### Navigation

- index
- modules |
- next |
- previous |
- Combinatorial Geometry »

# Double Description Algorithm for Cones

This module implements the double description algorithm for extremal vertex enumeration in a pointed cone following [FukudaProdon]. With a little bit of preprocessing (see `double_description_inhomogeneous`) this defines a backend for polyhedral computations. But as far as this module is concerned, inequality always means without a constant term and the origin is always a point of the cone.

EXAMPLES:

    sage: from sage.geometry.polyhedron.double_description import StandardAlgorithm
    sage: A = matrix(QQ, [(1,0,1), (0,1,1), (-1,-1,1)])
    sage: alg = StandardAlgorithm(A);  alg
    Pointed cone with inequalities
    (1, 0, 1)
    (0, 1, 1)
    (-1, -1, 1)
    sage: DD, _ = alg.initial_pair();  DD
    Double description pair (A, R) defined by
        [ 1  0  1]        [ 2/3 -1/3 -1/3]
    A = [ 0  1  1],   R = [-1/3  2/3 -1/3]
        [-1 -1  1]        [ 1/3  1/3  1/3]

The implementation works over any exact field that is embedded in \(\RR\), for example:

    sage: from sage.geometry.polyhedron.double_description import StandardAlgorithm
    sage: A = matrix(AA, [(1,0,1), (0,1,1), (-AA(2).sqrt(),-AA(3).sqrt(),1),
    ....:                 (-AA(3).sqrt(),-AA(2).sqrt(),1)])
    sage: alg = StandardAlgorithm(A)
    sage: alg.run().R
    ((-0.4177376677004119?, 0.5822623322995881?, 0.4177376677004119?),
     (-0.2411809548974793?, -0.2411809548974793?, 0.2411809548974793?),
     (0.07665629029830300?, 0.07665629029830300?, 0.2411809548974793?),
     (0.5822623322995881?, -0.4177376677004119?, 0.4177376677004119?))

REFERENCES:

[FukudaProdon] | (1, 2) Komei Fukuda , Alain Prodon: Double Description Method Revisited, Combinatorics and Computer Science, volume 1120 of Lecture Notes in Computer Science, page 91-111. Springer (1996)

class `sage.geometry.polyhedron.double_description.``DoubleDescriptionPair`(problem, A_rows, R_cols)

  Bases: `sage.structure.sage_object.SageObject`

  Base class for a double description pair \((A, R)\)

  Warning

  You should use the `Problem.initial_pair()` or `Problem.run()` to generate double description pairs for a set of inequalities, and not generate `DoubleDescriptionPair` instances directly.

  INPUT:

  - `problem` – instance of `Problem`.
  - `A_rows` – list of row vectors of the matrix \(A\). These encode the inequalities.
  - `R_cols` – list of column vectors of the matrix \(R\). These encode the rays.

  TESTS:

      sage: from sage.geometry.polyhedron.double_description import \
      ....:     DoubleDescriptionPair, Problem
      sage: A = matrix(QQ, [(1,0,1), (0,1,1), (-1,-1,1)])
      sage: alg = Problem(A)
      sage: DoubleDescriptionPair(alg,
      ....:     [(1, 0, 1), (0, 1, 1), (-1, -1, 1)],
      ....:     [(2/3, -1/3, 1/3), (-1/3, 2/3, 1/3), (-1/3, -1/3, 1/3)])
      Double description pair (A, R) defined by
          [ 1  0  1]        [ 2/3 -1/3 -1/3]
      A = [ 0  1  1],   R = [-1/3  2/3 -1/3]
          [-1 -1  1]        [ 1/3  1/3  1/3]

  `R_by_sign`(a)

    Classify the rays into those that are positive, zero, and negative on \(a\).

    INPUT:

    - `a` – vector. Coefficient vector of a homogeneous inequality.

    OUTPUT:

    A triple consisting of the rays (columns of \(R\)) that are positive, zero, and negative on \(a\). In that order.

    EXAMPLES:

        sage: from sage.geometry.polyhedron.double_description import StandardAlgorithm
        sage: A = matrix(QQ, [(1,0,1), (0,1,1), (-1,-1,1)])
        sage: DD, _ = StandardAlgorithm(A).initial_pair()
        sage: DD.R_by_sign(vector([1,-1,0]))
        ([(2/3, -1/3, 1/3)], [(-1/3, -1/3, 1/3)], [(-1/3, 2/3, 1/3)])
        sage: DD.R_by_sign(vector([1,1,1]))
        ([(2/3, -1/3, 1/3), (-1/3, 2/3, 1/3)], [], [(-1/3, -1/3, 1/3)])

  `are_adjacent`(r1, r2)

    Return whether the two rays are adjacent.

    INPUT:

    - `r1`, `r2` – two rays.

    OUTPUT:

    Boolean. Whether the two rays are adjacent.

    EXAMPLES:

        sage: from sage.geometry.polyhedron.double_description import StandardAlgorithm
        sage: A = matrix(QQ, [(0,1,0), (1,0,0), (0,-1,1), (-1,0,1)])
        sage: DD = StandardAlgorithm(A).run()
        sage: DD.are_adjacent(DD.R[0], DD.R[1])
        True
        sage: DD.are_adjacent(DD.R[0], DD.R[2])
        True
        sage: DD.are_adjacent(DD.R[0], DD.R[3])
        False

  `cone`()

    Return the cone defined by \(A\).

    This method is for debugging only. Assumes that the base ring is \(\QQ\).

    OUTPUT:

    The cone defined by the inequalities as a `Polyhedron()`, using the PPL backend.

    EXAMPLES:

        sage: from sage.geometry.polyhedron.double_description import StandardAlgorithm
        sage: A = matrix(QQ, [(1,0,1), (0,1,1), (-1,-1,1)])
        sage: DD, _ = StandardAlgorithm(A).initial_pair()
        sage: DD.cone().Hrepresentation()
        (An inequality (-1, -1, 1) x + 0 >= 0,
         An inequality (0, 1, 1) x + 0 >= 0,
         An inequality (1, 0, 1) x + 0 >= 0)

  `dual`()

    Return the dual.

    OUTPUT:

    For the double description pair \((A, R)\) this method returns the dual double description pair \((R^T, A^T)\)

    EXAMPLES:

        sage: from sage.geometry.polyhedron.double_description import Problem
        sage: A = matrix(QQ, [(0,1,0), (1,0,0), (0,-1,1), (-1,0,1)])
        sage: DD, _ = Problem(A).initial_pair()
        sage: DD
        Double description pair (A, R) defined by
            [ 0  1  0]        [0 1 0]
        A = [ 1  0  0],   R = [1 0 0]
            [ 0 -1  1]        [1 0 1]
        sage: DD.dual()
         Double description pair (A, R) defined by
            [0 1 1]        [ 0  1  0]
        A = [1 0 0],   R = [ 1  0 -1]
            [0 0 1]        [ 0  0  1]

  `first_coordinate_plane`()

    Restrict to the first coordinate plane.

    OUTPUT:

    A new double description pair with the constraint \(x_0 = 0\) added.

    EXAMPLES:

        sage: A = matrix([(1, 1), (-1, 1)])
        sage: from sage.geometry.polyhedron.double_description import StandardAlgorithm
        sage: DD, _ = StandardAlgorithm(A).initial_pair()
        sage: DD
        Double description pair (A, R) defined by
        A = [ 1  1],   R = [ 1/2 -1/2]
            [-1  1]        [ 1/2  1/2]
        sage: DD.first_coordinate_plane()
        Double description pair (A, R) defined by
            [ 1  1]
        A = [-1  1],   R = [  0]
            [-1  0]        [1/2]
            [ 1  0]

  `inner_product_matrix`()

    Return the inner product matrix between the rows of \(A\) and the columns of \(R\).

    OUTPUT:

    A matrix over the base ring. There is one row for each row of \(A\) and one column for each column of \(R\).

    EXAMPLES:

        sage: from sage.geometry.polyhedron.double_description import StandardAlgorithm
        sage: A = matrix(QQ, [(1,0,1), (0,1,1), (-1,-1,1)])
        sage: alg = StandardAlgorithm(A)
        sage: DD, _ = alg.initial_pair()
        sage: DD.inner_product_matrix()
        [1 0 0]
        [0 1 0]
        [0 0 1]

  `is_extremal`(ray)

    Test whether the ray is extremal.

    EXAMPLES:

        sage: from sage.geometry.polyhedron.double_description import StandardAlgorithm
        sage: A = matrix(QQ, [(0,1,0), (1,0,0), (0,-1,1), (-1,0,1)])
        sage: DD = StandardAlgorithm(A).run()
        sage: DD.is_extremal(DD.R[0])
        True

  `verify`()

    Validate the double description pair.

    This method used the PPL backend to check that the double description pair is valid. An assertion is triggered if it is not. Does nothing if the base ring is not \(\QQ\).

    EXAMPLES:

        sage: from sage.geometry.polyhedron.double_description import \
        ....:     DoubleDescriptionPair, Problem
        sage: A = matrix(QQ, [(1,0,1), (0,1,1), (-1,-1,1)])
        sage: alg = Problem(A)
        sage: DD = DoubleDescriptionPair(alg,
        ....:     [(1, 0, 3), (0, 1, 1), (-1, -1, 1)],
        ....:     [(2/3, -1/3, 1/3), (-1/3, 2/3, 1/3), (-1/3, -1/3, 1/3)])
        sage: DD.verify()
        Traceback (most recent call last):
        ...
            assert A_cone == R_cone
        AssertionError

  `zero_set`(ray)

    Return the zero set (active set) \(Z(r)\).

    INPUT:

    - `ray` – a ray vector.

    OUTPUT:

    A tuple containing the inequality vectors that are zero on `ray`.

    EXAMPLES:

        sage: from sage.geometry.polyhedron.double_description import Problem
        sage: A = matrix(QQ, [(1,0,1), (0,1,1), (-1,-1,1)])
        sage: DD, _ = Problem(A).initial_pair()
        sage: r = DD.R[0];  r
        (2/3, -1/3, 1/3)
        sage: DD.zero_set(r)
        ((0, 1, 1), (-1, -1, 1))

class `sage.geometry.polyhedron.double_description.``Problem`(A)

  Bases: `sage.structure.sage_object.SageObject`

  Base class for implementations of the double description algorithm

  It does not make sense to instantiate the base class directly, it just provides helpers for implementations.

  INPUT:

  - `A` – a matrix. The rows of the matrix are interpreted as homogeneous inequalities \(A x \geq 0\). Must have maximal rank.

  TESTS:

      sage: A = matrix([(1, 1), (-1, 1)])
      sage: from sage.geometry.polyhedron.double_description import Problem
      sage: Problem(A)
      Pointed cone with inequalities
      (1, 1)
      (-1, 1)

  `A`()

    Return the rows of the defining matrix \(A\).

    OUTPUT:

    The matrix \(A\) whose rows are the inequalities.

    EXAMPLES:

        sage: A = matrix([(1, 1), (-1, 1)])
        sage: from sage.geometry.polyhedron.double_description import Problem
        sage: Problem(A).A()
        ((1, 1), (-1, 1))

  `A_matrix`()

    Return the defining matrix \(A\).

    OUTPUT:

    Matrix whose rows are the inequalities.

    EXAMPLES:

        sage: A = matrix([(1, 1), (-1, 1)])
        sage: from sage.geometry.polyhedron.double_description import Problem
        sage: Problem(A).A_matrix()
        [ 1  1]
        [-1  1]

  `base_ring`()

    Return the base field.

    OUTPUT:

    A field.

    EXAMPLES:

        sage: A = matrix(AA, [(1, 1), (-1, 1)])
        sage: from sage.geometry.polyhedron.double_description import Problem
        sage: Problem(A).base_ring()
        Algebraic Real Field

  `dim`()

    Return the ambient space dimension.

    OUTPUT:

    Integer. The ambient space dimension of the cone.

    EXAMPLES:

        sage: A = matrix(QQ, [(1, 1), (-1, 1)])
        sage: from sage.geometry.polyhedron.double_description import Problem
        sage: Problem(A).dim()
        2

  `initial_pair`()

    Return an initial double description pair.

    Picks an initial set of rays by selecting a basis. This is probably the most efficient way to select the initial set.

    INPUT:

    - `pair_class` – subclass of `DoubleDescriptionPair`.

    OUTPUT:

    A pair consisting of a `DoubleDescriptionPair` instance and the tuple of remaining unused inequalities.

    EXAMPLES:

        sage: A = matrix([(-1, 1), (-1, 2), (1/2, -1/2), (1/2, 2)])
        sage: from sage.geometry.polyhedron.double_description import Problem
        sage: DD, remaining = Problem(A).initial_pair()
        sage: DD.verify()
        sage: remaining
        [(1/2, -1/2), (1/2, 2)]

  `pair_class`

    alias of `DoubleDescriptionPair`

class `sage.geometry.polyhedron.double_description.``StandardAlgorithm`(A)

  Bases: `sage.geometry.polyhedron.double_description.Problem`

  Standard implementation of the double description algorithm

  See [FukudaProdon] for the definition of the “Standard Algorithm”.

  EXAMPLES:

      sage: A = matrix(QQ, [(1, 1), (-1, 1)])
      sage: from sage.geometry.polyhedron.double_description import StandardAlgorithm
      sage: DD = StandardAlgorithm(A).run()
      sage: DD.R    # the extremal rays
      ((1/2, 1/2), (-1/2, 1/2))

  `pair_class`

    alias of `StandardDoubleDescriptionPair`

  `run`()

    Run the Standard Algorithm.

    OUTPUT:

    A double description pair \((A, R)\) of all inequalities as a `DoubleDescriptionPair`. By virtue of the double description algorithm, the columns of \(R\) are the extremal rays.

    EXAMPLES:

        sage: from sage.geometry.polyhedron.double_description import StandardAlgorithm
        sage: A = matrix(QQ, [(0,1,0), (1,0,0), (0,-1,1), (-1,0,1)])
        sage: StandardAlgorithm(A).run()
        Double description pair (A, R) defined by
            [ 0  1  0]        [0 0 1 1]
        A = [ 1  0  0],   R = [1 0 1 0]
            [ 0 -1  1]        [1 1 1 1]
            [-1  0  1]

class `sage.geometry.polyhedron.double_description.``StandardDoubleDescriptionPair`(problem, A_rows, R_cols)

  Bases: `sage.geometry.polyhedron.double_description.DoubleDescriptionPair`

  Double description pair for the “Standard Algorithm”.

  See `StandardAlgorithm`.

  TESTS:

      sage: A = matrix([(-1, 1, 0), (-1, 2, 1), (1/2, -1/2, -1)])
      sage: from sage.geometry.polyhedron.double_description import StandardAlgorithm
      sage: DD, _ = StandardAlgorithm(A).initial_pair()
      sage: type(DD)
      <class 'sage.geometry.polyhedron.double_description.StandardDoubleDescriptionPair'>

  `add_inequality`(a)

    Return a new double description pair with the inequality \(a\) added.

    INPUT:

    - `a` – vector. An inequality.

    OUTPUT:

    A new `StandardDoubleDescriptionPair` instance.

    EXAMPLES:

        sage: A = matrix([(-1, 1, 0), (-1, 2, 1), (1/2, -1/2, -1)])
        sage: from sage.geometry.polyhedron.double_description import StandardAlgorithm
        sage: DD, _ = StandardAlgorithm(A).initial_pair()
        sage: newDD = DD.add_inequality(vector([1,0,0]));  newDD
        Double description pair (A, R) defined by
            [  -1    1    0]        [   1    1    0    0]
        A = [  -1    2    1],   R = [   1    1    1    1]
            [ 1/2 -1/2   -1]        [   0   -1 -1/2   -2]
            [   1    0    0]

`sage.geometry.polyhedron.double_description.``random_inequalities`(d, n)

  Random collections of inequalities for testing purposes.

  INPUT:

  - `d` – integer. The dimension.
  - `n` – integer. The number of random inequalities to generate.

  OUTPUT:

  A random set of inequalites as a `StandardAlgorithm` instance.

  EXAMPLES:

      sage: from sage.geometry.polyhedron.double_description import random_inequalities
      sage: P = random_inequalities(5, 10)
      sage: P.run().verify()

### Navigation

- index
- modules |
- next |
- previous |
- Combinatorial Geometry »