Cross-polytope
| A 2-dimensional cross-polytope | A 3-dimensional cross-polytope |
| 2 dimensions square |
3 dimensions octahedron |
| A 4-dimensional cross-polytope | A 5-dimensional cross-polytope |
| 4 dimensions 16-cell |
5 dimensions 5-orthoplex |
In geometry, a cross-polytope,Template:Sfn hyperoctahedron, orthoplex,<ref>Template:Cite book</ref> staurotope,<ref>Template:Cite book</ref> or cocube is a regular, convex polytope that exists in n-dimensional Euclidean space. A 2-dimensional cross-polytope is a square, a 3-dimensional cross-polytope is a regular octahedron, and a 4-dimensional cross-polytope is a 16-cell. Its facets are simplexes of the previous dimension, while the cross-polytope's vertex figure is another cross-polytope from the previous dimension.
The vertices of a cross-polytope can be chosen as the unit vectors pointing along each co-ordinate axis – i.e. all the permutations of Template:Nowrap. The cross-polytope is the convex hull of its vertices. The n-dimensional cross-polytope can also be defined as the closed unit ball (or, according to some authors, its boundary) in the ℓ1-norm on Rn:
- <math>\{x\in\mathbb R^n : \|x\|_1 \le 1\}.</math>
In 1 dimension the cross-polytope is simply the line segment [−1, +1], in 2 dimensions it is a square (or diamond) with vertices {(±1, 0), (0, ±1)}. In 3 dimensions it is an octahedron—one of the five convex regular polyhedra known as the Platonic solids. This can be generalised to higher dimensions with an n-orthoplex being constructed as a bipyramid with an (n−1)-orthoplex base.
The cross-polytope is the dual polytope of the hypercube. The 1-skeleton of an n-dimensional cross-polytope is the Turán graph T(2n, n) (also known as a cocktail party graph <ref>Template:Mathworld</ref>).
4 dimensionsEdit
The 4-dimensional cross-polytope also goes by the name hexadecachoron or 16-cell. It is one of the six convex regular 4-polytopes. These 4-polytopes were first described by the Swiss mathematician Ludwig Schläfli in the mid-19th century.
The vertices of the 4-dimensional hypercube, or tesseract, can be divided into two sets of eight, the convex hull of each set forming a cross-polytope. Moreover, the polytope known as the 24-cell can be constructed by symmetrically arranging three cross-polytopes.<ref>Template:Cite book</ref>
Higher dimensionsEdit
The cross-polytope family is one of three regular polytope families, labeled by Coxeter as βn, the other two being the hypercube family, labeled as γn, and the simplex family, labeled as αn. A fourth family, the infinite tessellations of hypercubes, he labeled as δn.Template:Sfn
The n-dimensional cross-polytope has 2n vertices, and 2n facets ((n − 1)-dimensional components) all of which are (n − 1)-simplices. The vertex figures are all (n − 1)-cross-polytopes. The Schläfli symbol of the cross-polytope is {3,3,...,3,4}.
The dihedral angle of the n-dimensional cross-polytope is <math>\delta_n = \arccos\left(\frac{2-n}{n}\right)</math>. This gives: δ2 = arccos(0/2) = 90°, δ3 = arccos(−1/3) = 109.47°, δ4 = arccos(−2/4) = 120°, δ5 = arccos(−3/5) = 126.87°, ... δ∞ = arccos(−1) = 180°.
The hypervolume of the n-dimensional cross-polytope is
- <math>\frac{2^n}{n!}.</math>
For each pair of non-opposite vertices, there is an edge joining them. More generally, each set of k + 1 orthogonal vertices corresponds to a distinct k-dimensional component which contains them. The number of k-dimensional components (vertices, edges, faces, ..., facets) in an n-dimensional cross-polytope is thus given by (see binomial coefficient):
- <math>2^{k+1}{n \choose {k+1}}</math>Template:Sfn
The extended f-vector for an n-orthoplex can be computed by (1,2)n, like the coefficients of polynomial products. For example a 16-cell is (1,2)4 = (1,4,4)2 = (1,8,24,32,16).
There are many possible orthographic projections that can show the cross-polytopes as 2-dimensional graphs. Petrie polygon projections map the points into a regular 2n-gon or lower order regular polygons. A second projection takes the 2(n−1)-gon petrie polygon of the lower dimension, seen as a bipyramid, projected down the axis, with 2 vertices mapped into the center.
| n | βn k11 |
Name(s) Graph |
Graph 2n-gon |
Schläfli | Coxeter-Dynkin diagrams |
Vertices | Edges | Faces | Cells | 4-faces | 5-faces | 6-faces | 7-faces | 8-faces | 9-faces | 10-faces |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 0 | β0 | Point 0-orthoplex |
. | ( ) | Template:CDD |
1 | ||||||||||
| 1 | β1 | Line segment 1-orthoplex |
File:Cross graph 1.svg | { } | Template:CDD Template:CDD |
2 | 1 | |||||||||
| 2 | β2 −111 |
Square 2-orthoplex Bicross |
File:Cross graph 2.png | {4} 2{ } = { }+{ } |
Template:CDD Template:CDD |
4 | 4 | 1 | ||||||||
| 3 | β3 011 |
Octahedron 3-orthoplex Tricross |
File:3-orthoplex.svg | {3,4} {31,1} 3{ } |
Template:CDD Template:CDD Template:CDD |
6 | 12 | 8 | 1 | |||||||
| 4 | β4 111 |
16-cell 4-orthoplex Tetracross |
File:4-orthoplex.svg | {3,3,4} {3,31,1} 4{ } |
Template:CDD Template:CDD Template:CDD |
8 | 24 | 32 | 16 | 1 | ||||||
| 5 | β5 211 |
5-orthoplex Pentacross |
File:5-orthoplex.svg | {33,4} {3,3,31,1} 5{ } |
Template:CDD Template:CDD Template:CDD |
10 | 40 | 80 | 80 | 32 | 1 | |||||
| 6 | β6 311 |
6-orthoplex Hexacross |
File:6-orthoplex.svg | {34,4} {33,31,1} 6{ } |
Template:CDD Template:CDD Template:CDD |
12 | 60 | 160 | 240 | 192 | 64 | 1 | ||||
| 7 | β7 411 |
7-orthoplex Heptacross |
File:7-orthoplex.svg | {35,4} {34,31,1} 7{ } |
Template:CDD Template:CDD Template:CDD |
14 | 84 | 280 | 560 | 672 | 448 | 128 | 1 | |||
| 8 | β8 511 |
8-orthoplex Octacross |
File:8-orthoplex.svg | {36,4} {35,31,1} 8{ } |
Template:CDD Template:CDD Template:CDD |
16 | 112 | 448 | 1120 | 1792 | 1792 | 1024 | 256 | 1 | ||
| 9 | β9 611 |
9-orthoplex Enneacross |
File:9-orthoplex.svg | {37,4} {36,31,1} 9{ } |
Template:CDD Template:CDD Template:CDD |
18 | 144 | 672 | 2016 | 4032 | 5376 | 4608 | 2304 | 512 | 1 | |
| 10 | β10 711 |
10-orthoplex Decacross |
File:10-orthoplex.svg | {38,4} {37,31,1} 10{ } |
Template:CDD Template:CDD Template:CDD |
20 | 180 | 960 | 3360 | 8064 | 13440 | 15360 | 11520 | 5120 | 1024 | 1 |
| ... | ||||||||||||||||
| n | βn k11 |
n-orthoplex n-cross |
{3n − 2,4} {3n − 3,31,1} n{} |
Template:CDD...Template:CDD Template:CDD...Template:CDD Template:CDD...Template:CDD |
2n 0-faces, ... <math>2^{k+1}{n\choose k+1}</math> k-faces ..., 2n (n−1)-faces | |||||||||||
The vertices of an axis-aligned cross polytope are all at equal distance from each other in the Manhattan distance (L1 norm). Kusner's conjecture states that this set of 2d points is the largest possible equidistant set for this distance.<ref>Template:Citation.</ref>
Generalized orthoplexEdit
Regular complex polytopes can be defined in complex Hilbert space called generalized orthoplexes (or cross polytopes), βTemplate:Supsub = 2{3}2{3}...2{4}p, or Template:CDD..Template:CDD. Real solutions exist with p = 2, i.e. βTemplate:Supsub = βn = 2{3}2{3}...2{4}2 = {3,3,..,4}. For p > 2, they exist in <math>\mathbb{\Complex}^n</math>. A p-generalized n-orthoplex has pn vertices. Generalized orthoplexes have regular simplexes (real) as facets.<ref>Coxeter, Regular Complex Polytopes, p. 108</ref> Generalized orthoplexes make complete multipartite graphs, βTemplate:Supsub make Kp,p for complete bipartite graph, βTemplate:Supsub make Kp,p,p for complete tripartite graphs. βTemplate:Supsub creates Kpn. An orthogonal projection can be defined that maps all the vertices equally-spaced on a circle, with all pairs of vertices connected, except multiples of n. The regular polygon perimeter in these orthogonal projections is called a petrie polygon.
Related polytope familiesEdit
Cross-polytopes can be combined with their dual cubes to form compound polytopes:
- In two dimensions, we obtain the octagrammic star figure Template:Mset,
- In three dimensions we obtain the compound of cube and octahedron,
- In four dimensions we obtain the compound of tesseract and 16-cell.
See alsoEdit
- List of regular polytopes
- Hyperoctahedral group, the symmetry group of the cross-polytope
CitationsEdit
ReferencesEdit
- Template:Cite book
- pp. 121-122, §7.21. see illustration Fig 7.2B
- p. 296, Table I (iii): Regular Polytopes, three regular polytopes in n-dimensions (n≥5)