Fano toric varieties¶
This module provides support for (Crepant Partial Resolutions of) Fano toric
varieties, corresponding to crepant subdivisions of face fans of reflexive
lattice polytopes
.
The interface is provided via CPRFanoToricVariety()
.
A careful exposition of different flavours of Fano varieties can be found in the paper by Benjamin Nill [Nill2005]. The main goal of this module is to support work with Gorenstein weak Fano toric varieties. Such a variety corresponds to a coherent crepant refinement of the normal fan of a reflexive polytope \(\Delta\), where crepant means that primitive generators of the refining rays lie on the facets of the polar polytope \(\Delta^\circ\) and coherent (a.k.a. regular or projective) means that there exists a strictly upper convex piecewise linear function whose domains of linearity are precisely the maximal cones of the subdivision. These varieties are important for string theory in physics, as they serve as ambient spaces for mirror pairs of CalabiYau manifolds via constructions due to Victor V. Batyrev [Batyrev1994] and Lev A. Borisov [Borisov1993].
From the combinatorial point of view “crepant” requirement is much more simple and natural to work with than “coherent.” For this reason, the code in this module will allow work with arbitrary crepant subdivisions without checking whether they are coherent or not. We refer to corresponding toric varieties as CPRFano toric varieties.
REFERENCES:
[Batyrev1994]  Victor V. Batyrev, “Dual polyhedra and mirror symmetry for CalabiYau hypersurfaces in toric varieties”, J. Algebraic Geom. 3 (1994), no. 3, 493535. Arxiv alggeom/9310003v1 
[Borisov1993]  Lev A. Borisov, “Towards the mirror symmetry for CalabiYau complete intersections in Gorenstein Fano toric varieties”, 1993. Arxiv alggeom/9310001v1 
[CD2007]  Adrian Clingher and Charles F. Doran, “Modular invariants for lattice polarized K3 surfaces”, Michigan Math. J. 55 (2007), no. 2, 355393. Arxiv math/0602146v1 [math.AG] 
[Nill2005]  Benjamin Nill, “Gorenstein toric Fano varieties”, Manuscripta Math. 116 (2005), no. 2, 183210. Arxiv math/0405448v1 [math.AG] 
AUTHORS:
 Andrey Novoseltsev (20100518): initial version.
EXAMPLES:
Most of the functions available for Fano toric varieties are the same as for general toric varieties, so here we will concentrate only on CalabiYau subvarieties, which were the primary goal for creating this module.
For our first example we realize the projective plane as a Fano toric variety:
sage: simplex = LatticePolytope([(1,0), (0,1), (1,1)])
sage: P2 = CPRFanoToricVariety(Delta_polar=simplex)
Its anticanonical “hypersurface” is a onedimensional CalabiYau manifold:
sage: P2.anticanonical_hypersurface(
....: monomial_points="all")
Closed subscheme of 2d CPRFano toric variety
covered by 3 affine patches defined by:
a0*z0^3 + a9*z0^2*z1 + a7*z0*z1^2
+ a1*z1^3 + a8*z0^2*z2 + a6*z0*z1*z2
+ a4*z1^2*z2 + a5*z0*z2^2
+ a3*z1*z2^2 + a2*z2^3
In many cases it is sufficient to work with the “simplified polynomial moduli space” of anticanonical hypersurfaces:
sage: P2.anticanonical_hypersurface(
....: monomial_points="simplified")
Closed subscheme of 2d CPRFano toric variety
covered by 3 affine patches defined by:
a0*z0^3 + a1*z1^3 + a6*z0*z1*z2 + a2*z2^3
The mirror family to these hypersurfaces lives inside the Fano toric
variety obtained using simplex
as Delta
instead of Delta_polar
:
sage: FTV = CPRFanoToricVariety(Delta=simplex,
....: coordinate_points="all")
sage: FTV.anticanonical_hypersurface(
....: monomial_points="simplified")
Closed subscheme of 2d CPRFano toric variety
covered by 9 affine patches defined by:
a2*z2^3*z3^2*z4*z5^2*z8
+ a1*z1^3*z3*z4^2*z7^2*z9
+ a3*z0*z1*z2*z3*z4*z5*z7*z8*z9
+ a0*z0^3*z5*z7*z8^2*z9^2
Here we have taken the resolved version of the ambient space for the mirror family, but in fact we don’t have to resolve singularities corresponding to the interior points of facets  they are singular points which do not lie on a generic anticanonical hypersurface:
sage: FTV = CPRFanoToricVariety(Delta=simplex,
....: coordinate_points="all but facets")
sage: FTV.anticanonical_hypersurface(
....: monomial_points="simplified")
Closed subscheme of 2d CPRFano toric variety
covered by 3 affine patches defined by:
a0*z0^3 + a1*z1^3 + a3*z0*z1*z2 + a2*z2^3
This looks very similar to our second version of the anticanonical hypersurface of the projective plane, as expected, since all onedimensional CalabiYau manifolds are elliptic curves!
Now let’s take a look at a toric realization of \(M\)polarized K3 surfaces studied by Adrian Clingher and Charles F. Doran in [CD2007]:
sage: p4318 = ReflexivePolytope(3, 4318)
sage: FTV = CPRFanoToricVariety(Delta_polar=p4318)
sage: FTV.anticanonical_hypersurface()
Closed subscheme of 3d CPRFano toric variety
covered by 4 affine patches defined by:
a0*z2^12 + a4*z2^6*z3^6 + a3*z3^12
+ a8*z0*z1*z2*z3 + a2*z1^3 + a1*z0^2
Below you will find detailed descriptions of available functions. Current functionality of this module is very basic, but it is under active development and hopefully will improve in future releases of Sage. If there are some particular features that you would like to see implemented ASAP, please consider reporting them to the Sage Development Team or even implementing them on your own as a patch for inclusion!

class
sage.schemes.toric.fano_variety.
AnticanonicalHypersurface
(P_Delta, monomial_points=None, coefficient_names=None, coefficient_name_indices=None, coefficients=None)¶ Bases:
sage.schemes.toric.toric_subscheme.AlgebraicScheme_subscheme_toric
Construct an anticanonical hypersurface of a CPRFano toric variety.
INPUT:
P_Delta
–CPRFano toric variety
associated to a reflexive polytope \(\Delta\); see
CPRFanoToricVariety_field.anticanonical_hypersurface()
for documentation on all other acceptable parameters.
OUTPUT:
anticanonical hypersurface
ofP_Delta
(with the extended base field, if necessary).
EXAMPLES:
sage: P1xP1 = toric_varieties.P1xP1() sage: import sage.schemes.toric.fano_variety as ftv sage: ftv.AnticanonicalHypersurface(P1xP1) Closed subscheme of 2d CPRFano toric variety covered by 4 affine patches defined by: a0*s^2*x^2 + a3*t^2*x^2 + a6*s*t*x*y + a1*s^2*y^2 + a2*t^2*y^2
See
anticanonical_hypersurface()
for a more elaborate example.

sage.schemes.toric.fano_variety.
CPRFanoToricVariety
(Delta=None, Delta_polar=None, coordinate_points=None, charts=None, coordinate_names=None, names=None, coordinate_name_indices=None, make_simplicial=False, base_ring=None, base_field=None, check=True)¶ Construct a CPRFano toric variety.
Note
See documentation of the module
fano_variety
for the used definitions and supported varieties.Due to the large number of available options, it is recommended to always use keyword parameters.
INPUT:
Delta
– reflexivelattice polytope
. The fan of the constructed CPRFano toric variety will be a crepant subdivision of the normal fan ofDelta
. EitherDelta
orDelta_polar
must be given, but not both at the same time, since one is completely determined by another viapolar
method;Delta_polar
– reflexivelattice polytope
. The fan of the constructed CPRFano toric variety will be a crepant subdivision of the face fan ofDelta_polar
. EitherDelta
orDelta_polar
must be given, but not both at the same time, since one is completely determined by another viapolar
method;coordinate_points
– list of integers or string. A list will be interpreted as indices of (boundary) points ofDelta_polar
which should be used as rays of the underlying fan. It must include all vertices ofDelta_polar
and no repetitions are allowed. A string must be one of the following descriptions of points ofDelta_polar
: “vertices” (default),
 “all” (will not include the origin),
 “all but facets” (will not include points in the relative interior of facets);
charts
– list of lists of elements fromcoordinate_points
. Each of these lists must define a generating cone of a fan subdividing the normal fan ofDelta
. Defaultcharts
correspond to the normal fan ofDelta
without subdivision. The fan specified bycharts
will be subdivided to include all of the requestedcoordinate_points
;coordinate_names
– names of variables for the coordinate ring, seenormalize_names()
for acceptable formats. If not given, indexed variable names will be created automatically;names
– an alias ofcoordinate_names
for internal use. You may specify eithernames
orcoordinate_names
, but not both;coordinate_name_indices
– list of integers, indices for indexed variables. If not given, the index of each variable will coincide with the index of the corresponding point ofDelta_polar
;make_simplicial
– ifTrue
, the underlying fan will be made simplicial (default:False
);base_ring
– base field of the CPRFano toric variety (default: \(\QQ\));base_field
– alias forbase_ring
. Takes precedence if both are specified.check
– by default the input data will be checked for correctness (e.g. thatcharts
do form a subdivision of the normal fan ofDelta
). If you know for sure that the input is valid, you may significantly decrease construction time usingcheck=False
option.
OUTPUT:
EXAMPLES:
We start with the product of two projective lines:
sage: diamond = lattice_polytope.cross_polytope(2) sage: diamond.vertices() M( 1, 0), M( 0, 1), M(1, 0), M( 0, 1) in 2d lattice M sage: P1xP1 = CPRFanoToricVariety(Delta_polar=diamond) sage: P1xP1 2d CPRFano toric variety covered by 4 affine patches sage: P1xP1.fan() Rational polyhedral fan in 2d lattice M sage: P1xP1.fan().rays() M( 1, 0), M( 0, 1), M(1, 0), M( 0, 1) in 2d lattice M
“Unfortunately,” this variety is smooth to start with and we cannot perform any subdivisions of the underlying fan without leaving the category of CPRFano toric varieties. Our next example starts with a square:
sage: square = diamond.polar() sage: square.vertices() N( 1, 1), N( 1, 1), N(1, 1), N(1, 1) in 2d lattice N sage: square.points() N( 1, 1), N( 1, 1), N(1, 1), N(1, 1), N(1, 0), N( 0, 1), N( 0, 0), N( 0, 1), N( 1, 0) in 2d lattice N
We will construct several varieties associated to it:
sage: FTV = CPRFanoToricVariety(Delta_polar=square) sage: FTV.fan().rays() N( 1, 1), N( 1, 1), N(1, 1), N(1, 1) in 2d lattice N sage: FTV.gens() (z0, z1, z2, z3) sage: FTV = CPRFanoToricVariety(Delta_polar=square, ....: coordinate_points=[0,1,2,3,8]) sage: FTV.fan().rays() N( 1, 1), N( 1, 1), N(1, 1), N(1, 1), N( 1, 0) in 2d lattice N sage: FTV.gens() (z0, z1, z2, z3, z8) sage: FTV = CPRFanoToricVariety(Delta_polar=square, ....: coordinate_points=[8,0,2,1,3], ....: coordinate_names="x+") sage: FTV.fan().rays() N( 1, 0), N( 1, 1), N(1, 1), N( 1, 1), N(1, 1) in 2d lattice N sage: FTV.gens() (x8, x0, x2, x1, x3) sage: FTV = CPRFanoToricVariety(Delta_polar=square, ....: coordinate_points="all", ....: coordinate_names="x y Z+") sage: FTV.fan().rays() N( 1, 1), N( 1, 1), N(1, 1), N(1, 1), N(1, 0), N( 0, 1), N( 0, 1), N( 1, 0) in 2d lattice N sage: FTV.gens() (x, y, Z2, Z3, Z4, Z5, Z7, Z8)
Note that
Z6
is “missing”. This is due to the fact that the 6th point ofsquare
is the origin, and all automatically created names have the same indices as corresponding points ofDelta_polar()
. This is usually very convenient, especially if you have to work with several partial resolutions of the same Fano toric variety. However, you can change it, if you want:sage: FTV = CPRFanoToricVariety(Delta_polar=square, ....: coordinate_points="all", ....: coordinate_names="x y Z+", ....: coordinate_name_indices=list(range(8))) sage: FTV.gens() (x, y, Z2, Z3, Z4, Z5, Z6, Z7)
Note that you have to provide indices for all variables, including those that have “completely custom” names. Again, this is usually convenient, because you can add or remove “custom” variables without disturbing too much “automatic” ones:
sage: FTV = CPRFanoToricVariety(Delta_polar=square, ....: coordinate_points="all", ....: coordinate_names="x Z+", ....: coordinate_name_indices=list(range(8))) sage: FTV.gens() (x, Z1, Z2, Z3, Z4, Z5, Z6, Z7)
If you prefer to always start from zero, you will have to shift indices accordingly:
sage: FTV = CPRFanoToricVariety(Delta_polar=square, ....: coordinate_points="all", ....: coordinate_names="x Z+", ....: coordinate_name_indices=[0] + list(range(7))) sage: FTV.gens() (x, Z0, Z1, Z2, Z3, Z4, Z5, Z6) sage: FTV = CPRFanoToricVariety(Delta_polar=square, ....: coordinate_points="all", ....: coordinate_names="x y Z+", ....: coordinate_name_indices=[0]*2 + list(range(6))) sage: FTV.gens() (x, y, Z0, Z1, Z2, Z3, Z4, Z5)
So you always can get any names you want, somewhat complicated default behaviour was designed with the hope that in most cases you will have no desire to provide different names.
Now we will use the possibility to specify initial charts:
sage: charts = [(0,1), (1,2), (2,3), (3,0)]
(these charts actually form exactly the face fan of our square)
sage: FTV = CPRFanoToricVariety(Delta_polar=square, ....: coordinate_points=[0,1,2,3,4], ....: charts=charts) sage: FTV.fan().rays() N( 1, 1), N( 1, 1), N(1, 1), N(1, 1), N(1, 0) in 2d lattice N sage: [cone.ambient_ray_indices() for cone in FTV.fan()] [(0, 1), (1, 2), (3, 4), (2, 4), (0, 3)]
If charts are wrong, it should be detected:
sage: bad_charts = charts + [(3,0)] sage: FTV = CPRFanoToricVariety(Delta_polar=square, ....: coordinate_points=[0,1,2,3,4], ....: charts=bad_charts) Traceback (most recent call last): ... ValueError: you have provided 5 cones, but only 4 of them are maximal! Use discard_faces=True if you indeed need to construct a fan from these cones.
These charts are technically correct, they just happened to list one of them twice, but it is assumed that such a situation will not happen. It is especially important when you try to speed up your code:
sage: FTV = CPRFanoToricVariety(Delta_polar=square, ....: coordinate_points=[0,1,2,3,4], ....: charts=bad_charts, ....: check=False) Traceback (most recent call last): ... IndexError: list assignment index out of range
In this case you still get an error message, but it is harder to figure out what is going on. It may also happen that “everything will still work” in the sense of not crashing, but work with such an invalid variety may lead to mathematically wrong results, so use
check=False
carefully!Here are some other possible mistakes:
sage: bad_charts = charts + [(0,2)] sage: FTV = CPRFanoToricVariety(Delta_polar=square, ....: coordinate_points=[0,1,2,3,4], ....: charts=bad_charts) Traceback (most recent call last): ... ValueError: (0, 2) does not form a chart of a subdivision of the face fan of 2d reflexive polytope #14 in 2d lattice N! sage: bad_charts = charts[:1] sage: FTV = CPRFanoToricVariety(Delta_polar=square, ....: coordinate_points=[0,1,2,3,4], ....: charts=bad_charts) Traceback (most recent call last): ... ValueError: given charts do not form a complete fan! sage: FTV = CPRFanoToricVariety(Delta_polar=square, ....: coordinate_points=[1,2,3,4]) Traceback (most recent call last): ... ValueError: all 4 vertices of Delta_polar must be used for coordinates! Got: [1, 2, 3, 4] sage: FTV = CPRFanoToricVariety(Delta_polar=square, ....: coordinate_points=[0,0,1,2,3,4]) Traceback (most recent call last): ... ValueError: no repetitions are allowed for coordinate points! Got: [0, 0, 1, 2, 3, 4] sage: FTV = CPRFanoToricVariety(Delta_polar=square, ....: coordinate_points=[0,1,2,3,6]) Traceback (most recent call last): ... ValueError: the origin (point #6) cannot be used for a coordinate! Got: [0, 1, 2, 3, 6]
Here is a shorthand for defining the toric variety and homogeneous coordinates in one go:
sage: P1xP1.<a,b,c,d> = CPRFanoToricVariety(Delta_polar=diamond) sage: (a^2+b^2) * (c+d) a^2*c + b^2*c + a^2*d + b^2*d

class
sage.schemes.toric.fano_variety.
CPRFanoToricVariety_field
(Delta_polar, fan, coordinate_points, point_to_ray, coordinate_names, coordinate_name_indices, base_field)¶ Bases:
sage.schemes.toric.variety.ToricVariety_field
Construct a CPRFano toric variety associated to a reflexive polytope.
Warning
This class does not perform any checks of correctness of input and it does assume that the internal structure of the given parameters is coordinated in a certain way. Use
CPRFanoToricVariety()
to construct CPRFano toric varieties.Note
See documentation of the module
fano_variety
for the used definitions and supported varieties.INPUT:
Delta_polar
– reflexive polytope;fan
– rational polyhedral fan subdividing the face fan ofDelta_polar
;coordinate_points
– list of indices of points ofDelta_polar
used for rays offan
;point_to_ray
– dictionary mapping the index of a coordinate point to the index of the corresponding ray;coordinate_names
– names of the variables of the coordinate ring in the format accepted bynormalize_names()
;coordinate_name_indices
– indices for indexed variables, ifNone
, will be equal tocoordinate_points
;base_field
– base field of the CPRFano toric variety.
OUTPUT:

Delta
()¶ Return the reflexive polytope associated to
self
.OUTPUT:
 reflexive
lattice polytope
. The underlying fan ofself
is a coherent subdivision of the normal fan of this polytope.
EXAMPLES:
sage: diamond = lattice_polytope.cross_polytope(2) sage: P1xP1 = CPRFanoToricVariety(Delta_polar=diamond) sage: P1xP1.Delta() 2d reflexive polytope #14 in 2d lattice N sage: P1xP1.Delta() is diamond.polar() True
 reflexive

Delta_polar
()¶ Return polar of
Delta()
.OUTPUT:
 reflexive
lattice polytope
. The underlying fan ofself
is a coherent subdivision of the face fan of this polytope.
EXAMPLES:
sage: diamond = lattice_polytope.cross_polytope(2) sage: P1xP1 = CPRFanoToricVariety(Delta_polar=diamond) sage: P1xP1.Delta_polar() 2d reflexive polytope #3 in 2d lattice M sage: P1xP1.Delta_polar() is diamond True sage: P1xP1.Delta_polar() is P1xP1.Delta().polar() True
 reflexive

anticanonical_hypersurface
(**kwds)¶ Return an anticanonical hypersurface of
self
.Note
The returned hypersurface may be actually a subscheme of another CPRFano toric variety: if the base field of
self
does not include all of the required names for generic monomial coefficients, it will be automatically extended.Below \(\Delta\) is the reflexive polytope corresponding to
self
, i.e. the fan ofself
is a refinement of the normal fan of \(\Delta\). This function accepts only keyword parameters.INPUT:
monomial points
– a list of integers or a string. A list will be interpreted as indices of points of \(\Delta\) which should be used for monomials of this hypersurface. A string must be one of the following descriptions of points of \(\Delta\): “vertices”,
 “vertices+origin”,
 “all”,
 “simplified” (default) – all points of \(\Delta\) except for the interior points of facets, this choice corresponds to working with the “simplified polynomial moduli space” of anticanonical hypersurfaces;
coefficient_names
– names for the monomial coefficients, seenormalize_names()
for acceptable formats. If not given, indexed coefficient names will be created automatically;coefficient_name_indices
– a list of integers, indices for indexed coefficients. If not given, the index of each coefficient will coincide with the index of the corresponding point of \(\Delta\);coefficients
– as an alternative to specifying coefficient names and/or indices, you can give the coefficients themselves as arbitrary expressions and/or strings. Using strings allows you to easily add “parameters”: the base field ofself
will be extended to include all necessary names.
OUTPUT:
 an
anticanonical hypersurface
ofself
(with the extended base field, if necessary).
EXAMPLES:
We realize the projective plane as a Fano toric variety:
sage: simplex = LatticePolytope([(1,0), (0,1), (1,1)]) sage: P2 = CPRFanoToricVariety(Delta_polar=simplex)
Its anticanonical “hypersurface” is a onedimensional CalabiYau manifold:
sage: P2.anticanonical_hypersurface( ....: monomial_points="all") Closed subscheme of 2d CPRFano toric variety covered by 3 affine patches defined by: a0*z0^3 + a9*z0^2*z1 + a7*z0*z1^2 + a1*z1^3 + a8*z0^2*z2 + a6*z0*z1*z2 + a4*z1^2*z2 + a5*z0*z2^2 + a3*z1*z2^2 + a2*z2^3
In many cases it is sufficient to work with the “simplified polynomial moduli space” of anticanonical hypersurfaces:
sage: P2.anticanonical_hypersurface( ....: monomial_points="simplified") Closed subscheme of 2d CPRFano toric variety covered by 3 affine patches defined by: a0*z0^3 + a1*z1^3 + a6*z0*z1*z2 + a2*z2^3
The mirror family to these hypersurfaces lives inside the Fano toric variety obtained using
simplex
asDelta
instead ofDelta_polar
:sage: FTV = CPRFanoToricVariety(Delta=simplex, ....: coordinate_points="all") sage: FTV.anticanonical_hypersurface( ....: monomial_points="simplified") Closed subscheme of 2d CPRFano toric variety covered by 9 affine patches defined by: a2*z2^3*z3^2*z4*z5^2*z8 + a1*z1^3*z3*z4^2*z7^2*z9 + a3*z0*z1*z2*z3*z4*z5*z7*z8*z9 + a0*z0^3*z5*z7*z8^2*z9^2
Here we have taken the resolved version of the ambient space for the mirror family, but in fact we don’t have to resolve singularities corresponding to the interior points of facets  they are singular points which do not lie on a generic anticanonical hypersurface:
sage: FTV = CPRFanoToricVariety(Delta=simplex, ....: coordinate_points="all but facets") sage: FTV.anticanonical_hypersurface( ....: monomial_points="simplified") Closed subscheme of 2d CPRFano toric variety covered by 3 affine patches defined by: a0*z0^3 + a1*z1^3 + a3*z0*z1*z2 + a2*z2^3
This looks very similar to our second anticanonical hypersurface of the projective plane, as expected, since all onedimensional CalabiYau manifolds are elliptic curves!
All anticanonical hypersurfaces constructed above were generic with automatically generated coefficients. If you want, you can specify your own names
sage: FTV.anticanonical_hypersurface( ....: coefficient_names="a b c d") Closed subscheme of 2d CPRFano toric variety covered by 3 affine patches defined by: a*z0^3 + b*z1^3 + d*z0*z1*z2 + c*z2^3
or give concrete coefficients
sage: FTV.anticanonical_hypersurface( ....: coefficients=[1, 2, 3, 4]) Closed subscheme of 2d CPRFano toric variety covered by 3 affine patches defined by: z0^3 + 2*z1^3 + 4*z0*z1*z2 + 3*z2^3
or even mix numerical coefficients with some expressions
sage: H = FTV.anticanonical_hypersurface( ....: coefficients=[0, "t", "1/t", "psi/(psi^2 + phi)"]) sage: H Closed subscheme of 2d CPRFano toric variety covered by 3 affine patches defined by: t*z1^3 + (psi/(psi^2 + phi))*z0*z1*z2 + 1/t*z2^3 sage: R = H.ambient_space().base_ring() sage: R Fraction Field of Multivariate Polynomial Ring in phi, psi, t over Rational Field

cartesian_product
(other, coordinate_names=None, coordinate_indices=None)¶ Return the Cartesian product of
self
withother
.INPUT:
other
– a (possiblyCPRFano
)toric variety
;coordinate_names
– names of variables for the coordinate ring, seenormalize_names()
for acceptable formats. If not given, indexed variable names will be created automatically;coordinate_indices
– list of integers, indices for indexed variables. If not given, the index of each variable will coincide with the index of the corresponding ray of the fan.
OUTPUT:
 a
toric variety
, which isCPRFano
ifother
was.
EXAMPLES:
sage: P1 = toric_varieties.P1() sage: P2 = toric_varieties.P2() sage: P1xP2 = P1.cartesian_product(P2); P1xP2 3d CPRFano toric variety covered by 6 affine patches sage: P1xP2.fan().rays() N+N( 1, 0, 0), N+N(1, 0, 0), N+N( 0, 1, 0), N+N( 0, 0, 1), N+N( 0, 1, 1) in 3d lattice N+N sage: P1xP2.Delta_polar() 3d reflexive polytope in 3d lattice N+N

change_ring
(F)¶ Return a CPRFano toric variety over field
F
, otherwise the same asself
.INPUT:
F
– field.
OUTPUT:
CPRFano toric variety
overF
.
Note
There is no need to have any relation between
F
and the base field ofself
. If you do want to have such a relation, usebase_extend()
instead.EXAMPLES:
sage: P1xP1 = toric_varieties.P1xP1() sage: P1xP1.base_ring() Rational Field sage: P1xP1_RR = P1xP1.change_ring(RR) sage: P1xP1_RR.base_ring() Real Field with 53 bits of precision sage: P1xP1_QQ = P1xP1_RR.change_ring(QQ) sage: P1xP1_QQ.base_ring() Rational Field sage: P1xP1_RR.base_extend(QQ) Traceback (most recent call last): ... ValueError: no natural map from the base ring (=Real Field with 53 bits of precision) to R (=Rational Field)! sage: R = PolynomialRing(QQ, 2, 'a') sage: P1xP1.change_ring(R) Traceback (most recent call last): ... TypeError: need a field to construct a Fano toric variety! Got Multivariate Polynomial Ring in a0, a1 over Rational Field

coordinate_point_to_coordinate
(point)¶ Return the variable of the coordinate ring corresponding to
point
.INPUT:
point
– integer from the list ofcoordinate_points()
.
OUTPUT:
 the corresponding generator of the coordinate ring of
self
.
EXAMPLES:
sage: diamond = lattice_polytope.cross_polytope(2) sage: FTV = CPRFanoToricVariety(diamond, ....: coordinate_points=[0,1,2,3,8]) sage: FTV.coordinate_points() (0, 1, 2, 3, 8) sage: FTV.gens() (z0, z1, z2, z3, z8) sage: FTV.coordinate_point_to_coordinate(8) z8

coordinate_points
()¶ Return indices of points of
Delta_polar()
used for coordinates.OUTPUT:
tuple
of integers.
EXAMPLES:
sage: diamond = lattice_polytope.cross_polytope(2) sage: square = diamond.polar() sage: FTV = CPRFanoToricVariety(Delta_polar=square, ....: coordinate_points=[0,1,2,3,8]) sage: FTV.coordinate_points() (0, 1, 2, 3, 8) sage: FTV.gens() (z0, z1, z2, z3, z8) sage: FTV = CPRFanoToricVariety(Delta_polar=square, ....: coordinate_points="all") sage: FTV.coordinate_points() (0, 1, 2, 3, 4, 5, 7, 8) sage: FTV.gens() (z0, z1, z2, z3, z4, z5, z7, z8)
Note that one point is missing, namely
sage: square.origin() 6

nef_complete_intersection
(nef_partition, **kwds)¶ Return a nef complete intersection in
self
.Note
The returned complete intersection may be actually a subscheme of another CPRFano toric variety: if the base field of
self
does not include all of the required names for monomial coefficients, it will be automatically extended.Below \(\Delta\) is the reflexive polytope corresponding to
self
, i.e. the fan ofself
is a refinement of the normal fan of \(\Delta\). Other polytopes are described in the documentation ofnefpartitions
ofreflexive polytopes
.Except for the first argument,
nef_partition
, this method accepts only keyword parameters.INPUT:
nef_partition
– a \(k\)partnefpartition
of \(\Delta^\circ\), all other parameters (if given) must be lists of length \(k\);monomial_points
– the \(i\)th element of this list is either a list of integers or a string. A list will be interpreted as indices of points of \(\Delta_i\) which should be used for monomials of the \(i\)th polynomial of this complete intersection. A string must be one of the following descriptions of points of \(\Delta_i\): “vertices”,
 “vertices+origin”,
 “all” (default),
when using this description, it is also OK to pass a single string as
monomial_points
instead of repeating it \(k\) times;coefficient_names
– the \(i\)th element of this list specifies names for the monomial coefficients of the \(i\)th polynomial, seenormalize_names()
for acceptable formats. If not given, indexed coefficient names will be created automatically;coefficient_name_indices
– the \(i\)th element of this list specifies indices for indexed coefficients of the \(i\)th polynomial. If not given, the index of each coefficient will coincide with the index of the corresponding point of \(\Delta_i\);coefficients
– as an alternative to specifying coefficient names and/or indices, you can give the coefficients themselves as arbitrary expressions and/or strings. Using strings allows you to easily add “parameters”: the base field ofself
will be extended to include all necessary names.
OUTPUT:
 a
nef complete intersection
ofself
(with the extended base field, if necessary).
EXAMPLES:
We construct several complete intersections associated to the same nefpartition of the 3dimensional reflexive polytope #2254:
sage: p = ReflexivePolytope(3, 2254) sage: np = p.nef_partitions()[1] sage: np Nefpartition {2, 3, 4, 7, 8} U {0, 1, 5, 6} sage: X = CPRFanoToricVariety(Delta_polar=p) sage: X.nef_complete_intersection(np) Closed subscheme of 3d CPRFano toric variety covered by 10 affine patches defined by: a0*z1*z4^2*z5^2*z7^3 + a2*z2*z4*z5*z6*z7^2*z8^2 + a3*z2*z3*z4*z7*z8 + a1*z0*z2, b3*z1*z4*z5^2*z6^2*z7^2*z8^2 + b0*z2*z5*z6^3*z7*z8^4 + b5*z1*z3*z4*z5*z6*z7*z8 + b2*z2*z3*z6^2*z8^3 + b1*z1*z3^2*z4 + b4*z0*z1*z5*z6
Now we include only monomials associated to vertices of \(\Delta_i\):
sage: X.nef_complete_intersection(np, monomial_points="vertices") Closed subscheme of 3d CPRFano toric variety covered by 10 affine patches defined by: a0*z1*z4^2*z5^2*z7^3 + a2*z2*z4*z5*z6*z7^2*z8^2 + a3*z2*z3*z4*z7*z8 + a1*z0*z2, b3*z1*z4*z5^2*z6^2*z7^2*z8^2 + b0*z2*z5*z6^3*z7*z8^4 + b2*z2*z3*z6^2*z8^3 + b1*z1*z3^2*z4 + b4*z0*z1*z5*z6
(effectively, we set
b5=0
). Next we provide coefficients explicitly instead of using default generic names:sage: X.nef_complete_intersection(np, ....: monomial_points="vertices", ....: coefficients=[("a", "a^2", "a/e", "c_i"), list(range(1,6))]) Closed subscheme of 3d CPRFano toric variety covered by 10 affine patches defined by: a*z1*z4^2*z5^2*z7^3 + a/e*z2*z4*z5*z6*z7^2*z8^2 + c_i*z2*z3*z4*z7*z8 + a^2*z0*z2, 4*z1*z4*z5^2*z6^2*z7^2*z8^2 + z2*z5*z6^3*z7*z8^4 + 3*z2*z3*z6^2*z8^3 + 2*z1*z3^2*z4 + 5*z0*z1*z5*z6
Finally, we take a look at the generic representative of these complete intersections in a completely resolved ambient toric variety:
sage: X = CPRFanoToricVariety(Delta_polar=p, ....: coordinate_points="all") sage: X.nef_complete_intersection(np) Closed subscheme of 3d CPRFano toric variety covered by 22 affine patches defined by: a2*z2*z4*z5*z6*z7^2*z8^2*z9^2*z10^2*z11*z12*z13 + a0*z1*z4^2*z5^2*z7^3*z9*z10^2*z12*z13 + a3*z2*z3*z4*z7*z8*z9*z10*z11*z12 + a1*z0*z2, b0*z2*z5*z6^3*z7*z8^4*z9^3*z10^2*z11^2*z12*z13^2 + b3*z1*z4*z5^2*z6^2*z7^2*z8^2*z9^2*z10^2*z11*z12*z13^2 + b2*z2*z3*z6^2*z8^3*z9^2*z10*z11^2*z12*z13 + b5*z1*z3*z4*z5*z6*z7*z8*z9*z10*z11*z12*z13 + b1*z1*z3^2*z4*z11*z12 + b4*z0*z1*z5*z6*z13

resolve
(**kwds)¶ Construct a toric variety whose fan subdivides the fan of
self
.This function accepts only keyword arguments, none of which are mandatory.
INPUT:
new_points
– list of integers, indices of boundary points ofDelta_polar()
, which should be added as rays to the subdividing fan; all other arguments will be passed to
resolve()
method of (general) toric varieties, see its documentation for details.
OUTPUT:
CPRFano toric variety
if there was nonew_rays
argument andtoric variety
otherwise.
EXAMPLES:
sage: diamond = lattice_polytope.cross_polytope(2) sage: FTV = CPRFanoToricVariety(Delta=diamond) sage: FTV.coordinate_points() (0, 1, 2, 3) sage: FTV.gens() (z0, z1, z2, z3) sage: FTV_res = FTV.resolve(new_points=[6,8]) Traceback (most recent call last): ... ValueError: the origin (point #6) cannot be used for subdivision! sage: FTV_res = FTV.resolve(new_points=[8,5]) sage: FTV_res 2d CPRFano toric variety covered by 6 affine patches sage: FTV_res.coordinate_points() (0, 1, 2, 3, 8, 5) sage: FTV_res.gens() (z0, z1, z2, z3, z8, z5) sage: TV_res = FTV.resolve(new_rays=[(1,2)]) sage: TV_res 2d toric variety covered by 5 affine patches sage: TV_res.gens() (z0, z1, z2, z3, z4)

class
sage.schemes.toric.fano_variety.
NefCompleteIntersection
(P_Delta, nef_partition, monomial_points='all', coefficient_names=None, coefficient_name_indices=None, coefficients=None)¶ Bases:
sage.schemes.toric.toric_subscheme.AlgebraicScheme_subscheme_toric
Construct a nef complete intersection in a CPRFano toric variety.
INPUT:
P_Delta
– aCPRFano toric variety
associated to a reflexive polytope \(\Delta\); see
CPRFanoToricVariety_field.nef_complete_intersection()
for documentation on all other acceptable parameters.
OUTPUT:
 a
nef complete intersection
ofP_Delta
(with the extended base field, if necessary).
EXAMPLES:
sage: o = lattice_polytope.cross_polytope(3) sage: np = o.nef_partitions()[0] sage: np Nefpartition {0, 1, 3} U {2, 4, 5} sage: X = CPRFanoToricVariety(Delta_polar=o) sage: X.nef_complete_intersection(np) Closed subscheme of 3d CPRFano toric variety covered by 8 affine patches defined by: a2*z0^2*z1 + a5*z0*z1*z3 + a1*z1*z3^2 + a3*z0^2*z4 + a4*z0*z3*z4 + a0*z3^2*z4, b1*z1*z2^2 + b2*z2^2*z4 + b5*z1*z2*z5 + b4*z2*z4*z5 + b3*z1*z5^2 + b0*z4*z5^2
See
CPRFanoToricVariety_field.nef_complete_intersection()
for a more elaborate example.
cohomology_class
()¶ Return the class of
self
in the ambient space cohomology ring.OUTPUT:
 a
cohomology class
.
EXAMPLES:
sage: o = lattice_polytope.cross_polytope(3) sage: np = o.nef_partitions()[0] sage: np Nefpartition {0, 1, 3} U {2, 4, 5} sage: X = CPRFanoToricVariety(Delta_polar=o) sage: CI = X.nef_complete_intersection(np) sage: CI Closed subscheme of 3d CPRFano toric variety covered by 8 affine patches defined by: a2*z0^2*z1 + a5*z0*z1*z3 + a1*z1*z3^2 + a3*z0^2*z4 + a4*z0*z3*z4 + a0*z3^2*z4, b1*z1*z2^2 + b2*z2^2*z4 + b5*z1*z2*z5 + b4*z2*z4*z5 + b3*z1*z5^2 + b0*z4*z5^2 sage: CI.cohomology_class() [2*z3*z4 + 4*z3*z5 + 2*z4*z5]
 a

nef_partition
()¶ Return the nefpartition associated to
self
.OUTPUT:
EXAMPLES:
sage: o = lattice_polytope.cross_polytope(3) sage: np = o.nef_partitions()[0] sage: np Nefpartition {0, 1, 3} U {2, 4, 5} sage: X = CPRFanoToricVariety(Delta_polar=o) sage: CI = X.nef_complete_intersection(np) sage: CI Closed subscheme of 3d CPRFano toric variety covered by 8 affine patches defined by: a2*z0^2*z1 + a5*z0*z1*z3 + a1*z1*z3^2 + a3*z0^2*z4 + a4*z0*z3*z4 + a0*z3^2*z4, b1*z1*z2^2 + b2*z2^2*z4 + b5*z1*z2*z5 + b4*z2*z4*z5 + b3*z1*z5^2 + b0*z4*z5^2 sage: CI.nef_partition() Nefpartition {0, 1, 3} U {2, 4, 5} sage: CI.nef_partition() is np True

sage.schemes.toric.fano_variety.
add_variables
(field, variables)¶ Extend
field
to include allvariables
.INPUT:
field
 a field;variables
 a list of strings.
OUTPUT:
 a fraction field extending the original
field
, which has allvariables
among its generators.
EXAMPLES:
We start with the rational field and slowly add more variables:
sage: from sage.schemes.toric.fano_variety import * sage: F = add_variables(QQ, []); F # No extension Rational Field sage: F = add_variables(QQ, ["a"]); F Fraction Field of Univariate Polynomial Ring in a over Rational Field sage: F = add_variables(F, ["a"]); F Fraction Field of Univariate Polynomial Ring in a over Rational Field sage: F = add_variables(F, ["b", "c"]); F Fraction Field of Multivariate Polynomial Ring in a, b, c over Rational Field sage: F = add_variables(F, ["c", "d", "b", "c", "d"]); F Fraction Field of Multivariate Polynomial Ring in a, b, c, d over Rational Field

sage.schemes.toric.fano_variety.
is_CPRFanoToricVariety
(x)¶ Check if
x
is a CPRFano toric variety.INPUT:
x
– anything.
OUTPUT:
True
ifx
is aCPRFano toric variety
andFalse
otherwise.
Note
While projective spaces are Fano toric varieties mathematically, they are not toric varieties in Sage due to efficiency considerations, so this function will return
False
.EXAMPLES:
sage: from sage.schemes.toric.fano_variety import ( ....: is_CPRFanoToricVariety) sage: is_CPRFanoToricVariety(1) False sage: FTV = toric_varieties.P2() sage: FTV 2d CPRFano toric variety covered by 3 affine patches sage: is_CPRFanoToricVariety(FTV) True sage: is_CPRFanoToricVariety(ProjectiveSpace(2)) False