The Chow group of a toric variety¶
In general, the Chow group is an algebraic version of a homology theory. That is, the objects are formal linear combinations of submanifolds modulo relations. In particular, the objects of the Chow group are formal linear combinations of algebraic subvarieties and the equivalence relation is rational equivalence. There is no relative version of the Chow group, so it is not a generalized homology theory.
The Chow groups of smooth or mildly singular toric varieties are almost the same as the homology groups:
 For smooth toric varieties, \(A_{k}(X) = H_{2k}(X,\ZZ)\). While they are the same, using the cohomology ring instead of the Chow group will be much faster! The cohomology ring does not try to keep track of torsion and uses Groebner bases to encode the cup product.
 For simplicial toric varieties, \(A_{k}(X)\otimes \QQ = H_{2k}(X,\QQ)\).
Note that in these cases the odddimensional (co)homology groups vanish. But for sufficiently singular toric varieties the Chow group differs from the homology groups (and the odddimensional homology groups no longer vanish). For singular varieties the Chow group is much easier to compute than the (co)homology groups.
The toric Chow group of a toric variety is the Chow group generated by the toric subvarieties, that is, closures of orbits under the torus action. These are in onetoone correspondence with the cones of the fan and, therefore, the toric Chow group is a quotient of the free Abelian group generated by the cones. In particular, the toric Chow group has finite rank. One can show [FMSS1] that the toric Chow groups equal the “full” Chow group of a toric variety, so there is no need to distinguish these in the following.
AUTHORS:
 Volker Braun (20100809): Initial version
REFERENCES:
[wp:ChowRing]  Wikipedia article Chow_ring 
[FMSS1]  Fulton, MacPherson, Sottile, Sturmfels: Intersection theory on spherical varieties, J. of Alg. Geometry 4 (1995), 181193. http://www.math.tamu.edu/~frank.sottile/research/ps/spherical.ps.gz 
[FultonChow]  Chapter 5.1 “Chow Groups” of Fulton, William: Introduction to Toric Varieties, Princeton University Press 
EXAMPLES:
sage: X = toric_varieties.Cube_deformation(7)
sage: X.is_smooth()
False
sage: X.is_orbifold()
False
sage: A = X.Chow_group()
sage: A.degree()
(Z, C7, C2 x C2 x Z^5, Z)
sage: A.degree(2).ngens()
7
sage: a = sum( A.gen(i) * (i+1) for i in range(A.ngens()) ) # an element of A
sage: a # long time (2s on sage.math, 2011)
( 3  1 mod 7  0 mod 2, 1 mod 2, 4, 5, 6, 7, 8  9 )
The Chow group elements are printed as ( a0  a1 mod 7  a2 mod 2,
a3 mod 2, a4, a5, a6, a7, a8  a9 )
, which denotes the element of
the Chow group in the same basis as A.degree()
. The 
separates individual degrees, so the example means:
 The degree0 part is \(3 \in \ZZ\).
 The degree1 part is \(1 \in \ZZ_7\).
 The torsion of the degree2 Chow group is \((0, 1) \in \ZZ_2\oplus\ZZ_2\).
 The free part of the degree2 Chow group is \((4, 5, 6, 7, 8) \in \ZZ^5\).
 The degree3 part is \(9 \in \ZZ\).
Note that the generators A.gens()
are not sorted in any way. In
fact, they may be of mixed degree. Use A.gens(degree=d)
to obtain
the generators in a fixed degree d
. See
ChowGroup_class.gens()
for more details.
Cones of toric varieties can determine their own Chow cycle:
sage: A = X.Chow_group(); A
Chow group of 3d toric variety covered by 6 affine patches
sage: cone = X.fan(dim=2)[3]; cone
2d cone of Rational polyhedral fan in 3d lattice N
sage: A_cone = A(cone); A_cone
( 0  1 mod 7  0 mod 2, 0 mod 2, 0, 0, 0, 0, 0  0 )
sage: A_cone.degree()
1
sage: 2 * A_cone
( 0  2 mod 7  0 mod 2, 0 mod 2, 0, 0, 0, 0, 0  0 )
sage: A_cone + A.gen(0)
( 0  1 mod 7  0 mod 2, 1 mod 2, 0, 0, 0, 0, 0  0 )
Chow cycles can be of mixed degrees:
sage: mixed = sum(A.gens()); mixed
( 1  4 mod 7  1 mod 2, 1 mod 2, 1, 1, 1, 1, 1  1 )
sage: mixed.project_to_degree(1)
( 0  4 mod 7  0 mod 2, 0 mod 2, 0, 0, 0, 0, 0  0 )
sage: sum( mixed.project_to_degree(i) for i in range(X.dimension()+1) ) == mixed
True

class
sage.schemes.toric.chow_group.
ChowCycle
(parent, v, check=True)¶ Bases:
sage.modules.fg_pid.fgp_element.FGP_Element
The elements of the Chow group.
Warning
Do not construct
ChowCycle
objects manually. Instead, use the parentChowGroup
to obtain generators or Chow cycles corresponding to cones of the fan.EXAMPLES:
sage: P2 = toric_varieties.P2() sage: A = P2.Chow_group() sage: A.gens() (( 1  0  0 ), ( 0  1  0 ), ( 0  0  1 )) sage: cone = P2.fan(1)[0] sage: A(cone) ( 0  1  0 ) sage: A( Cone([(1,0)]) ) ( 0  1  0 )

cohomology_class
()¶ Return the (Poincarédual) cohomology class.
Consider a simplicial cone of the fan, that is, a \(d\)dimensional cone spanned by \(d\) rays. Take the product of the corresponding \(d\) homogeneous coordinates. This monomial represents a cohomology classes of the toric variety \(X\), see
cohomology_ring()
. Its cohomological degree is \(2d\), which is the same degree as the Poincarédual of the (real) \(\dim(X)2d\)dimensional torus orbit associated to the simplicial cone. By linearity, we can associate a cohomology class to each Chow cycle of a simplicial toric variety.If the toric variety is compact and smooth, the associated cohomology class actually is the Poincaré dual (over the integers) of the Chow cycle. In particular, integrals of dual cohomology classes perform intersection computations.
If the toric variety is compact and has at most orbifold singularities, the torsion parts in cohomology and the Chow group can differ. But they are still isomorphic as rings over the rationals. Moreover, the normalization of integration (
volume_class
) andcount_points()
are chosen to agree.OUTPUT:
The
CohomologyClass
which is associated to the Chow cycle.If the toric variety is not simplicial, that is, has worse than orbifold singularities, there is no way to associate a cohomology class of the correct degree. In this case,
cohomology_class()
raises aValueError
.EXAMPLES:
sage: dP6 = toric_varieties.dP6() sage: cone = dP6.fan().cone_containing(2,3) sage: HH = dP6.cohomology_ring() sage: A = dP6.Chow_group() sage: HH(cone) [w^2] sage: A(cone) ( 1  0, 0, 0, 0  0 ) sage: A(cone).cohomology_class() [w^2]
Here is an example of a toric variety with orbifold singularities, where we can also use the isomorphism with the rational cohomology ring:
sage: WP4 = toric_varieties.P4_11169() sage: A = WP4.Chow_group() sage: HH = WP4.cohomology_ring() sage: cone3d = Cone([(0,0,1,0), (0,0,0,1), (9,6,1,1)]) sage: A(cone3d) ( 0  1  0  0  0 ) sage: HH(cone3d) [3*z4^3] sage: D = WP4.K() # the anticanonical divisor sage: A(D) ( 0  0  0  18  0 ) sage: HH(D) [18*z4] sage: WP4.integrate( A(cone3d).cohomology_class() * D.cohomology_class() ) 1 sage: WP4.integrate( HH(cone3d) * D.cohomology_class() ) 1 sage: A(cone3d).intersection_with_divisor(D).count_points() 1

count_points
()¶ Return the number of points in the Chow cycle.
OUTPUT:
An element of
self.base_ring()
, which is usually \(\ZZ\). The number of points in the Chow cycle.EXAMPLES:
sage: P2 = toric_varieties.P2() sage: A = P2.Chow_group() sage: a = 5*A.gen(0) + 7*A.gen(1); a ( 5  7  0 ) sage: a.count_points() 5
In the case of a smooth complete toric variety, the Chow (homology) groups are Poincaré dual to the integral cohomology groups. Here is such a smooth example:
sage: D = P2.divisor(1) sage: a = D.Chow_cycle() sage: aD = a.intersection_with_divisor(D) sage: aD.count_points() 1 sage: P2.integrate( aD.cohomology_class() ) 1
For toric varieties with at most orbifold singularities, the isomorphism only holds over \(\QQ\). But the normalization of the integral is still chosen such that the intersection numbers (which are potentially rational) computed both ways agree:
sage: P1xP1_Z2 = toric_varieties.P1xP1_Z2() sage: Dt = P1xP1_Z2.divisor(1); Dt V(t) sage: Dy = P1xP1_Z2.divisor(3); Dy V(y) sage: Dt.Chow_cycle(QQ).intersection_with_divisor(Dy).count_points() 1/2 sage: P1xP1_Z2.integrate( Dt.cohomology_class() * Dy.cohomology_class() ) 1/2

degree
()¶ The degree of the Chow cycle.
OUTPUT:
Integer. The complex dimension of the subvariety representing the Chow cycle. Raises a
ValueError
if the Chow cycle is a sum of mixed degree cycles.EXAMPLES:
sage: P2 = toric_varieties.P2() sage: A = P2.Chow_group() sage: [ a.degree() for a in A.gens() ] [0, 1, 2]

intersection_with_divisor
(divisor)¶ Intersect the Chow cycle with
divisor
.See [FultonChow] for a description of the toric algorithm.
INPUT:
divisor
– aToricDivisor
that can be moved away from the Chow cycle. For example, any Cartier divisor. See alsoToricDivisor.move_away_from
.
OUTPUT:
A new
ChowCycle
. If the divisor is not Cartier then this method potentially raises aValueError
, indicating that the divisor cannot be made transversal to the Chow cycle.EXAMPLES:
sage: dP6 = toric_varieties.dP6() sage: cone = dP6.fan().cone_containing(2); cone 1d cone of Rational polyhedral fan in 2d lattice N sage: D = dP6.divisor(cone); D V(y) sage: A = dP6.Chow_group() sage: A(cone) ( 0  0, 0, 0, 1  0 ) sage: intersection = A(cone).intersection_with_divisor(D); intersection ( 1  0, 0, 0, 0  0 ) sage: intersection.count_points() 1
You can do the same computation over the rational Chow group since there is no torsion in this case:
sage: A_QQ = dP6.Chow_group(base_ring=QQ) sage: A_QQ(cone) ( 0  0, 0, 0, 1  0 ) sage: intersection_QQ = A_QQ(cone).intersection_with_divisor(D); intersection ( 1  0, 0, 0, 0  0 ) sage: intersection_QQ.count_points() 1 sage: type(intersection_QQ.count_points()) <... 'sage.rings.rational.Rational'> sage: type(intersection.count_points()) <... 'sage.rings.integer.Integer'>

project_to_degree
(degree)¶ Project a (mixeddegree) Chow cycle to the given
degree
.INPUT:
degree
– integer. The degree to project to.
OUTPUT:
The projection of the Chow class to the given degree as a new
ChowCycle
of the same Chow group.EXAMPLES:
sage: A = toric_varieties.P2().Chow_group() sage: cycle = 10*A.gen(0) + 11*A.gen(1) + 12*A.gen(2) sage: cycle ( 10  11  12 ) sage: cycle.project_to_degree(2) ( 0  0  12 )


class
sage.schemes.toric.chow_group.
ChowGroupFactory
¶ Bases:
sage.structure.factory.UniqueFactory
Factory for
ChowGroup_class
.
create_key_and_extra_args
(toric_variety, base_ring=Integer Ring, check=True)¶ Create a key that uniquely determines the
ChowGroup_class
.INPUT:
toric_variety
– a toric variety.base_ring
– either \(\ZZ\) (default) or \(\QQ\). The coefficient ring of the Chow group.check
– boolean (default:True
).
EXAMPLES:
sage: from sage.schemes.toric.chow_group import * sage: P2 = toric_varieties.P2() sage: ChowGroup(P2, ZZ, check=True) == ChowGroup(P2, ZZ, check=False) # indirect doctest True

create_object
(version, key, **extra_args)¶ Create a
ChowGroup_class
.INPUT:
version
– object version. Currently not used.key
– a key created bycreate_key_and_extra_args()
.**extra_args
– Currently not used.
EXAMPLES:
sage: from sage.schemes.toric.chow_group import * sage: P2 = toric_varieties.P2() sage: ChowGroup(P2) # indirect doctest Chow group of 2d CPRFano toric variety covered by 3 affine patches


class
sage.schemes.toric.chow_group.
ChowGroup_class
(toric_variety, base_ring, check)¶ Bases:
sage.modules.fg_pid.fgp_module.FGP_Module_class
,sage.misc.fast_methods.WithEqualityById
The Chow group of a toric variety.
EXAMPLES:
sage: P2=toric_varieties.P2() sage: from sage.schemes.toric.chow_group import ChowGroup_class sage: A = ChowGroup_class(P2,ZZ,True); A Chow group of 2d CPRFano toric variety covered by 3 affine patches sage: A.an_element() ( 1  0  0 )

coordinate_vector
(chow_cycle, degree=None, reduce=True)¶ Return the coordinate vector of the
chow_cycle
.INPUT:
chow_cycle
– aChowCycle
.degree
– None (default) or an integer.reduce
– boolean (default:True
). Whether to reduce modulo the invariants.
OUTPUT:
 If
degree is None
(default), the coordinate vector relative to the basisself.gens()
is returned.  If some integer
degree=d
is specified, the chow cycle is projected to the given degree and the coordinate vector relative to the basisself.gens(degree=d)
is returned.
EXAMPLES:
sage: A = toric_varieties.P2().Chow_group() sage: a = A.gen(0) + 2*A.gen(1) + 3*A.gen(2) sage: A.coordinate_vector(a) (1, 2, 3) sage: A.coordinate_vector(a, degree=1) (2)

degree
(k=None)¶ Return the degree\(k\) Chow group.
INPUT:
k
– an integer orNone
(default). The degree of the Chow group.
OUTPUT:
 if \(k\) was specified, the Chow group \(A_k\) as an Abelian group.
 if \(k\) was not specified, a tuple containing the Chow groups in all degrees.
Note
 For a smooth toric variety, this is the same as the Poincarédual cohomology group \(H^{d2k}(X,\ZZ)\).
 For a simplicial toric variety (“orbifold”), \(A_k(X)\otimes \QQ = H^{d2k}(X,\QQ)\).
EXAMPLES:
Four exercises from page 65 of [FultonP65]. First, an example with \(A_1(X)=\ZZ\oplus\ZZ/3\ZZ\):
sage: X = ToricVariety(Fan(cones=[[0,1],[1,2],[2,0]], ....: rays=[[2,1],[1,2],[1,1]])) sage: A = X.Chow_group() sage: A.degree(1) C3 x Z
Second, an example with \(A_2(X)=\ZZ^2\):
sage: points = [[1,0,0],[0,1,0],[0,0,1],[1,1,1],[1,0,1]] sage: l = LatticePolytope(points) sage: l.show3d() sage: X = ToricVariety(FaceFan(l)) sage: A = X.Chow_group() sage: A.degree(2) Z^2
Third, an example with \(A_2(X)=\ZZ^5\):
sage: cube = [[ 1,0,0],[0, 1,0],[0,0, 1],[1, 1, 1], ....: [1,0,0],[0,1,0],[0,0,1],[ 1,1,1]] sage: lat_cube = LatticePolytope(cube) sage: X = ToricVariety(FaceFan((LatticePolytope(lat_cube)))) sage: X.Chow_group().degree(2) Z^5
Fourth, a fan that is not the fan over a polytope. Combinatorially, the fan is the same in the third example, only the coordinates of the first point are different. But the resulting fan is not the face fan of a cube, so the variety is “more singular”. Its Chow group has torsion, \(A_2(X)=\ZZ^5 \oplus \ZZ/2\):
sage: rays = [[ 1, 2, 3],[ 1,1, 1],[1, 1, 1],[1,1, 1], ....: [1,1,1],[1, 1,1],[ 1,1,1],[ 1, 1,1]] sage: cones = [[0,1,2,3],[4,5,6,7],[0,1,7,6], ....: [4,5,3,2],[0,2,5,7],[4,6,1,3]] sage: X = ToricVariety(Fan(cones, rays)) sage: X.Chow_group().degree(2) # long time (2s on sage.math, 2011) C2 x Z^5
Finally, Example 1.3 of [FS]:
sage: points_mod = lambda k: matrix([[ 1, 1, 2*k+1],[ 1,1, 1], ....: [1, 1, 1],[1,1, 1],[1,1,1], ....: [1, 1,1],[ 1,1,1],[ 1, 1,1]]) sage: rays = lambda k: matrix([[1,1,1],[1,1,1],[1,1,1]] ....: ).solve_left(points_mod(k)).rows() sage: cones = [[0,1,2,3],[4,5,6,7],[0,1,7,6], ....: [4,5,3,2],[0,2,5,7],[4,6,1,3]] sage: X_Delta = lambda k: ToricVariety(Fan(cones=cones, rays=rays(k))) sage: X_Delta(0).Chow_group().degree() # long time (3s on sage.math, 2011) (Z, Z, Z^5, Z) sage: X_Delta(1).Chow_group().degree() # long time (3s on sage.math, 2011) (Z, 0, Z^5, Z) sage: X_Delta(2).Chow_group().degree() # long time (3s on sage.math, 2011) (Z, C2, Z^5, Z) sage: X_Delta(2).Chow_group(base_ring=QQ).degree() # long time (4s on sage.math, 2011) (Q, 0, Q^5, Q)

gens
(degree=None)¶ Return the generators of the Chow group.
INPUT:
degree
– integer (optional). The degree of the Chow group.
OUTPUT:
 if no degree is specified, the generators of the whole Chow group. The chosen generators may be of mixed degree.
 if
degree=
\(k\) was specified, the generators of the degree\(k\) part \(A_k\) of the Chow group.
EXAMPLES:
sage: A = toric_varieties.P2().Chow_group() sage: A.gens() (( 1  0  0 ), ( 0  1  0 ), ( 0  0  1 )) sage: A.gens(degree=1) (( 0  1  0 ),)

relation_gens
()¶ Return the Chow cycles equivalent to zero.
For each \(dk1\)dimensional cone \(\rho \in \Sigma^{(dk1)}\), the relations in \(A_k(X)\), that is the cycles equivalent to zero, are generated by
\[0 \stackrel{!}{=} \mathop{\mathrm{div}}(u) = \sum_{\rho < \sigma \in \Sigma^{(np)} } \big< u, n_{\rho,\sigma} \big> V(\sigma) ,\qquad u \in M(\rho)\]where \(n_{\rho,\sigma}\) is a (randomly chosen) lift of the generator of \(N_\sigma/N_\rho \simeq \ZZ\). See also Exercise 12.5.7 of [CLS].
See also
relations()
to obtain the relations as submodule of the free module generated by the cones. Or useself.relations().gens()
to list the relations in the free module.OUTPUT:
A tuple of Chow cycles, each rationally equivalent to zero, that generates the rational equivalence.
EXAMPLES:
sage: P2 = toric_varieties.P2() sage: A = P2.Chow_group() sage: first = A.relation_gens()[0] sage: first ( 0  0  0 ) sage: first.is_zero() True sage: first.lift() (0, 1, 0, 1, 0, 0, 0)

scheme
()¶ Return the underlying toric variety.
OUTPUT:
A
ToricVariety
.EXAMPLES:
sage: P2 = toric_varieties.P2() sage: A = P2.Chow_group() sage: A.scheme() 2d CPRFano toric variety covered by 3 affine patches sage: A.scheme() is P2 True


class
sage.schemes.toric.chow_group.
ChowGroup_degree_class
(A, d)¶ Bases:
sage.structure.sage_object.SageObject
A fixeddegree subgroup of the Chow group of a toric variety.
Warning
Use
degree()
to constructChowGroup_degree_class
instances.EXAMPLES:
sage: P2 = toric_varieties.P2() sage: A = P2.Chow_group() sage: A Chow group of 2d CPRFano toric variety covered by 3 affine patches sage: A.degree() (Z, Z, Z) sage: A.degree(2) Z sage: type(_) <class 'sage.schemes.toric.chow_group.ChowGroup_degree_class'>

gen
(i)¶ Return the
i
th generator of the Chow group of fixed degree.INPUT:
i
– integer. The index of the generator to be returned.
OUTPUT:
A tuple of Chow cycles of fixed degree generating
module()
.EXAMPLES:
sage: projective_plane = toric_varieties.P2() sage: A2 = projective_plane.Chow_group().degree(2) sage: A2.gen(0) ( 0  0  1 )

gens
()¶ Return the generators of the Chow group of fixed degree.
OUTPUT:
A tuple of Chow cycles of fixed degree generating
module()
.EXAMPLES:
sage: projective_plane = toric_varieties.P2() sage: A2 = projective_plane.Chow_group().degree(2) sage: A2.gens() (( 0  0  1 ),)

module
()¶ Return the submodule of the toric Chow group generated.
OUTPUT:
A
sage.modules.fg_pid.fgp_module.FGP_Module_class
EXAMPLES:
sage: projective_plane = toric_varieties.P2() sage: A2 = projective_plane.Chow_group().degree(2) sage: A2.module() Finitely generated module V/W over Integer Ring with invariants (0)

ngens
()¶ Return the number of generators.
OUTPUT:
An integer.
EXAMPLES:
sage: projective_plane = toric_varieties.P2() sage: A2 = projective_plane.Chow_group().degree(2) sage: A2.ngens() 1


sage.schemes.toric.chow_group.
is_ChowCycle
(x)¶ Return whether
x
is aChowGroup_class
INPUT:
x
– anything.
OUTPUT:
True
orFalse
.EXAMPLES:
sage: P2=toric_varieties.P2() sage: A = P2.Chow_group() sage: from sage.schemes.toric.chow_group import * sage: is_ChowCycle(A) False sage: is_ChowCycle(A.an_element()) True sage: is_ChowCycle('Victoria') False

sage.schemes.toric.chow_group.
is_ChowGroup
(x)¶ Return whether
x
is aChowGroup_class
INPUT:
x
– anything.
OUTPUT:
True
orFalse
.EXAMPLES:
sage: P2 = toric_varieties.P2() sage: A = P2.Chow_group() sage: from sage.schemes.toric.chow_group import is_ChowGroup sage: is_ChowGroup(A) True sage: is_ChowGroup('Victoria') False