# Line bundle cohomology with cohomCalg

We employ the cohomCalg algorithm (2010) to compute the dimension of line bundle cohomologies as well as vanishing sets.

## Dimensions of line bundle cohomology

all_cohomologiesMethod
all_cohomologies(l::ToricLineBundle)

Computes the dimension of all sheaf cohomologies of the toric line bundle l by use of the cohomCalg algorithm Ralph Blumenhagen, Benjamin Jurke, Thorsten Rahn, Helmut Roschy (2010), [cohomCalg:Implementation(@cite), Helmut Roschy, Thorsten Rahn (2010), Shin-Yao Jow (2011), Ralph Blumenhagen, Benjamin Jurke, Thorsten Rahn, Helmut Roschy (2012).

Examples

julia> dP3 = del_pezzo_surface(3)
A normal, non-affine, smooth, projective, gorenstein, fano, 2-dimensional toric variety without torusfactor

julia> all_cohomologies(ToricLineBundle(dP3, [1, 2, 3, 4]))
3-element Vector{fmpz}:
0
16
0
source
cohomologyMethod
cohomology(l::ToricLineBundle, i::Int)

Computes the dimension of the i-th sheaf cohomology of the toric line bundle l by use of the cohomCalg algorithm Ralph Blumenhagen, Benjamin Jurke, Thorsten Rahn, Helmut Roschy (2010), [cohomCalg:Implementation(@cite), Helmut Roschy, Thorsten Rahn (2010), Shin-Yao Jow (2011), Ralph Blumenhagen, Benjamin Jurke, Thorsten Rahn, Helmut Roschy (2012).

Examples

julia> dP3 = del_pezzo_surface(3)
A normal, non-affine, smooth, projective, gorenstein, fano, 2-dimensional toric variety without torusfactor

julia> cohomology(ToricLineBundle(dP3, [4, 1, 1, 1]), 0)
12
source

## Toric vanishing sets

Vanishing sets describe subsets of the Picard group of toric varieties. Their computations is based on (2010), i.e. this functionality is only available if the toric variety in question is either smooth and complete or alternatively, simplicial and projective. This approach to identify vanishing sets on toric varieties was originally introduced in Martin Bies (2018). As described there, on a technical level, a vanishing set is the complement of a finite family of polyhedra.

For a toric variety, all vanishing sets are computed as follows:

vanishing_setsMethod
vanishing_sets(variety::AbstractNormalToricVariety)

Compute the vanishing sets of an abstract toric variety v by use of the cohomCalg algorithm.

source

The return value is a vector of vanishing sets. This vector has length one larger than the dimension of the variety in question. The first vanishing set in this vector describes all line bundles for which the zero-th sheaf cohomology vanishes. More generally, if a line bundle is contained in the n-th vanishing set, then its n-1-th sheaf cohomology vanishes. The following method checks if a line bundle is contained in a vanishing set:

containsMethod
contains(tvs::ToricVanishingSet, l::ToricLineBundle)

Checks if the toric line bundle l is contained in the toric vanishing set tvs.

Examples

julia> dP1 = del_pezzo_surface(1)
A normal, non-affine, smooth, projective, gorenstein, fano, 2-dimensional toric variety without torusfactor

julia> l = ToricLineBundle(dP1, [3, 2])
A toric line bundle on a normal toric variety

julia> all_cohomologies(l)
3-element Vector{fmpz}:
7
0
0

julia> vs = vanishing_sets(dP1)
3-element Vector{ToricVanishingSet}:
A toric vanishing set for cohomology index 0
A toric vanishing set for cohomology index 1
A toric vanishing set for cohomology index 2

julia> contains(vs[1], l)
false

julia> contains(vs[2], l)
true

julia> contains(vs[3], l)
true
source

A vanishing set can in principle cover the entire Picard group. This can be checked with isfull. This methods returns true if the vanishing set is the entire Picard group and false otherwise. Beyond this, we support the following attributes for vanishing sets:

toric_varietyMethod
toric_variety(tvs::ToricVanishingSet)

Return the toric variety of the vanishing set tvs.

Examples

julia> dP1 = del_pezzo_surface(1)
A normal, non-affine, smooth, projective, gorenstein, fano, 2-dimensional toric variety without torusfactor

julia> vs = vanishing_sets(dP1)
3-element Vector{ToricVanishingSet}:
A toric vanishing set for cohomology index 0
A toric vanishing set for cohomology index 1
A toric vanishing set for cohomology index 2

julia> toric_variety(vs[3])
A normal, non-affine, smooth, projective, gorenstein, fano, 2-dimensional toric variety without torusfactor
source
polyhedraMethod
polyhedra(tvs::ToricVanishingSet)

Return the vector of the polyhedra whose complement defines the vanishing set tvs.

Examples

jldoctest julia> dP1 = delpezzosurface(1) A normal, non-affine, smooth, projective, gorenstein, fano, 2-dimensional toric variety without torusfactor

julia> vs = vanishing_sets(dP1) 3-element Vector{ToricVanishingSet}: A toric vanishing set for cohomology index 0 A toric vanishing set for cohomology index 1 A toric vanishing set for cohomology index 2

julia> polyhedra(vs[3]) 1-element Vector{Polyhedra{fmpq}}: A polyhedron in ambient dimension 2

source
cohomology_indexMethod
cohomology_index(tvs::ToricVanishingSet)

Return the cohomology index of the toric vanishing set tvs.

Examples

julia> dP1 = del_pezzo_surface(1)
A normal, non-affine, smooth, projective, gorenstein, fano, 2-dimensional toric variety without torusfactor

julia> vs = vanishing_sets(dP1)
3-element Vector{ToricVanishingSet}:
A toric vanishing set for cohomology index 0
A toric vanishing set for cohomology index 1
A toric vanishing set for cohomology index 2

julia> cohomology_index(vs[3])
2`
source