Normal Toric Varieties
Introduction
We introduce two main types of normal toric varieties, distinguishing between the affine and non-affine case:
AffineNormalToricVarietyis the type for toric varieties associated to a cone $\sigma$, denoted by $U_{\sigma}$ in [CLS11]NormalToricVarietyis the type for toric varieties associated to a polyhedral fan $\Sigma$, denoted by $X_{\Sigma}$ in [CLS11]
The lattice is always assumed to be the standard lattice $\mathbb{Z}^n$. Transformations for non-standard lattices will have to be done by the user.
Equality of Normal Toric Varieties
Equality of normal toric varieties is computationally very demanding. We have therefore made special design decisions for the == method.
In OSCAR, the == operator is reserved to check if two normal toric varieties are identical, meaning their underlying polyhedral fans are the same. However, this check is computationally challenging due to several reasons:
- The ray generators might be scaled.
- The ray generators might be stored in different orders.
- The maximal (polyhedral) cones of the polyhedral fan might be stored in different orders.
- If we fall back on polyhedral fan equality, lineality of the cones must also be considered.
To avoid this computational bottleneck, we have specially designed the == method. It checks if the memory locations of the two objects in question are identical. If so, our == method returns true. Otherwise, it raise an error.
Note that triple equality === (i.e. the check of equal the memory locations) is always supported for normal toric varieties. We recommend using it.
However, if you truly need to check for two normal toric varieties to be mathematically identical, then you will need to add a custom method. This method could look as follows:
function slow_equal(tv1::NormalToricVariety, tv2::NormalToricVariety)
tv1 === tv2 && return true
ambient_dim(tv1) == ambient_dim(tv2) || return false
f_vector(tv1) == f_vector(tv2) || return false
return Set(maximal_cones(tv1)) == Set(maximal_cones(tv2))
endPlease note that this method slow_equal is not performant, that we currently (summer 2024) have no intentions in adding this function to OSCAR nor to make improvements to its performance. Rather, expect this method to be slow, potentially painfully so.
Constructors
Affine Toric Varieties
affine_normal_toric_variety — Methodaffine_normal_toric_variety(C::Cone)Construct the affine normal toric variety $U_{C}$ corresponding to a polyhedral cone C.
Examples
Set C to be the positive orthant in two dimensions.
julia> C = positive_hull([1 0; 0 1])
Polyhedral cone in ambient dimension 2
julia> antv = affine_normal_toric_variety(C)
Normal toric varietynormal_toric_variety — Methodnormal_toric_variety(C::Cone)Construct the (affine) normal toric variety $X_{\Sigma}$ corresponding to a polyhedral fan $\Sigma = C$ consisting only of the cone C.
Examples
Set C to be the positive orthant in two dimensions.
julia> C = positive_hull([1 0; 0 1])
Polyhedral cone in ambient dimension 2
julia> ntv = normal_toric_variety(C)
Normal toric varietyaffine_normal_toric_variety — Methodaffine_normal_toric_variety(v::NormalToricVariety)For internal design, we make a strict distinction between normal toric varieties and affine toric varieties. Given an affine, normal toric variety v, this method turns it into an affine toric variety.
Examples
julia> v = normal_toric_variety(positive_hull([1 0; 0 1]))
Normal toric variety
julia> affineVariety = affine_normal_toric_variety(v)
Normal toric varietyNormal Toric Varieties
normal_toric_variety — Functionnormal_toric_variety(C::Cone)Construct the (affine) normal toric variety $X_{\Sigma}$ corresponding to a polyhedral fan $\Sigma = C$ consisting only of the cone C.
Examples
Set C to be the positive orthant in two dimensions.
julia> C = positive_hull([1 0; 0 1])
Polyhedral cone in ambient dimension 2
julia> ntv = normal_toric_variety(C)
Normal toric varietynormal_toric_variety(max_cones::IncidenceMatrix, rays::AbstractCollection[RayVector]; non_redundant::Bool = false)Construct a normal toric variety $X$ by providing the rays and maximal cones as vector of vectors. By default, this method allows redundancies in the input, e.g. duplicate rays and non-maximal cones. If the user is certain that no redundancy exists in the entered information, one can pass non_redundant = true as third argument. This will bypass these consistency checks. In addition, this will ensure that the order of the rays is not altered by the constructor.
Examples
julia> ray_generators = [[1,0], [0, 1], [-1, 5], [0, -1]]
4-element Vector{Vector{Int64}}:
[1, 0]
[0, 1]
[-1, 5]
[0, -1]
julia> max_cones = incidence_matrix([[1, 2], [2, 3], [3, 4], [4, 1]])
4×4 IncidenceMatrix
[1, 2]
[2, 3]
[3, 4]
[1, 4]
julia> normal_toric_variety(max_cones, ray_generators)
Normal toric variety
julia> normal_toric_variety(max_cones, ray_generators; non_redundant = true)
Normal toric varietynormal_toric_variety(PF::PolyhedralFan)Construct the normal toric variety $X_{PF}$ corresponding to a polyhedral fan PF.
Examples
Take PF to be the normal fan of the square.
julia> square = cube(2)
Polytope in ambient dimension 2
julia> nf = normal_fan(square)
Polyhedral fan in ambient dimension 2
julia> ntv = normal_toric_variety(nf)
Normal toric varietynormal_toric_variety(P::Polyhedron)Construct the normal toric variety $X_{\Sigma_P}$ corresponding to the normal fan $\Sigma_P$ of the given polyhedron P.
Note that this only coincides with the projective variety associated to P from the affine relations of the lattice points in P, if P is very ample.
Examples
Set P to be a square.
julia> square = cube(2)
Polytope in ambient dimension 2
julia> ntv = normal_toric_variety(square)
Normal toric varietyFamous Toric Vareties
The constructors of del_pezzo_surface, hirzebruch_surface, projective_space and weighted_projective_space always make a default/standard choice for the grading of the Cox ring.
affine_space — Methodaffine_space(::Type{NormalToricVariety}, d::Int)Construct the (toric) affine space of dimension d.
Examples
julia> affine_space(NormalToricVariety, 2)
Normal toric varietydel_pezzo_surface — Methoddel_pezzo_surface(::Type{NormalToricVariety}, b::Int)Constructs the del Pezzo surface with b blowups for b at most 3.
Examples
julia> del_pezzo_surface(NormalToricVariety, 3)
Normal toric varietyhirzebruch_surface — Methodhirzebruch_surface(::Type{NormalToricVariety}, r::Int)Construct the r-th Hirzebruch surface.
Examples
julia> hirzebruch_surface(NormalToricVariety, 5)
Normal toric varietyprojective_space — Methodprojective_space(::Type{NormalToricVariety}, d::Int)Construct the projective space of dimension d.
Examples
julia> projective_space(NormalToricVariety, 2)
Normal toric varietyweighted_projective_space — Methodweighted_projective_space(::Type{NormalToricVariety}, w::Vector{T}) where {T <: IntegerUnion}Construct the weighted projective space corresponding to the weights w.
Examples
julia> weighted_projective_space(NormalToricVariety, [2, 3, 1])
Normal toric varietyConstructions based on triangulations
It is possible to associate toric varieties to star triangulations of the lattice points of polyhedrons. Specifically, we can associate to any full star triangulation of the lattice points of the polyhedron in question a toric variety. For this task, we provide the following constructors.
normal_toric_variety_from_star_triangulation — Methodnormal_toric_variety_from_star_triangulation(P::Polyhedron)Return a toric variety that was obtained from a fine regular star triangulation of the lattice points of the polyhedron P. This is particularly useful when the lattice points of the polyhedron in question admit many triangulations.
Examples
julia> P = convex_hull([0 0 0; 0 0 1; 1 0 1; 1 1 1; 0 1 1])
Polyhedron in ambient dimension 3
julia> v = normal_toric_variety_from_star_triangulation(P)
Normal toric varietynormal_toric_varieties_from_star_triangulations — Methodnormal_toric_varieties_from_star_triangulations(P::Polyhedron)Return the list of toric varieties obtained from fine regular star triangulations of the polyhedron P. With this we can compute the two phases of the famous conifold transition.
Examples
julia> P = convex_hull([0 0 0; 0 0 1; 1 0 1; 1 1 1; 0 1 1])
Polyhedron in ambient dimension 3
julia> (v1, v2) = normal_toric_varieties_from_star_triangulations(P)
2-element Vector{NormalToricVariety}:
Normal toric variety
Normal toric variety
julia> stanley_reisner_ideal(v1)
Ideal generated by
x1*x4
julia> stanley_reisner_ideal(v2)
Ideal generated by
x2*x3An application of this functionality exists in the physics. Witten's Generalized-Sigma models (GLSM) [Wit88] originally sparked interest in the physics community in toric varieties. On a mathematical level, this establishes a construction of toric varieties for which a Z^n grading of the Cox ring is provided. See for example [FJR17], which describes this as GIT construction [CLS11].
Explicitly, given the grading of the Cox ring, the map from the group of torus invariant Weil divisors to the class group is known. Under the assumption that the variety in question has no torus factor, we can then identify the map from the lattice to the group of torus invariant Weil divisors as the kernel of the map from the torus invariant Weil divisor to the class group. The latter is a map between free Abelian groups, i.e. is provided by an integer valued matrix. The rows of this matrix are nothing but the ray generators of the fan of the toric variety. It then remains to triangulate these rays, hence in general for a GLSM the toric variety is only unique up to fine regular star triangulations. We provide the following two constructors:
normal_toric_variety_from_glsm — Methodnormal_toric_variety_from_glsm(charges::ZZMatrix)Return one toric variety with the desired GLSM charges. This can be particularly useful provided that there are many such toric varieties.
Examples
julia> charges = [[1, 1, 1]]
1-element Vector{Vector{Int64}}:
[1, 1, 1]
julia> normal_toric_variety_from_glsm(charges)
Normal toric varietyFor convenience, we also support:
normal_toric_variety_from_glsm(charges::Vector{Vector{Int}})normal_toric_variety_from_glsm(charges::Vector{Vector{ZZRingElem}})
normal_toric_varieties_from_glsm — Methodnormal_toric_varieties_from_glsm(charges::ZZMatrix)Return all toric variety with the desired GLSM charges. This computation may take a long time if there are many such toric varieties.
Examples
julia> charges = [[1, 1, 1]]
1-element Vector{Vector{Int64}}:
[1, 1, 1]
julia> normal_toric_varieties_from_glsm(charges)
1-element Vector{NormalToricVariety}:
Normal toric variety
julia> varieties = normal_toric_varieties_from_glsm(matrix(ZZ, [1 2 3 4 6 0; -1 -1 -2 -2 -3 1]))
1-element Vector{NormalToricVariety}:
Normal toric variety
julia> cox_ring(varieties[1])
Multivariate polynomial ring in 6 variables over QQ graded by
x1 -> [1 -1]
x2 -> [2 -1]
x3 -> [3 -2]
x4 -> [4 -2]
x5 -> [6 -3]
x6 -> [0 1]For convenience, we also support:
normal_toric_varieties_from_glsm(charges::Vector{Vector{Int}})normal_toric_varieties_from_glsm(charges::Vector{Vector{ZZRingElem}})
Further Constructions
* — MethodBase.:*(v::NormalToricVarietyType, w::NormalToricVarietyType)Return the Cartesian/direct product of two normal toric varieties v and w.
By default, we prepend an "x" to all homogeneous coordinate names of the first factor v and a "y" to all homogeneous coordinate names of the second factor w. This default can be overwritten by invoking set_coordinate_names after creating the variety (cf. set_coordinate_names(v::NormalToricVarietyType, coordinate_names::Vector{String})).
Important: Recall that the coordinate names can only be changed as long as the toric variety in question is not finalized (cf. is_finalized(v::NormalToricVarietyType)).
Crucially, the order of the homogeneous coordinates is not shuffled. To be more specific, assume that v has $n_1$ and w has $n_2$ homogeneous coordinates. Then v * w has $n_1 + n_2$ homogeneous coordinates. The first $n_1$ of these coordinates are those of v and appear in the very same order as they do for v. The remaining $n_2$ homogeneous coordinates are those of w and appear in the very same order as they do for w.
Examples
julia> P2 = projective_space(NormalToricVariety, 2)
Normal toric variety
julia> v1 = P2 * P2
Normal toric variety
julia> cox_ring(v1)
Multivariate polynomial ring in 6 variables over QQ graded by
xx1 -> [1 0]
xx2 -> [1 0]
xx3 -> [1 0]
yx1 -> [0 1]
yx2 -> [0 1]
yx3 -> [0 1]
julia> v2 = P2 * P2
Normal toric variety
julia> set_coordinate_names(v2, ["x1", "x2", "x3", "y1", "y2", "y3"])
julia> cox_ring(v2)
Multivariate polynomial ring in 6 variables over QQ graded by
x1 -> [1 0]
x2 -> [1 0]
x3 -> [1 0]
y1 -> [0 1]
y2 -> [0 1]
y3 -> [0 1]projectivization — Methodprojectivization(E::ToricLineBundle...)This function computes the projectivization of a direct sum of line bundles or divisors. Please see [OM78] for more background information.
Examples
Let us construct the projective bundles $X=\mathbb{P}(\mathcal{O}_{\mathbb{P}^1}\oplus\mathcal{O}_{\mathbb{P}^1}(1))$ and $Y=\mathbb{P}(\mathcal{O}_{\mathbb{P}^1}\oplus\mathcal{O}_{\mathbb{P}^1}(2))$.
julia> P1 = projective_space(NormalToricVariety, 1);
julia> D0 = toric_divisor(P1, [0,0]);
julia> D1 = toric_divisor(P1, [1,0]);
julia> X = projectivization(D0, D1)
Normal toric variety
julia> L0 = toric_line_bundle(P1, [0]);
julia> L1 = toric_line_bundle(P1, [2]);
julia> Y = projectivization(L0, L1)
Normal toric varietytotal_space — Methodtotal_space(E::ToricLineBundle...)This function computes the total space of a direct sum of line bundles or divisors. Please see [OM78] for more background information.
Examples
Let us construct the toric Calabi-Yau varieties given by the total space of $\mathcal{O}_{\mathbb{P}^1}(2)\oplus\mathcal{O}_{\mathbb{P}^1}(-4)$ and $\omega_{\mathbb{P}^2}$.
julia> P1 = projective_space(NormalToricVariety, 1);
julia> L1 = toric_line_bundle(P1, [2]);
julia> L2 = toric_line_bundle(P1, [-4]);
julia> X = total_space(L1, L2)
Normal toric variety
julia> degree(canonical_bundle(X))
0
julia> P2 = projective_space(NormalToricVariety, 2);
julia> D = canonical_divisor(P2);
julia> Y = total_space(D)
Normal toric variety
julia> degree(canonical_bundle(Y))
0Properties of Toric Varieties
has_torusfactor — Methodhas_torusfactor(v::NormalToricVarietyType)Checks if the normal toric variety v has a torus factor.
Examples
julia> has_torusfactor(projective_space(NormalToricVariety, 2))
falseis_affine — Methodis_affine(v::NormalToricVarietyType)Checks if the normal toric variety v is affine.
Examples
julia> is_affine(projective_space(NormalToricVariety, 2))
falseis_complete — Methodis_complete(v::NormalToricVarietyType)Checks if the normal toric variety v is complete.
Examples
julia> is_complete(projective_space(NormalToricVariety, 2))
trueis_fano — Methodis_fano(v::NormalToricVarietyType)Checks if the normal toric variety v is fano.
Examples
julia> is_fano(projective_space(NormalToricVariety, 2))
trueis_gorenstein — Methodis_gorenstein(v::NormalToricVarietyType)Checks if the normal toric variety v is Gorenstein.
Examples
julia> is_gorenstein(projective_space(NormalToricVariety, 2))
trueis_simplicial — Methodis_simplicial(v::NormalToricVarietyType)Checks if the normal toric variety v is simplicial. Hence, this function works just as is_orbifold. It is implemented for user convenience.
Examples
julia> is_simplicial(projective_space(NormalToricVariety, 2))
trueis_smooth — Methodis_smooth(v::NormalToricVarietyType)Checks if the normal toric variety v is smooth.
Examples
julia> is_smooth(projective_space(NormalToricVariety, 2))
trueis_normal — Methodis_normal(v::NormalToricVarietyType)Checks if the normal toric variety v is normal. (This function is somewhat tautological at this point.)
Examples
julia> is_normal(projective_space(NormalToricVariety, 2))
trueis_orbifold — Methodis_orbifold(v::NormalToricVarietyType)Checks if the normal toric variety v is an orbifold.
Examples
julia> is_orbifold(projective_space(NormalToricVariety, 2))
trueis_projective — Methodis_projective(v::NormalToricVarietyType)Checks if the normal toric variety v is projective, i.e. if the fan of v is the the normal fan of a polytope.
Examples
julia> is_projective(projective_space(NormalToricVariety, 2))
trueis_projective_space — Methodis_projective_space(v::NormalToricVarietyType)Decides if the normal toric varieties v is a projective space.
Examples
julia> F5 = hirzebruch_surface(NormalToricVariety, 5)
Normal toric variety
julia> is_projective_space(F5)
false
julia> is_projective_space(projective_space(NormalToricVariety, 2))
trueis_q_gorenstein — Methodis_q_gorenstein(v::NormalToricVarietyType)Checks if the normal toric variety v is Q-Gorenstein.
Examples
julia> is_q_gorenstein(projective_space(NormalToricVariety, 2))
trueOperations for Toric Varieties
Affine Open Covering
affine_open_covering — Methodaffine_open_covering(v::NormalToricVarietyType)Compute an affine open cover of the normal toric variety v, i.e. returns a list of affine toric varieties.
Examples
julia> p2 = projective_space(NormalToricVariety, 2)
Normal toric variety
julia> affine_open_covering(p2)
3-element Vector{AffineNormalToricVariety}:
Normal toric variety
Normal toric variety
Normal toric varietyCharacters, Weil Divisors, Cartier Divisors, Class Group and Picard Group
torusinvariant_cartier_divisor_group — Methodtorusinvariant_cartier_divisor_group(v::NormalToricVarietyType)Return the Cartier divisor group of an abstract normal toric variety v.
Examples
julia> p2 = projective_space(NormalToricVariety, 2)
Normal toric variety
julia> torusinvariant_cartier_divisor_group(p2)
Z^3character_lattice — Methodcharacter_lattice(v::NormalToricVarietyType)Return the character lattice of a normal toric variety v.
Examples
julia> p2 = projective_space(NormalToricVariety, 2);
julia> character_lattice(p2)
Z^2class_group — Methodclass_group(v::NormalToricVarietyType)Return the class group of the normal toric variety v.
Examples
julia> p2 = projective_space(NormalToricVariety, 2);
julia> class_group(p2)
Zmap_from_torusinvariant_cartier_divisor_group_to_torusinvariant_weil_divisor_group — Methodmap_from_torusinvariant_cartier_divisor_group_to_torusinvariant_weil_divisor_group(v::NormalToricVarietyType)Return the embedding of the group of Cartier divisors into the group of torus-invariant Weil divisors of an abstract normal toric variety v.
Examples
julia> p2 = projective_space(NormalToricVariety, 2)
Normal toric variety
julia> map_from_torusinvariant_cartier_divisor_group_to_torusinvariant_weil_divisor_group(p2)
Map
from Z^3
to Z^3map_from_torusinvariant_cartier_divisor_group_to_picard_group — Methodmap_from_torusinvariant_cartier_divisor_group_to_picard_group(v::NormalToricVarietyType)Return the map from the Cartier divisors to the Picard group of an abstract normal toric variety v.
Examples
julia> p2 = projective_space(NormalToricVariety, 2)
Normal toric variety
julia> map_from_torusinvariant_cartier_divisor_group_to_picard_group(p2)
Map
from Z^3
to Zmap_from_character_lattice_to_torusinvariant_weil_divisor_group — Methodmap_from_character_lattice_to_torusinvariant_weil_divisor_group(v::NormalToricVarietyType)Return the map from the character lattice to the group of principal divisors of a normal toric variety v.
Examples
julia> p2 = projective_space(NormalToricVariety, 2);
julia> map_from_character_lattice_to_torusinvariant_weil_divisor_group(p2)
Map
from Z^2
to Z^3map_from_torusinvariant_weil_divisor_group_to_class_group — Methodmap_from_torusinvariant_weil_divisor_group_to_class_group(v::NormalToricVarietyType)Return the map from the group of Weil divisors to the class of group of a normal toric variety v.
Examples
julia> p2 = projective_space(NormalToricVariety, 2);
julia> map_from_torusinvariant_weil_divisor_group_to_class_group(p2)
Map
from Z^3
to Zpicard_group — Methodpicard_group(v::NormalToricVarietyType)Return the Picard group of an abstract normal toric variety v.
Examples
julia> p2 = projective_space(NormalToricVariety, 2)
Normal toric variety
julia> picard_group(p2)
Ztorusinvariant_weil_divisor_group — Methodtorusinvariant_weil_divisor_group(v::NormalToricVarietyType)Return the torusinvariant divisor group of a normal toric variety v.
Examples
julia> p2 = projective_space(NormalToricVariety, 2);
julia> torusinvariant_weil_divisor_group(p2)
Z^3torusinvariant_prime_divisors — Methodtorusinvariant_prime_divisors(v::NormalToricVarietyType)Return the list of all torus invariant prime divisors in a normal toric variety v.
Examples
julia> p2 = projective_space(NormalToricVariety, 2);
julia> torusinvariant_prime_divisors(p2)
3-element Vector{ToricDivisor}:
Torus-invariant, prime divisor on a normal toric variety
Torus-invariant, prime divisor on a normal toric variety
Torus-invariant, prime divisor on a normal toric varietyGorenstein and Picard index
gorenstein_index — Methodgorenstein_index(v::NormalToricVarietyType)Return the Gorenstein index of a $\mathbb{Q}$-Gorenstein normal toric variety v. This is the smallest positive integer $l$ such that $-l K$ is Cartier, where $K$ is a canonical divisor on v. See exercise 8.3.10 and 8.3.11 in [CLS11] for more details.
Examples
julia> gorenstein_index(weighted_projective_space(NormalToricVariety, [2,3,5]))
3picard_index — Methodpicard_index(v::NormalToricVarietyType)Return the index of the Picard group in the class group of a simplicial normal toric variety v. Here, the Picard group embeds as the group of Cartier divisor classes into the class group via map_from_picard_group_to_class_group. See [HHS11] for more details.
Examples
julia> picard_index(weighted_projective_space(NormalToricVariety, [2,3,5]))
30Cones and Fans
polyhedral_fan — Methodpolyhedral_fan(v::NormalToricVarietyType)Return the fan of an abstract normal toric variety v.
Examples
julia> p2 = projective_space(NormalToricVariety, 2)
Normal toric variety
julia> polyhedral_fan(p2)
Polyhedral fan in ambient dimension 2cone — Methodcone(v::AffineNormalToricVariety)Return the cone of the affine normal toric variety v.
Examples
julia> cone(affine_normal_toric_variety(Oscar.positive_hull([1 1; -1 1])))
Polyhedral cone in ambient dimension 2weight_cone — Methodweight_cone(v::AffineNormalToricVariety)Return the dual cone of the affine normal toric variety v.
Examples
julia> C = positive_hull([1 0; 0 1])
Polyhedral cone in ambient dimension 2
julia> antv = affine_normal_toric_variety(C)
Normal toric variety
julia> weight_cone(antv)
Polyhedral cone in ambient dimension 2
julia> polarize(cone(antv)) == weight_cone(antv)
truehilbert_basis — Methodhilbert_basis(v::AffineNormalToricVariety)For an affine toric variety $v$, this returns the Hilbert basis of the cone dual to the cone of $v$.
Examples
julia> C = positive_hull([-1 1; 1 1])
Polyhedral cone in ambient dimension 2
julia> antv = affine_normal_toric_variety(C)
Normal toric variety
julia> hilbert_basis(antv)
[-1 1]
[ 1 1]
[ 0 1]mori_cone — Methodmori_cone(v::NormalToricVariety)Return the mori cone of the normal toric variety v.
Examples
julia> p2 = projective_space(NormalToricVariety, 2)
Normal toric variety
julia> mori = mori_cone(p2)
Polyhedral cone in ambient dimension 1
julia> dim(mori)
1nef_cone — Methodnef_cone(v::NormalToricVariety)Return the nef cone of the normal toric variety v.
Examples
julia> p2 = projective_space(NormalToricVariety, 2)
Normal toric variety
julia> nef = nef_cone(p2)
Polyhedral cone in ambient dimension 1
julia> dim(nef)
1Dimensions
dim — Methoddim(v::NormalToricVarietyType)Return the dimension of the normal toric variety v.
Examples
julia> C = Oscar.positive_hull([1 0]);
julia> antv = affine_normal_toric_variety(C);
julia> dim(antv)
1dim_of_torusfactor — Methoddim_of_torusfactor(v::NormalToricVarietyType)Return the dimension of the torus factor of the normal toric variety v.
Examples
julia> C = Oscar.positive_hull([1 0]);
julia> antv = affine_normal_toric_variety(C);
julia> dim_of_torusfactor(antv)
1euler_characteristic — Methodeuler_characteristic(v::NormalToricVarietyType)Return the Euler characteristic of the normal toric variety v.
Examples
julia> C = Oscar.positive_hull([1 0]);
julia> antv = affine_normal_toric_variety(C);
julia> euler_characteristic(antv)
1betti_number — Methodbetti_number(v::NormalToricVarietyType, i::Int)Compute the i-th Betti number of the normal toric variety v. Specifically, this method returns the dimension of the i-th simplicial homology group (with rational coefficients) of v. The employed algorithm is derived from theorem 12.3.12 in [CLS11]. Note that this theorem requires that the normal toric variety v is both complete and simplicial.
Examples
julia> P3 = projective_space(NormalToricVariety, 3)
Normal toric variety
julia> betti_number(P3,0)
1
julia> betti_number(P3, 1)
0Rings and ideals
We support the following rings and ideals for toric varieties:
- Cox ring (also termed the "total coordinate ring" in [CLS11]),
- coordinate ring of torus,
- cohomology_ring,
- Chow ring,
- irrelevant ideal,
- Stanley-Reisner ideal,
- ideal of linear relations,
- toric ideal.
Of course, for any of these coordinate names and the coefficient ring have to be chosen. The coefficient ring is fixed to Q. Therefore, the method coefficient_ring(v::NormalToricVarietyType) always return the field of rational numbers. For the coordinate names, we provide the following setter functions:
set_coordinate_names — Methodset_coordinate_names(v::NormalToricVarietyType, coordinate_names::AbstractVector{<:VarName})Allows to set the names of the homogeneous coordinates as long as the toric variety in question is not yet finalized (cf. is_finalized(v::NormalToricVarietyType)).
Examples
julia> C = Oscar.positive_hull([1 0]);
julia> antv = affine_normal_toric_variety(C);
julia> set_coordinate_names(antv, [:u])
julia> coordinate_names(antv)
1-element Vector{String}:
"u"
julia> set_coordinate_names(antv, ["v"])
julia> coordinate_names(antv)
1-element Vector{String}:
"v"
julia> set_coordinate_names(antv, ['w'])
julia> coordinate_names(antv)
1-element Vector{String}:
"w"set_coordinate_names_of_torus — Methodset_coordinate_names_of_torus(v::NormalToricVarietyType, coordinate_names::AbstractVector{<:VarName})Allows to set the names of the coordinates of the torus.
Examples
julia> F3 = hirzebruch_surface(NormalToricVariety, 3);
julia> set_coordinate_names_of_torus(F3, ["u", "v"])
julia> coordinate_names_of_torus(F3)
2-element Vector{String}:
"u"
"v"The following methods allow to etract the chosen coordinates:
coordinate_names — Methodcoordinate_names(v::NormalToricVarietyType)Return the names of the homogeneous coordinates of the normal toric variety v. The default is x1, ..., xn.
Examples
julia> C = Oscar.positive_hull([1 0]);
julia> antv = affine_normal_toric_variety(C);
julia> coordinate_names(antv)
1-element Vector{String}:
"x1"coordinate_names_of_torus — Methodcoordinate_names_of_torus(v::NormalToricVarietyType)Return the names of the coordinates of the torus of the normal toric variety v. The default is x1, ..., xn.
In order to efficiently construct algebraic cycles (elements of the Chox ring), cohomology classes (elements of the cohomology ring), or in order to compare ideals, it is imperative to fix choices of the coordinate names. The default value for coordinate names is [x1, x2, ... ]. The choice of coordinate names is fixed, once one of the above-mentioned rings is computed via one the following methods:
cox_ring — Methodcox_ring(v::NormalToricVarietyType)Computes the Cox ring of the normal toric variety v. Note that [CLS11] refers to this ring as the "total coordinate ring". For uniformity with schemes, we also support the function coordinate_ring to refer to the Cox ring.
Examples
julia> p2 = projective_space(NormalToricVariety, 2);
julia> set_coordinate_names(p2, ["y1", "y2", "y3"])
julia> cox_ring(p2)
Multivariate polynomial ring in 3 variables over QQ graded by
y1 -> [1]
y2 -> [1]
y3 -> [1]
julia> cox_ring(p2) == coordinate_ring(p2)
trueirrelevant_ideal — Methodirrelevant_ideal(v::NormalToricVarietyType)Return the irrelevant ideal of a normal toric variety v.
Examples
julia> p2 = projective_space(NormalToricVariety, 2);
julia> length(gens(irrelevant_ideal(p2)))
3ideal_of_linear_relations — Methodideal_of_linear_relations(v::NormalToricVarietyType)Return the ideal of linear relations of the simplicial and complete toric variety v.
Examples
julia> p2 = projective_space(NormalToricVariety, 2);
julia> ngens(ideal_of_linear_relations(p2))
2stanley_reisner_ideal — Methodstanley_reisner_ideal(v::NormalToricVarietyType)Return the Stanley-Reisner ideal of a normal toric variety v.
Examples
julia> p2 = projective_space(NormalToricVariety, 2);
julia> ngens(stanley_reisner_ideal(p2))
1toric_ideal — Methodtoric_ideal(antv::AffineNormalToricVariety)Return the toric ideal defining the affine normal toric variety.
Examples
Take the cone over the square at height one. The resulting toric variety has one defining equation. In projective space this corresponds to $\mathbb{P}^1\times\mathbb{P}^1$. Note that this cone is self-dual, the toric ideal comes from the dual cone.
julia> C = positive_hull([1 0 0; 1 1 0; 1 0 1; 1 1 1])
Polyhedral cone in ambient dimension 3
julia> antv = affine_normal_toric_variety(C)
Normal toric variety
julia> toric_ideal(antv)
Ideal generated by
-x1*x2 + x3*x4coordinate_ring_of_torus — Methodcoordinate_ring_of_torus(v::NormalToricVarietyType)Computes the coordinate ring of the torus of the normal toric variety v.
Examples
julia> p2 = projective_space(NormalToricVariety, 2);
julia> set_coordinate_names_of_torus(p2, ["y1", "y2"])
julia> coordinate_ring_of_torus(p2)
Quotient
of multivariate polynomial ring in 4 variables y1, y2, y1_, y2_
over rational field
by ideal (y1*y1_ - 1, y2*y2_ - 1)One can check the status as follows:
is_finalized — Methodis_finalized(v::NormalToricVarietyType)Checks if the Cox ring, the coordinate ring of the torus, the cohomology_ring, the Chow ring, the Stanley-Reisner ideal, the irrelevant ideal, the ideal of linear relations or the toric ideal has been cached. If any of these has been cached, then this function returns true and otherwise false.
Examples
julia> is_finalized(del_pezzo_surface(NormalToricVariety, 3))
falseAfter the variety finalized, one can enforce to obtain the above ideals in different rings. Also, one can opt to compute the above rings with a different choice of coordinate names and different coefficient ring. To this end, onc provides a custom ring (which reflects the desired choice of coordinate names and coefficient ring) as first argument. However, note that the cached ideals and rings are not altered.
cox_ring — Methodcox_ring(R::MPolyRing, v::NormalToricVarietyType)Computes the Cox ring of the normal toric variety v, in this case by adding the Cox grading to the given ring R. Note that [CLS11] refers to this ring as the "total coordinate ring".
Examples
julia> p2 = projective_space(NormalToricVariety, 2);
julia> R, _ = polynomial_ring(QQ, 3);
julia> cox_ring(R, p2)
Multivariate polynomial ring in 3 variables over QQ graded by
x1 -> [1]
x2 -> [1]
x3 -> [1]irrelevant_ideal — Methodirrelevant_ideal(R::MPolyRing, v::NormalToricVarietyType)Return the irrelevant ideal of a normal toric variety v as an ideal in R.
Examples
julia> p2 = projective_space(NormalToricVariety, 2);
julia> R, _ = polynomial_ring(QQ, 3);
julia> length(gens(irrelevant_ideal(R, p2)))
3ideal_of_linear_relations — Methodideal_of_linear_relations(R::MPolyRing, v::NormalToricVarietyType)Return the ideal of linear relations of the simplicial and complete toric variety v in the ring R.
Examples
julia> p2 = projective_space(NormalToricVariety, 2);
julia> R, _ = polynomial_ring(QQ, 3);
julia> ngens(ideal_of_linear_relations(R, p2))
2stanley_reisner_ideal — Methodstanley_reisner_ideal(R::MPolyRing, v::NormalToricVarietyType)Return the Stanley-Reisner ideal of a normal toric variety v as an ideal of R.
Examples
julia> p2 = projective_space(NormalToricVariety, 2);
julia> R, _ = polynomial_ring(QQ, 3);
julia> ngens(stanley_reisner_ideal(R, p2))
1toric_ideal — Methodtoric_ideal(R::MPolyRing, antv::AffineNormalToricVariety)Return the toric ideal defining the affine normal toric variety as an ideal in R.
Examples
Take the cone over the square at height one. The resulting toric variety has one defining equation. In projective space this corresponds to $\mathbb{P}^1\times\mathbb{P}^1$. Note that this cone is self-dual, the toric ideal comes from the dual cone.
julia> C = positive_hull([1 0 0; 1 1 0; 1 0 1; 1 1 1])
Polyhedral cone in ambient dimension 3
julia> antv = affine_normal_toric_variety(C)
Normal toric variety
julia> R, _ = polynomial_ring(QQ, 4);
julia> toric_ideal(R, antv)
Ideal generated by
-x1*x2 + x3*x4coordinate_ring_of_torus — Methodcoordinate_ring_of_torus(R::MPolyRing, v::NormalToricVarietyType)Computes the coordinate ring of the torus of the normal toric variety v in the given polynomial ring R.
Along the same lines, characters can be turned into rational functions:
character_to_rational_function — Methodcharacter_to_rational_function(v::NormalToricVarietyType, character::Vector{ZZRingElem})Computes the rational function corresponding to a character of the normal toric variety v.
Examples
julia> p2 = projective_space(NormalToricVariety, 2);
julia> character_to_rational_function(p2, [-1, 2])
x2^2*x1_character_to_rational_function — Methodcharacter_to_rational_function(R::MPolyRing, v::NormalToricVarietyType, character::Vector{ZZRingElem})Computes the rational function corresponding to a character of the normal toric variety v.
Examples
julia> p2 = projective_space(NormalToricVariety, 2);
julia> R, _ = polynomial_ring(QQ, 4);
julia> character_to_rational_function(R, p2, [-1, 2])
x2^2*x3Auxiliary Methods
binomial_exponents_to_ideal — Methodbinomial_exponents_to_ideal(binoms::Union{AbstractMatrix, ZZMatrix})This function converts the rows of a matrix to binomials. Each row $r$ is written as $r=u-v$ with $u, v\ge 0$ by splitting into positive and negative entries. Then the row $r$ corresponds to $x^u-x^v$. The resulting ideal is returned.
Examples
julia> A = [-1 -1 0 2; 2 3 -2 -1]
2×4 Matrix{Int64}:
-1 -1 0 2
2 3 -2 -1
julia> binomial_exponents_to_ideal(A)
Ideal generated by
-x1*x2 + x4^2
x1^2*x2^3 - x3^2*x4toric_ideal — Methodtoric_ideal(pts::ZZMatrix)Return the toric ideal generated from the linear relations between the points pts. This is the ideal generated by the set of binomials $\{x^u-x^v\ |\ u, v\in\mathbb{Z}^n_{\ge 0}\ (pts)^T\cdot(u-v) = 0\}$
Examples
julia> C = positive_hull([-2 5; 1 0]);
julia> H = hilbert_basis(C);
julia> toric_ideal(H)
Ideal generated by
x2*x3 - x4^2
-x1*x3 + x2^2*x4
-x1*x4 + x2^3
-x1*x3^2 + x2*x4^3
-x1*x3^3 + x4^5