve over the algebraic closure of the base field; the base field must be a prime field.

Note

This calls Singular’s genus command.

EXAMPLES:

Examples of projective curves.

sage: P2 = ProjectiveSpace(2, GF(5), names=['x','y','z'])
sage: x, y, z = P2.coordinate_ring().gens()
sage: C = Curve(y^2*z - x^3 - 17*x*z^2 + y*z^2)
sage: C.geometric_genus()
1
sage: C = Curve(y^2*z - x^3)
sage: C.geometric_genus()
0
sage: C = Curve(x^10 + y^7*z^3 + z^10)
sage: C.geometric_genus()
3


Examples of affine curves.

sage: x, y = PolynomialRing(GF(5), 2, 'xy').gens()
sage: C = Curve(y^2 - x^3 - 17*x + y)
sage: C.geometric_genus()
1
sage: C = Curve(y^2 - x^3)
sage: C.geometric_genus()
0
sage: C = Curve(x^10 + y^7 + 1)
sage: C.geometric_genus()
3

intersection_points(C, F=None)

Return the points in the intersection of this curve and the curve C.

If the intersection of these two curves has dimension greater than zero, and if the base ring of this curve is not a finite field, then an error is returned.

INPUT:

• C – a curve in the same ambient space as this curve

• F – (default: None); field over which to compute the intersection points; if not specified, the base ring of this curve is used

OUTPUT: a list of points in the ambient space of this curve

EXAMPLES:

sage: R.<a> = QQ[]
sage: K.<b> = NumberField(a^2 + a + 1)
sage: P.<x,y,z,w> = ProjectiveSpace(QQ, 3)
sage: C = Curve([y^2 - w*z, w^3 - y^3], P)
sage: D = Curve([x*y - w*z, z^3 - y^3], P)
sage: C.intersection_points(D, F=K)
[(-b - 1 : -b - 1 : b : 1), (b : b : -b - 1 : 1), (1 : 0 : 0 : 0),
(1 : 1 : 1 : 1)]

sage: A.<x,y> = AffineSpace(GF(7), 2)
sage: C = Curve([y^3 - x^3], A)
sage: D = Curve([-x*y^3 + y^4 - 2*x^3 + 2*x^2*y], A)
sage: C.intersection_points(D)
[(0, 0), (1, 1), (2, 2), (3, 3), (4, 4), (5, 3), (5, 5), (5, 6),
(6, 6)]

sage: A.<x,y> = AffineSpace(QQ, 2)
sage: C = Curve([y^3 - x^3], A)
sage: D = Curve([-x*y^3 + y^4 - 2*x^3 + 2*x^2*y], A)
sage: C.intersection_points(D)
Traceback (most recent call last):
...
NotImplementedError: the intersection must have dimension zero or
(=Rational Field) must be a finite field

intersects_at(C, P)

Return whether the point P is or is not in the intersection of this curve with the curve C.

INPUT:

• C – a curve in the same ambient space as this curve.

• P – a point in the ambient space of this curve.

EXAMPLES:

sage: P.<x,y,z,w> = ProjectiveSpace(QQ, 3)
sage: C = Curve([x^2 - z^2, y^3 - w*x^2], P)
sage: D = Curve([w^2 - 2*x*y + z^2, y^2 - w^2], P)
sage: Q1 = P([1,1,-1,1])
sage: C.intersects_at(D, Q1)
True
sage: Q2 = P([0,0,1,-1])
sage: C.intersects_at(D, Q2)
False

sage: A.<x,y> = AffineSpace(GF(13), 2)
sage: C = Curve([y + 12*x^5 + 3*x^3 + 7], A)
sage: D = Curve([y^2 + 7*x^2 + 8], A)
sage: Q1 = A([9,6])
sage: C.intersects_at(D, Q1)
True
sage: Q2 = A([3,7])
sage: C.intersects_at(D, Q2)
False

is_singular(P=None)

Return whether P is a singular point of this curve, or if no point is passed, whether this curve is singular or not.

This just uses the is_smooth function for algebraic subschemes.

INPUT:

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

OUTPUT:

A boolean. If a point P is provided, and if P lies on this curve, returns True if P is a singular point of this curve, and False otherwise. If no point is provided, returns True or False depending on whether this curve is or is not singular, respectively.

EXAMPLES:

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

sage: A.<x,y,z> = AffineSpace(GF(11), 3)
sage: C = A.curve([y^3 - z^5, x^5 - y + 1])
sage: Q = A([7,0,0])
sage: C.is_singular(Q)
True

singular_points(F=None)

Return the set of singular points of this curve.

INPUT:

• F – (default: None) field over which to find the singular points; if not given, the base ring of this curve is used

OUTPUT: a list of points in the ambient space of this curve

EXAMPLES:

sage: A.<x,y,z> = AffineSpace(QQ, 3)
sage: C = Curve([y^2 - x^5, x - z], A)
sage: C.singular_points()
[(0, 0, 0)]

sage: R.<a> = QQ[]
sage: K.<b> = NumberField(a^8 - a^4 + 1)
sage: P.<x,y,z> = ProjectiveSpace(QQ, 2)
sage: C = Curve([359/12*x*y^2*z^2 + 2*y*z^4 + 187/12*y^3*z^2 + x*z^4\
+ 67/3*x^2*y*z^2 + 117/4*y^5 + 9*x^5 + 6*x^3*z^2 + 393/4*x*y^4\
+ 145*x^2*y^3 + 115*x^3*y^2 + 49*x^4*y], P)
sage: sorted(C.singular_points(K), key=str)
[(-1/2*b^5 - 1/2*b^3 + 1/2*b - 1 : 1 : 0),
(-2/3*b^4 + 1/3 : 0 : 1),
(-b^6 : b^6 : 1),
(1/2*b^5 + 1/2*b^3 - 1/2*b - 1 : 1 : 0),
(2/3*b^4 - 1/3 : 0 : 1),
(b^6 : -b^6 : 1)]

singular_subscheme()

Return the subscheme of singular points of this curve.

OUTPUT:

• a subscheme in the ambient space of this curve.

EXAMPLES:

sage: A.<x,y> = AffineSpace(CC, 2)
sage: C = Curve([y^4 - 2*x^5 - x^2*y], A)
sage: C.singular_subscheme()
Closed subscheme of Affine Space of dimension 2 over Complex Field
with 53 bits of precision defined by:
(-2.00000000000000)*x^5 + y^4 - x^2*y,
(-10.0000000000000)*x^4 + (-2.00000000000000)*x*y,
4.00000000000000*y^3 - x^2

sage: P.<x,y,z,w> = ProjectiveSpace(QQ, 3)
sage: C = Curve([y^8 - x^2*z*w^5, w^2 - 2*y^2 - x*z], P)
sage: C.singular_subscheme()
Closed subscheme of Projective Space of dimension 3 over Rational
Field defined by:
y^8 - x^2*z*w^5,
-2*y^2 - x*z + w^2,
-x^3*y*z^4 + 3*x^2*y*z^3*w^2 - 3*x*y*z^2*w^4 + 8*x*y*z*w^5 + y*z*w^6,
x^2*z*w^5,
-5*x^2*z^2*w^4 - 4*x*z*w^6,
x^4*y*z^3 - 3*x^3*y*z^2*w^2 + 3*x^2*y*z*w^4 - 4*x^2*y*w^5 - x*y*w^6,
-2*x^3*y*z^3*w + 6*x^2*y*z^2*w^3 - 20*x^2*y*z*w^4 - 6*x*y*z*w^5 +
2*y*w^7,
-5*x^3*z*w^4 - 2*x^2*w^6

union(other)

Return the union of self and other.

EXAMPLES:

sage: x,y,z = PolynomialRing(QQ, 3, names='x,y,z').gens()
sage: C1 = Curve(z - x)
sage: C2 = Curve(y - x)
sage: C1.union(C2).defining_polynomial()
x^2 - x*y - x*z + y*z