A triangulation¶
In Sage, the
PointConfiguration
and Triangulation
satisfy a parent/element relationship. In
particular, each triangulation refers back to its point
configuration. If you want to triangulate a point configuration, you
should construct a point configuration first and then use one of its
methods to triangulate it according to your requirements. You should
never have to construct a Triangulation
object directly.
EXAMPLES:
First, we select the internal implementation for enumerating triangulations:
sage: PointConfiguration.set_engine('internal') # to make doctests independent of TOPCOM
Here is a simple example of how to triangulate a point configuration:
sage: p = [[0,1,1],[0,0,1],[0,1,0], [1,1,1],[1,0,1],[1,1,0]]
sage: points = PointConfiguration(p)
sage: triang = points.triangulate(); triang
(<0,1,2,5>, <0,1,3,5>, <1,3,4,5>)
sage: triang.plot(axes=False)
Graphics3d Object
See sage.geometry.triangulation.point_configuration
for more details.

class
sage.geometry.triangulation.element.
Triangulation
(triangulation, parent, check=True)¶ Bases:
sage.structure.element.Element
A triangulation of a
PointConfiguration
.Warning
You should never create
Triangulation
objects manually. Seetriangulate()
andtriangulations()
to triangulate point configurations.
adjacency_graph
()¶ Returns a graph showing which simplices are adjacent in the triangulation
OUTPUT:
A graph consisting of vertices referring to the simplices in the triangulation, and edges showing which simplices are adjacent to each other.
See also
 To obtain the triangulation’s 1skeleton, use
SimplicialComplex.graph()
throughMyTriangulation.simplicial_complex().graph()
.
AUTHORS:
 Stephen Farley (20130810): initial version
EXAMPLES:
sage: p = PointConfiguration([[1,0,0], [0,1,0], [0,0,1], [1,0,1], ....: [1,0,1], [1,0,0], [0,1,0], [0,0,1]]) sage: t = p.triangulate() sage: t.adjacency_graph() Graph on 8 vertices
 To obtain the triangulation’s 1skeleton, use

boundary
()¶ Return the boundary of the triangulation.
OUTPUT:
The outwardfacing boundary simplices (of dimension \(d1\)) of the \(d\)dimensional triangulation as a set. Each boundary is returned by a tuple of point indices.
EXAMPLES:
sage: triangulation = polytopes.cube().triangulate(engine='internal') sage: triangulation (<0,1,2,7>, <0,1,4,7>, <0,2,4,7>, <1,2,3,7>, <1,4,5,7>, <2,4,6,7>) sage: triangulation.boundary() frozenset({(0, 1, 2), (0, 1, 4), (0, 2, 4), (1, 2, 3), (1, 3, 7), (1, 4, 5), (1, 5, 7), (2, 3, 7), (2, 4, 6), (2, 6, 7), (4, 5, 7), (4, 6, 7)}) sage: triangulation.interior_facets() frozenset({(0, 1, 7), (0, 2, 7), (0, 4, 7), (1, 2, 7), (1, 4, 7), (2, 4, 7)})

enumerate_simplices
()¶ Return the enumerated simplices.
OUTPUT:
A tuple of integers that uniquely specifies the triangulation.
EXAMPLES:
sage: pc = PointConfiguration(matrix([ ....: [ 0, 0, 0, 0, 0, 2, 4,1, 1, 1, 0, 0, 1, 0], ....: [ 0, 0, 0, 1, 0, 0,1, 0, 0, 0, 0, 0, 0, 0], ....: [ 0, 2, 0, 0, 0, 0,1, 0, 1, 0, 1, 0, 0, 1], ....: [ 0, 1, 1, 0, 0, 1, 0,2, 1, 0, 0,1, 1, 1], ....: [ 0, 0, 0, 0, 1, 0,1, 0, 0, 0, 0, 0, 0, 0] ....: ]).columns()) sage: triangulation = pc.lexicographic_triangulation() sage: triangulation.enumerate_simplices() (1678, 1688, 1769, 1779, 1895, 1905, 2112, 2143, 2234, 2360, 2555, 2580, 2610, 2626, 2650, 2652, 2654, 2661, 2663, 2667, 2685, 2755, 2757, 2759, 2766, 2768, 2772, 2811, 2881, 2883, 2885, 2892, 2894, 2898)
You can recreate the triangulation from this list by passing it to the constructor:
sage: from sage.geometry.triangulation.point_configuration import Triangulation sage: Triangulation([1678, 1688, 1769, 1779, 1895, 1905, 2112, 2143, ....: 2234, 2360, 2555, 2580, 2610, 2626, 2650, 2652, 2654, 2661, 2663, ....: 2667, 2685, 2755, 2757, 2759, 2766, 2768, 2772, 2811, 2881, 2883, ....: 2885, 2892, 2894, 2898], pc) (<1,3,4,7,10,13>, <1,3,4,8,10,13>, <1,3,6,7,10,13>, <1,3,6,8,10,13>, <1,4,6,7,10,13>, <1,4,6,8,10,13>, <2,3,4,6,7,12>, <2,3,4,7,12,13>, <2,3,6,7,12,13>, <2,4,6,7,12,13>, <3,4,5,6,9,12>, <3,4,5,8,9,12>, <3,4,6,7,11,12>, <3,4,6,9,11,12>, <3,4,7,10,11,13>, <3,4,7,11,12,13>, <3,4,8,9,10,12>, <3,4,8,10,12,13>, <3,4,9,10,11,12>, <3,4,10,11,12,13>, <3,5,6,8,9,12>, <3,6,7,10,11,13>, <3,6,7,11,12,13>, <3,6,8,9,10,12>, <3,6,8,10,12,13>, <3,6,9,10,11,12>, <3,6,10,11,12,13>, <4,5,6,8,9,12>, <4,6,7,10,11,13>, <4,6,7,11,12,13>, <4,6,8,9,10,12>, <4,6,8,10,12,13>, <4,6,9,10,11,12>, <4,6,10,11,12,13>)

fan
(origin=None)¶ Construct the fan of cones over the simplices of the triangulation.
INPUT:
origin
–None
(default) or coordinates of a point. The common apex of all cones of the fan. IfNone
, the triangulation must be a star triangulation and the distinguished central point is used as the origin.
OUTPUT:
A
RationalPolyhedralFan
. The coordinates of the points are shifted so that the apex of the fan is the origin of the coordinate system.Note
If the set of cones over the simplices is not a fan, a suitable exception is raised.
EXAMPLES:
sage: pc = PointConfiguration([(0,0), (1,0), (0,1), (1,1)], star=0, fine=True) sage: triangulation = pc.triangulate() sage: fan = triangulation.fan(); fan Rational polyhedral fan in 2d lattice N sage: fan.is_equivalent( toric_varieties.P2().fan() ) True
Toric diagrams (the \(\ZZ_5\) hyperconifold):
sage: vertices=[(0, 1, 0), (0, 3, 1), (0, 2, 3), (0, 0, 2)] sage: interior=[(0, 1, 1), (0, 1, 2), (0, 2, 1), (0, 2, 2)] sage: points = vertices+interior sage: pc = PointConfiguration(points, fine=True) sage: triangulation = pc.triangulate() sage: fan = triangulation.fan( (1,0,0) ) sage: fan Rational polyhedral fan in 3d lattice N sage: fan.rays() N(1, 1, 0), N(1, 3, 1), N(1, 2, 3), N(1, 0, 2), N(1, 1, 1), N(1, 1, 2), N(1, 2, 1), N(1, 2, 2) in 3d lattice N

gkz_phi
()¶ Calculate the GKZ phi vector of the triangulation.
The phi vector is a vector of length equals to the number of points in the point configuration. For a fixed triangulation \(T\), the entry corresponding to the \(i\)th point \(p_i\) is
\[\phi_T(p_i) = \sum_{t\in T, t\owns p_i} Vol(t)\]that is, the total volume of all simplices containing \(p_i\). See also [GKZ1994] page 220 equation 1.4.
OUTPUT:
The phi vector of self.
EXAMPLES:
sage: p = PointConfiguration([[0,0],[1,0],[2,1],[1,2],[0,1]]) sage: p.triangulate().gkz_phi() (3, 1, 5, 2, 4) sage: p.lexicographic_triangulation().gkz_phi() (1, 3, 4, 2, 5)

interior_facets
()¶ Return the interior facets of the triangulation.
OUTPUT:
The inwardfacing boundary simplices (of dimension \(d1\)) of the \(d\)dimensional triangulation as a set. Each boundary is returned by a tuple of point indices.
EXAMPLES:
sage: triangulation = polytopes.cube().triangulate(engine='internal') sage: triangulation (<0,1,2,7>, <0,1,4,7>, <0,2,4,7>, <1,2,3,7>, <1,4,5,7>, <2,4,6,7>) sage: triangulation.boundary() frozenset({(0, 1, 2), (0, 1, 4), (0, 2, 4), (1, 2, 3), (1, 3, 7), (1, 4, 5), (1, 5, 7), (2, 3, 7), (2, 4, 6), (2, 6, 7), (4, 5, 7), (4, 6, 7)}) sage: triangulation.interior_facets() frozenset({(0, 1, 7), (0, 2, 7), (0, 4, 7), (1, 2, 7), (1, 4, 7), (2, 4, 7)})

normal_cone
()¶ Return the (closure of the) normal cone of the triangulation.
Recall that a regular triangulation is one that equals the “crease lines” of a convex piecewiselinear function. This support function is not unique, for example, you can scale it by a positive constant. The set of all piecewiselinear functions with fixed creases forms an open cone. This cone can be interpreted as the cone of normal vectors at a point of the secondary polytope, which is why we call it normal cone. See [GKZ1994] Section 7.1 for details.
OUTPUT:
The closure of the normal cone. The \(i\)th entry equals the value of the piecewiselinear function at the \(i\)th point of the configuration.
For an irregular triangulation, the normal cone is empty. In this case, a single point (the origin) is returned.
EXAMPLES:
sage: triangulation = polytopes.hypercube(2).triangulate(engine='internal') sage: triangulation (<0,1,3>, <0,2,3>) sage: N = triangulation.normal_cone(); N 4d cone in 4d lattice sage: N.rays() (1, 0, 0, 0), ( 1, 0, 1, 0), (1, 0, 1, 0), ( 1, 0, 0, 1), (1, 0, 0, 1), ( 1, 1, 0, 0), (1, 1, 0, 0) in Ambient free module of rank 4 over the principal ideal domain Integer Ring sage: N.dual().rays() (1, 1, 1, 1) in Ambient free module of rank 4 over the principal ideal domain Integer Ring

plot
(**kwds)¶ Produce a graphical representation of the triangulation.
EXAMPLES:
sage: p = PointConfiguration([[0,0],[0,1],[1,0],[1,1],[1,1]]) sage: triangulation = p.triangulate() sage: triangulation (<1,3,4>, <2,3,4>) sage: triangulation.plot(axes=False) Graphics object consisting of 12 graphics primitives

point_configuration
()¶ Returns the point configuration underlying the triangulation.
EXAMPLES:
sage: pconfig = PointConfiguration([[0,0],[0,1],[1,0]]) sage: pconfig A point configuration in affine 2space over Integer Ring consisting of 3 points. The triangulations of this point configuration are assumed to be connected, not necessarily fine, not necessarily regular. sage: triangulation = pconfig.triangulate() sage: triangulation (<0,1,2>) sage: triangulation.point_configuration() A point configuration in affine 2space over Integer Ring consisting of 3 points. The triangulations of this point configuration are assumed to be connected, not necessarily fine, not necessarily regular. sage: pconfig == triangulation.point_configuration() True

simplicial_complex
()¶ Return a simplicial complex from a triangulation of the point configuration.
OUTPUT:
EXAMPLES:
sage: p = polytopes.cuboctahedron() sage: sc = p.triangulate(engine='internal').simplicial_complex() sage: sc Simplicial complex with 12 vertices and 16 facets
Any convex set is contractable, so its reduced homology groups vanish:
sage: sc.homology() {0: 0, 1: 0, 2: 0, 3: 0}


sage.geometry.triangulation.element.
triangulation_render_2d
(triangulation, **kwds)¶ Return a graphical representation of a 2d triangulation.
INPUT:
triangulation
– aTriangulation
.**kwds
– keywords that are passed on to the graphics primitives.
OUTPUT:
A 2d graphics object.
EXAMPLES:
sage: points = PointConfiguration([[0,0],[0,1],[1,0],[1,1],[1,1]]) sage: triang = points.triangulate() sage: triang.plot(axes=False, aspect_ratio=1) # indirect doctest Graphics object consisting of 12 graphics primitives

sage.geometry.triangulation.element.
triangulation_render_3d
(triangulation, **kwds)¶ Return a graphical representation of a 3d triangulation.
INPUT:
triangulation
– aTriangulation
.**kwds
– keywords that are passed on to the graphics primitives.
OUTPUT:
A 3d graphics object.
EXAMPLES:
sage: p = [[0,1,1],[0,0,1],[0,1,0], [1,1,1],[1,0,1],[1,1,0]] sage: points = PointConfiguration(p) sage: triang = points.triangulate() sage: triang.plot(axes=False) # indirect doctest Graphics3d Object