Subschemes of projective space

AUTHORS:

• David Kohel (2005): initial version.
• William Stein (2005): initial version.
• Volker Braun (2010-12-24): documentation of schemes and refactoring. Added coordinate neighborhoods and is_smooth()
• Ben Hutz (2013) refactoring

class sage.schemes.projective.projective_subscheme.AlgebraicScheme_subscheme_projective(A, polynomials)

Bases: sage.schemes.generic.algebraic_scheme.AlgebraicScheme_subscheme

Construct an algebraic subscheme of projective space.

Warning

You should not create objects of this class directly. The preferred method to construct such subschemes is to use subscheme() method of projective space.

INPUT:

• A – ambient projective space.
• polynomials – single polynomial, ideal or iterable of defining homogeneous polynomials.

EXAMPLES:

sage: P.<x, y, z> = ProjectiveSpace(2, QQ)
sage: P.subscheme([x^2-y*z])
Closed subscheme of Projective Space of dimension 2 over Rational Field defined by:
  x^2 - y*z

Chow_form()

Returns the Chow form associated to this subscheme.

For a \(k\)-dimensional subvariety of \(\mathbb{P}^N\) of degree \(D\). The \((N-k-1)\)-dimensional projective linear subspaces of \(\mathbb{P}^N\) meeting \(X\) form a hypersurface in the Grassmannian \(G(N-k-1,N)\). The homogeneous form of degree \(D\) defining this hypersurface in Plucker coordinates is called the Chow form of \(X\).

The base ring needs to be a number field, finite field, or \(\QQbar\).

ALGORITHM:

For a \(k\)-dimension subscheme \(X\) consider the \(k+1\) linear forms \(l_i = u_{i0}x_0 + \cdots + u_{in}x_n\). Let \(J\) be the ideal in the polynomial ring \(K[x_i,u_{ij}]\) defined by the equations of \(X\) and the \(l_i\). Let \(J'\) be the saturation of \(J\) with respect to the irrelevant ideal of the ambient projective space of \(X\). The elimination ideal \(I = J' \cap K[u_{ij}]\) is a principal ideal, let \(R\) be its generator. The Chow form is obtained by writing \(R\) as a polynomial in Plucker coordinates (i.e. bracket polynomials). [DS1994].

OUTPUT: a homogeneous polynomial.

EXAMPLES:

sage: P.<x0,x1,x2,x3> = ProjectiveSpace(GF(17), 3)
sage: X = P.subscheme([x3+x1,x2-x0,x2-x3])
sage: X.Chow_form()
t0 - t1 + t2 + t3

sage: P.<x0,x1,x2,x3> = ProjectiveSpace(QQ,3)
sage: X = P.subscheme([x3^2 -101*x1^2 - 3*x2*x0])
sage: X.Chow_form()
t0^2 - 101*t2^2 - 3*t1*t3

sage: P.<x0,x1,x2,x3>=ProjectiveSpace(QQ,3)
sage: X = P.subscheme([x0*x2-x1^2, x0*x3-x1*x2, x1*x3-x2^2])
sage: Ch = X.Chow_form(); Ch
t2^3 + 2*t2^2*t3 + t2*t3^2 - 3*t1*t2*t4 - t1*t3*t4 + t0*t4^2 + t1^2*t5
sage: Y = P.subscheme_from_Chow_form(Ch, 1); Y
Closed subscheme of Projective Space of dimension 3 over Rational Field
defined by:
  x2^2*x3 - x1*x3^2,
  -x2^3 + x0*x3^2,
  -x2^2*x3 + x1*x3^2,
  x1*x2*x3 - x0*x3^2,
  3*x1*x2^2 - 3*x0*x2*x3,
  -2*x1^2*x3 + 2*x0*x2*x3,
  -3*x1^2*x2 + 3*x0*x1*x3,
  x1^3 - x0^2*x3,
  x2^3 - x1*x2*x3,
  -3*x1*x2^2 + 2*x1^2*x3 + x0*x2*x3,
  2*x0*x2^2 - 2*x0*x1*x3,
  3*x1^2*x2 - 2*x0*x2^2 - x0*x1*x3,
  -x0*x1*x2 + x0^2*x3,
  -x0*x1^2 + x0^2*x2,
  -x1^3 + x0*x1*x2,
  x0*x1^2 - x0^2*x2
sage: I = Y.defining_ideal()
sage: I.saturation(I.ring().ideal(list(I.ring().gens())))[0]
Ideal (x2^2 - x1*x3, x1*x2 - x0*x3, x1^2 - x0*x2) of Multivariate
Polynomial Ring in x0, x1, x2, x3 over Rational Field

affine_patch(i, AA=None)

Return the \(i^{th}\) affine patch of this projective scheme. This is the intersection with this \(i^{th}\) affine patch of its ambient space.

INPUT:

• i – integer between 0 and dimension of self, inclusive.

• AA – (default: None) ambient affine space, this is constructed

  if it is not given.

OUTPUT:

An affine algebraic scheme with fixed embedding_morphism() equal to the default projective_embedding() map`.

EXAMPLES:

sage: PP = ProjectiveSpace(2, QQ, names='X,Y,Z')
sage: X,Y,Z = PP.gens()
sage: C = PP.subscheme(X^3*Y + Y^3*Z + Z^3*X)
sage: U = C.affine_patch(0)
sage: U
Closed subscheme of Affine Space of dimension 2 over Rational Field defined by:
  Y^3*Z + Z^3 + Y
sage: U.embedding_morphism()
Scheme morphism:
  From: Closed subscheme of Affine Space of dimension 2 over Rational Field defined by:
  Y^3*Z + Z^3 + Y
  To:   Closed subscheme of Projective Space of dimension 2 over Rational Field defined by:
  X^3*Y + Y^3*Z + X*Z^3
  Defn: Defined on coordinates by sending (Y, Z) to
        (1 : Y : Z)
sage: U.projective_embedding() is U.embedding_morphism()
True

sage: A.<x,y,z> = AffineSpace(QQ,3)
sage: X = A.subscheme([x-y*z])
sage: Y = X.projective_embedding(1).codomain()
sage: Y.affine_patch(1,A).ambient_space() == A
True

sage: P.<u,v,w> = ProjectiveSpace(2,ZZ)
sage: S = P.subscheme([u^2-v*w])
sage: A.<x, y> = AffineSpace(2, ZZ)
sage: S.affine_patch(1, A)
Closed subscheme of Affine Space of dimension 2 over Integer Ring
defined by:
  x^2 - y

degree()

Return the degree of this projective subscheme.

If \(P(t) = a_{m}t^m + \ldots + a_{0}\) is the Hilbert polynomial of this subscheme, then the degree is \(a_{m} m!\).

OUTPUT: Integer.

EXAMPLES:

sage: P.<x,y,z,w,t,u> = ProjectiveSpace(QQ, 5)
sage: X = P.subscheme([x^7 + x*y*z*t^4 - u^7])
sage: X.degree()
7

sage: P.<x,y,z,w> = ProjectiveSpace(GF(13), 3)
sage: X = P.subscheme([y^3 - w^3, x + 7*z])
sage: X.degree()
3

sage: P.<x,y,z,w,u> = ProjectiveSpace(QQ, 4)
sage: C = P.curve([x^7 - y*z^3*w^2*u, w*x^2 - y*u^2, z^3 + y^3])
sage: C.degree()
63

dimension()

Return the dimension of the projective algebraic subscheme.

OUTPUT:

Integer.

EXAMPLES:

sage: P2.<x,y,z> = ProjectiveSpace(2, QQ)
sage: P2.subscheme([]).dimension()
2
sage: P2.subscheme([x]).dimension()
1
sage: P2.subscheme([x^5]).dimension()
1
sage: P2.subscheme([x^2 + y^2 - z^2]).dimension()
1
sage: P2.subscheme([x*(x-z), y*(y-z)]).dimension()
0

Something less obvious:

sage: P3.<x,y,z,w,t> = ProjectiveSpace(4, QQ)
sage: X = P3.subscheme([x^2, x^2*y^2 + z^2*t^2, z^2 - w^2, 10*x^2 + w^2 - z^2])
sage: X
Closed subscheme of Projective Space of dimension 4 over Rational Field defined by:
  x^2,
  x^2*y^2 + z^2*t^2,
  z^2 - w^2,
  10*x^2 - z^2 + w^2
sage: X.dimension()
1

dual()

Return the projective dual of the given subscheme of projective space.

INPUT:

• X – A subscheme of projective space. At present, X is required to be an irreducible and reduced hypersurface defined over \(\QQ\) or a finite field.

OUTPUT:

• The dual of X as a subscheme of the dual projective space.

EXAMPLES:

The dual of a smooth conic in the plane is also a smooth conic:

sage: R.<x, y, z> = QQ[]
sage: P.<x, y, z> = ProjectiveSpace(2, QQ)
sage: I = R.ideal(x^2 + y^2 + z^2)
sage: X = P.subscheme(I)
sage: X.dual()
Closed subscheme of Projective Space of dimension 2 over Rational Field defined by:
  y0^2 + y1^2 + y2^2

The dual of the twisted cubic curve in projective 3-space is a singular quartic surface. In the following example, we compute the dual of this surface, which by double duality is equal to the twisted cubic itself. The output is the twisted cubic as an intersection of three quadrics:

sage: R.<x, y, z, w> = QQ[]
sage: P.<x, y, z, w> = ProjectiveSpace(3, QQ)
sage: I = R.ideal(y^2*z^2 - 4*x*z^3 - 4*y^3*w + 18*x*y*z*w - 27*x^2*w^2)
sage: X = P.subscheme(I)
sage: X.dual()
Closed subscheme of Projective Space of dimension 3 over
Rational Field defined by:
  y2^2 - y1*y3,
  y1*y2 - y0*y3,
  y1^2 - y0*y2

The singular locus of the quartic surface in the last example is itself supported on a twisted cubic:

sage: X.Jacobian().radical()
Ideal (z^2 - 3*y*w, y*z - 9*x*w, y^2 - 3*x*z) of Multivariate
Polynomial Ring in x, y, z, w over Rational Field

An example over a finite field:

sage: R = PolynomialRing(GF(61), 'a,b,c')
sage: P.<a, b, c> = ProjectiveSpace(2, R.base_ring())
sage: X = P.subscheme(R.ideal(a*a+2*b*b+3*c*c))
sage: X.dual()
Closed subscheme of Projective Space of dimension 2 over
Finite Field of size 61 defined by:
y0^2 - 30*y1^2 - 20*y2^2

intersection_multiplicity(X, P)

Return the intersection multiplicity of this subscheme and the subscheme X at the point P.

This uses the intersection_multiplicity function for affine subschemes on affine patches of this subscheme and X that contain P.

INPUT:

• X – subscheme in the same ambient space as this subscheme.
• P – a point in the intersection of this subscheme with X.

OUTPUT: An integer.

EXAMPLES:

sage: P.<x,y,z> = ProjectiveSpace(GF(5), 2)
sage: C = Curve([x^4 - z^2*y^2], P)
sage: D = Curve([y^4*z - x^5 - x^3*z^2], P)
sage: Q1 = P([0,1,0])
sage: C.intersection_multiplicity(D, Q1)
4
sage: Q2 = P([0,0,1])
sage: C.intersection_multiplicity(D, Q2)
6

sage: R.<a> = QQ[]
sage: K.<b> = NumberField(a^4 + 1)
sage: P.<x,y,z,w> = ProjectiveSpace(K, 3)
sage: X = P.subscheme([x^2 + y^2 - z*w])
sage: Y = P.subscheme([y*z - x*w, z - w])
sage: Q1 = P([b^2,1,0,0])
sage: X.intersection_multiplicity(Y, Q1)
1
sage: Q2 = P([1/2*b^3-1/2*b,1/2*b^3-1/2*b,1,1])
sage: X.intersection_multiplicity(Y, Q2)
1

sage: P.<x,y,z,w> = ProjectiveSpace(QQ, 3)
sage: X = P.subscheme([x^2 - z^2, y^3 - w*x^2])
sage: Y = P.subscheme([w^2 - 2*x*y + z^2, y^2 - w^2])
sage: Q = P([1,1,-1,1])
sage: X.intersection_multiplicity(Y, Q)
Traceback (most recent call last):
...
TypeError: the intersection of this subscheme and (=Closed subscheme of Affine Space of dimension 3
over Rational Field defined by:
  z^2 + w^2 - 2*y,
  y^2 - w^2) must be proper and finite

is_smooth(point=None)

Test whether the algebraic subscheme is smooth.

INPUT:

• point – A point or None (default). The point to test smoothness at.

OUTPUT:

Boolean. If no point was specified, returns whether the algebraic subscheme is smooth everywhere. Otherwise, smoothness at the specified point is tested.

EXAMPLES:

sage: P2.<x,y,z> = ProjectiveSpace(2,QQ)
sage: cuspidal_curve = P2.subscheme([y^2*z-x^3])
sage: cuspidal_curve
Closed subscheme of Projective Space of dimension 2 over Rational Field defined by:
  -x^3 + y^2*z
sage: cuspidal_curve.is_smooth([1,1,1])
True
sage: cuspidal_curve.is_smooth([0,0,1])
False
sage: cuspidal_curve.is_smooth()
False
sage: P2.subscheme([y^2*z-x^3+z^3+1/10*x*y*z]).is_smooth()
True

multiplicity(P)

Return the multiplicity of P on this subscheme.

This is computed as the multiplicity of the corresponding point on an affine patch of this subscheme that contains P. This subscheme must be defined over a field. An error is returned if P not a point on this subscheme.

INPUT:

• P – a point on this subscheme.

OUTPUT:

An integer.

EXAMPLES:

sage: P.<x,y,z,w,t> = ProjectiveSpace(QQ, 4)
sage: X = P.subscheme([y^2 - x*t, w^7 - t*w*x^5 - z^7])
sage: Q1 = P([0,0,1,1,1])
sage: X.multiplicity(Q1)
1
sage: Q2 = P([1,0,0,0,0])
sage: X.multiplicity(Q2)
3
sage: Q3 = P([0,0,0,0,1])
sage: X.multiplicity(Q3)
7

sage: P.<x,y,z,w> = ProjectiveSpace(CC, 3)
sage: X = P.subscheme([z^5*x^2*w - y^8])
sage: Q = P([2,0,0,1])
sage: X.multiplicity(Q)
5

sage: P.<x,y,z,w> = ProjectiveSpace(GF(29), 3)
sage: C = Curve([y^17 - x^5*w^4*z^8, x*y - z^2], P)
sage: Q = P([3,0,0,1])
sage: C.multiplicity(Q)
8

neighborhood(point)

Return an affine algebraic subscheme isomorphic to a neighborhood of the point.

INPUT:

• point – a point of the projective subscheme.

OUTPUT:

An affine algebraic scheme (polynomial equations in affine space) result such that

• embedding_morphism is an isomorphism to a neighborhood of point
• embedding_center is mapped to point.

EXAMPLES:

sage: P.<x,y,z>= ProjectiveSpace(QQ,2)
sage: S = P.subscheme(x+2*y+3*z)
sage: s = S.point([0,-3,2]); s
(0 : -3/2 : 1)
sage: patch = S.neighborhood(s); patch
Closed subscheme of Affine Space of dimension 2 over Rational Field defined by:
  x + 3*z
sage: patch.embedding_morphism()
Scheme morphism:
  From: Closed subscheme of Affine Space of dimension 2 over Rational Field defined by:
  x + 3*z
  To:   Closed subscheme of Projective Space of dimension 2 over Rational Field defined by:
  x + 2*y + 3*z
  Defn: Defined on coordinates by sending (x, z) to
        (x : -3/2 : z + 1)
sage: patch.embedding_center()
(0, 0)
sage: patch.embedding_morphism()([0,0])
(0 : -3/2 : 1)
sage: patch.embedding_morphism()(patch.embedding_center())
(0 : -3/2 : 1)

nth_iterate(f, n)

The nth forward image of this scheme by the map f.

INPUT:

• f – a DynamicalSystem_projective with self in f.domain()
• n – a positive integer.

OUTPUT:

• A subscheme in f.codomain()

EXAMPLES:

sage: P.<x,y,z,w> = ProjectiveSpace(QQ, 3)
sage: f = DynamicalSystem_projective([y^2, z^2, x^2, w^2])
sage: f.nth_iterate(P.subscheme([x-w,y-z]), 3)
Closed subscheme of Projective Space of dimension 3 over Rational Field
defined by:
  y - z,
  x - w

sage: PS.<x,y,z> = ProjectiveSpace(ZZ, 2)
sage: f = DynamicalSystem_projective([x^2, y^2, z^2])
sage: X = PS.subscheme([x-y])
sage: X.nth_iterate(f,-2)
Traceback (most recent call last):
...
TypeError: must be a forward orbit

sage: PS.<x,y,z> = ProjectiveSpace(ZZ, 2)
sage: P2.<u,v,w>=ProjectiveSpace(QQ, 2)
sage: H = Hom(PS, P2)
sage: f = H([x^2, y^2, z^2])
sage: X = PS.subscheme([x-y])
sage: X.nth_iterate(f,2)
Traceback (most recent call last):
...
TypeError: map must be a dynamical system for iteration

sage: PS.<x,y,z> = ProjectiveSpace(QQ, 2)
sage: f = DynamicalSystem_projective([x^2, y^2, z^2])
sage: X = PS.subscheme([x-y])
sage: X.nth_iterate(f,2.5)
Traceback (most recent call last):
...
TypeError: Attempt to coerce non-integral RealNumber to Integer

orbit(f, N)

Returns the orbit of this scheme by f.

If \(N\) is an integer it returns \([self,f(self),\ldots,f^N(self)]\). If \(N\) is a list or tuple \(N=[m,k]\) it returns \([f^m(self),\ldots,f^k(self)\)].

INPUT:

• f – a DynamicalSystem_projective with self in f.domain()
• N – a non-negative integer or list or tuple of two non-negative integers

OUTPUT:

• a list of projective subschemes

EXAMPLES:

sage: P.<x,y,z,w> = ProjectiveSpace(QQ, 3)
sage: f = DynamicalSystem_projective([(x-2*y)^2,(x-2*z)^2,(x-2*w)^2,x^2])
sage: f.orbit(P.subscheme([x]),5)
[Closed subscheme of Projective Space of dimension 3 over Rational Field
defined by:
   x,
 Closed subscheme of Projective Space of dimension 3 over Rational Field
defined by:
   w,
 Closed subscheme of Projective Space of dimension 3 over Rational Field
defined by:
   z - w,
 Closed subscheme of Projective Space of dimension 3 over Rational Field
defined by:
   y - z,
 Closed subscheme of Projective Space of dimension 3 over Rational Field
defined by:
   x - y,
 Closed subscheme of Projective Space of dimension 3 over Rational Field
defined by:
   x - w]

sage: PS.<x,y,z> = ProjectiveSpace(QQ, 2)
sage: P1.<u,v> = ProjectiveSpace(QQ, 1)
sage: H = Hom(PS, P1)
sage: f = H([x^2, y^2])
sage: X = PS.subscheme([x-y])
sage: X.orbit(f,2)
Traceback (most recent call last):
...
TypeError: map must be a dynamical system for iteration

sage: PS.<x,y,z> = ProjectiveSpace(QQ, 2)
sage: f = DynamicalSystem_projective([x^2, y^2, z^2])
sage: X = PS.subscheme([x-y])
sage: X.orbit(f,[-1,2])
Traceback (most recent call last):
...
TypeError: orbit bounds must be non-negative

point(v, check=True)

Create a point on this projective subscheme.

INPUT:

• v – anything that defines a point
• check – boolean (optional, default: True); whether to check the defining data for consistency

OUTPUT: A point of the subscheme.

EXAMPLES:

sage: P2.<x,y,z> = ProjectiveSpace(QQ, 2)
sage: X = P2.subscheme([x-y,y-z])
sage: X.point([1,1,1])
(1 : 1 : 1)

sage: P2.<x,y> = ProjectiveSpace(QQ, 1)
sage: X = P2.subscheme([y])
sage: X.point(infinity)
(1 : 0)

sage: P.<x,y> = ProjectiveSpace(QQ, 1)
sage: X = P.subscheme(x^2+2*y^2)
sage: X.point(infinity)
Traceback (most recent call last):
...
TypeError: Coordinates [1, 0] do not define a point on Closed subscheme
of Projective Space of dimension 1 over Rational Field defined by:
  x^2 + 2*y^2

preimage(f, k=1, check=True)

The subscheme that maps to this scheme by the map \(f^k\).

In particular, \(f^{-k}(V(h_1,\ldots,h_t)) = V(h_1 \circ f^k, \ldots, h_t \circ f^k)\). Map must be a morphism and also must be an endomorphism for \(k > 1\).

INPUT:

• f - a map whose codomain contains this scheme
• k - a positive integer
• check – Boolean, if False no input checking is done

OUTPUT:

a subscheme in the domain of f

EXAMPLES:

sage: PS.<x,y,z> = ProjectiveSpace(ZZ, 2)
sage: H = End(PS)
sage: f = H([y^2, x^2, z^2])
sage: X = PS.subscheme([x-y])
sage: X.preimage(f)
Closed subscheme of Projective Space of dimension 2 over Integer Ring
defined by:
  -x^2 + y^2

sage: P.<x,y,z,w,t> = ProjectiveSpace(QQ, 4)
sage: H = End(P)
sage: f = H([x^2-y^2, y^2, z^2, w^2, t^2+w^2])
sage: f.rational_preimages(P.subscheme([x-z, t^2, w-t]))
Closed subscheme of Projective Space of dimension 4 over Rational Field
defined by:
  x^2 - y^2 - z^2,
  w^4 + 2*w^2*t^2 + t^4,
  -t^2

sage: P1.<x,y> = ProjectiveSpace(QQ, 1)
sage: P3.<u,v,w,t> = ProjectiveSpace(QQ, 3)
sage: H = Hom(P1, P3)
sage: X = P3.subscheme([u-v, 2*u-w, u+t])
sage: f = H([x^2,y^2, x^2+y^2, x*y])
sage: X.preimage(f)
Closed subscheme of Projective Space of dimension 1 over Rational Field
defined by:
  x^2 - y^2,
  x^2 - y^2,
  x^2 + x*y

sage: P1.<x,y> = ProjectiveSpace(QQ, 1)
sage: P3.<u,v,w,t> = ProjectiveSpace(QQ, 3)
sage: H = Hom(P3, P1)
sage: X = P1.subscheme([x-y])
sage: f = H([u^2, v^2])
sage: X.preimage(f)
Traceback (most recent call last):
...
TypeError: map must be a morphism

sage: PS.<x,y,z> = ProjectiveSpace(ZZ, 2)
sage: H = End(PS)
sage: f = H([x^2, x^2, x^2])
sage: X = PS.subscheme([x-y])
sage: X.preimage(f)
Traceback (most recent call last):
...
TypeError: map must be a morphism

sage: PS.<x,y,z> = ProjectiveSpace(ZZ, 2)
sage: P1.<u,v> = ProjectiveSpace(ZZ, 1)
sage: Y = P1.subscheme([u^2-v^2])
sage: H = End(PS)
sage: f = H([x^2, y^2, z^2])
sage: Y.preimage(f)
Traceback (most recent call last):
...
TypeError: subscheme must be in ambient space of codomain

sage: P.<x,y,z> = ProjectiveSpace(QQ, 2)
sage: Y = P.subscheme([x-y])
sage: H = End(P)
sage: f = H([x^2, y^2, z^2])
sage: Y.preimage(f, k=2)
Closed subscheme of Projective Space of dimension 2 over Rational Field
defined by:
  x^4 - y^4

veronese_embedding(d, CS=None, order='lex')

Return the degree d Veronese embedding of this projective subscheme.

INPUT:

• d – a positive integer.
• CS – a projective ambient space to embed into. If the projective ambient space of this subscheme is of dimension \(N\), the dimension of CS must be \(\binom{N + d}{d} - 1\). This is constructed if not specified. Default: None.
• order – a monomial order to use to arrange the monomials defining the embedding. The monomials will be arranged from greatest to least with respect to this order. Default: 'lex'.

OUTPUT:

• a scheme morphism from this subscheme to its image by the degree d Veronese embedding.

EXAMPLES:

sage: P.<x,y,z> = ProjectiveSpace(QQ, 2)
sage: L = P.subscheme([y - x])
sage: v = L.veronese_embedding(2)
sage: v
Scheme morphism:
  From: Closed subscheme of Projective Space of dimension 2 over
Rational Field defined by:
  -x + y
  To:   Closed subscheme of Projective Space of dimension 5 over
Rational Field defined by:
  -x4^2 + x3*x5,
  x2 - x4,
  x1 - x3,
  x0 - x3
  Defn: Defined on coordinates by sending (x : y : z) to
        (x^2 : x*y : x*z : y^2 : y*z : z^2)
sage: v.codomain().degree()
2
sage: C = P.subscheme([y*z - x^2])
sage: C.veronese_embedding(2).codomain().degree()
4

twisted cubic:

sage: P.<x,y> = ProjectiveSpace(QQ, 1)
sage: Q.<u,v,s,t> = ProjectiveSpace(QQ, 3)
sage: P.subscheme([]).veronese_embedding(3, Q)
Scheme morphism:
  From: Closed subscheme of Projective Space of dimension 1 over
Rational Field defined by:
  (no polynomials)
  To:   Closed subscheme of Projective Space of dimension 3 over
Rational Field defined by:
  -s^2 + v*t,
  -v*s + u*t,
  -v^2 + u*s
  Defn: Defined on coordinates by sending (x : y) to
        (x^3 : x^2*y : x*y^2 : y^3)