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# Groebner Fans

Sage provides much of the functionality of gfan, which is a software package whose main function is to enumerate all reduced Groebner bases of a polynomial ideal. The reduced Groebner bases yield the maximal cones in the Groebner fan of the ideal. Several subcomputations can be issued and additional tools are included. Among these the highlights are:

- Commands for computing tropical varieties.
- Interactive walks in the Groebner fan of an ideal.
- Commands for graphical renderings of Groebner fans and monomial ideals.

AUTHORS:

- Anders Nedergaard Jensen: Wrote the gfan C++ program, which implements algorithms many of which were invented by Jensen, Komei Fukuda, and Rekha Thomas. All the underlying hard work of the Groebner fans functionality of Sage depends on this C++ program.
- William Stein (2006-04-20): Wrote first version of the Sage code for working with Groebner fans.
- Tristram Bogart: the design of the Sage interface to gfan is joint work with Tristram Bogart, who also supplied numerous examples.
- Marshall Hampton (2008-03-25): Rewrote various functions to use gfan-0.3. This is still a work in progress, comments are appreciated on sage-devel@googlegroups.com (or personally at hamptonio@gmail.com).

EXAMPLES:

    sage: x,y = QQ['x,y'].gens()
    sage: i = ideal(x^2 - y^2 + 1)
    sage: g = i.groebner_fan()
    sage: g.reduced_groebner_bases()
    [[x^2 - y^2 + 1], [-x^2 + y^2 - 1]]

TESTS:

    sage: x,y = QQ['x,y'].gens()
    sage: i = ideal(x^2 - y^2 + 1)
    sage: g = i.groebner_fan()
    sage: g == loads(dumps(g))
    True

REFERENCES:

- Anders N. Jensen; Gfan, a software system for Groebner fans; available at http://www.math.tu-berlin.de/~jensen/software/gfan/gfan.html

class `sage.rings.polynomial.groebner_fan.``GroebnerFan`(I, is_groebner_basis=False, symmetry=None, verbose=False)

  Bases: `sage.structure.sage_object.SageObject`

  This class is used to access capabilities of the program Gfan. In addition to computing Groebner fans, Gfan can compute other things in tropical geometry such as tropical prevarieties.

  INPUT:

  - `I` - ideal in a multivariate polynomial ring
  - `is_groebner_basis` - bool (default False). if True, then I.gens() must be a Groebner basis with respect to the standard degree lexicographic term order.
  - `symmetry` - default: None; if not None, describes symmetries of the ideal
  - `verbose` - default: False; if True, printout useful info during computations

  EXAMPLES:

      sage: R.<x,y,z> = QQ[]
      sage: I = R.ideal([x^2*y - z, y^2*z - x, z^2*x - y])
      sage: G = I.groebner_fan(); G
      Groebner fan of the ideal:
      Ideal (x^2*y - z, y^2*z - x, x*z^2 - y) of Multivariate Polynomial Ring in x, y, z over Rational Field

  Here is an example of the use of the tropical_intersection command, and then using the RationalPolyhedralFan class to compute the Stanley-Reisner ideal of the tropical prevariety:

      sage: R.<x,y,z> = QQ[]
      sage: I = R.ideal([(x+y+z)^3-1,(x+y+z)^3-x,(x+y+z)-3])
      sage: GF = I.groebner_fan()
      sage: PF = GF.tropical_intersection()
      sage: PF.rays()
      [[-1, 0, 0], [0, -1, 0], [0, 0, -1], [1, 1, 1]]
      sage: RPF = PF.to_RationalPolyhedralFan()
      sage: RPF.Stanley_Reisner_ideal(PolynomialRing(QQ,4,'A, B, C, D'))
      Ideal (A*B, A*C, B*C*D) of Multivariate Polynomial Ring in A, B, C, D over Rational Field

  `buchberger`()

    Computes and returns a lexicographic reduced Groebner basis for the ideal.

    EXAMPLES:

        sage: R.<x,y,z> = PolynomialRing(QQ,3)
        sage: G = R.ideal([x - z^3, y^2 - x + x^2 - z^3*x]).groebner_fan()
        sage: G.buchberger()
        [-z^3 + y^2, -z^3 + x]

  `characteristic`()

    Return the characteristic of the base ring.

    EXAMPLES:

        sage: R.<x,y,z> = PolynomialRing(QQ,3)
        sage: i1 = ideal(x*z + 6*y*z - z^2, x*y + 6*x*z + y*z - z^2, y^2 + x*z + y*z)
        sage: gf = i1.groebner_fan()
        sage: gf.characteristic()
        0

  `dimension_of_homogeneity_space`()

    Return the dimension of the homogeneity space.

    EXAMPLES:

        sage: R.<x,y> = PolynomialRing(QQ,2)
        sage: G = R.ideal([y^3 - x^2, y^2 - 13*x]).groebner_fan()
        sage: G.dimension_of_homogeneity_space()
        0

  `gfan`(cmd='bases', I=None, format=True)

    Returns the gfan output as a string given an input cmd; the default is to produce the list of reduced Groebner bases in gfan format.

    EXAMPLES:

        sage: R.<x,y> = PolynomialRing(QQ,2)
        sage: gf = R.ideal([x^3-y,y^3-x-1]).groebner_fan()
        sage: gf.gfan()
        'Q[x,y]\n{{\ny^9-1-y+3*y^3-3*y^6,\nx+1-y^3}\n,\n{\ny^3-1-x,\nx^3-y}\n,\n{\ny-x^3,\nx^9-1-x}\n}\n'

  `homogeneity_space`()

    Return the homogeneity space of a the list of polynomials that define this Groebner fan.

    EXAMPLES:

        sage: R.<x,y> = PolynomialRing(QQ,2)
        sage: G = R.ideal([y^3 - x^2, y^2 - 13*x]).groebner_fan()
        sage: H = G.homogeneity_space()

  `ideal`()

    Return the ideal the was used to define this Groebner fan.

    EXAMPLES:

        sage: R.<x1,x2> = PolynomialRing(QQ,2)
        sage: gf = R.ideal([x1^3-x2,x2^3-2*x1-2]).groebner_fan()
        sage: gf.ideal()
        Ideal (x1^3 - x2, x2^3 - 2*x1 - 2) of Multivariate Polynomial Ring in x1, x2 over Rational Field

  `interactive`(*args, **kwds)

    See the documentation for self[0].interactive(). This does not work with the notebook.

    EXAMPLES:

        sage: print "This is not easily doc-testable; please write a good one!"
        This is not easily doc-testable; please write a good one!

  `maximal_total_degree_of_a_groebner_basis`()

    Return the maximal total degree of any Groebner basis.

    EXAMPLES:

        sage: R.<x,y> = PolynomialRing(QQ,2)
        sage: G = R.ideal([y^3 - x^2, y^2 - 13*x]).groebner_fan()
        sage: G.maximal_total_degree_of_a_groebner_basis()
        4

  `minimal_total_degree_of_a_groebner_basis`()

    Return the minimal total degree of any Groebner basis.

    EXAMPLES:

        sage: R.<x,y> = PolynomialRing(QQ,2)
        sage: G = R.ideal([y^3 - x^2, y^2 - 13*x]).groebner_fan()
        sage: G.minimal_total_degree_of_a_groebner_basis()
        2

  `mixed_volume`()

    Returns the mixed volume of the generators of this ideal (i.e. this is not really an ideal property, it can depend on the generators used). The generators must give a square system (as many polynomials as variables).

    EXAMPLES:

        sage: R.<x,y,z> = QQ[]
        sage: example_ideal = R.ideal([x^2-y-1,y^2-z-1,z^2-x-1])
        sage: gf = example_ideal.groebner_fan()
        sage: mv = gf.mixed_volume()
        sage: mv
        8

        sage: R2.<x,y> = QQ[]
        sage: g1 = 1 - x + x^7*y^3 + 2*x^8*y^4
        sage: g2 = 2 + y + 3*x^7*y^3 + x^8*y^4
        sage: example2 = R2.ideal([g1,g2])
        sage: example2.groebner_fan().mixed_volume()
        15

  `number_of_reduced_groebner_bases`()

    Return the number of reduced Groebner bases.

    EXAMPLES:

        sage: R.<x,y> = PolynomialRing(QQ,2)
        sage: G = R.ideal([y^3 - x^2, y^2 - 13*x]).groebner_fan()
        sage: G.number_of_reduced_groebner_bases()
        3

  `number_of_variables`()

    Return the number of variables.

    EXAMPLES:

        sage: R.<x,y> = PolynomialRing(QQ,2)
        sage: G = R.ideal([y^3 - x^2, y^2 - 13*x]).groebner_fan()
        sage: G.number_of_variables()
        2

        sage: R = PolynomialRing(QQ,'x',10)
        sage: R.inject_variables(globals())
        Defining x0, x1, x2, x3, x4, x5, x6, x7, x8, x9
        sage: G = ideal([x0 - x9, sum(R.gens())]).groebner_fan()
        sage: G.number_of_variables()
        10

  `polyhedralfan`()

    Returns a polyhedral fan object corresponding to the reduced Groebner bases.

    EXAMPLES:

        sage: R3.<x,y,z> = PolynomialRing(QQ,3)
        sage: gf = R3.ideal([x^8-y^4,y^4-z^2,z^2-1]).groebner_fan()
        sage: pf = gf.polyhedralfan()
        sage: pf.rays()
        [[0, 0, 1], [0, 1, 0], [1, 0, 0]]

  `reduced_groebner_bases`()

    EXAMPLES:

        sage: R.<x,y,z> = PolynomialRing(QQ, 3, order='lex')
        sage: G = R.ideal([x^2*y - z, y^2*z - x, z^2*x - y]).groebner_fan()
        sage: X = G.reduced_groebner_bases()
        sage: len(X)
        33
        sage: X[0]
        [z^15 - z, y - z^11, x - z^9]
        sage: X[0].ideal()
        Ideal (z^15 - z, y - z^11, x - z^9) of Multivariate Polynomial Ring in x, y, z over Rational Field
        sage: X[:5]
        [[z^15 - z, y - z^11, x - z^9],
        [-y + z^11, y*z^4 - z, y^2 - z^8, x - z^9],
        [-y^2 + z^8, y*z^4 - z, y^2*z^3 - y, y^3 - z^5, x - y^2*z],
        [-y^3 + z^5, y*z^4 - z, y^2*z^3 - y, y^4 - z^2, x - y^2*z],
        [-y^4 + z^2, y^6*z - y, y^9 - z, x - y^2*z]]
        sage: R3.<x,y,z> = PolynomialRing(GF(2477),3)
        sage: gf = R3.ideal([300*x^3-y,y^2-z,z^2-12]).groebner_fan()
        sage: gf.reduced_groebner_bases()
        [[z^2 - 12, y^2 - z, x^3 + 933*y],
        [-y^2 + z, y^4 - 12, x^3 + 933*y],
        [z^2 - 12, -300*x^3 + y, x^6 - 1062*z],
        [-828*x^6 + z, -300*x^3 + y, x^12 + 200]]

  `render`(file=None, larger=False, shift=0, rgbcolor=(0, 0, 0), polyfill=<function max_degree at 0xe30c2064>, scale_colors=True)

    Render a Groebner fan as sage graphics or save as an xfig file.

    More precisely, the output is a drawing of the Groebner fan intersected with a triangle. The corners of the triangle are (1,0,0) to the right, (0,1,0) to the left and (0,0,1) at the top. If there are more than three variables in the ring we extend these coordinates with zeros.

    INPUT:

    - `file` - a filename if you prefer the output saved to a file. This will be in xfig format.
    - `shift` - shift the positions of the variables in the drawing. For example, with shift=1, the corners will be b (right), c (left), and d (top). The shifting is done modulo the number of variables in the polynomial ring. The default is 0.
    - `larger` - bool (default: False); if True, make the triangle larger so that the shape of of the Groebner region appears. Affects the xfig file but probably not the sage graphics (?)
    - `rgbcolor` - This will not affect the saved xfig file, only the sage graphics produced.
    - `polyfill` - Whether or not to fill the cones with a color determined by the highest degree in each reduced Groebner basis for that cone.
    - `scale_colors` - if True, this will normalize color values to try to maximize the range

    EXAMPLES:

        sage: R.<x,y,z> = PolynomialRing(QQ,3)
        sage: G = R.ideal([y^3 - x^2, y^2 - 13*x,z]).groebner_fan()
        sage: test_render = G.render()

        sage: R.<x,y,z> = PolynomialRing(QQ,3)
        sage: G = R.ideal([x^2*y - z, y^2*z - x, z^2*x - y]).groebner_fan()
        sage: test_render = G.render(larger=True)

    TESTS:

    Testing the case where the number of generators is < 3. Currently, this should raise a `NotImplementedError` error.

        sage: R.<x,y> = PolynomialRing(QQ, 2)
        sage: R.ideal([y^3 - x^2, y^2 - 13*x]).groebner_fan().render()
        Traceback (most recent call last):
        ...
        NotImplementedError

  `render3d`(verbose=False)

    For a Groebner fan of an ideal in a ring with four variables, this function intersects the fan with the standard simplex perpendicular to (1,1,1,1), creating a 3d polytope, which is then projected into 3 dimensions. The edges of this projected polytope are returned as lines.

    EXAMPLES:

        sage: R4.<w,x,y,z> = PolynomialRing(QQ,4)
        sage: gf = R4.ideal([w^2-x,x^2-y,y^2-z,z^2-x]).groebner_fan()
        sage: three_d = gf.render3d()

    TESTS:

    Now test the case where the number of generators is not 4. Currently, this should raise a `NotImplementedError` error.

        sage: P.<a,b,c> = PolynomialRing(QQ, 3, order="lex")
        sage: sage.rings.ideal.Katsura(P, 3).groebner_fan().render3d()
        Traceback (most recent call last):
        ...
        NotImplementedError

  `ring`()

    Return the multivariate polynomial ring.

    EXAMPLES:

        sage: R.<x1,x2> = PolynomialRing(QQ,2)
        sage: gf = R.ideal([x1^3-x2,x2^3-x1-2]).groebner_fan()
        sage: gf.ring()
        Multivariate Polynomial Ring in x1, x2 over Rational Field

  `tropical_basis`(check=True, verbose=False)

    Return a tropical basis for the tropical curve associated to this ideal.

    INPUT:

    - `check` - bool (default: True); if True raises a ValueError exception if this ideal does not define a tropical curve (i.e., the condition that R/I has dimension equal to 1 + the dimension of the homogeneity space is not satisfied).

    EXAMPLES:

        sage: R.<x,y,z> = PolynomialRing(QQ,3, order='lex')
        sage: G = R.ideal([y^3-3*x^2, z^3-x-y-2*y^3+2*x^2]).groebner_fan()
        sage: G
        Groebner fan of the ideal:
        Ideal (-3*x^2 + y^3, 2*x^2 - x - 2*y^3 - y + z^3) of Multivariate Polynomial Ring in x, y, z over Rational Field
        sage: G.tropical_basis()
        [-3*x^2 + y^3, 2*x^2 - x - 2*y^3 - y + z^3, 3/4*x + y^3 + 3/4*y - 3/4*z^3]

  `tropical_intersection`(parameters=[], symmetry_generators=[], *args, **kwds)

    Returns information about the tropical intersection of the polynomials defining the ideal. This is the common refinement of the outward-pointing normal fans of the Newton polytopes of the generators of the ideal. Note that some people use the inward-pointing normal fans.

    INPUT:

    - `parameters` (optional) - a list of variables to be considered as parameters
    - `symmetry_generators` (optional) - generators of the symmetry group

    OUTPUT: a TropicalPrevariety object

    EXAMPLES:

        sage: R.<x,y,z> = PolynomialRing(QQ,3)
        sage: I = R.ideal(x*z + 6*y*z - z^2, x*y + 6*x*z + y*z - z^2, y^2 + x*z + y*z)
        sage: gf = I.groebner_fan()
        sage: pf = gf.tropical_intersection()
        sage: pf.rays()
        [[-2, 1, 1]]

        sage: R.<x,y,z> = PolynomialRing(QQ,3)
        sage: f1 = x*y*z - 1
        sage: f2 = f1*(x^2 + y^2 + z^2)
        sage: f3 = f2*(x + y + z - 1)
        sage: I = R.ideal([f1,f2,f3])
        sage: gf = I.groebner_fan()
        sage: pf = gf.tropical_intersection(symmetry_generators = '(1,2,0),(1,0,2)')
        sage: pf.rays()
        [[-2, 1, 1], [1, -2, 1], [1, 1, -2]]

        sage: R.<x,y,z> = QQ[]
        sage: I = R.ideal([(x+y+z)^2-1,(x+y+z)-x,(x+y+z)-3])
        sage: GF = I.groebner_fan()
        sage: TI = GF.tropical_intersection()
        sage: TI.rays()
        [[-1, 0, 0], [0, -1, -1], [1, 1, 1]]
        sage: GF = I.groebner_fan()
        sage: TI = GF.tropical_intersection(parameters=[y])
        sage: TI.rays()
        [[-1, 0, 0]]

  `weight_vectors`()

    Returns the weight vectors corresponding to the reduced Groebner bases.

    EXAMPLES:

        sage: r3.<x,y,z> = PolynomialRing(QQ,3)
        sage: g = r3.ideal([x^3+y,y^3-z,z^2-x]).groebner_fan()
        sage: g.weight_vectors()
        [(3, 7, 1), (5, 1, 2), (7, 1, 4), (5, 1, 4), (1, 1, 1), (1, 4, 8), (1, 4, 10)]
        sage: r4.<x,y,z,w> = PolynomialRing(QQ,4)
        sage: g4 = r4.ideal([x^3+y,y^3-z,z^2-x,z^3 - w]).groebner_fan()
        sage: len(g4.weight_vectors())
        23

class `sage.rings.polynomial.groebner_fan.``InitialForm`(cone, rays, initial_forms)

  Bases: `sage.structure.sage_object.SageObject`

  A system of initial forms from a polynomial system. To each form is associated a cone and a list of polynomials (the initial form system itself).

  This class is intended for internal use inside of the TropicalPrevariety class.

  EXAMPLES:

      sage: from sage.rings.polynomial.groebner_fan import InitialForm
      sage: R.<x,y> = QQ[]
      sage: inform = InitialForm([0], [[-1, 0]], [y^2 - 1, y^2 - 2, y^2 - 3])
      sage: inform._cone
      [0]

  `cone`()

    The cone associated with the initial form system.

    EXAMPLES:

        sage: R.<x,y> = QQ[]
        sage: I = R.ideal([(x+y)^2-1,(x+y)^2-2,(x+y)^2-3])
        sage: GF = I.groebner_fan()
        sage: PF = GF.tropical_intersection()
        sage: pfi0 = PF.initial_form_systems()[0]
        sage: pfi0.cone()
        [0]

  `initial_forms`()

    The initial forms (polynomials).

    EXAMPLES:

        sage: R.<x,y> = QQ[]
        sage: I = R.ideal([(x+y)^2-1,(x+y)^2-2,(x+y)^2-3])
        sage: GF = I.groebner_fan()
        sage: PF = GF.tropical_intersection()
        sage: pfi0 = PF.initial_form_systems()[0]
        sage: pfi0.initial_forms()
        [y^2 - 1, y^2 - 2, y^2 - 3]

  `internal_ray`()

    A ray internal to the cone associated with the initial form system.

    EXAMPLES:

        sage: R.<x,y> = QQ[]
        sage: I = R.ideal([(x+y)^2-1,(x+y)^2-2,(x+y)^2-3])
        sage: GF = I.groebner_fan()
        sage: PF = GF.tropical_intersection()
        sage: pfi0 = PF.initial_form_systems()[0]
        sage: pfi0.internal_ray()
        (-1, 0)

  `rays`()

    The rays of the cone associated with the initial form system.

    EXAMPLES:

        sage: R.<x,y> = QQ[]
        sage: I = R.ideal([(x+y)^2-1,(x+y)^2-2,(x+y)^2-3])
        sage: GF = I.groebner_fan()
        sage: PF = GF.tropical_intersection()
        sage: pfi0 = PF.initial_form_systems()[0]
        sage: pfi0.rays()
        [[-1, 0]]

class `sage.rings.polynomial.groebner_fan.``PolyhedralCone`(gfan_polyhedral_cone, ring=Rational Field)

  Bases: `sage.structure.sage_object.SageObject`

  Converts polymake/gfan data on a polyhedral cone into a sage class. Currently (18-03-2008) needs a lot of work.

  EXAMPLES:

      sage: R3.<x,y,z> = PolynomialRing(QQ,3)
      sage: gf = R3.ideal([x^8-y^4,y^4-z^2,z^2-2]).groebner_fan()
      sage: a = gf[0].groebner_cone()
      sage: a.facets()
      [[0, 0, 1], [0, 1, 0], [1, 0, 0]]

  `ambient_dim`()

    Returns the ambient dimension of the Groebner cone.

    EXAMPLES:

        sage: R3.<x,y,z> = PolynomialRing(QQ,3)
        sage: gf = R3.ideal([x^8-y^4,y^4-z^2,z^2-2]).groebner_fan()
        sage: a = gf[0].groebner_cone()
        sage: a.ambient_dim()
        3

  `dim`()

    Returns the dimension of the Groebner cone.

    EXAMPLES:

        sage: R3.<x,y,z> = PolynomialRing(QQ,3)
        sage: gf = R3.ideal([x^8-y^4,y^4-z^2,z^2-2]).groebner_fan()
        sage: a = gf[0].groebner_cone()
        sage: a.dim()
        3

  `facets`()

    Returns the inward facet normals of the Groebner cone.

    EXAMPLES:

        sage: R3.<x,y,z> = PolynomialRing(QQ,3)
        sage: gf = R3.ideal([x^8-y^4,y^4-z^2,z^2-2]).groebner_fan()
        sage: a = gf[0].groebner_cone()
        sage: a.facets()
        [[0, 0, 1], [0, 1, 0], [1, 0, 0]]

  `lineality_dim`()

    Returns the lineality dimension of the Groebner cone. This is just the difference between the ambient dimension and the dimension of the cone.

    EXAMPLES:

        sage: R3.<x,y,z> = PolynomialRing(QQ,3)
        sage: gf = R3.ideal([x^8-y^4,y^4-z^2,z^2-2]).groebner_fan()
        sage: a = gf[0].groebner_cone()
        sage: a.lineality_dim()
        0

  `relative_interior_point`()

    Returns a point in the relative interior of the Groebner cone.

    EXAMPLES:

        sage: R3.<x,y,z> = PolynomialRing(QQ,3)
        sage: gf = R3.ideal([x^8-y^4,y^4-z^2,z^2-2]).groebner_fan()
        sage: a = gf[0].groebner_cone()
        sage: a.relative_interior_point()
        [1, 1, 1]

class `sage.rings.polynomial.groebner_fan.``PolyhedralFan`(gfan_polyhedral_fan, parameter_indices=[])

  Bases: `sage.structure.sage_object.SageObject`

  Converts polymake/gfan data on a polyhedral fan into a sage class.

  INPUT:

  - `gfan_polyhedral_fan` - output from gfan of a polyhedral fan.

  EXAMPLES:

      sage: R.<x,y,z> = PolynomialRing(QQ,3)
      sage: i2 = ideal(x*z + 6*y*z - z^2, x*y + 6*x*z + y*z - z^2, y^2 + x*z + y*z)
      sage: gf2 = i2.groebner_fan(verbose = False)
      sage: pf = gf2.polyhedralfan()
      sage: pf.rays()
      [[-1, 0, 1], [-1, 1, 0], [1, -2, 1], [1, 1, -2], [2, -1, -1]]

  `ambient_dim`()

    Returns the ambient dimension of the Groebner fan.

    EXAMPLES:

        sage: R3.<x,y,z> = PolynomialRing(QQ,3)
        sage: gf = R3.ideal([x^8-y^4,y^4-z^2,z^2-2]).groebner_fan()
        sage: a = gf.polyhedralfan()
        sage: a.ambient_dim()
        3

  `cones`()

    A dictionary of cones in which the keys are the cone dimensions. For each dimension, the value is a list of the cones, where each element consists of a list of ray indices.

    EXAMPLES:

        sage: R.<x,y,z> = QQ[]
        sage: f = 1+x+y+x*y
        sage: I = R.ideal([f+z*f, 2*f+z*f, 3*f+z^2*f])
        sage: GF = I.groebner_fan()
        sage: PF = GF.tropical_intersection()
        sage: PF.cones()
        {1: [[0], [1], [2], [3], [4], [5]], 2: [[0, 1], [0, 2], [0, 3], [0, 4], [1, 2], [1, 3], [2, 4], [3, 4], [1, 5], [2, 5], [3, 5], [4, 5]]}

  `dim`()

    Returns the dimension of the Groebner fan.

    EXAMPLES:

        sage: R3.<x,y,z> = PolynomialRing(QQ,3)
        sage: gf = R3.ideal([x^8-y^4,y^4-z^2,z^2-2]).groebner_fan()
        sage: a = gf.polyhedralfan()
        sage: a.dim()
        3

  `f_vector`()

    The f-vector of the fan.

    EXAMPLES:

        sage: R.<x,y,z> = QQ[]
        sage: f = 1+x+y+x*y
        sage: I = R.ideal([f+z*f, 2*f+z*f, 3*f+z^2*f])
        sage: GF = I.groebner_fan()
        sage: PF = GF.tropical_intersection()
        sage: PF.f_vector()
        [1, 6, 12]

  `is_simplicial`()

    Whether the fan is simplicial or not.

    EXAMPLES:

        sage: R.<x,y,z> = QQ[]
        sage: f = 1+x+y+x*y
        sage: I = R.ideal([f+z*f, 2*f+z*f, 3*f+z^2*f])
        sage: GF = I.groebner_fan()
        sage: PF = GF.tropical_intersection()
        sage: PF.is_simplicial()
        True

  `lineality_dim`()

    Returns the lineality dimension of the fan. This is the dimension of the largest subspace contained in the fan.

    EXAMPLES:

        sage: R3.<x,y,z> = PolynomialRing(QQ,3)
        sage: gf = R3.ideal([x^8-y^4,y^4-z^2,z^2-2]).groebner_fan()
        sage: a = gf.polyhedralfan()
        sage: a.lineality_dim()
        0

  `maximal_cones`()

    A dictionary of the maximal cones in which the keys are the cone dimensions. For each dimension, the value is a list of the maximal cones, where each element consists of a list of ray indices.

    EXAMPLES:

        sage: R.<x,y,z> = QQ[]
        sage: f = 1+x+y+x*y
        sage: I = R.ideal([f+z*f, 2*f+z*f, 3*f+z^2*f])
        sage: GF = I.groebner_fan()
        sage: PF = GF.tropical_intersection()
        sage: PF.maximal_cones()
        {2: [[0, 1], [0, 2], [0, 3], [0, 4], [1, 2], [1, 3], [2, 4], [3, 4], [1, 5], [2, 5], [3, 5], [4, 5]]}

  `rays`()

    A list of rays of the polyhedral fan.

    EXAMPLES:

        sage: R.<x,y,z> = PolynomialRing(QQ,3)
        sage: i2 = ideal(x*z + 6*y*z - z^2, x*y + 6*x*z + y*z - z^2, y^2 + x*z + y*z)
        sage: gf2 = i2.groebner_fan(verbose = False)
        sage: pf = gf2.polyhedralfan()
        sage: pf.rays()
        [[-1, 0, 1], [-1, 1, 0], [1, -2, 1], [1, 1, -2], [2, -1, -1]]

  `to_RationalPolyhedralFan`()

    Converts to the RationalPolyhedralFan class, which is more actively maintained. While the information in each class is essentially the same, the methods and implementation are different.

    EXAMPLES:

        sage: R.<x,y,z> = QQ[]
        sage: f = 1+x+y+x*y
        sage: I = R.ideal([f+z*f, 2*f+z*f, 3*f+z^2*f])
        sage: GF = I.groebner_fan()
        sage: PF = GF.tropical_intersection()
        sage: fan = PF.to_RationalPolyhedralFan()
        sage: [tuple(q.facet_normals()) for q in fan]
        [(M(0, -1, 0), M(-1, 0, 0)), (M(0, 0, -1), M(-1, 0, 0)), (M(0, 0, 1), M(-1, 0, 0)), (M(0, 1, 0), M(-1, 0, 0)), (M(0, 0, -1), M(0, -1, 0)), (M(0, 0, 1), M(0, -1, 0)), (M(0, 1, 0), M(0, 0, -1)), (M(0, 1, 0), M(0, 0, 1)), (M(1, 0, 0), M(0, -1, 0)), (M(1, 0, 0), M(0, 0, -1)), (M(1, 0, 0), M(0, 0, 1)), (M(1, 0, 0), M(0, 1, 0))]

    Here we use the RationalPolyhedralFan’s Gale_transform method on a tropical prevariety.

        sage: fan.Gale_transform()
        [ 1  0  0  0  0  1 -2]
        [ 0  1  0  0  1  0 -2]
        [ 0  0  1  1  0  0 -2]

class `sage.rings.polynomial.groebner_fan.``ReducedGroebnerBasis`(groebner_fan, gens, gfan_gens)

  Bases: `sage.structure.sage_object.SageObject`, `list`

  A class for representing reduced Groebner bases as produced by gfan.

  INPUT:

  - `groebner_fan` - a GroebnerFan object from an ideal
  - `gens` - the generators of the ideal
  - `gfan_gens` - the generators as a gfan string

  EXAMPLES:

      sage: R.<a,b> = PolynomialRing(QQ,2)
      sage: gf = R.ideal([a^2-b^2,b-a-1]).groebner_fan()
      sage: from sage.rings.polynomial.groebner_fan import ReducedGroebnerBasis
      sage: ReducedGroebnerBasis(gf,gf[0],gf[0]._gfan_gens())
      [b - 1/2, a + 1/2]

  `groebner_cone`(restrict=False)

    Return defining inequalities for the full-dimensional Groebner cone associated to this marked minimal reduced Groebner basis.

    INPUT:

    - `restrict` - bool (default: False); if True, add an inequality for each coordinate, so that the cone is restricted to the positive orthant.

    OUTPUT: tuple of integer vectors

    EXAMPLES:

        sage: R.<x,y> = PolynomialRing(QQ,2)
        sage: G = R.ideal([y^3 - x^2, y^2 - 13*x]).groebner_fan()
        sage: poly_cone = G[1].groebner_cone()
        sage: poly_cone.facets()
        [[-1, 2], [1, -1]]
        sage: [g.groebner_cone().facets() for g in G]
        [[[0, 1], [1, -2]], [[-1, 2], [1, -1]], [[-1, 1], [1, 0]]]
        sage: G[1].groebner_cone(restrict=True).facets()
        [[-1, 2], [1, -1]]

  `ideal`()

    Return the ideal generated by this basis.

    EXAMPLES:

        sage: R.<x,y,z> = PolynomialRing(QQ,3)
        sage: G = R.ideal([x - z^3, y^2 - 13*x]).groebner_fan()
        sage: G[0].ideal()
        Ideal (-13*z^3 + y^2, -z^3 + x) of Multivariate Polynomial Ring in x, y, z over Rational Field

  `interactive`(latex=False, flippable=False, wall=False, inequalities=False, weight=False)

    Do an interactive walk of the Groebner fan starting at this reduced Groebner basis.

    EXAMPLES:

        sage: R.<x,y> = PolynomialRing(QQ,2)
        sage: G = R.ideal([y^3 - x^2, y^2 - 13*x]).groebner_fan()
        sage: G[0].interactive()      # not tested
        Initializing gfan interactive mode
        *********************************************
        *     Press control-C to return to Sage     *
        *********************************************
        ....

class `sage.rings.polynomial.groebner_fan.``TropicalPrevariety`(gfan_polyhedral_fan, polynomial_system, poly_ring, parameters=[])

  Bases: `sage.rings.polynomial.groebner_fan.PolyhedralFan`

  This class is a subclass of the PolyhedralFan class, with some additional methods for tropical prevarieties.

  INPUT:

  - `gfan_polyhedral_fan` - output from gfan of a polyhedral fan.
  - `polynomial_system` - a list of polynomials
  - `poly_ring` - the polynomial ring of the list of polynomials
  - `parameters` (optional) - a list of variables to be considered as parameters

  EXAMPLES:

      sage: R.<x,y,z> = QQ[]
      sage: I = R.ideal([(x+y+z)^2-1,(x+y+z)-x,(x+y+z)-3])
      sage: GF = I.groebner_fan()
      sage: TI = GF.tropical_intersection()
      sage: TI._polynomial_system
      [x^2 + 2*x*y + y^2 + 2*x*z + 2*y*z + z^2 - 1, y + z, x + y + z - 3]

  `initial_form_systems`()

    Returns a list of systems of initial forms for each cone in the tropical prevariety.

    EXAMPLES:

        sage: R.<x,y> = QQ[]
        sage: I = R.ideal([(x+y)^2-1,(x+y)^2-2,(x+y)^2-3])
        sage: GF = I.groebner_fan()
        sage: PF = GF.tropical_intersection()
        sage: pfi = PF.initial_form_systems()
        sage: for q in pfi:
        ...     print q.initial_forms()
        [y^2 - 1, y^2 - 2, y^2 - 3]
        [x^2 - 1, x^2 - 2, x^2 - 3]
        [x^2 + 2*x*y + y^2, x^2 + 2*x*y + y^2, x^2 + 2*x*y + y^2]

`sage.rings.polynomial.groebner_fan.``ideal_to_gfan_format`(input_ring, polys)

  Return the ideal in gfan’s notation.

  > EXAMPLES:

  >     sage: R.<x,y,z> = PolynomialRing(QQ,3)
  >     sage: polys = [x^2*y - z, y^2*z - x, z^2*x - y]
  >     sage: from sage.rings.polynomial.groebner_fan import ideal_to_gfan_format
  >     sage: ideal_to_gfan_format(R, polys)
  >     'Q[x, y, z]{x^2*y-z,y^2*z-x,x*z^2-y}'

`sage.rings.polynomial.groebner_fan.``max_degree`(list_of_polys)

  Computes the maximum degree of a list of polynomials

  EXAMPLES:

      sage: from sage.rings.polynomial.groebner_fan import max_degree
      sage: R.<x,y> = PolynomialRing(QQ,2)
      sage: p_list = [x^2-y,x*y^10-x]
      sage: max_degree(p_list)
      11.0

`sage.rings.polynomial.groebner_fan.``prefix_check`(str_list)

  Checks if any strings in a list are prefixes of another string in the list.

  EXAMPLES:

      sage: from sage.rings.polynomial.groebner_fan import prefix_check
      sage: prefix_check(['z1','z1z1'])
      False
      sage: prefix_check(['z1','zz1'])
      True

`sage.rings.polynomial.groebner_fan.``ring_to_gfan_format`(input_ring)

  Converts a ring to gfan’s format.

  EXAMPLES:

      sage: R.<w,x,y,z> = QQ[]
      sage: from sage.rings.polynomial.groebner_fan import ring_to_gfan_format
      sage: ring_to_gfan_format(R)
      'Q[w, x, y, z]'
      sage: R2.<x,y> = GF(2)[]
      sage: ring_to_gfan_format(R2)
      'Z/2Z[x, y]'

`sage.rings.polynomial.groebner_fan.``verts_for_normal`(normal, poly)

  Returns the exponents of the vertices of a newton polytope that make up the supporting hyperplane for the given outward normal.

  EXAMPLES:

      sage: from sage.rings.polynomial.groebner_fan import verts_for_normal
      sage: R.<x,y,z> = PolynomialRing(QQ,3)
      sage: f1 = x*y*z - 1
      sage: f2 = f1*(x^2 + y^2 + 1)
      sage: verts_for_normal([1,1,1],f2)
      [(3, 1, 1), (1, 3, 1)]

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