# Orthoplex

(Redirected from Cross polytope)
n -orthoplex
Rankn
TypeRegular
Notation
Coxeter diagramo4o3o3...3o3x
(...)
Schläfli symbol{3,3, ... 3,4}
Elements
Facets${\displaystyle 2^{n}}$ (n  − 1)-simplices
Vertices2n
Vertex figure(n  − 1)-orthoplex, edge length 1
Measures (edge length 1)
Circumradius${\displaystyle {\frac {\sqrt {2}}{2}}}$
Inradius${\displaystyle {\frac {1}{\sqrt {2n}}}}$
Volume${\displaystyle {\frac {\sqrt {2^{n}}}{n!}}}$
Height${\displaystyle {\frac {2}{\sqrt {2n}}}}$
Central density1
Number of external pieces${\displaystyle 2^{n}}$
Level of complexity1
Related polytopes
DualHypercube
ConjugateNone
Abstract & topological properties
Flag count${\displaystyle n!*2^{n}}$
Euler characteristic0 if n  even
2 if n  odd
OrientableYes
Properties
SymmetryBn, order ${\displaystyle n!*2^{n}}$
ConvexYes
NatureTame

An orthoplex (plural orthoplices or orthoplexes), cross-polytope, or hyperoctahedron is the simplest center-symmetric polytope in each respective dimension, by vertex count. The n -dimensional orthoplex, or simply the n -orthoplex, has 2n  vertices lying in n  opposite pairs, connected by each of the 2n  (n  − 1)-simplices containing exactly one vertex from each pair. Alternatively, one can construct each orthoplex as the bipyramid of the orthoplex of the lower dimension. The tegum product of an m -orthoplex and an n -orthoplex is an (m  + n )-orthoplex. An n -orthoplex can also be constructed as the antiprism of the (n  − 1)-simplex. They are the only polytopes is 3+ dimensions that are both quasiregular and regular other than the hypercubic honeycombs.

Every orthoplex can be made regular; in fact, it’s rare for the term to be used to refer to non-regular shapes. As such, the orthoplexes comprise one of the three infinite families of regular polytopes that exist in every dimension, the other two being the simplexes and the hypercubes (the duals of the orthoplexes).

All orthoplexes are step prisms derived from the compound of two simplices in two inverted positions.

## Naming

The name orthoplex was coined by John Horton Conway and Neil Sloane from "orthant complex", alluding to the fact that each facet of a centered orthoplex in usual orientation lies in a different orthant (the generalization of a quadrant to higher dimensions) of its space.[1] Alternate names include:

• Cross-polytope, alluding to the cross shape formed by the lines from the center of an orthoplex to its vertices.
• Hyperoctahedron, as a generalized octahedron.

## Elements

All of the elements of an orthoplex, besides the orthoplex itself, are simplices. For n  < d , the number of d -elements in an n -orthoplex is given by the the binomial coefficient 2d  + 1C(n , d  + 1). This is because any choice of d  + 1 vertices, no two opposite, define a unique d -simplex through them.

In particular, an n -orthoplex has 2n  vertices and 2n  facets, each shaped like an (n  − 1)-simplex, with the vertex figure being the orthoplex of the previous dimension. In total, an n -orthoplex has 3n  + 1 elements, including both improper elements.

## Examples

Excluding the point, the orthoplexes up to 10D are the following:

Rank Name Picture Rank 1 Dyad 6 Hexacontatetrapeton 2 Square 7 Hecatonicosoctaexon 3 Octahedron 8 Diacosipentacontahexazetton 4 Hexadecachoron 9 Pentacosidodecayotton 5 Triacontaditeron 10 Chiliaicositetraxennon

## Vertex coordinates

Coordinates for the vertices of an n -orthoplex with edge length 1 are given by all permutations of:

• 2/2, 0, ..., 0),

where the last n –1 entries are zeros.

## Measures

• The circumradius of an n -orthoplex of unit edge length is ${\displaystyle {\frac {\sqrt {2}}{2}}}$, regardless of n .
• This same orthoplex's inradius is given by ${\displaystyle {\frac {1}{\sqrt {2n}}}}$.
• Its height from a facet to the opposite facet is given by twice the inradius, that is ${\displaystyle {\frac {2}{\sqrt {2n}}}}$.
• Its hypervolume is given by ${\displaystyle {\frac {\sqrt {2^{n}}}{n!}}}$.
• The angle between two facet hyperplanes is ${\displaystyle \arccos {\left({\frac {2}{n}}-1\right)}}$.

## References

1. Conway, J. H.; Sloane, N. J. A. (1991). "The Cell Structures of Certain Lattices". In Hilton, P.; Hirzebruch, F.; Remmert, R. (eds.). Miscellanea Mathematica. Berlin: Springer. pp. 89–90. doi:10.1007/978-3-642-76709-8_5. ISBN 978-3-642-76711-1.