View source for 6-simplex

In [[Geometry|geometry]], a 6-[[Simplex|simplex]] is a [[Duality (mathematics)|self-dual]] [[Regular polytope|regular]] [[6-polytope|6-polytope]]. It has 7 [[Vertex (geometry)|vertices]], 21 [[Edge (geometry)|edge]]s, 35 triangle [[Face (geometry)|faces]], 35 [[Tetrahedron|tetrahedral]] cells,  21 [[5-cell|5-cell]] 4-faces, and 7 [[5-simplex|5-simplex]] 5-faces. Its [[Dihedral angle|dihedral angle]] is cos<sup>−1</sup>(1/6), or approximately 80.41°.

== Alternate names ==

It can also be called a '''heptapeton''', or '''hepta-6-tope''', as a 7-[[Facet (geometry)|facetted]] polytope in 6-dimensions. The [[5-polytope#A note on generality of terms for n-polytopes and elements|name]] ''heptapeton'' is derived from ''hepta'' for seven facets in [[Social:Greek language|Greek]] and [[Peta-|''-peta'']] for having five-dimensional facets, and ''-on''. Jonathan Bowers gives a heptapeton the acronym '''hop'''.<ref>{{KlitzingPolytopes|polypeta.htm|6D uniform polytopes (polypeta)|x3o3o3o3o3o — hop}}</ref>

== As a configuration==
This [[Regular 4-polytope#As configurations|configuration matrix]] represents the 6-simplex. The rows and columns correspond to vertices, edges, faces, cells, 4-faces and 5-faces. The diagonal numbers say how many of each element occur in the whole 6-simplex. The nondiagonal numbers say how many of the column's element occur in or at the row's element. This self-dual simplex's matrix is identical to its 180 degree rotation.<ref>{{harvnb|Coxeter|1973|loc=§1.8 Configurations}}</ref><ref>{{cite book |first=H.S.M. |last=Coxeter |title=Regular Complex Polytopes |url=https://books.google.com/books?id=9BY9AAAAIAAJ&pg=PA117 |pages=117 |edition=2nd |publisher=Cambridge University Press |year=1991 |isbn=9780521394901}}</ref>

<math>\begin{bmatrix}\begin{matrix}7 & 6 & 15 & 20 & 15 & 6 \\ 2 & 21 & 5 & 10 & 10 & 5 \\ 3 & 3 & 35 & 4 & 6 & 4 \\ 4 & 6 & 4 & 35 & 3 & 3 \\ 5 & 10 & 10 & 5 & 21 & 2 \\ 6 & 15 & 20 & 15 & 6 & 7 \end{matrix}\end{bmatrix}</math>

== Coordinates ==

The Cartesian coordinates for an origin-centered regular heptapeton having edge length&nbsp;2 are:

:<math>\left(\sqrt{1/21},\ \sqrt{1/15},\ \sqrt{1/10},\ \sqrt{1/6},\ \sqrt{1/3},\ \pm1\right)</math>
:<math>\left(\sqrt{1/21},\ \sqrt{1/15},\ \sqrt{1/10},\ \sqrt{1/6},\ -2\sqrt{1/3},\ 0\right)</math>
:<math>\left(\sqrt{1/21},\ \sqrt{1/15},\ \sqrt{1/10},\ -\sqrt{3/2},\ 0,\ 0\right)</math>
:<math>\left(\sqrt{1/21},\ \sqrt{1/15},\ -2\sqrt{2/5},\ 0,\ 0,\ 0\right)</math>
:<math>\left(\sqrt{1/21},\ -\sqrt{5/3},\ 0,\ 0,\ 0,\ 0\right)</math>
:<math>\left(-\sqrt{12/7},\ 0,\ 0,\ 0,\ 0,\ 0\right)</math>

The vertices of the ''6-simplex'' can be more simply positioned in 7-space as permutations of:
: (0,0,0,0,0,0,1)

This construction is based on [[Facet (geometry)|facets]] of the [[7-orthoplex|7-orthoplex]].

== Images ==

{{6-simplex Coxeter plane graphs|t0|150}}

== Related uniform 6-polytopes ==
The regular 6-simplex is one of 35 [[Uniform 6-polytope#The A6 .5B3.2C3.2C3.2C3.2C3.5D family .286-simplex.29|uniform 6-polytopes]] based on the [3,3,3,3,3] [[Coxeter group]], all shown here in A<sub>6</sub> Coxeter plane [[Orthographic projection|orthographic projection]]s.


==Notes ==
{{reflist}}

== References==
* [[Biography:Harold Scott MacDonald Coxeter|Coxeter, H.S.M.]]: 
** {{cite book |title-link=Regular Polytopes (book) |author-mask=1 |first=H.S.M. |last=Coxeter |chapter=Table I (iii): Regular Polytopes, three regular polytopes in n-dimensions (n≥5) |title=Regular Polytopes |publisher=Dover |edition=3rd |year=1973 |isbn=0-486-61480-8 |pages=296 }}
** {{cite book |editor-first=F. Arthur |editor-last=Sherk |editor2-first=Peter |editor2-last=McMullen |editor3-first=Anthony C. |editor3-last=Thompson |editor4-first=Asia Ivic |editor4-last=Weiss |title=Kaleidoscopes: Selected Writings of H.S.M. Coxeter |publisher=Wiley  |year=1995 |isbn=978-0-471-01003-6 |url=https://books.google.com/books?id=fUm5Mwfx8rAC&pg=PP1}}
*** (Paper 22) {{cite journal |author-mask=1 |first=H.S.M. |last=Coxeter |title=Regular and Semi Regular Polytopes I |journal=Math. Zeit. |volume=46 |pages=380–407 |year=1940 |doi=10.1007/BF01181449 |s2cid=186237114 |url=https://books.google.com/books?id=fUm5Mwfx8rAC&pg=PA251 }}
*** (Paper 23) {{cite journal |author-mask=1 |first=H.S.M. |last=Coxeter |title=Regular and Semi-Regular Polytopes II |journal=Math. Zeit. |volume=188 |pages=559–591 |year=1985 |issue=4 |doi=10.1007/BF01161657 |s2cid=120429557 |url=https://books.google.com/books?id=fUm5Mwfx8rAC&pg=PA279}}
*** (Paper 24) {{cite journal |author-mask=1 |first=H.S.M. |last=Coxeter |title=Regular and Semi-Regular Polytopes III |journal=Math. Zeit. |volume=200 |pages=3–45 |year=1988 |doi=10.1007/BF01161745 |s2cid=186237142 |url=https://books.google.com/books?id=fUm5Mwfx8rAC&pg=PA313}}
* {{cite book |first1=John H. |last1=Conway |first2=Heidi |last2=Burgiel |first3=Chaim |last3=Goodman-Strauss |chapter=26. Hemicubes: 1<sub>n1</sub> |title=The Symmetries of Things |year=2008 |isbn=978-1-56881-220-5 |pages=409 }}
* {{cite document |first=Norman |last=Johnson |title=Uniform Polytopes |date=1991|publisher= Norman Johnson (mathematician) |type=Manuscript }}
**  {{cite thesis |first=N.W. |last=Johnson |title=The Theory of Uniform Polytopes and Honeycombs |date=1966 |type=PhD |publisher=University of Toronto |url=https://search.library.utoronto.ca/details?402790 |oclc=258527038}}

== External links ==
*{{GlossaryForHyperspace | anchor=Simplex | title=Simplex }}
* [http://www.polytope.net/hedrondude/topes.htm Polytopes of Various Dimensions]
* [http://tetraspace.alkaline.org/glossary.htm Multi-dimensional Glossary]
{{Polytopes}}

[[Category:6-polytopes]]

{{Sourceattribution|6-simplex}}