Rational polyhedral fans

This module was designed as a part of the framework for toric varieties (variety, fano_variety). While the emphasis is on complete full-dimensional fans, arbitrary fans are supported. Work with distinct lattices. The default lattice is ToricLattice \(N\) of the appropriate dimension. The only case when you must specify lattice explicitly is creation of a 0-dimensional fan, where dimension of the ambient space cannot be guessed.

A rational polyhedral fan is a finite collection of strictly convex rational polyhedral cones, such that the intersection of any two cones of the fan is a face of each of them and each face of each cone is also a cone of the fan.

AUTHORS:

• Andrey Novoseltsev (2010-05-15): initial version.
• Andrey Novoseltsev (2010-06-17): substantial improvement during review by Volker Braun.

EXAMPLES:

Use Fan() to construct fans “explicitly”:

sage: fan = Fan(cones=[(0,1), (1,2)],
....:           rays=[(1,0), (0,1), (-1,0)])
sage: fan
Rational polyhedral fan in 2-d lattice N

In addition to giving such lists of cones and rays you can also create cones first using Cone() and then combine them into a fan. See the documentation of Fan() for details.

In 2 dimensions there is a unique maximal fan determined by rays, and you can use Fan2d() to construct it:

sage: fan2d = Fan2d(rays=[(1,0), (0,1), (-1,0)])
sage: fan2d.is_equivalent(fan)
True

But keep in mind that in higher dimensions the cone data is essential and cannot be omitted. Instead of building a fan from scratch, for this tutorial we will use an easy way to get two fans associated to lattice polytopes: FaceFan() and NormalFan():

sage: fan1 = FaceFan(lattice_polytope.cross_polytope(3))
sage: fan2 = NormalFan(lattice_polytope.cross_polytope(3))

Given such “automatic” fans, you may wonder what are their rays and cones:

sage: fan1.rays()
M( 1,  0,  0),
M( 0,  1,  0),
M( 0,  0,  1),
M(-1,  0,  0),
M( 0, -1,  0),
M( 0,  0, -1)
in 3-d lattice M
sage: fan1.generating_cones()
(3-d cone of Rational polyhedral fan in 3-d lattice M,
 3-d cone of Rational polyhedral fan in 3-d lattice M,
 3-d cone of Rational polyhedral fan in 3-d lattice M,
 3-d cone of Rational polyhedral fan in 3-d lattice M,
 3-d cone of Rational polyhedral fan in 3-d lattice M,
 3-d cone of Rational polyhedral fan in 3-d lattice M,
 3-d cone of Rational polyhedral fan in 3-d lattice M,
 3-d cone of Rational polyhedral fan in 3-d lattice M)

The last output is not very illuminating. Let’s try to improve it:

sage: for cone in fan1: print(cone.rays())
M( 0, 1, 0),
M( 0, 0, 1),
M(-1, 0, 0)
in 3-d lattice M
M( 0,  0, 1),
M(-1,  0, 0),
M( 0, -1, 0)
in 3-d lattice M
M(-1,  0,  0),
M( 0, -1,  0),
M( 0,  0, -1)
in 3-d lattice M
M( 0, 1,  0),
M(-1, 0,  0),
M( 0, 0, -1)
in 3-d lattice M
M(1, 0,  0),
M(0, 1,  0),
M(0, 0, -1)
in 3-d lattice M
M(1, 0, 0),
M(0, 1, 0),
M(0, 0, 1)
in 3-d lattice M
M(1,  0, 0),
M(0,  0, 1),
M(0, -1, 0)
in 3-d lattice M
M(1,  0,  0),
M(0, -1,  0),
M(0,  0, -1)
in 3-d lattice M

You can also do

sage: for cone in fan1: print(cone.ambient_ray_indices())
(1, 2, 3)
(2, 3, 4)
(3, 4, 5)
(1, 3, 5)
(0, 1, 5)
(0, 1, 2)
(0, 2, 4)
(0, 4, 5)

to see indices of rays of the fan corresponding to each cone.

While the above cycles were over “cones in fan”, it is obvious that we did not get ALL the cones: every face of every cone in a fan must also be in the fan, but all of the above cones were of dimension three. The reason for this behaviour is that in many cases it is enough to work with generating cones of the fan, i.e. cones which are not faces of bigger cones. When you do need to work with lower dimensional cones, you can easily get access to them using cones():

sage: [cone.ambient_ray_indices() for cone in fan1.cones(2)]
[(0, 1), (0, 2), (1, 2), (1, 3), (2, 3), (0, 4),
 (2, 4), (3, 4), (1, 5), (3, 5), (4, 5), (0, 5)]

In fact, you do not have to type .cones:

sage: [cone.ambient_ray_indices() for cone in fan1(2)]
[(0, 1), (0, 2), (1, 2), (1, 3), (2, 3), (0, 4),
 (2, 4), (3, 4), (1, 5), (3, 5), (4, 5), (0, 5)]

You may also need to know the inclusion relations between all of the cones of the fan. In this case check out cone_lattice():

sage: L = fan1.cone_lattice()
sage: L
Finite lattice containing 28 elements with distinguished linear extension
sage: L.bottom()
0-d cone of Rational polyhedral fan in 3-d lattice M
sage: L.top()
Rational polyhedral fan in 3-d lattice M
sage: cone = L.level_sets()[2][0]
sage: cone
2-d cone of Rational polyhedral fan in 3-d lattice M
sage: sorted(L.hasse_diagram().neighbors(cone))
[1-d cone of Rational polyhedral fan in 3-d lattice M,
 1-d cone of Rational polyhedral fan in 3-d lattice M,
 3-d cone of Rational polyhedral fan in 3-d lattice M,
 3-d cone of Rational polyhedral fan in 3-d lattice M]

You can check how “good” a fan is:

sage: fan1.is_complete()
True
sage: fan1.is_simplicial()
True
sage: fan1.is_smooth()
True

The face fan of the octahedron is really good! Time to remember that we have also constructed its normal fan:

sage: fan2.is_complete()
True
sage: fan2.is_simplicial()
False
sage: fan2.is_smooth()
False

This one does have some “problems,” but we can fix them:

sage: fan3 = fan2.make_simplicial()
sage: fan3.is_simplicial()
True
sage: fan3.is_smooth()
False

Note that we had to save the result of make_simplicial() in a new fan. Fans in Sage are immutable, so any operation that does change them constructs a new fan.

We can also make fan3 smooth, but it will take a bit more work:

sage: cube = lattice_polytope.cross_polytope(3).polar()
sage: sk = cube.skeleton_points(2)
sage: rays = [cube.point(p) for p in sk]
sage: fan4 = fan3.subdivide(new_rays=rays)
sage: fan4.is_smooth()
True

Let’s see how “different” are fan2 and fan4:

sage: fan2.ngenerating_cones()
6
sage: fan2.nrays()
8
sage: fan4.ngenerating_cones()
48
sage: fan4.nrays()
26

Smoothness does not come for free!

Please take a look at the rest of the available functions below and their complete descriptions. If you need any features that are missing, feel free to suggest them. (Or implement them on your own and submit a patch to Sage for inclusion!)

class sage.geometry.fan.Cone_of_fan(ambient, ambient_ray_indices)

Bases: sage.geometry.cone.ConvexRationalPolyhedralCone

Construct a cone belonging to a fan.

Warning

This class does not check that the input defines a valid cone of a fan. You must not construct objects of this class directly.

In addition to all of the properties of “regular” cones, such cones know their relation to the fan.

INPUT:

• ambient – fan whose cone is constructed;
• ambient_ray_indices – increasing list or tuple of integers, indices of rays of ambient generating this cone.

OUTPUT:

• cone of ambient.

EXAMPLES:

The intended way to get objects of this class is the following:

sage: fan = toric_varieties.P1xP1().fan()
sage: cone = fan.generating_cone(0)
sage: cone
2-d cone of Rational polyhedral fan in 2-d lattice N
sage: cone.ambient_ray_indices()
(0, 2)
sage: cone.star_generator_indices()
(0,)

star_generator_indices()

Return indices of generating cones of the “ambient fan” containing self.

OUTPUT:

• increasing tuple of integers.

EXAMPLES:

sage: P1xP1 = toric_varieties.P1xP1()
sage: cone = P1xP1.fan().generating_cone(0)
sage: cone.star_generator_indices()
(0,)

star_generators()

Return indices of generating cones of the “ambient fan” containing self.

OUTPUT:

• increasing tuple of integers.

EXAMPLES:

sage: P1xP1 = toric_varieties.P1xP1()
sage: cone = P1xP1.fan().generating_cone(0)
sage: cone.star_generators()
(2-d cone of Rational polyhedral fan in 2-d lattice N,)

sage.geometry.fan.FaceFan(polytope, lattice=None)

Construct the face fan of the given rational polytope.

INPUT:

• polytope – a polytope over \(\QQ\) or a lattice polytope. A (not necessarily full-dimensional) polytope containing the origin in its relative interior.
• lattice – ToricLattice, \(\ZZ^n\), or any other object that behaves like these. If not specified, an attempt will be made to determine an appropriate toric lattice automatically.

OUTPUT:

• rational polyhedral fan.

See also NormalFan().

EXAMPLES:

Let’s construct the fan corresponding to the product of two projective lines:

sage: diamond = lattice_polytope.cross_polytope(2)
sage: P1xP1 = FaceFan(diamond)
sage: P1xP1.rays()
M( 1,  0),
M( 0,  1),
M(-1,  0),
M( 0, -1)
in 2-d lattice M
sage: for cone in P1xP1: print(cone.rays())
M(-1,  0),
M( 0, -1)
in 2-d lattice M
M( 0, 1),
M(-1, 0)
in 2-d lattice M
M(1, 0),
M(0, 1)
in 2-d lattice M
M(1,  0),
M(0, -1)
in 2-d lattice M

sage.geometry.fan.Fan(cones, rays=None, lattice=None, check=True, normalize=True, is_complete=None, virtual_rays=None, discard_faces=False)

Construct a rational polyhedral fan.

Note

Approximate time to construct a fan consisting of \(n\) cones is \(n^2/5\) seconds. That is half an hour for 100 cones. This time can be significantly reduced in the future, but it is still likely to be \(\sim n^2\) (with, say, \(/500\) instead of \(/5\)). If you know that your input does form a valid fan, use check=False option to skip consistency checks.

INPUT:

• cones – list of either Cone objects or lists of integers interpreted as indices of generating rays in rays. These must be only maximal cones of the fan, unless discard_faces=True option is specified;
• rays – list of rays given as list or vectors convertible to the rational extension of lattice. If cones are given by Cone objects rays may be determined automatically. You still may give them explicitly to ensure a particular order of rays in the fan. In this case you must list all rays that appear in cones. You can give “extra” ones if it is convenient (e.g. if you have a big list of rays for several fans), but all “extra” rays will be discarded;
• lattice – ToricLattice, \(\ZZ^n\), or any other object that behaves like these. If not specified, an attempt will be made to determine an appropriate toric lattice automatically;
• check – by default the input data will be checked for correctness (e.g. that intersection of any two given cones is a face of each). If you know for sure that the input is correct, you may significantly decrease construction time using check=False option;
• normalize – you can further speed up construction using normalize=False option. In this case cones must be a list of sorted tuples and rays must be immutable primitive vectors in lattice. In general, you should not use this option, it is designed for code optimization and does not give as drastic improvement in speed as the previous one;
• is_complete – every fan can determine on its own if it is complete or not, however it can take quite a bit of time for “big” fans with many generating cones. On the other hand, in some situations it is known in advance that a certain fan is complete. In this case you can pass is_complete=True option to speed up some computations. You may also pass is_complete=False option, although it is less likely to be beneficial. Of course, passing a wrong value can compromise the integrity of data structures of the fan and lead to wrong results, so you should be very careful if you decide to use this option;
• virtual_rays – (optional, computed automatically if needed) a list of ray generators to be used for virtual_rays();
• discard_faces – by default, the fan constructor expects the list of maximal cones. If you provide “extra” ones and leave check=True (default), an exception will be raised. If you provide “extra” cones and set check=False, you may get wrong results as assumptions on internal data structures will be invalid. If you want the fan constructor to select the maximal cones from the given input, you may provide discard_faces=True option (it works both for check=True and check=False).

OUTPUT:

• a fan.

See also

In 2 dimensions you can cyclically order the rays. Hence the rays determine a unique maximal fan without having to specify the cones, and you can use Fan2d() to construct this fan from just the rays.

EXAMPLES:

Let’s construct a fan corresponding to the projective plane in several ways:

sage: cone1 = Cone([(1,0), (0,1)])
sage: cone2 = Cone([(0,1), (-1,-1)])
sage: cone3 = Cone([(-1,-1), (1,0)])
sage: P2 = Fan([cone1, cone2, cone2])
Traceback (most recent call last):
...
ValueError: you have provided 3 cones, but only 2 of them are maximal!
Use discard_faces=True if you indeed need to construct a fan from
these cones.

Oops! There was a typo and cone2 was listed twice as a generating cone of the fan. If it was intentional (e.g. the list of cones was generated automatically and it is possible that it contains repetitions or faces of other cones), use discard_faces=True option:

sage: P2 = Fan([cone1, cone2, cone2], discard_faces=True)
sage: P2.ngenerating_cones()
2

However, in this case it was definitely a typo, since the fan of \(\mathbb{P}^2\) has 3 maximal cones:

sage: P2 = Fan([cone1, cone2, cone3])
sage: P2.ngenerating_cones()
3

Looks better. An alternative way is

sage: rays = [(1,0), (0,1), (-1,-1)]
sage: cones = [(0,1), (1,2), (2,0)]
sage: P2a = Fan(cones, rays)
sage: P2a.ngenerating_cones()
3
sage: P2 == P2a
False

That may seem wrong, but it is not:

sage: P2.is_equivalent(P2a)
True

See is_equivalent() for details.

Yet another way to construct this fan is

sage: P2b = Fan(cones, rays, check=False)
sage: P2b.ngenerating_cones()
3
sage: P2a == P2b
True

If you try the above examples, you are likely to notice the difference in speed, so when you are sure that everything is correct, it is a good idea to use check=False option. On the other hand, it is usually NOT a good idea to use normalize=False option:

sage: P2c = Fan(cones, rays, check=False, normalize=False)
Traceback (most recent call last):
...
AttributeError: 'tuple' object has no attribute 'parent'

Yet another way is to use functions FaceFan() and NormalFan() to construct fans from lattice polytopes.

We have not yet used lattice argument, since if was determined automatically:

sage: P2.lattice()
2-d lattice N
sage: P2b.lattice()
2-d lattice N

However, it is necessary to specify it explicitly if you want to construct a fan without rays or cones:

sage: Fan([], [])
Traceback (most recent call last):
...
ValueError: you must specify the lattice
when you construct a fan without rays and cones!
sage: F = Fan([], [], lattice=ToricLattice(2, "L"))
sage: F
Rational polyhedral fan in 2-d lattice L
sage: F.lattice_dim()
2
sage: F.dim()
0

sage.geometry.fan.Fan2d(rays, lattice=None)

Construct the maximal 2-d fan with given rays.

In two dimensions we can uniquely construct a fan from just rays, just by cyclically ordering the rays and constructing as many cones as possible. This is why we implement a special constructor for this case.

INPUT:

• rays – list of rays given as list or vectors convertible to the rational extension of lattice. Duplicate rays are removed without changing the ordering of the remaining rays.
• lattice – ToricLattice, \(\ZZ^n\), or any other object that behaves like these. If not specified, an attempt will be made to determine an appropriate toric lattice automatically.

EXAMPLES:

sage: Fan2d([(0,1), (1,0)])
Rational polyhedral fan in 2-d lattice N
sage: Fan2d([], lattice=ToricLattice(2, 'myN'))
Rational polyhedral fan in 2-d lattice myN

The ray order is as specified, even if it is not the cyclic order:

sage: fan1 = Fan2d([(0,1), (1,0)])
sage: fan1.rays()
N(0, 1),
N(1, 0)
in 2-d lattice N

sage: fan2 = Fan2d([(1,0), (0,1)])
sage: fan2.rays()
N(1, 0),
N(0, 1)
in 2-d lattice N

sage: fan1 == fan2, fan1.is_equivalent(fan2)
(False, True)

sage: fan = Fan2d([(1,1), (-1,-1), (1,-1), (-1,1)])
sage: [ cone.ambient_ray_indices() for cone in fan ]
[(2, 1), (1, 3), (3, 0), (0, 2)]
sage: fan.is_complete()
True

sage.geometry.fan.NormalFan(polytope, lattice=None)

Construct the normal fan of the given rational polytope.

This returns the inner normal fan. For the outer normal fan, use NormalFan(-P).

INPUT:

• polytope – a full-dimensional polytope over \(\QQ\) or:class:\(lattice polytope <sage.geometry.lattice_polytope.LatticePolytopeClass>\).
• lattice – ToricLattice, \(\ZZ^n\), or any other object that behaves like these. If not specified, an attempt will be made to determine an appropriate toric lattice automatically.

OUTPUT:

• rational polyhedral fan.

See also FaceFan().

EXAMPLES:

Let’s construct the fan corresponding to the product of two projective lines:

sage: square = LatticePolytope([(1,1), (-1,1), (-1,-1), (1,-1)])
sage: P1xP1 = NormalFan(square)
sage: P1xP1.rays()
N( 1,  0),
N( 0,  1),
N(-1,  0),
N( 0, -1)
in 2-d lattice N
sage: for cone in P1xP1: print(cone.rays())
N(-1,  0),
N( 0, -1)
in 2-d lattice N
N(1,  0),
N(0, -1)
in 2-d lattice N
N(1, 0),
N(0, 1)
in 2-d lattice N
N( 0, 1),
N(-1, 0)
in 2-d lattice N

sage: cuboctahed = polytopes.cuboctahedron()
sage: NormalFan(cuboctahed)
Rational polyhedral fan in 3-d lattice N

class sage.geometry.fan.RationalPolyhedralFan(cones, rays, lattice, is_complete=None, virtual_rays=None)

Bases: sage.geometry.cone.IntegralRayCollection, _abcoll.Callable, _abcoll.Container

Create a rational polyhedral fan.

Warning

This class does not perform any checks of correctness of input nor does it convert input into the standard representation. Use Fan() to construct fans from “raw data” or FaceFan() and NormalFan() to get fans associated to polytopes.

Fans are immutable, but they cache most of the returned values.

INPUT:

• cones – list of generating cones of the fan, each cone given as a list of indices of its generating rays in rays;
• rays – list of immutable primitive vectors in lattice consisting of exactly the rays of the fan (i.e. no “extra” ones);
• lattice – ToricLattice, \(\ZZ^n\), or any other object that behaves like these. If None, it will be determined as parent() of the first ray. Of course, this cannot be done if there are no rays, so in this case you must give an appropriate lattice directly;
• is_complete – if given, must be True or False depending on whether this fan is complete or not. By default, it will be determined automatically if necessary;
• virtual_rays – if given, must be a list of immutable primitive vectors in lattice, see virtual_rays() for details. By default, it will be determined automatically if necessary.

OUTPUT:

• rational polyhedral fan.

Gale_transform()

Return the Gale transform of self.

OUTPUT:

A matrix over \(ZZ\).

EXAMPLES:

sage: fan = toric_varieties.P1xP1().fan()
sage: fan.Gale_transform()
[ 1  1  0  0 -2]
[ 0  0  1  1 -2]
sage: _.base_ring()
Integer Ring

Stanley_Reisner_ideal(ring)

Return the Stanley-Reisner ideal.

INPUT:

• A polynomial ring in self.nrays() variables.

OUTPUT:

• The Stanley-Reisner ideal in the given polynomial ring.

EXAMPLES:

sage: fan = Fan([[0,1,3],[3,4],[2,0],[1,2,4]], [(-3, -2, 1), (0, 0, 1), (3, -2, 1), (-1, -1, 1), (1, -1, 1)])
sage: fan.Stanley_Reisner_ideal( PolynomialRing(QQ,5,'A, B, C, D, E') )
Ideal (A*E, C*D, A*B*C, B*D*E) of Multivariate Polynomial Ring in A, B, C, D, E over Rational Field

cartesian_product(other, lattice=None)

Return the Cartesian product of self with other.

INPUT:

• other – a rational polyhedral fan;
• lattice – (optional) the ambient lattice for the Cartesian product fan. By default, the direct sum of the ambient lattices of self and other is constructed.

OUTPUT:

• a fan whose cones are all pairwise Cartesian products of the cones of self and other.

EXAMPLES:

sage: K = ToricLattice(1, 'K')
sage: fan1 = Fan([[0],[1]],[(1,),(-1,)], lattice=K)
sage: L = ToricLattice(2, 'L')
sage: fan2 = Fan(rays=[(1,0),(0,1),(-1,-1)],
....:        cones=[[0,1],[1,2],[2,0]], lattice=L)
sage: fan1.cartesian_product(fan2)
Rational polyhedral fan in 3-d lattice K+L
sage: _.ngenerating_cones()
6

common_refinement(other)

Return the common refinement of this fan and other.

INPUT:

• other – a fan in the same lattice() and with the same support as this fan

OUTPUT:

• a fan

EXAMPLES:

Refining a fan with itself gives itself:

sage: F0 = Fan2d([(1,0),(0,1),(-1,0),(0,-1)])
sage: F0.common_refinement(F0) == F0
True

A more complex example with complete fans:

sage: F1 = Fan([[0],[1]],[(1,),(-1,)])
sage: F2 = Fan2d([(1,0),(1,1),(0,1),(-1,0),(0,-1)])
sage: F3 = F2.cartesian_product(F1)
sage: F4 = F1.cartesian_product(F2)
sage: FF = F3.common_refinement(F4)
sage: F3.ngenerating_cones()
10
sage: F4.ngenerating_cones()
10
sage: FF.ngenerating_cones()
13

An example with two non-complete fans with the same support:

sage: F5 = Fan2d([(1,0),(1,2),(0,1)])
sage: F6 = Fan2d([(1,0),(2,1),(0,1)])
sage: F5.common_refinement(F6).ngenerating_cones()
3

Both fans must live in the same lattice:

sage: F0.common_refinement(F1)
Traceback (most recent call last):
...
ValueError: the fans are not in the same lattice

complex(base_ring=Integer Ring, extended=False)

Return the chain complex of the fan.

To a \(d\)-dimensional fan \(\Sigma\), one can canonically associate a chain complex \(K^\bullet\)

\[0 \longrightarrow \ZZ^{\Sigma(d)} \longrightarrow \ZZ^{\Sigma(d-1)} \longrightarrow \cdots \longrightarrow \ZZ^{\Sigma(0)} \longrightarrow 0\]

where the leftmost non-zero entry is in degree \(0\) and the rightmost entry in degree \(d\). See [Kly1990], eq. (3.2). This complex computes the homology of \(|\Sigma|\subset N_\RR\) with arbitrary support,

\[H_i(K) = H_{d-i}(|\Sigma|, \ZZ)_{\text{non-cpct}}\]

For a complete fan, this is just the non-compactly supported homology of \(\RR^d\). In this case, \(H_0(K)=\ZZ\) and \(0\) in all non-zero degrees.

For a complete fan, there is an extended chain complex

\[0 \longrightarrow \ZZ \longrightarrow \ZZ^{\Sigma(d)} \longrightarrow \ZZ^{\Sigma(d-1)} \longrightarrow \cdots \longrightarrow \ZZ^{\Sigma(0)} \longrightarrow 0\]

where we take the first \(\ZZ\) term to be in degree -1. This complex is an exact sequence, that is, all homology groups vanish.

The orientation of each cone is chosen as in oriented_boundary().

INPUT:

• extended – Boolean (default:False). Whether to construct the extended complex, that is, including the \(\ZZ\)-term at degree -1 or not.
• base_ring – A ring (default: ZZ). The ring to use instead of \(\ZZ\).

OUTPUT:

The complex associated to the fan as a ChainComplex. Raises a ValueError if the extended complex is requested for a non-complete fan.

EXAMPLES:

sage: fan = toric_varieties.P(3).fan()
sage: K_normal = fan.complex(); K_normal
Chain complex with at most 4 nonzero terms over Integer Ring
sage: K_normal.homology()
{0: Z, 1: 0, 2: 0, 3: 0}
sage: K_extended = fan.complex(extended=True); K_extended
Chain complex with at most 5 nonzero terms over Integer Ring
sage: K_extended.homology()
{-1: 0, 0: 0, 1: 0, 2: 0, 3: 0}

Homology computations are much faster over \(\QQ\) if you do not care about the torsion coefficients:

sage: toric_varieties.P2_123().fan().complex(extended=True, base_ring=QQ)
Chain complex with at most 4 nonzero terms over Rational Field
sage: _.homology()
{-1: Vector space of dimension 0 over Rational Field,
 0: Vector space of dimension 0 over Rational Field,
 1: Vector space of dimension 0 over Rational Field,
 2: Vector space of dimension 0 over Rational Field}

The extended complex is only defined for complete fans:

sage: fan = Fan([ Cone([(1,0)]) ])
sage: fan.is_complete()
False
sage: fan.complex(extended=True)
Traceback (most recent call last):
...
ValueError: The extended complex is only defined for complete fans!

The definition of the complex does not refer to the ambient space of the fan, so it does not distinguish a fan from the same fan embedded in a subspace:

sage: K1 = Fan([Cone([(-1,)]), Cone([(1,)])]).complex()
sage: K2 = Fan([Cone([(-1,0,0)]), Cone([(1,0,0)])]).complex()
sage: K1 == K2
True

Things get more complicated for non-complete fans:

sage: fan = Fan([Cone([(1,1,1)]),
....:            Cone([(1,0,0),(0,1,0)]),
....:            Cone([(-1,0,0),(0,-1,0),(0,0,-1)])])
sage: fan.complex().homology()
{0: 0, 1: 0, 2: Z x Z, 3: 0}
sage: fan = Fan([Cone([(1,0,0),(0,1,0)]),
....:            Cone([(-1,0,0),(0,-1,0),(0,0,-1)])])
sage: fan.complex().homology()
{0: 0, 1: 0, 2: Z, 3: 0}
sage: fan = Fan([Cone([(-1,0,0),(0,-1,0),(0,0,-1)])])
sage: fan.complex().homology()
{0: 0, 1: 0, 2: 0, 3: 0}

cone_containing(*points)

Return the smallest cone of self containing all given points.

INPUT:

• either one or more indices of rays of self, or one or more objects representing points of the ambient space of self, or a list of such objects (you CANNOT give a list of indices).

OUTPUT:

• A cone of fan whose ambient fan is self.

Note

We think of the origin as of the smallest cone containing no rays at all. If there is no ray in self that contains all rays, a ValueError exception will be raised.

EXAMPLES:

sage: cone1 = Cone([(0,-1), (1,0)])
sage: cone2 = Cone([(1,0), (0,1)])
sage: f = Fan([cone1, cone2])
sage: f.rays()
N(0,  1),
N(0, -1),
N(1,  0)
in 2-d lattice N
sage: f.cone_containing(0)  # ray index
1-d cone of Rational polyhedral fan in 2-d lattice N
sage: f.cone_containing(0, 1) # ray indices
Traceback (most recent call last):
...
ValueError: there is no cone in
Rational polyhedral fan in 2-d lattice N
containing all of the given rays! Ray indices: [0, 1]
sage: f.cone_containing(0, 2) # ray indices
2-d cone of Rational polyhedral fan in 2-d lattice N
sage: f.cone_containing((0,1))  # point
1-d cone of Rational polyhedral fan in 2-d lattice N
sage: f.cone_containing([(0,1)]) # point
1-d cone of Rational polyhedral fan in 2-d lattice N
sage: f.cone_containing((1,1))
2-d cone of Rational polyhedral fan in 2-d lattice N
sage: f.cone_containing((1,1), (1,0))
2-d cone of Rational polyhedral fan in 2-d lattice N
sage: f.cone_containing()
0-d cone of Rational polyhedral fan in 2-d lattice N
sage: f.cone_containing((0,0))
0-d cone of Rational polyhedral fan in 2-d lattice N
sage: f.cone_containing((-1,1))
Traceback (most recent call last):
...
ValueError: there is no cone in
Rational polyhedral fan in 2-d lattice N
containing all of the given points! Points: [N(-1, 1)]

cone_lattice()

Return the cone lattice of self.

This lattice will have the origin as the bottom (we do not include the empty set as a cone) and the fan itself as the top.

OUTPUT:

• finite poset <sage.combinat.posets.posets.FinitePoset of cones of fan, behaving like “regular” cones, but also containing the information about their relation to this fan, namely, the contained rays and containing generating cones. The top of the lattice will be this fan itself (which is not a cone of fan).

See also cones().

EXAMPLES:

Cone lattices can be computed for arbitrary fans:

sage: cone1 = Cone([(1,0), (0,1)])
sage: cone2 = Cone([(-1,0)])
sage: fan = Fan([cone1, cone2])
sage: fan.rays()
N( 0, 1),
N( 1, 0),
N(-1, 0)
in 2-d lattice N
sage: for cone in fan: print(cone.ambient_ray_indices())
(0, 1)
(2,)
sage: L = fan.cone_lattice()
sage: L
Finite poset containing 6 elements with distinguished linear extension

These 6 elements are the origin, three rays, one two-dimensional cone, and the fan itself. Since we do add the fan itself as the largest face, you should be a little bit careful with this last element:

sage: for face in L: print(face.ambient_ray_indices())
Traceback (most recent call last):
...
AttributeError: 'RationalPolyhedralFan'
object has no attribute 'ambient_ray_indices'
sage: L.top()
Rational polyhedral fan in 2-d lattice N

For example, you can do

sage: for l in L.level_sets()[:-1]:
....:     print([f.ambient_ray_indices() for f in l])
[()]
[(0,), (1,), (2,)]
[(0, 1)]

If the fan is complete, its cone lattice is atomic and coatomic and can (and will!) be computed in a much more efficient way, but the interface is exactly the same:

sage: fan = toric_varieties.P1xP1().fan()
sage: L = fan.cone_lattice()
sage: for l in L.level_sets()[:-1]:
....:     print([f.ambient_ray_indices() for f in l])
[()]
[(0,), (1,), (2,), (3,)]
[(0, 2), (1, 2), (0, 3), (1, 3)]

Let’s also consider the cone lattice of a fan generated by a single cone:

sage: fan = Fan([cone1])
sage: L = fan.cone_lattice()
sage: L
Finite poset containing 5 elements with distinguished linear extension

Here these 5 elements correspond to the origin, two rays, one generating cone of dimension two, and the whole fan. While this single cone “is” the whole fan, it is consistent and convenient to distinguish them in the cone lattice.

cones(dim=None, codim=None)

Return the specified cones of self.

INPUT:

• dim – dimension of the requested cones;
• codim – codimension of the requested cones.

Note

You can specify at most one input parameter.

OUTPUT:

• tuple of cones of self of the specified (co)dimension, if either dim or codim is given. Otherwise tuple of such tuples for all existing dimensions.

EXAMPLES:

sage: cone1 = Cone([(1,0), (0,1)])
sage: cone2 = Cone([(-1,0)])
sage: fan = Fan([cone1, cone2])
sage: fan(dim=0)
(0-d cone of Rational polyhedral fan in 2-d lattice N,)
sage: fan(codim=2)
(0-d cone of Rational polyhedral fan in 2-d lattice N,)
sage: for cone in fan.cones(1): cone.ray(0)
N(0, 1)
N(1, 0)
N(-1, 0)
sage: fan.cones(2)
(2-d cone of Rational polyhedral fan in 2-d lattice N,)

You cannot specify both dimension and codimension, even if they “agree”:

sage: fan(dim=1, codim=1)
Traceback (most recent call last):
...
ValueError: dimension and codimension
cannot be specified together!

But it is OK to ask for cones of too high or low (co)dimension:

sage: fan(-1)
()
sage: fan(3)
()
sage: fan(codim=4)
()

contains(cone)

Check if a given cone is equivalent to a cone of the fan.

INPUT:

• cone – anything.

OUTPUT:

• False if cone is not a cone or if cone is not equivalent to a cone of the fan. True otherwise.

Note

Recall that a fan is a (finite) collection of cones. A cone is contained in a fan if it is equivalent to one of the cones of the fan. In particular, it is possible that all rays of the cone are in the fan, but the cone itself is not.

If you want to know whether a point is in the support of the fan, you should use support_contains().

EXAMPLES:

We first construct a simple fan:

sage: cone1 = Cone([(0,-1), (1,0)])
sage: cone2 = Cone([(1,0), (0,1)])
sage: f = Fan([cone1, cone2])

Now we check if some cones are in this fan. First, we make sure that the order of rays of the input cone does not matter (check=False option ensures that rays of these cones will be listed exactly as they are given):

sage: f.contains(Cone([(1,0), (0,1)], check=False))
True
sage: f.contains(Cone([(0,1), (1,0)], check=False))
True

Now we check that a non-generating cone is in our fan:

sage: f.contains(Cone([(1,0)]))
True
sage: Cone([(1,0)]) in f   # equivalent to the previous command
True

Finally, we test some cones which are not in this fan:

sage: f.contains(Cone([(1,1)]))
False
sage: f.contains(Cone([(1,0), (-0,1)]))
True

A point is not a cone:

sage: n = f.lattice()(1,1); n
N(1, 1)
sage: f.contains(n)
False

embed(cone)

Return the cone equivalent to the given one, but sitting in self.

You may need to use this method before calling methods of cone that depend on the ambient structure, such as ambient_ray_indices() or facet_of(). The cone returned by this method will have self as ambient. If cone does not represent a valid cone of self, ValueError exception is raised.

Note

This method is very quick if self is already the ambient structure of cone, so you can use without extra checks and performance hit even if cone is likely to sit in self but in principle may not.

INPUT:

• cone – a cone.

OUTPUT:

• a cone of fan, equivalent to cone but sitting inside self.

EXAMPLES:

Let’s take a 3-d fan generated by a cone on 4 rays:

sage: f = Fan([Cone([(1,0,1), (0,1,1), (-1,0,1), (0,-1,1)])])

Then any ray generates a 1-d cone of this fan, but if you construct such a cone directly, it will not “sit” inside the fan:

sage: ray = Cone([(0,-1,1)])
sage: ray
1-d cone in 3-d lattice N
sage: ray.ambient_ray_indices()
(0,)
sage: ray.adjacent()
()
sage: ray.ambient()
1-d cone in 3-d lattice N

If we want to operate with this ray as a part of the fan, we need to embed it first:

sage: e_ray = f.embed(ray)
sage: e_ray
1-d cone of Rational polyhedral fan in 3-d lattice N
sage: e_ray.rays()
N(0, -1, 1)
in 3-d lattice N
sage: e_ray is ray
False
sage: e_ray.is_equivalent(ray)
True
sage: e_ray.ambient_ray_indices()
(3,)
sage: e_ray.adjacent()
(1-d cone of Rational polyhedral fan in 3-d lattice N,
 1-d cone of Rational polyhedral fan in 3-d lattice N)
sage: e_ray.ambient()
Rational polyhedral fan in 3-d lattice N

Not every cone can be embedded into a fixed fan:

sage: f.embed(Cone([(0,0,1)]))
Traceback (most recent call last):
...
ValueError: 1-d cone in 3-d lattice N does not belong
to Rational polyhedral fan in 3-d lattice N!
sage: f.embed(Cone([(1,0,1), (-1,0,1)]))
Traceback (most recent call last):
...
ValueError: 2-d cone in 3-d lattice N does not belong
to Rational polyhedral fan in 3-d lattice N!

generating_cone(n)

Return the n-th generating cone of self.

INPUT:

• n – integer, the index of a generating cone.

OUTPUT:

• cone of fan.

EXAMPLES:

sage: fan = toric_varieties.P1xP1().fan()
sage: fan.generating_cone(0)
2-d cone of Rational polyhedral fan in 2-d lattice N

generating_cones()

Return generating cones of self.

OUTPUT:

• tuple of cones of fan.

EXAMPLES:

sage: fan = toric_varieties.P1xP1().fan()
sage: fan.generating_cones()
(2-d cone of Rational polyhedral fan in 2-d lattice N,
 2-d cone of Rational polyhedral fan in 2-d lattice N,
 2-d cone of Rational polyhedral fan in 2-d lattice N,
 2-d cone of Rational polyhedral fan in 2-d lattice N)
sage: cone1 = Cone([(1,0), (0,1)])
sage: cone2 = Cone([(-1,0)])
sage: fan = Fan([cone1, cone2])
sage: fan.generating_cones()
(2-d cone of Rational polyhedral fan in 2-d lattice N,
 1-d cone of Rational polyhedral fan in 2-d lattice N)

is_complete()

Check if self is complete.

A rational polyhedral fan is complete if its cones fill the whole space.

OUTPUT:

• True if self is complete and False otherwise.

EXAMPLES:

sage: fan = toric_varieties.P1xP1().fan()
sage: fan.is_complete()
True
sage: cone1 = Cone([(1,0), (0,1)])
sage: cone2 = Cone([(-1,0)])
sage: fan = Fan([cone1, cone2])
sage: fan.is_complete()
False

is_equivalent(other)

Check if self is “mathematically” the same as other.

INPUT:

• other - fan.

OUTPUT:

• True if self and other define the same fans as collections of equivalent cones in the same lattice, False otherwise.

There are three different equivalences between fans \(F_1\) and \(F_2\) in the same lattice:

1. They have the same rays in the same order and the same generating cones in the same order. This is tested by F1 == F2.
2. They have the same rays and the same generating cones without taking into account any order. This is tested by F1.is_equivalent(F2).
3. They are in the same orbit of \(GL(n,\ZZ)\) (and, therefore, correspond to isomorphic toric varieties). This is tested by F1.is_isomorphic(F2).

Note that virtual_rays() are included into consideration for all of the above equivalences.

EXAMPLES:

sage: fan1 = Fan(cones=[(0,1), (1,2)],
....:            rays=[(1,0), (0,1), (-1,-1)],
....:            check=False)
sage: fan2 = Fan(cones=[(2,1), (0,2)],
....:            rays=[(1,0), (-1,-1), (0,1)],
....:            check=False)
sage: fan3 = Fan(cones=[(0,1), (1,2)],
....:            rays=[(1,0), (0,1), (-1,1)],
....:            check=False)
sage: fan1 == fan2
False
sage: fan1.is_equivalent(fan2)
True
sage: fan1 == fan3
False
sage: fan1.is_equivalent(fan3)
False

is_isomorphic(other)

Check if self is in the same \(GL(n, \ZZ)\)-orbit as other.

There are three different equivalences between fans \(F_1\) and \(F_2\) in the same lattice:

1. They have the same rays in the same order and the same generating cones in the same order. This is tested by F1 == F2.
2. They have the same rays and the same generating cones without taking into account any order. This is tested by F1.is_equivalent(F2).
3. They are in the same orbit of \(GL(n,\ZZ)\) (and, therefore, correspond to isomorphic toric varieties). This is tested by F1.is_isomorphic(F2).

Note that virtual_rays() are included into consideration for all of the above equivalences.

INPUT:

• other – a fan.

OUTPUT:

• True if self and other are in the same \(GL(n, \ZZ)\)-orbit, False otherwise.

See also

If you want to obtain the actual fan isomorphism, use isomorphism().

EXAMPLES:

Here we pick an \(SL(2,\ZZ)\) matrix m and then verify that the image fan is isomorphic:

sage: rays = ((1, 1), (0, 1), (-1, -1), (1, 0))
sage: cones = [(0,1), (1,2), (2,3), (3,0)]
sage: fan1 = Fan(cones, rays)
sage: m = matrix([[-2,3],[1,-1]])
sage: fan2 = Fan(cones, [vector(r)*m for r in rays])
sage: fan1.is_isomorphic(fan2)
True
sage: fan1.is_equivalent(fan2)
False
sage: fan1 == fan2
False

These fans are “mirrors” of each other:

sage: fan1 = Fan(cones=[(0,1), (1,2)],
....:            rays=[(1,0), (0,1), (-1,-1)],
....:            check=False)
sage: fan2 = Fan(cones=[(0,1), (1,2)],
....:            rays=[(1,0), (0,-1), (-1,1)],
....:            check=False)
sage: fan1 == fan2
False
sage: fan1.is_equivalent(fan2)
False
sage: fan1.is_isomorphic(fan2)
True
sage: fan1.is_isomorphic(fan1)
True

is_simplicial()

Check if self is simplicial.

A rational polyhedral fan is simplicial if all of its cones are, i.e. primitive vectors along generating rays of every cone form a part of a rational basis of the ambient space.

OUTPUT:

• True if self is simplicial and False otherwise.

EXAMPLES:

sage: fan = toric_varieties.P1xP1().fan()
sage: fan.is_simplicial()
True
sage: cone1 = Cone([(1,0), (0,1)])
sage: cone2 = Cone([(-1,0)])
sage: fan = Fan([cone1, cone2])
sage: fan.is_simplicial()
True

In fact, any fan in a two-dimensional ambient space is simplicial. This is no longer the case in dimension three:

sage: fan = NormalFan(lattice_polytope.cross_polytope(3))
sage: fan.is_simplicial()
False
sage: fan.generating_cone(0).nrays()
4

is_smooth(codim=None)

Check if self is smooth.

A rational polyhedral fan is smooth if all of its cones are, i.e. primitive vectors along generating rays of every cone form a part of an integral basis of the ambient space. In this case the corresponding toric variety is smooth.

A fan in an \(n\)-dimensional lattice is smooth up to codimension \(c\) if all cones of codimension greater than or equal to \(c\) are smooth, i.e. if all cones of dimension less than or equal to \(n-c\) are smooth. In this case the singular set of the corresponding toric variety is of dimension less than \(c\).

INPUT:

• codim – codimension in which smoothness has to be checked, by default complete smoothness will be checked.

OUTPUT:

• True if self is smooth (in codimension codim, if it was given) and False otherwise.

EXAMPLES:

sage: fan = toric_varieties.P1xP1().fan()
sage: fan.is_smooth()
True
sage: cone1 = Cone([(1,0), (0,1)])
sage: cone2 = Cone([(-1,0)])
sage: fan = Fan([cone1, cone2])
sage: fan.is_smooth()
True
sage: fan = NormalFan(lattice_polytope.cross_polytope(2))
sage: fan.is_smooth()
False
sage: fan.is_smooth(codim=1)
True
sage: fan.generating_cone(0).rays()
N(-1, -1),
N(-1,  1)
in 2-d lattice N
sage: fan.generating_cone(0).rays().matrix().det()
-2

isomorphism(other)

Return a fan isomorphism from self to other.

INPUT:

• other – fan.

OUTPUT:

A fan isomorphism. If no such isomorphism exists, a FanNotIsomorphicError is raised.

EXAMPLES:

sage: rays = ((1, 1), (0, 1), (-1, -1), (3, 1))
sage: cones = [(0,1), (1,2), (2,3), (3,0)]
sage: fan1 = Fan(cones, rays)
sage: m = matrix([[-2,3],[1,-1]])
sage: fan2 = Fan(cones, [vector(r)*m for r in rays])

sage: fan1.isomorphism(fan2)
Fan morphism defined by the matrix
[-2  3]
[ 1 -1]
Domain fan: Rational polyhedral fan in 2-d lattice N
Codomain fan: Rational polyhedral fan in 2-d lattice N

sage: fan2.isomorphism(fan1)
Fan morphism defined by the matrix
[1 3]
[1 2]
Domain fan: Rational polyhedral fan in 2-d lattice N
Codomain fan: Rational polyhedral fan in 2-d lattice N

sage: fan1.isomorphism(toric_varieties.P2().fan())
Traceback (most recent call last):
...
FanNotIsomorphicError

linear_equivalence_ideal(ring)

Return the ideal generated by linear relations.

INPUT:

• A polynomial ring in self.nrays() variables.

OUTPUT:

Returns the ideal, in the given ring, generated by the linear relations of the rays. In toric geometry, this corresponds to rational equivalence of divisors.

EXAMPLES:

sage: fan = Fan([[0,1,3],[3,4],[2,0],[1,2,4]], [(-3, -2, 1), (0, 0, 1), (3, -2, 1), (-1, -1, 1), (1, -1, 1)])
sage: fan.linear_equivalence_ideal( PolynomialRing(QQ,5,'A, B, C, D, E') )
Ideal (-3*A + 3*C - D + E, -2*A - 2*C - D - E, A + B + C + D + E) of Multivariate Polynomial Ring in A, B, C, D, E over Rational Field

make_simplicial(**kwds)

Construct a simplicial fan subdividing self.

It is a synonym for subdivide() with make_simplicial=True option.

INPUT:

• this functions accepts only keyword arguments. See subdivide() for documentation.

OUTPUT:

• rational polyhedral fan.

EXAMPLES:

sage: fan = NormalFan(lattice_polytope.cross_polytope(3))
sage: fan.is_simplicial()
False
sage: fan.ngenerating_cones()
6
sage: new_fan = fan.make_simplicial()
sage: new_fan.is_simplicial()
True
sage: new_fan.ngenerating_cones()
12

ngenerating_cones()

Return the number of generating cones of self.

OUTPUT:

• integer.

EXAMPLES:

sage: fan = toric_varieties.P1xP1().fan()
sage: fan.ngenerating_cones()
4
sage: cone1 = Cone([(1,0), (0,1)])
sage: cone2 = Cone([(-1,0)])
sage: fan = Fan([cone1, cone2])
sage: fan.ngenerating_cones()
2

oriented_boundary(cone)

Return the facets bounding cone with their induced orientation.

INPUT:

• cone – a cone of the fan or the whole fan.

OUTPUT:

The boundary cones of cone as a formal linear combination of cones with coefficients \(\pm 1\). Each summand is a facet of cone and the coefficient indicates whether their (chosen) orientation agrees or disagrees with the “outward normal first” boundary orientation. Note that the orientation of any individual cone is arbitrary. This method once and for all picks orientations for all cones and then computes the boundaries relative to that chosen orientation.

If cone is the fan itself, the generating cones with their orientation relative to the ambient space are returned.

See complex() for the associated chain complex. If you do not require the orientation, use cone.facets() instead.

EXAMPLES:

sage: fan = toric_varieties.P(3).fan()
sage: cone = fan(2)[0]
sage: bdry = fan.oriented_boundary(cone);  bdry
-1-d cone of Rational polyhedral fan in 3-d lattice N + 1-d cone of Rational polyhedral fan in 3-d lattice N
sage: bdry[0]
(-1, 1-d cone of Rational polyhedral fan in 3-d lattice N)
sage: bdry[1]
(1, 1-d cone of Rational polyhedral fan in 3-d lattice N)
sage: fan.oriented_boundary(bdry[0][1])
-0-d cone of Rational polyhedral fan in 3-d lattice N
sage: fan.oriented_boundary(bdry[1][1])
-0-d cone of Rational polyhedral fan in 3-d lattice N

If you pass the fan itself, this method returns the orientation of the generating cones which is determined by the order of the rays in cone.ray_basis()

sage: fan.oriented_boundary(fan)
-3-d cone of Rational polyhedral fan in 3-d lattice N
+ 3-d cone of Rational polyhedral fan in 3-d lattice N
- 3-d cone of Rational polyhedral fan in 3-d lattice N
+ 3-d cone of Rational polyhedral fan in 3-d lattice N
sage: [cone.rays().basis().matrix().det()
....:  for cone in fan.generating_cones()]
[-1, 1, -1, 1]

A non-full dimensional fan:

sage: cone = Cone([(4,5)])
sage: fan = Fan([cone])
sage: fan.oriented_boundary(cone)
0-d cone of Rational polyhedral fan in 2-d lattice N
sage: fan.oriented_boundary(fan)
1-d cone of Rational polyhedral fan in 2-d lattice N

plot(**options)

Plot self.

INPUT:

• any options for toric plots (see toric_plotter.options), none are mandatory.

OUTPUT:

• a plot.

EXAMPLES:

sage: fan = toric_varieties.dP6().fan()
sage: fan.plot()
Graphics object consisting of 31 graphics primitives

primitive_collections()

Return the primitive collections.

OUTPUT:

Returns the subsets \(\{i_1,\dots,i_k\} \subset \{ 1,\dots,n\}\) such that

• The points \(\{p_{i_1},\dots,p_{i_k}\}\) do not span a cone of the fan.
• If you remove any one \(p_{i_j}\) from the set, then they do span a cone of the fan.

Note

By replacing the multiindices \(\{i_1,\dots,i_k\}\) of each primitive collection with the monomials \(x_{i_1}\cdots x_{i_k}\) one generates the Stanley-Reisner ideal in \(\ZZ[x_1,\dots]\).

REFERENCES:

• [Bat1991]

EXAMPLES:

sage: fan = Fan([[0,1,3],[3,4],[2,0],[1,2,4]], [(-3, -2, 1), (0, 0, 1), (3, -2, 1), (-1, -1, 1), (1, -1, 1)])
sage: fan.primitive_collections()
[frozenset({0, 4}),
 frozenset({2, 3}),
 frozenset({0, 1, 2}),
 frozenset({1, 3, 4})]

subdivide(new_rays=None, make_simplicial=False, algorithm='default', verbose=False)

Construct a new fan subdividing self.

INPUT:

• new_rays - list of new rays to be added during subdivision, each ray must be a list or a vector. May be empty or None (default);
• make_simplicial - if True, the returned fan is guaranteed to be simplicial, default is False;
• algorithm - string with the name of the algorithm used for subdivision. Currently there is only one available algorithm called “default”;
• verbose - if True, some timing information may be printed during the process of subdivision.

OUTPUT:

• rational polyhedral fan.

Currently the “default” algorithm corresponds to iterative stellar subdivision for each ray in new_rays.

EXAMPLES:

sage: fan = NormalFan(lattice_polytope.cross_polytope(3))
sage: fan.is_simplicial()
False
sage: fan.ngenerating_cones()
6
sage: fan.nrays()
8
sage: new_fan = fan.subdivide(new_rays=[(1,0,0)])
sage: new_fan.is_simplicial()
False
sage: new_fan.ngenerating_cones()
9
sage: new_fan.nrays()
9

support_contains(*args)

Check if a point is contained in the support of the fan.

The support of a fan is the union of all cones of the fan. If you want to know whether the fan contains a given cone, you should use contains() instead.

INPUT:

• *args – an element of self.lattice() or something that can be converted to it (for example, a list of coordinates).

OUTPUT:

• True if point is contained in the support of the fan, False otherwise.

toric_variety(*args, **kwds)

Return the associated toric variety.

INPUT:

same arguments as ToricVariety()

OUTPUT:

a toric variety

This is equivalent to the command ToricVariety(self) and is provided only as a convenient alternative method to go from the fan to the associated toric variety.

EXAMPLES:

sage: Fan([Cone([(1,0)]), Cone([(0,1)])]).toric_variety()
2-d toric variety covered by 2 affine patches

vertex_graph()

Return the graph of 1- and 2-cones.

OUTPUT:

An edge-colored graph. The vertices correspond to the 1-cones (i.e. rays) of the fan. Two vertices are joined by an edge iff the rays span a 2-cone of the fan. The edges are colored by pairs of integers that classify the 2-cones up to \(GL(2,\ZZ)\) transformation, see classify_cone_2d().

EXAMPLES:

sage: dP8 = toric_varieties.dP8()
sage: g = dP8.fan().vertex_graph()
sage: g
Graph on 4 vertices
sage: set(dP8.fan(1)) == set(g.vertices())
True
sage: g.edge_labels()  # all edge labels the same since every cone is smooth
[(1, 0), (1, 0), (1, 0), (1, 0)]

sage: g = toric_varieties.Cube_deformation(10).fan().vertex_graph()
sage: g.automorphism_group().order()
48
sage: g.automorphism_group(edge_labels=True).order()
4

virtual_rays(*args)

Return (some of the) virtual rays of self.

Let \(N\) be the \(D\)-dimensional lattice() of a \(d\)-dimensional fan \(\Sigma\) in \(N_\RR\). Then the corresponding toric variety is of the form \(X \times (\CC^*)^{D-d}\). The actual rays() of \(\Sigma\) give a canonical choice of homogeneous coordinates on \(X\). This function returns an arbitrary but fixed choice of virtual rays corresponding to a (non-canonical) choice of homogeneous coordinates on the torus factor. Combinatorially primitive integral generators of virtual rays span the \(D-d\) dimensions of \(N_\QQ\) “missed” by the actual rays. (In general addition of virtual rays is not sufficient to span \(N\) over \(\ZZ\).)

Note

You may use a particular choice of virtual rays by passing optional argument virtual_rays to the Fan() constructor.

INPUT:

• ray_list – a list of integers, the indices of the requested virtual rays. If not specified, all virtual rays of self will be returned.

OUTPUT:

• a PointCollection of primitive integral ray generators. Usually (if the fan is full-dimensional) this will be empty.

EXAMPLES:

sage: f = Fan([Cone([(1,0,1,0), (0,1,1,0)])])
sage: f.virtual_rays()
N(0, 0, 0, 1),
N(0, 0, 1, 0)
in 4-d lattice N

sage: f.rays()
N(1, 0, 1, 0),
N(0, 1, 1, 0)
in 4-d lattice N

sage: f.virtual_rays([0])
N(0, 0, 0, 1)
in 4-d lattice N

You can also give virtual ray indices directly, without packing them into a list:

sage: f.virtual_rays(0)
N(0, 0, 0, 1)
in 4-d lattice N

Make sure that trac ticket #16344 is fixed and one can compute the virtual rays of fans in non-saturated lattices:

sage: N = ToricLattice(1)
sage: B = N.submodule([(2,)]).basis()
sage: f = Fan([Cone([B[0]])])
sage: len(f.virtual_rays())
0

sage.geometry.fan.discard_faces(cones)

Return the cones of the given list which are not faces of each other.

INPUT:

• cones – a list of cones.

OUTPUT:

• a list of cones, sorted by dimension in decreasing order.

EXAMPLES:

Consider all cones of a fan:

sage: Sigma = toric_varieties.P2().fan()
sage: cones = flatten(Sigma.cones())
sage: len(cones)
7

Most of them are not necessary to generate this fan:

sage: from sage.geometry.fan import discard_faces
sage: len(discard_faces(cones))
3
sage: Sigma.ngenerating_cones()
3

sage.geometry.fan.is_Fan(x)

Check if x is a Fan.

INPUT:

• x – anything.

OUTPUT:

• True if x is a fan and False otherwise.

EXAMPLES:

sage: from sage.geometry.fan import is_Fan
sage: is_Fan(1)
False
sage: fan = toric_varieties.P2().fan()
sage: fan
Rational polyhedral fan in 2-d lattice N
sage: is_Fan(fan)
True