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# Double Description for Arbitrary Polyhedra

This module is part of the python backend for polyhedra. It uses the double description method for cones `double_description` to find minimal H/V-representations of polyhedra. The latter works with cones only. This is sufficient to treat general polyhedra by the following construction: Any polyhedron can be embedded in one dimension higher in the hyperplane \((1,*,\dots,*)\). The cone over the embedded polyhedron will be called the homogenized cone in the following. Conversely, intersecting the homogenized cone with the hyperplane \(x_0=1\) gives you back the original polyhedron.

While slower than specialized C/C++ implementations, the implementation is general and works with any field in Sage that allows you to define polyhedra.

Note

If you just want polyhedra over arbitrary fields then you should just use the `Polyhedron()` constructor.

EXAMPLES:

    sage: from sage.geometry.polyhedron.double_description_inhomogeneous     ....:     import Hrep2Vrep, Vrep2Hrep
    sage: Hrep2Vrep(QQ, 2, [(1,2,3), (2,4,3)], [])
    [-1/2|-1/2  1/2|]
    [   0| 2/3 -1/3|]

Note that the columns of the printed matrix are the vertices, rays, and lines of the minimal V-representation. Dually, the rows of the following are the inequalities and equations:

    sage: Vrep2Hrep(QQ, 2, [(-1/2,0)], [(-1/2,2/3), (1/2,-1/3)], [])
    [1 2 3]
    [2 4 3]
    [-----]

class `sage.geometry.polyhedron.double_description_inhomogeneous.``Hrep2Vrep`(base_ring, dim, inequalities, equations)

  Bases: `sage.geometry.polyhedron.double_description_inhomogeneous.PivotedInequalities`

  Convert H-representation to a minimal V-representation.

  INPUT:

  - `base_ring` – a field.
  - `dim` – integer. The ambient space dimension.
  - `inequalities` – list of inequalities. Each inequalitiy is given as constant term, `dim` coefficients.
  - `equations` – list of equations. Same notation as for inequalities.

  EXAMPLES:

      sage: from sage.geometry.polyhedron.double_description_inhomogeneous import Hrep2Vrep
      sage: Hrep2Vrep(QQ, 2, [(1,2,3), (2,4,3)], [])
      [-1/2|-1/2  1/2|]
      [   0| 2/3 -1/3|]
      sage: Hrep2Vrep(QQ, 2, [(1,2,3), (2,-2,-3)], [])
      [   1 -1/2||   1]
      [   0    0||-2/3]
      sage: Hrep2Vrep(QQ, 2, [(1,2,3), (2,2,3)], [])
      [-1/2| 1/2|   1]
      [   0|   0|-2/3]
      sage: Hrep2Vrep(QQ, 2, [(8,7,-2), (1,-4,3), (4,-3,-1)], [])
      [ 1  0 -2||]
      [ 1  4 -3||]
      sage: Hrep2Vrep(QQ, 2, [(1,2,3), (2,4,3), (5,-1,-2)], [])
      [-19/5  -1/2| 2/33  1/11|]
      [ 22/5     0|-1/33 -2/33|]
      sage: Hrep2Vrep(QQ, 2, [(0,2,3), (0,4,3), (0,-1,-2)], [])
      [   0| 1/2  1/3|]
      [   0|-1/3 -1/6|]
      sage: Hrep2Vrep(QQ, 2, [], [(1,2,3), (7,8,9)])
      [-2||]
      [ 1||]
      sage: Hrep2Vrep(QQ, 2, [(1,0,0)], [])    # universe
      [0||1 0]
      [0||0 1]
      sage: Hrep2Vrep(QQ, 2, [(-1,0,0)], [])   # empty
      []
      sage: Hrep2Vrep(QQ, 2, [], [])   # empty
      []

  `verify`(inequalities, equations)

    Compare result to PPL if the base ring is QQ.

    This method is for debugging purposes and compares the computation with another backend if available.

    INPUT:

    - `inequalities`, `equations` – see `Hrep2Vrep`.

    EXAMPLES:

        sage: from sage.geometry.polyhedron.double_description_inhomogeneous import Hrep2Vrep
        sage: H = Hrep2Vrep(QQ, 1, [(1,2)], [])
        sage: H.verify([(1,2)], [])

class `sage.geometry.polyhedron.double_description_inhomogeneous.``PivotedInequalities`(base_ring, dim)

  Bases: `sage.structure.sage_object.SageObject`

  Base class for inequalities that may contain linear subspaces

  INPUT:

  - `base_ring` – a field.
  - `dim` – integer. The ambient space dimension.

  EXAMPLES:

      sage: from sage.geometry.polyhedron.double_description_inhomogeneous             ....:     import PivotedInequalities
      sage: piv = PivotedInequalities(QQ, 2)
      sage: piv._pivot_inequalities(matrix([(1,1,3), (5,5,7)]))
      [1 3]
      [5 7]
      sage: piv._pivots
      (0, 2)
      sage: piv._linear_subspace
      Free module of degree 3 and rank 1 over Integer Ring
      Echelon basis matrix:
      [ 1 -1  0]

class `sage.geometry.polyhedron.double_description_inhomogeneous.``Vrep2Hrep`(base_ring, dim, vertices, rays, lines)

  Bases: `sage.geometry.polyhedron.double_description_inhomogeneous.PivotedInequalities`

  Convert V-representation to a minimal H-representation.

  INPUT:

  - `base_ring` – a field.
  - `dim` – integer. The ambient space dimension.
  - `vertices` – list of vertices. Each vertex is given as list of `dim` coordinates.
  - `rays` – list of rays. Each ray is given as list of `dim` coordinates, not all zero.
  - `lines` – list of line generators. Each line is given as list of `dim` coordinates, not all zero.

  EXAMPLES:

      sage: from sage.geometry.polyhedron.double_description_inhomogeneous import Vrep2Hrep
      sage: Vrep2Hrep(QQ, 2, [(-1/2,0)], [(-1/2,2/3), (1/2,-1/3)], [])
      [1 2 3]
      [2 4 3]
      [-----]

      sage: Vrep2Hrep(QQ, 2, [(1,0), (-1/2,0)], [], [(1,-2/3)])
      [ 1/3  2/3    1]
      [ 2/3 -2/3   -1]
      [--------------]

      sage: Vrep2Hrep(QQ, 2, [(-1/2,0)], [(1/2,0)], [(1,-2/3)])
      [1 2 3]
      [-----]

      sage: Vrep2Hrep(QQ, 2, [(1,1), (0,4), (-2,-3)], [], [])
      [ 8/13  7/13 -2/13]
      [ 1/13 -4/13  3/13]
      [ 4/13 -3/13 -1/13]
      [-----------------]

      sage: Vrep2Hrep(QQ, 2, [(-19/5,22/5), (-1/2,0)], [(2/33,-1/33), (1/11,-2/33)], [])
      [10/11 -2/11 -4/11]
      [ 66/5 132/5  99/5]
      [ 2/11  4/11  6/11]
      [-----------------]

      sage: Vrep2Hrep(QQ, 2, [(0,0)], [(1/2,-1/3), (1/3,-1/6)], [])
      [  0  -6 -12]
      [  0  12  18]
      [-----------]

      sage: Vrep2Hrep(QQ, 2, [(-1/2,0)], [], [(1,-2/3)])
      [-----]
      [1 2 3]

      sage: Vrep2Hrep(QQ, 2, [(-1/2,0)], [], [(1,-2/3), (1,0)])
      []

  `verify`(vertices, rays, lines)

    Compare result to PPL if the base ring is QQ.

    This method is for debugging purposes and compares the computation with another backend if available.

    INPUT:

    - `vertices`, `rays`, `lines` – see `Vrep2Hrep`.

    EXAMPLES:

        sage: from sage.geometry.polyhedron.double_description_inhomogeneous import Vrep2Hrep
        sage: vertices = [(-1/2,0)]
        sage: rays = [(-1/2,2/3), (1/2,-1/3)]
        sage: lines = []
        sage: V2H = Vrep2Hrep(QQ, 2, vertices, rays, lines)
        sage: V2H.verify(vertices, rays, lines)

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