# 6-polytope

 6-simplex 6-orthoplex, 311 6-cube (Hexeract) 221 Expanded 6-simplex Rectified 6-orthoplex 6-demicube 131 (Demihexeract) 122

In six-dimensional geometry, a six-dimensional polytope or 6-polytope is a polytope, bounded by 5-polytope facets.

## Definition

A 6-polytope is a closed six-dimensional figure with vertices, edges, faces, cells (3-faces), 4-faces, and 5-faces. A vertex is a point where six or more edges meet. An edge is a line segment where four or more faces meet, and a face is a polygon where three or more cells meet. A cell is a polyhedron. A 4-face is a polychoron, and a 5-face is a 5-polytope. Furthermore, the following requirements must be met:

• Each 4-face must join exactly two 5-faces (facets).
• Adjacent facets are not in the same five-dimensional hyperplane.
• The figure is not a compound of other figures which meet the requirements.

## Characteristics

The topology of any given 6-polytope is defined by its Betti numbers and torsion coefficients.[1]

The value of the Euler characteristic used to characterise polyhedra does not generalize usefully to higher dimensions, and is zero for all 6-polytopes, whatever their underlying topology. This inadequacy of the Euler characteristic to reliably distinguish between different topologies in higher dimensions led to the discovery of the more sophisticated Betti numbers.[1]

Similarly, the notion of orientability of a polyhedron is insufficient to characterise the surface twistings of toroidal polytopes, and this led to the use of torsion coefficients.[1]

## Classification

6-polytopes may be classified by properties like "convexity" and "symmetry".

• A 6-polytope is convex if its boundary (including its 5-faces, 4-faces, cells, faces and edges) does not intersect itself and the line segment joining any two points of the 6-polytope is contained in the 6-polytope or its interior; otherwise, it is non-convex. Self-intersecting 6-polytope are also known as star 6-polytopes, from analogy with the star-like shapes of the non-convex Kepler-Poinsot polyhedra.
• A regular 6-polytope has all identical regular 5-polytope facets. All regular 6-polytope are convex.
Main article: Uniform 6-polytope
• A prismatic 6-polytope is constructed by the Cartesian product of two lower-dimensional polytopes. A prismatic 6-polytope is uniform if its factors are uniform. The 6-cube is prismatic (product of a squares and a cube), but is considered separately because it has symmetries other than those inherited from its factors.
• A 5-space tessellation is the division of five-dimensional Euclidean space into a regular grid of 5-polytope facets. Strictly speaking, tessellations are not 6-polytopes as they do not bound a "6D" volume, but we include them here for the sake of completeness because they are similar in many ways to 6-polytope. A uniform 5-space tessellation is one whose vertices are related by a space group and whose facets are uniform 5-polytopes.

## Regular 6-polytopes

Regular 6-polytopes can be generated from Coxeter groups represented by the Schläfli symbol {p,q,r,s,t} with t {p,q,r,s} 5-polytope facets around each cell.

There are only three such convex regular 6-polytopes:

There are no nonconvex regular polytopes of 5 or more dimensions.

For the 3 convex regular 6-polytopes, their elements are:

Name Schläfli
symbol
Coxeter
diagram
Vertices Edges Faces Cells 4-faces 5-faces Symmetry (order)
6-simplex {3,3,3,3,3} 7 21 35 35 21 7 A6 (720)
6-orthoplex {3,3,3,3,4} 12 60 160 240 192 64 B6 (46080)
6-cube {4,3,3,3,3} 64 192 240 160 60 12 B6 (46080)

## Uniform 6-polytopes

Main article: Uniform 6-polytope

Here are six simple uniform convex 6-polytopes, including the 6-orthoplex repeated with its alternate construction.

Name Schläfli
symbol(s)
Coxeter
diagram(s)
Vertices Edges Faces Cells 4-faces 5-faces Symmetry (order)
Expanded 6-simplex t0,5{3,3,3,3,3} 42 210 490 630 434 126 2×A6 (1440)
6-orthoplex, 311
(alternate construction)
{3,3,3,31,1} 12 60 160 240 192 64 D6 (23040)
6-demicube {3,33,1}
h{4,3,3,3,3}

32 240 640 640 252 44 D6 (23040)
½B6
Rectified 6-orthoplex t1{3,3,3,3,4}
t1{3,3,3,31,1}

60 480 1120 1200 576 76 B6 (46080)
2×D6
221 polytope {3,3,32,1} 27 216 720 1080 648 99 E6 (51840)
122 polytope {3,32,2}
or
72 720 2160 2160 702 54 2×E6 (103680)

The expanded 6-simplex is the vertex figure of the uniform 6-simplex honeycomb, . The 6-demicube honeycomb, , vertex figure is a rectified 6-orthoplex and facets are the 6-orthoplex and 6-demicube. The uniform 222 honeycomb,, has 122 polytope is the vertex figure and 221 facets.

## References

1. ^ a b c Richeson, D.; Euler's Gem: The Polyhedron Formula and the Birth of Topoplogy, Princeton, 2008.
• T. Gosset: On the Regular and Semi-Regular Figures in Space of n Dimensions, Messenger of Mathematics, Macmillan, 1900
• A. Boole Stott: Geometrical deduction of semiregular from regular polytopes and space fillings, Verhandelingen of the Koninklijke academy van Wetenschappen width unit Amsterdam, Eerste Sectie 11,1, Amsterdam, 1910
• H.S.M. Coxeter:
• H.S.M. Coxeter, M.S. Longuet-Higgins und J.C.P. Miller: Uniform Polyhedra, Philosophical Transactions of the Royal Society of London, Londne, 1954
• H.S.M. Coxeter, Regular Polytopes, 3rd Edition, Dover New York, 1973
• Kaleidoscopes: Selected Writings of H.S.M. Coxeter, edited by F. Arthur Sherk, Peter McMullen, Anthony C. Thompson, Asia Ivic Weiss, Wiley-Interscience Publication, 1995, ISBN 978-0-471-01003-6 [1]
• (Paper 22) H.S.M. Coxeter, Regular and Semi Regular Polytopes I, [Math. Zeit. 46 (1940) 380-407, MR 2,10]
• (Paper 23) H.S.M. Coxeter, Regular and Semi-Regular Polytopes II, [Math. Zeit. 188 (1985) 559-591]
• (Paper 24) H.S.M. Coxeter, Regular and Semi-Regular Polytopes III, [Math. Zeit. 200 (1988) 3-45]
• N.W. Johnson: The Theory of Uniform Polytopes and Honeycombs, Ph.D. Dissertation, University of Toronto, 1966
• Klitzing, Richard. "6D uniform polytopes (polypeta)".