Coverage for local/lib/python2.7/site-packages/sage/geometry/polyhedron/constructor.py : 90%
Hot-keys on this page
r m x p toggle line displays
j k next/prev highlighted chunk
0 (zero) top of page
1 (one) first highlighted chunk
|
r""" Polyhedra
In this module, a polyhedron is a convex (possibly unbounded) set in Euclidean space cut out by a finite set of linear inequalities and linear equations. Note that the dimension of the polyhedron can be less than the dimension of the ambient space. There are two complementary representations of the same data:
**H(alf-space/Hyperplane)-representation** This describes a polyhedron as the common solution set of a finite number of
* linear **inequalities** `A \vec{x} + b \geq 0`, and
* linear **equations** `C \vec{x} + d = 0`.
**V(ertex)-representation** The other representation is as the convex hull of vertices (and rays and lines to all for unbounded polyhedra) as generators. The polyhedron is then the Minkowski sum
.. MATH::
P = \text{conv}\{v_1,\dots,v_k\} + \sum_{i=1}^m \RR_+ r_i + \sum_{j=1}^n \RR \ell_j
where
* **vertices** `v_1`, `\dots`, `v_k` are a finite number of points. Each vertex is specified by an arbitrary vector, and two points are equal if and only if the vector is the same.
* **rays** `r_1`, `\dots`, `r_m` are a finite number of directions (directions of infinity). Each ray is specified by a non-zero vector, and two rays are equal if and only if the vectors are the same up to rescaling with a positive constant.
* **lines** `\ell_1`, `\dots`, `\ell_n` are a finite number of unoriented directions. In other words, a line is equivalent to the set `\{r, -r\}` for a ray `r`. Each line is specified by a non-zero vector, and two lines are equivalent if and only if the vectors are the same up to rescaling with a non-zero (possibly negative) constant.
When specifying a polyhedron, you can input a non-minimal set of inequalities/equations or generating vertices/rays/lines. The non-minimal generators are usually called points, non-extremal rays, and non-extremal lines, but for our purposes it is more convenient to always talk about vertices/rays/lines. Sage will remove any superfluous representation objects and always return a minimal representation. For example, `(0,0)` is a superfluous vertex here::
sage: triangle = Polyhedron(vertices=[(0,2), (-1,0), (1,0), (0,0)]) sage: triangle.vertices() (A vertex at (-1, 0), A vertex at (1, 0), A vertex at (0, 2))
.. SEEALSO::
If one only needs to keep track of a system of linear system of inequalities, one should also consider the class for mixed integer linear programming.
- :mod:`Mixed Integer Linear Programming <sage.numerical.mip>`
Unbounded Polyhedra -------------------
A polytope is defined as a bounded polyhedron. In this case, the minimal representation is unique and a vertex of the minimal representation is equivalent to a 0-dimensional face of the polytope. This is why one generally does not distinguish vertices and 0-dimensional faces. But for non-bounded polyhedra we have to allow for a more general notion of "vertex" in order to make sense of the Minkowsi sum presentation::
sage: half_plane = Polyhedron(ieqs=[(0,1,0)]) sage: half_plane.Hrepresentation() (An inequality (1, 0) x + 0 >= 0,) sage: half_plane.Vrepresentation() (A line in the direction (0, 1), A ray in the direction (1, 0), A vertex at (0, 0))
Note how we need a point in the above example to anchor the ray and line. But any point on the boundary of the half-plane would serve the purpose just as well. Sage picked the origin here, but this choice is not unique. Similarly, the choice of ray is arbitrary but necessary to generate the half-plane.
Finally, note that while rays and lines generate unbounded edges of the polyhedron they are not in a one-to-one correspondence with them. For example, the infinite strip has two infinite edges (1-faces) but only one generating line::
sage: strip = Polyhedron(vertices=[(1,0),(-1,0)], lines=[(0,1)]) sage: strip.Hrepresentation() (An inequality (1, 0) x + 1 >= 0, An inequality (-1, 0) x + 1 >= 0) sage: strip.lines() (A line in the direction (0, 1),) sage: strip.faces(1) (<0,1>, <0,2>) sage: for face in strip.faces(1): ....: print("{} = {}".format(face, face.as_polyhedron().Vrepresentation())) <0,1> = (A line in the direction (0, 1), A vertex at (-1, 0)) <0,2> = (A line in the direction (0, 1), A vertex at (1, 0))
EXAMPLES::
sage: trunc_quadr = Polyhedron(vertices=[[1,0],[0,1]], rays=[[1,0],[0,1]]) sage: trunc_quadr A 2-dimensional polyhedron in ZZ^2 defined as the convex hull of 2 vertices and 2 rays sage: v = next(trunc_quadr.vertex_generator()) # the first vertex in the internal enumeration sage: v A vertex at (0, 1) sage: v.vector() (0, 1) sage: list(v) [0, 1] sage: len(v) 2 sage: v[0] + v[1] 1 sage: v.is_vertex() True sage: type(v) <class 'sage.geometry.polyhedron.representation.Vertex'> sage: type( v() ) <type 'sage.modules.vector_integer_dense.Vector_integer_dense'> sage: v.polyhedron() A 2-dimensional polyhedron in ZZ^2 defined as the convex hull of 2 vertices and 2 rays sage: r = next(trunc_quadr.ray_generator()) sage: r A ray in the direction (0, 1) sage: r.vector() (0, 1) sage: list( v.neighbors() ) [A ray in the direction (0, 1), A vertex at (1, 0)]
Inequalities `A \vec{x} + b \geq 0` (and, similarly, equations) are specified by a list ``[b, A]``::
sage: Polyhedron(ieqs=[(0,1,0),(0,0,1),(1,-1,-1)]).Hrepresentation() (An inequality (-1, -1) x + 1 >= 0, An inequality (1, 0) x + 0 >= 0, An inequality (0, 1) x + 0 >= 0)
See :func:`Polyhedron` for a detailed description of all possible ways to construct a polyhedron.
Base Rings ----------
The base ring of the polyhedron can be specified by the ``base_ring`` optional keyword argument. If not specified, a suitable common base ring for all coordinates/coefficients will be chosen automatically. Important cases are:
* ``base_ring=QQ`` uses a fast implementation for exact rational numbers.
* ``base_ring=ZZ`` is similar to ``QQ``, but the resulting polyhedron object will have extra methods for lattice polyhedra.
* ``base_ring=RDF`` uses floating point numbers, this is fast but susceptible to numerical errors.
Polyhedra with symmetries often are defined over some algebraic field extension of the rationals. As a simple example, consider the equilateral triangle whose vertex coordinates involve `\sqrt{3}`. An exact way to work with roots in Sage is the :mod:`Algebraic Real Field <sage.rings.qqbar>` ::
sage: triangle = Polyhedron([(0,0), (1,0), (1/2, sqrt(3)/2)], base_ring=AA) sage: triangle.Hrepresentation() (An inequality (-1, -0.5773502691896258?) x + 1 >= 0, An inequality (1, -0.5773502691896258?) x + 0 >= 0, An inequality (0, 1.154700538379252?) x + 0 >= 0)
Without specifying the ``base_ring``, the ``sqrt(3)`` would be a symbolic ring element and, therefore, the polyhedron defined over the symbolic ring. This is currently not supported as SR is not exact::
sage: Polyhedron([(0,0), (1,0), (1/2, sqrt(3)/2)]) Traceback (most recent call last): ... ValueError: for polyhedra with floating point numbers, the only allowed ring is RDF with backend 'cdd'
sage: SR.is_exact() False
Even faster than all algebraic real numbers (the field ``AA``) is to take the smallest extension field. For the equilateral triangle, that would be::
sage: K.<sqrt3> = NumberField(x^2 - 3, embedding=AA(3)**(1/2)) sage: Polyhedron([(0,0), (1,0), (1/2, sqrt3/2)]) A 2-dimensional polyhedron in (Number Field in sqrt3 with defining polynomial x^2 - 3)^2 defined as the convex hull of 3 vertices
.. WARNING::
Be careful when you construct polyhedra with floating point numbers. The only available backend for such computation is `cdd` which uses machine floating point numbers which have have limited precision. If the input consists of floating point numbers and the `base_ring` is not specified, the base ring is set to be the `RealField` with the precision given by the minimal bit precision of the input. Then, if the obtained minimum is 53 bits of precision, the constructor converts automatically the base ring to `RDF`. Otherwise, it returns an error::
sage: Polyhedron(vertices = [[1.12345678901234, 2.12345678901234]]) A 0-dimensional polyhedron in RDF^2 defined as the convex hull of 1 vertex sage: Polyhedron(vertices = [[1.12345678901234, 2.123456789012345]]) A 0-dimensional polyhedron in RDF^2 defined as the convex hull of 1 vertex sage: Polyhedron(vertices = [[1.123456789012345, 2.123456789012345]]) Traceback (most recent call last): ... ValueError: for polyhedra with floating point numbers, the only allowed ring is RDF with backend 'cdd'
The strongly suggested method to input floating point numbers is to specify the `base_ring` to be `RDF`::
sage: Polyhedron(vertices = [[1.123456789012345, 2.123456789012345]], base_ring=RDF) A 0-dimensional polyhedron in RDF^2 defined as the convex hull of 1 vertex
.. SEEALSO::
:mod:`Parents for polyhedra <sage.geometry.polyhedron.parent.Polyhedra>`
Base classes ------------
Depending on the chosen base ring, a specific class is used to represent the polyhedron object.
.. SEEALSO::
- :mod:`Base class for polyhedra <sage.geometry.polyhedron.base.Polyhedron_base>` - :mod:`Base class for polyhedra over integers <sage.geometry.polyhedron.base_ZZ.Polyhedron_ZZ>` - :mod:`Base class for polyhedra over rationals <sage.geometry.polyhedron.base_QQ.Polyhedron_QQ>` - :mod:`Base class for polyhedra over RDF <sage.geometry.polyhedron.base_RDF.Polyhedron_RDF>`
The most important base class is **Base class for polyhedra** from which other base classes and backends inherit.
Backends --------
There are different backends available to deal with polyhedron objects.
.. SEEALSO::
- :mod:`cdd backend for polyhedra <sage.geometry.polyhedron.backend_cdd.Polyhedron_cdd>` - :mod:`field backend for polyhedra <sage.geometry.polyhedron.backend_field.Polyhedron_field>` - :mod:`normaliz backend for polyhedra <sage.geometry.polyhedron.backend_normaliz.Polyhedron_normaliz>` - :mod:`ppl backend for polyhedra <sage.geometry.polyhedron.backend_ppl.Polyhedron_ppl>`
.. NOTE::
Depending on the backend used, it may occur that different methods are available or not.
Appendix --------
REFERENCES:
Komei Fukuda's `FAQ in Polyhedral Computation <https://www.inf.ethz.ch/personal/fukudak/polyfaq/polyfaq.html>`_
AUTHORS:
- Marshall Hampton: first version, bug fixes, and various improvements, 2008 and 2009 - Arnaud Bergeron: improvements to triangulation and rendering, 2008 - Sebastien Barthelemy: documentation improvements, 2008 - Volker Braun: refactoring, handle non-compact case, 2009 and 2010 - Andrey Novoseltsev: added Hasse_diagram_from_incidences, 2010 - Volker Braun: rewrite to use PPL instead of cddlib, 2011 - Volker Braun: Add support for arbitrary subfields of the reals """
######################################################################## # Copyright (C) 2008 Marshall Hampton <hamptonio@gmail.com> # Copyright (C) 2014 Volker Braun <vbraun.name@gmail.com> # # Distributed under the terms of the GNU General Public License (GPL) # # http://www.gnu.org/licenses/ ######################################################################## from __future__ import print_function from __future__ import absolute_import
from sage.rings.all import QQ, ZZ, RDF, RR
from .misc import _make_listlist, _common_length_of
######################################################################### def Polyhedron(vertices=None, rays=None, lines=None, ieqs=None, eqns=None, ambient_dim=None, base_ring=None, minimize=True, verbose=False, backend=None): """ Construct a polyhedron object.
You may either define it with vertex/ray/line or inequalities/equations data, but not both. Redundant data will automatically be removed (unless ``minimize=False``), and the complementary representation will be computed.
INPUT:
- ``vertices`` -- list of point. Each point can be specified as any iterable container of ``base_ring`` elements. If ``rays`` or ``lines`` are specified but no ``vertices``, the origin is taken to be the single vertex.
- ``rays`` -- list of rays. Each ray can be specified as any iterable container of ``base_ring`` elements.
- ``lines`` -- list of lines. Each line can be specified as any iterable container of ``base_ring`` elements.
- ``ieqs`` -- list of inequalities. Each line can be specified as any iterable container of ``base_ring`` elements. An entry equal to ``[-1,7,3,4]`` represents the inequality `7x_1+3x_2+4x_3\geq 1`.
- ``eqns`` -- list of equalities. Each line can be specified as any iterable container of ``base_ring`` elements. An entry equal to ``[-1,7,3,4]`` represents the equality `7x_1+3x_2+4x_3= 1`.
- ``base_ring`` -- a sub-field of the reals implemented in Sage. The field over which the polyhedron will be defined. For ``QQ`` and algebraic extensions, exact arithmetic will be used. For ``RDF``, floating point numbers will be used. Floating point arithmetic is faster but might give the wrong result for degenerate input.
- ``ambient_dim`` -- integer. The ambient space dimension. Usually can be figured out automatically from the H/Vrepresentation dimensions.
- ``backend`` -- string or ``None`` (default). The backend to use. Valid choices are
* ``'cdd'``: use cdd (:mod:`~sage.geometry.polyhedron.backend_cdd`) with `\QQ` or `\RDF` coefficients depending on ``base_ring``.
* ``'normaliz'``: use normaliz (:mod:`~sage.geometry.polyhedron.backend_normaliz`) with `\ZZ` or `\QQ` coefficients depending on ``base_ring``.
* ``'polymake'``: use polymake (:mod:`~sage.geometry.polyhedron.backend_polymake`) with `\QQ`, `\RDF` or ``QuadraticField`` coefficients depending on ``base_ring``.
* ``'ppl'``: use ppl (:mod:`~sage.geometry.polyhedron.backend_ppl`) with `\ZZ` or `\QQ` coefficients depending on ``base_ring``.
* ``'field'``: use python implementation (:mod:`~sage.geometry.polyhedron.backend_field`) for any field
Some backends support further optional arguments:
- ``minimize`` -- boolean (default: ``True``). Whether to immediately remove redundant H/V-representation data. Currently not used.
- ``verbose`` -- boolean (default: ``False``). Whether to print verbose output for debugging purposes. Only supported by the cdd backends.
OUTPUT:
The polyhedron defined by the input data.
EXAMPLES:
Construct some polyhedra::
sage: square_from_vertices = Polyhedron(vertices = [[1, 1], [1, -1], [-1, 1], [-1, -1]]) sage: square_from_ieqs = Polyhedron(ieqs = [[1, 0, 1], [1, 1, 0], [1, 0, -1], [1, -1, 0]]) sage: list(square_from_ieqs.vertex_generator()) [A vertex at (1, -1), A vertex at (1, 1), A vertex at (-1, 1), A vertex at (-1, -1)] sage: list(square_from_vertices.inequality_generator()) [An inequality (1, 0) x + 1 >= 0, An inequality (0, 1) x + 1 >= 0, An inequality (-1, 0) x + 1 >= 0, An inequality (0, -1) x + 1 >= 0] sage: p = Polyhedron(vertices = [[1.1, 2.2], [3.3, 4.4]], base_ring=RDF) sage: p.n_inequalities() 2
The same polyhedron given in two ways::
sage: p = Polyhedron(ieqs = [[0,1,0,0],[0,0,1,0]]) sage: p.Vrepresentation() (A line in the direction (0, 0, 1), A ray in the direction (1, 0, 0), A ray in the direction (0, 1, 0), A vertex at (0, 0, 0)) sage: q = Polyhedron(vertices=[[0,0,0]], rays=[[1,0,0],[0,1,0]], lines=[[0,0,1]]) sage: q.Hrepresentation() (An inequality (1, 0, 0) x + 0 >= 0, An inequality (0, 1, 0) x + 0 >= 0)
Finally, a more complicated example. Take `\mathbb{R}_{\geq 0}^6` with coordinates `a, b, \dots, f` and
* The inequality `e+b \geq c+d` * The inequality `e+c \geq b+d` * The equation `a+b+c+d+e+f = 31`
::
sage: positive_coords = Polyhedron(ieqs=[ ....: [0, 1, 0, 0, 0, 0, 0], [0, 0, 1, 0, 0, 0, 0], [0, 0, 0, 1, 0, 0, 0], ....: [0, 0, 0, 0, 1, 0, 0], [0, 0, 0, 0, 0, 1, 0], [0, 0, 0, 0, 0, 0, 1]]) sage: P = Polyhedron(ieqs=positive_coords.inequalities() + ( ....: [0,0,1,-1,-1,1,0], [0,0,-1,1,-1,1,0]), eqns=[[-31,1,1,1,1,1,1]]) sage: P A 5-dimensional polyhedron in QQ^6 defined as the convex hull of 7 vertices sage: P.dim() 5 sage: P.Vrepresentation() (A vertex at (31, 0, 0, 0, 0, 0), A vertex at (0, 0, 0, 0, 0, 31), A vertex at (0, 0, 0, 0, 31, 0), A vertex at (0, 0, 31/2, 0, 31/2, 0), A vertex at (0, 31/2, 31/2, 0, 0, 0), A vertex at (0, 31/2, 0, 0, 31/2, 0), A vertex at (0, 0, 0, 31/2, 31/2, 0))
When the input contains elements of a Number Field, they require an embedding::
sage: K = NumberField(x^2-2,'s') sage: s = K.0 sage: L = NumberField(x^3-2,'t') sage: t = L.0 sage: P = Polyhedron(vertices = [[0,s],[t,0]]) Traceback (most recent call last): ... ValueError: invalid base ring
.. NOTE::
* Once constructed, a ``Polyhedron`` object is immutable.
* Although the option ``base_ring=RDF`` allows numerical data to be used, it might not give the right answer for degenerate input data - the results can depend upon the tolerance setting of cdd.
TESTS:
Check that giving ``float`` input gets converted to ``RDF`` (see :trac:`22605`)::
sage: f = float(1.1) sage: Polyhedron(vertices=[[f]]) A 0-dimensional polyhedron in RDF^1 defined as the convex hull of 1 vertex
Check that giving ``int`` input gets converted to ``ZZ`` (see :trac:`22605`)::
sage: Polyhedron(vertices=[[int(42)]]) A 0-dimensional polyhedron in ZZ^1 defined as the convex hull of 1 vertex
Check that giving ``Fraction`` input gets converted to ``QQ`` (see :trac:`22605`)::
sage: from fractions import Fraction sage: f = Fraction(int(6), int(8)) sage: Polyhedron(vertices=[[f]]) A 0-dimensional polyhedron in QQ^1 defined as the convex hull of 1 vertex
Check that input with too many bits of precision returns an error (see :trac:`22552`)::
sage: Polyhedron(vertices=[(8.3319544851638732, 7.0567045956967727), (6.4876921900819049, 4.8435898415984129)]) Traceback (most recent call last): ... ValueError: for polyhedra with floating point numbers, the only allowed ring is RDF with backend 'cdd'
Check that setting ``base_ring`` to a ``RealField`` returns an error (see :trac:`22552`)::
sage: Polyhedron(vertices =[(8.3, 7.0), (6.4, 4.8)], base_ring=RealField(40)) Traceback (most recent call last): ... ValueError: no appropriate backend for computations with Real Field with 40 bits of precision sage: Polyhedron(vertices =[(8.3, 7.0), (6.4, 4.8)], base_ring=RealField(53)) Traceback (most recent call last): ... ValueError: no appropriate backend for computations with Real Field with 53 bits of precision """ # Clean up the arguments
raise ValueError('cannot specify both H- and V-representation.') else: else:
# set ambient_dim raise ValueError('ambient space dimension mismatch. Try removing the "ambient_dim" parameter.')
# figure out base_ring
else: else:
else:
# TODO: remove this hack?
# Add the origin if necessary
# Specific backends can override the base_ring
# finally, construct the Polyhedron |