# Subschemes of products of projective spaces#

AUTHORS:

• Ben Hutz (2014): subschemes of Cartesian products of projective space

class sage.schemes.product_projective.subscheme.AlgebraicScheme_subscheme_product_projective(A, polynomials, category=None)[source]#

Construct an algebraic subscheme of a product of projective spaces.

Warning

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

INPUT:

EXAMPLES:

sage: P.<x, y, u, v> = ProductProjectiveSpaces([1, 1], QQ)
sage: P.subscheme([u*x^2 - v*y*x])
Closed subscheme of Product of projective spaces P^1 x P^1 over Rational Field
defined by:
x^2*u - x*y*v

>>> from sage.all import *
>>> P = ProductProjectiveSpaces([Integer(1), Integer(1)], QQ, names=('x', 'y', 'u', 'v',)); (x, y, u, v,) = P._first_ngens(4)
>>> P.subscheme([u*x**Integer(2) - v*y*x])
Closed subscheme of Product of projective spaces P^1 x P^1 over Rational Field
defined by:
x^2*u - x*y*v

affine_patch(I, return_embedding=False)[source]#

Return the $$I^{th}$$ affine patch of this projective scheme where $$I$$ is a multi-index.

INPUT:

• I – a list or tuple of positive integers

• return_embedding – Boolean, if true the projective embedding is also returned

OUTPUT:

• An affine algebraic scheme

• An embedding into a product of projective space (optional)

EXAMPLES:

sage: PP.<x,y,z,w,u,v> = ProductProjectiveSpaces([3, 1],QQ)
sage: W = PP.subscheme([y^2*z - x^3, z^2 - w^2, u^3 - v^3])
sage: W.affine_patch([0, 1], True)
(Closed subscheme of Affine Space of dimension 4 over Rational Field defined by:
x0^2*x1 - 1,
x1^2 - x2^2,
x3^3 - 1,
Scheme morphism:
From: Closed subscheme of Affine Space of dimension 4
over Rational Field defined by: x0^2*x1 - 1, x1^2 - x2^2, x3^3 - 1
To:   Closed subscheme of Product of projective spaces P^3 x P^1
over Rational Field defined by: -x^3 + y^2*z, z^2 - w^2, u^3 - v^3
Defn: Defined on coordinates by sending (x0, x1, x2, x3) to
(1 : x0 : x1 : x2 , x3 : 1))

>>> from sage.all import *
>>> PP = ProductProjectiveSpaces([Integer(3), Integer(1)],QQ, names=('x', 'y', 'z', 'w', 'u', 'v',)); (x, y, z, w, u, v,) = PP._first_ngens(6)
>>> W = PP.subscheme([y**Integer(2)*z - x**Integer(3), z**Integer(2) - w**Integer(2), u**Integer(3) - v**Integer(3)])
>>> W.affine_patch([Integer(0), Integer(1)], True)
(Closed subscheme of Affine Space of dimension 4 over Rational Field defined by:
x0^2*x1 - 1,
x1^2 - x2^2,
x3^3 - 1,
Scheme morphism:
From: Closed subscheme of Affine Space of dimension 4
over Rational Field defined by: x0^2*x1 - 1, x1^2 - x2^2, x3^3 - 1
To:   Closed subscheme of Product of projective spaces P^3 x P^1
over Rational Field defined by: -x^3 + y^2*z, z^2 - w^2, u^3 - v^3
Defn: Defined on coordinates by sending (x0, x1, x2, x3) to
(1 : x0 : x1 : x2 , x3 : 1))

dimension()[source]#

Return the dimension of the algebraic subscheme.

OUTPUT: An integer.

EXAMPLES:

sage: X.<x,y,z,w,u,v> = ProductProjectiveSpaces([2, 2], QQ)
sage: L = (-w - v)*x + (-w*y - u*z)
sage: Q = ((-u*w - v^2)*x^2 + ((-w^2 - u*w + (-u*v - u^2))*y + (-w^2 - u*v)*z)*x
....:      + ((-w^2 - u*w - u^2)*y^2 + (-u*w - v^2)*z*y + (-w^2 + (-v - u)*w)*z^2))
sage: W = X.subscheme([L, Q])
sage: W.dimension()                                                         # needs sage.libs.singular
2

>>> from sage.all import *
>>> X = ProductProjectiveSpaces([Integer(2), Integer(2)], QQ, names=('x', 'y', 'z', 'w', 'u', 'v',)); (x, y, z, w, u, v,) = X._first_ngens(6)
>>> L = (-w - v)*x + (-w*y - u*z)
>>> Q = ((-u*w - v**Integer(2))*x**Integer(2) + ((-w**Integer(2) - u*w + (-u*v - u**Integer(2)))*y + (-w**Integer(2) - u*v)*z)*x
...      + ((-w**Integer(2) - u*w - u**Integer(2))*y**Integer(2) + (-u*w - v**Integer(2))*z*y + (-w**Integer(2) + (-v - u)*w)*z**Integer(2)))
>>> W = X.subscheme([L, Q])
>>> W.dimension()                                                         # needs sage.libs.singular
2

sage: PP.<x,y,z,u,v,s,t> = ProductProjectiveSpaces([2, 1, 1], QQ)
sage: X = PP.subscheme([x^3, x^5 + y^5, z^6, x*u - v*y, s^2 - t^2])
sage: X.dimension()                                                         # needs sage.libs.singular
-1

>>> from sage.all import *
>>> PP = ProductProjectiveSpaces([Integer(2), Integer(1), Integer(1)], QQ, names=('x', 'y', 'z', 'u', 'v', 's', 't',)); (x, y, z, u, v, s, t,) = PP._first_ngens(7)
>>> X = PP.subscheme([x**Integer(3), x**Integer(5) + y**Integer(5), z**Integer(6), x*u - v*y, s**Integer(2) - t**Integer(2)])
>>> X.dimension()                                                         # needs sage.libs.singular
-1

sage: PP = ProductProjectiveSpaces([2, 1, 3], CC, 't')                      # needs sage.rings.real_mpfr
sage: PP.subscheme([]).dimension()                                          # needs sage.libs.singular sage.rings.real_mpfr
6

>>> from sage.all import *
>>> PP = ProductProjectiveSpaces([Integer(2), Integer(1), Integer(3)], CC, 't')                      # needs sage.rings.real_mpfr
>>> PP.subscheme([]).dimension()                                          # needs sage.libs.singular sage.rings.real_mpfr
6

sage: PP = ProductProjectiveSpaces([1, 3, 1], ZZ, 't')
sage: PP.subscheme([]).dimension()                                          # needs sage.libs.singular
5

>>> from sage.all import *
>>> PP = ProductProjectiveSpaces([Integer(1), Integer(3), Integer(1)], ZZ, 't')
>>> PP.subscheme([]).dimension()                                          # needs sage.libs.singular
5

sage: PP.<x,y,u,v,s,t> = ProductProjectiveSpaces([1,1,1], CC)               # needs sage.rings.real_mpfr
sage: X = PP.subscheme([x^2 - y^2, u - v, s^2 - t^2])                       # needs sage.libs.singular sage.rings.real_mpfr
sage: X.dimension()                                                         # needs sage.libs.singular sage.rings.real_mpfr
0

>>> from sage.all import *
>>> PP = ProductProjectiveSpaces([Integer(1),Integer(1),Integer(1)], CC, names=('x', 'y', 'u', 'v', 's', 't',)); (x, y, u, v, s, t,) = PP._first_ngens(6)# needs sage.rings.real_mpfr
>>> X = PP.subscheme([x**Integer(2) - y**Integer(2), u - v, s**Integer(2) - t**Integer(2)])                       # needs sage.libs.singular sage.rings.real_mpfr
>>> X.dimension()                                                         # needs sage.libs.singular sage.rings.real_mpfr
0

intersection_multiplicity(X, P)[source]#

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:

Multiplicity of a fixed point of the map $$z^2 + \frac{1}{4}$$:

sage: PP.<x,y,u,v> = ProductProjectiveSpaces(QQ, [1, 1])
sage: G = PP.subscheme([(x^2 + 1/4*y^2)*v - y^2*u])
sage: D = PP.subscheme([x*v - y*u])
sage: sorted(G.intersection(D).rational_points())                           # needs sage.libs.singular
[(1/2 : 1 , 1/2 : 1), (1 : 0 , 1 : 0)]
sage: Q = PP([1/2,1,1/2,1])
sage: G.intersection_multiplicity(D, Q)                                     # needs sage.libs.singular
2

>>> from sage.all import *
>>> PP = ProductProjectiveSpaces(QQ, [Integer(1), Integer(1)], names=('x', 'y', 'u', 'v',)); (x, y, u, v,) = PP._first_ngens(4)
>>> G = PP.subscheme([(x**Integer(2) + Integer(1)/Integer(4)*y**Integer(2))*v - y**Integer(2)*u])
>>> D = PP.subscheme([x*v - y*u])
>>> sorted(G.intersection(D).rational_points())                           # needs sage.libs.singular
[(1/2 : 1 , 1/2 : 1), (1 : 0 , 1 : 0)]
>>> Q = PP([Integer(1)/Integer(2),Integer(1),Integer(1)/Integer(2),Integer(1)])
>>> G.intersection_multiplicity(D, Q)                                     # needs sage.libs.singular
2

sage: # needs sage.rings.finite_rings
sage: F.<a> = GF(4)
sage: PP.<x,y,z,u,v,w> = ProductProjectiveSpaces(F, [2, 2])
sage: X = PP.subscheme([z^5 + 3*x*y^4 + 8*y^5, u^2 - v^2])
sage: Y = PP.subscheme([x^6 + z^6, w*z - v*y])
sage: Q = PP([a,a+1,1,a,a,1])
sage: X.intersection_multiplicity(Y, Q)                                     # needs sage.libs.singular
16

>>> from sage.all import *
>>> # needs sage.rings.finite_rings
>>> F = GF(Integer(4), names=('a',)); (a,) = F._first_ngens(1)
>>> PP = ProductProjectiveSpaces(F, [Integer(2), Integer(2)], names=('x', 'y', 'z', 'u', 'v', 'w',)); (x, y, z, u, v, w,) = PP._first_ngens(6)
>>> X = PP.subscheme([z**Integer(5) + Integer(3)*x*y**Integer(4) + Integer(8)*y**Integer(5), u**Integer(2) - v**Integer(2)])
>>> Y = PP.subscheme([x**Integer(6) + z**Integer(6), w*z - v*y])
>>> Q = PP([a,a+Integer(1),Integer(1),a,a,Integer(1)])
>>> X.intersection_multiplicity(Y, Q)                                     # needs sage.libs.singular
16

sage: PP.<x,y,z,u,v,w> = ProductProjectiveSpaces(QQ, [2, 2])
sage: X = PP.subscheme([x^2*u^3 + y*z*u*v^2, x - y])
sage: Y = PP.subscheme([u^3 - w^3, x*v - y*w, z^3*w^2 - y^3*u*v])
sage: Q = PP([0,0,1,0,1,0])
sage: X.intersection_multiplicity(Y, Q)                                     # needs sage.libs.singular
Traceback (most recent call last):
...
TypeError: the intersection of this subscheme and (=Closed subscheme of Affine Space of dimension 4
over Rational Field defined by: x2^3 - x3^3, -x1*x3 + x0, -x1^3*x2 + x3^2) must be proper and finite

>>> from sage.all import *
>>> PP = ProductProjectiveSpaces(QQ, [Integer(2), Integer(2)], names=('x', 'y', 'z', 'u', 'v', 'w',)); (x, y, z, u, v, w,) = PP._first_ngens(6)
>>> X = PP.subscheme([x**Integer(2)*u**Integer(3) + y*z*u*v**Integer(2), x - y])
>>> Y = PP.subscheme([u**Integer(3) - w**Integer(3), x*v - y*w, z**Integer(3)*w**Integer(2) - y**Integer(3)*u*v])
>>> Q = PP([Integer(0),Integer(0),Integer(1),Integer(0),Integer(1),Integer(0)])
>>> X.intersection_multiplicity(Y, Q)                                     # needs sage.libs.singular
Traceback (most recent call last):
...
TypeError: the intersection of this subscheme and (=Closed subscheme of Affine Space of dimension 4
over Rational Field defined by: x2^3 - x3^3, -x1*x3 + x0, -x1^3*x2 + x3^2) must be proper and finite

is_smooth(point=None)[source]#

Test whether the algebraic subscheme is smooth.

EXAMPLES:

sage: X.<x,y,z,w,u,v> = ProductProjectiveSpaces([2, 2],QQ)
sage: L = (-w - v)*x + (-w*y - u*z)
sage: Q = ((-u*w - v^2)*x^2 + ((-w^2 - u*w + (-u*v - u^2))*y + (-w^2 - u*v)*z)*x
....:      + ((-w^2 - u*w - u^2)*y^2 + (-u*w - v^2)*z*y + (-w^2 + (-v - u)*w)*z^2))
sage: W = X.subscheme([L, Q])
sage: W.is_smooth()
Traceback (most recent call last):
...
NotImplementedError: Not Implemented

>>> from sage.all import *
>>> X = ProductProjectiveSpaces([Integer(2), Integer(2)],QQ, names=('x', 'y', 'z', 'w', 'u', 'v',)); (x, y, z, w, u, v,) = X._first_ngens(6)
>>> L = (-w - v)*x + (-w*y - u*z)
>>> Q = ((-u*w - v**Integer(2))*x**Integer(2) + ((-w**Integer(2) - u*w + (-u*v - u**Integer(2)))*y + (-w**Integer(2) - u*v)*z)*x
...      + ((-w**Integer(2) - u*w - u**Integer(2))*y**Integer(2) + (-u*w - v**Integer(2))*z*y + (-w**Integer(2) + (-v - u)*w)*z**Integer(2)))
>>> W = X.subscheme([L, Q])
>>> W.is_smooth()
Traceback (most recent call last):
...
NotImplementedError: Not Implemented

multiplicity(P)[source]#

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: PP.<x,y,z,w> = ProductProjectiveSpaces(QQ, [1, 1])
sage: X = PP.subscheme([x^4*z^3 - y^4*w^3])
sage: Q1 = PP([1,1,1,1])
sage: X.multiplicity(Q1)                                                    # needs sage.libs.singular
1
sage: Q2 = PP([0,1,1,0])
sage: X.multiplicity(Q2)                                                    # needs sage.libs.singular
3

>>> from sage.all import *
>>> PP = ProductProjectiveSpaces(QQ, [Integer(1), Integer(1)], names=('x', 'y', 'z', 'w',)); (x, y, z, w,) = PP._first_ngens(4)
>>> X = PP.subscheme([x**Integer(4)*z**Integer(3) - y**Integer(4)*w**Integer(3)])
>>> Q1 = PP([Integer(1),Integer(1),Integer(1),Integer(1)])
>>> X.multiplicity(Q1)                                                    # needs sage.libs.singular
1
>>> Q2 = PP([Integer(0),Integer(1),Integer(1),Integer(0)])
>>> X.multiplicity(Q2)                                                    # needs sage.libs.singular
3

sage: PP.<x,y,z,w,u> = ProductProjectiveSpaces(GF(11), [1,2])
sage: X = PP.subscheme([x^7*u - y^7*z, u^6*x^2 - w^3*z^3*x*y - w^6*y^2])
sage: Q1 = PP([1,0,10,1,0])
sage: X.multiplicity(Q1)                                                    # needs sage.libs.singular sage.rings.finite_rings
1
sage: Q2 = PP([1,0,1,0,0])
sage: X.multiplicity(Q2)                                                    # needs sage.libs.singular sage.rings.finite_rings
4

>>> from sage.all import *
>>> PP = ProductProjectiveSpaces(GF(Integer(11)), [Integer(1),Integer(2)], names=('x', 'y', 'z', 'w', 'u',)); (x, y, z, w, u,) = PP._first_ngens(5)
>>> X = PP.subscheme([x**Integer(7)*u - y**Integer(7)*z, u**Integer(6)*x**Integer(2) - w**Integer(3)*z**Integer(3)*x*y - w**Integer(6)*y**Integer(2)])
>>> Q1 = PP([Integer(1),Integer(0),Integer(10),Integer(1),Integer(0)])
>>> X.multiplicity(Q1)                                                    # needs sage.libs.singular sage.rings.finite_rings
1
>>> Q2 = PP([Integer(1),Integer(0),Integer(1),Integer(0),Integer(0)])
>>> X.multiplicity(Q2)                                                    # needs sage.libs.singular sage.rings.finite_rings
4

segre_embedding(PP=None)[source]#

Return the Segre embedding of this subscheme into the appropriate projective space.

INPUT:

• PP – (default: None) ambient image projective space; this is constructed if it is not given.

OUTPUT:

Hom from this subscheme to the appropriate subscheme of projective space

EXAMPLES:

sage: X.<x,y,z,w,u,v> = ProductProjectiveSpaces([2, 2], QQ)
sage: P = ProjectiveSpace(QQ, 8, 't')
sage: L = (-w - v)*x + (-w*y - u*z)
sage: Q = ((-u*w - v^2)*x^2 + ((-w^2 - u*w + (-u*v - u^2))*y + (-w^2 - u*v)*z)*x
....:      + ((-w^2 - u*w - u^2)*y^2 + (-u*w - v^2)*z*y + (-w^2 + (-v - u)*w)*z^2))
sage: W = X.subscheme([L,Q])
sage: phi = W.segre_embedding(P)                                            # needs sage.libs.singular
sage: phi.codomain().ambient_space() == P                                   # needs sage.libs.singular
True

>>> from sage.all import *
>>> X = ProductProjectiveSpaces([Integer(2), Integer(2)], QQ, names=('x', 'y', 'z', 'w', 'u', 'v',)); (x, y, z, w, u, v,) = X._first_ngens(6)
>>> P = ProjectiveSpace(QQ, Integer(8), 't')
>>> L = (-w - v)*x + (-w*y - u*z)
>>> Q = ((-u*w - v**Integer(2))*x**Integer(2) + ((-w**Integer(2) - u*w + (-u*v - u**Integer(2)))*y + (-w**Integer(2) - u*v)*z)*x
...      + ((-w**Integer(2) - u*w - u**Integer(2))*y**Integer(2) + (-u*w - v**Integer(2))*z*y + (-w**Integer(2) + (-v - u)*w)*z**Integer(2)))
>>> W = X.subscheme([L,Q])
>>> phi = W.segre_embedding(P)                                            # needs sage.libs.singular
>>> phi.codomain().ambient_space() == P                                   # needs sage.libs.singular
True

sage: PP.<x,y,u,v,s,t> = ProductProjectiveSpaces([1, 1, 1], CC)             # needs sage.rings.real_mpfr
sage: PP.subscheme([]).segre_embedding()                                    # needs sage.libs.singular sage.rings.real_mpfr
Scheme morphism:
From: Closed subscheme of Product of projective spaces P^1 x P^1 x P^1
over Complex Field with 53 bits of precision defined by:
(no polynomials)
To:   Closed subscheme of Projective Space of dimension 7
over Complex Field with 53 bits of precision defined by:
-u5*u6 + u4*u7,       -u3*u6 + u2*u7,       -u3*u4 + u2*u5,
-u3*u5 + u1*u7,       -u3*u4 + u1*u6,       -u3*u4 + u0*u7,
-u2*u4 + u0*u6,       -u1*u4 + u0*u5,       -u1*u2 + u0*u3
Defn: Defined by sending (x : y , u : v , s : t) to
(x*u*s : x*u*t : x*v*s : x*v*t : y*u*s : y*u*t : y*v*s : y*v*t).

>>> from sage.all import *
>>> PP = ProductProjectiveSpaces([Integer(1), Integer(1), Integer(1)], CC, names=('x', 'y', 'u', 'v', 's', 't',)); (x, y, u, v, s, t,) = PP._first_ngens(6)# needs sage.rings.real_mpfr
>>> PP.subscheme([]).segre_embedding()                                    # needs sage.libs.singular sage.rings.real_mpfr
Scheme morphism:
From: Closed subscheme of Product of projective spaces P^1 x P^1 x P^1
over Complex Field with 53 bits of precision defined by:
(no polynomials)
To:   Closed subscheme of Projective Space of dimension 7
over Complex Field with 53 bits of precision defined by:
-u5*u6 + u4*u7,       -u3*u6 + u2*u7,       -u3*u4 + u2*u5,
-u3*u5 + u1*u7,       -u3*u4 + u1*u6,       -u3*u4 + u0*u7,
-u2*u4 + u0*u6,       -u1*u4 + u0*u5,       -u1*u2 + u0*u3
Defn: Defined by sending (x : y , u : v , s : t) to
(x*u*s : x*u*t : x*v*s : x*v*t : y*u*s : y*u*t : y*v*s : y*v*t).

sage: PP.<x,y,z,u,v,s,t> = ProductProjectiveSpaces([2, 1, 1], ZZ)
sage: PP.subscheme([x^3, u - v, s^2 - t^2]).segre_embedding()               # needs sage.libs.singular
Scheme morphism:
From: Closed subscheme of Product of projective spaces P^2 x P^1 x P^1
over Integer Ring defined by:
x^3,                  u - v,                s^2 - t^2
To:   Closed subscheme of Projective Space of dimension 11
over Integer Ring defined by:
u10^2 - u11^2,        u9 - u11,             u8 - u10,
-u7*u10 + u6*u11,     u6*u10 - u7*u11,      u6^2 - u7^2,
u5 - u7,              u4 - u6,              u3^3,
-u3*u10 + u2*u11,     u2*u10 - u3*u11,      -u3*u6 + u2*u7,
u2*u6 - u3*u7,        u2*u3^2,              u2^2 - u3^2,
u1 - u3,              u0 - u2
Defn: Defined by sending (x : y : z , u : v , s : t) to
(x*u*s : x*u*t : x*v*s : x*v*t : y*u*s : y*u*t : y*v*s : y*v*t
: z*u*s : z*u*t : z*v*s : z*v*t).

>>> from sage.all import *
>>> PP = ProductProjectiveSpaces([Integer(2), Integer(1), Integer(1)], ZZ, names=('x', 'y', 'z', 'u', 'v', 's', 't',)); (x, y, z, u, v, s, t,) = PP._first_ngens(7)
>>> PP.subscheme([x**Integer(3), u - v, s**Integer(2) - t**Integer(2)]).segre_embedding()               # needs sage.libs.singular
Scheme morphism:
From: Closed subscheme of Product of projective spaces P^2 x P^1 x P^1
over Integer Ring defined by:
x^3,                  u - v,                s^2 - t^2
To:   Closed subscheme of Projective Space of dimension 11
over Integer Ring defined by:
u10^2 - u11^2,        u9 - u11,             u8 - u10,
-u7*u10 + u6*u11,     u6*u10 - u7*u11,      u6^2 - u7^2,
u5 - u7,              u4 - u6,              u3^3,
-u3*u10 + u2*u11,     u2*u10 - u3*u11,      -u3*u6 + u2*u7,
u2*u6 - u3*u7,        u2*u3^2,              u2^2 - u3^2,
u1 - u3,              u0 - u2
Defn: Defined by sending (x : y : z , u : v , s : t) to
(x*u*s : x*u*t : x*v*s : x*v*t : y*u*s : y*u*t : y*v*s : y*v*t
: z*u*s : z*u*t : z*v*s : z*v*t).