# Simplex

n -simplex
Rankn
TypeRegular
Notation
Coxeter diagramx3o3o3...3o (...)
Schläfli symbol{3,3,3, ..., ,3}
Elements
Vertices${\displaystyle n+1}$
Vertex figure(n  – 1)-simplex, edge length 1
Petrie polygons${\displaystyle (n-1)!/2}$ simplicially embedded n -gons
Measures (edge length 1)
Circumradius${\displaystyle {\sqrt {\frac {n}{2n+2}}}}$
Inradius${\displaystyle {\frac {1}{\sqrt {2n\left(n+1\right)}}}}$
Volume${\displaystyle {\frac {\sqrt {(n+1)/2^{n}}}{n!}}}$
Height${\displaystyle {\sqrt {\frac {n+1}{2n}}}}$
Central density1
Number of external pieces${\displaystyle n+1}$
Level of complexity1
Related polytopes
Dualn -simplex
ConjugateNone
Abstract & topological properties
Flag count${\displaystyle \left(n+1\right)!}$
Euler characteristic0 if n  even
2 if n  odd
OrientableYes
Properties
SymmetryAn , order ${\displaystyle \left(n+1\right)!}$
Flag orbits1
ConvexYes
NatureTame

A simplex (plural simplices or simplexes) is a polytope generalizing the notion of the triangle, tetrahedron, pentachoron, etc. to arbitrary dimensions. It is the simplest possible non-degenerate n-polytope in n-dimensional Euclidean space.

The n -dimensional simplex, or simply n -simplex, consists of n  + 1 vertices, with each n  of them joined in the unique manner by a simplex of the lower dimension. Alternatively, one may construct an n -simplex as the pyramid of the (n  – 1)-simplex. All possible realizations of simplexes in Euclidean space produce convex and self-dual polytopes.

Every simplex is abstractly regular and can also be realized as geometrically regular. As such, the simplexes comprise one of the three infinite families of regular polytopes that exist in every dimension, the other two being the hypercubes and the orthoplexes.

## Elements

All of the elements of a simplex are simplexes themselves. The number of d -dimensional elements in an n -dimensional simplex is given by the binomial coefficient ${\displaystyle C(n+1,d+1)}$. This is because any choice of d +1 vertices defines a unique d -dimensional simplex. In particular, an n -dimensional simplex has n +1 vertices and n +1 facets (each shaped like an (n -1)-dimensional simplex), and its vertex figure is also the simplex of the previous dimension.

In total, an n -simplex has 2n  elements, including the nullitope and the bulk of the polytope.

## Examples

Excluding the degenerate nullitope and the point, the simplexes up to 10D are the following:

Dimension Name Picture Dimension 1 Dyad 6 Heptapeton 2 Triangle 7 Octaexon 3 Tetrahedron 8 Enneazetton 4 Pentachoron 9 Decayotton 5 Hexateron 10 Hendecaxennon

## Vertex coordinates

Coordinates for the vertices of an n -simplex of edge length 1, centered at the origin, are given by:

• ${\displaystyle \left(-{\frac {1}{2}},\,-{\frac {\sqrt {3}}{6}},\,-{\frac {\sqrt {6}}{12}},\,-{\frac {\sqrt {10}}{20}},\,\ldots ,\,-{\frac {1}{\sqrt {2n\left(n+1\right)}}}\right)}$,
• ${\displaystyle \left({\frac {1}{2}},\,-{\frac {\sqrt {3}}{6}},\,-{\frac {\sqrt {6}}{12}},\,-{\frac {\sqrt {10}}{20}},\,\ldots ,\,-{\frac {1}{\sqrt {2n\left(n+1\right)}}}\right)}$,
• ${\displaystyle \left(0,\,{\frac {\sqrt {3}}{3}},\,-{\frac {\sqrt {6}}{12}},\,-{\frac {\sqrt {10}}{20}},\,\ldots ,\,-{\frac {1}{\sqrt {2n\left(n+1\right)}}}\right)}$,
• ${\displaystyle \left(0,\,0,\,{\frac {\sqrt {6}}{4}},\,-{\frac {\sqrt {10}}{20}},\,\ldots ,\,-{\frac {1}{\sqrt {2n\left(n+1\right)}}}\right)}$,
• ${\displaystyle \left(0,\,0,\,0,\,{\frac {\sqrt {10}}{5}},\,\ldots ,\,-{\frac {1}{\sqrt {2n\left(n+1\right)}}}\right)}$,
• ${\displaystyle \ldots }$
• ${\displaystyle \left(0,\,0,\,0,\,0,\,\ldots ,\,{\sqrt {\frac {n}{2n+2}}}\right)}$.

Alternatively, there are simpler coordinates but the simplex is not centered at the origin:

• all permutations of ${\displaystyle \left({\frac {\sqrt {2}}{2}},\,0,\,0,\,...,\,0\right)}$
• ${\displaystyle (a,\,a,\,a,\,...,\,a)}$ where ${\displaystyle a={\frac {\left(1-{\sqrt {n+1}}\right){\sqrt {2}}}{2n}}}$.

Much simpler coordinates can be given in n  + 1 dimensions, as all permutations of

• ${\displaystyle \left({\frac {\sqrt {2}}{2}},\,0,\,0,\,0,\,...,\,0\right)}$.

## Related polytopes

The pyramid product of an m -dimensional simplex and an n -dimensional simplex is an (m  + n  + 1)-dimensional simplex.

In even dimensions greater than 2, simplexes can be thought of as step prisms, such as the pentachoron (5-2 step prism) and the heptapeton (7-2-3 step prism). In odd dimensions greater than 1, simplexes can be thought of as disphenoids of an ${\displaystyle {\tfrac {n-1}{2}}}$-dimensional simplex, such as the tetrahedron (digonal disphenoid) and the hexateron (triangular disphenoid).

## Measures

The values below, except for the last one, are rational if and only if n  is a member of A001108.

• The circumradius of a regular n -dimensional simplex of unit edge length is given by ${\displaystyle {\sqrt {\frac {n}{2n+2}}}}$.
• Its inradius is given by ${\displaystyle {\frac {1}{\sqrt {2n(n+1)}}}}$
• Its height from a vertex to the opposite facet is given by ${\displaystyle {\sqrt {\frac {n+1}{2n}}}}$.
• Its hypervolume is given by ${\displaystyle {\frac {\sqrt {\frac {n+1}{2^{n}}}}{n!}}}$.
• The angle between two facet hyperplanes is ${\displaystyle \arccos \left({\frac {1}{n}}\right)}$.