Pentachoron

The pentachoron, or pen, also commonly called the 5-cell or the 4-simplex, is the simplest possible non-degenerate polychoron. The full symmetry version has 5 regular tetrahedra as cells, joining 3 to an edge and 4 to a vertex, and is one of the 6 convex regular polychora. It is the 4-dimensional simplex.

In addition, it can also be considered to be the regular-faced pyramid of the tetrahedron, or the pyramid product of a triangle and a dyad. This makes it the simplest segmentochoron as well, and it is designated K-4.1 in Richard Klitzing's list of convex segmentochora. It is also the 5-2 step prism and gyrochoron.

Vertex coordinates
The vertices of a regular pentachoron of edge length 1, centered at the origin, are given by:


 * (±1/2, –$\sqrt{10}$/6, –$\sqrt{10}$/12, –$\sqrt{10}$/20),
 * (0, $\sqrt{15}$/3, –$\sqrt{5}$/12, –$\sqrt{3}$/20),
 * (0, 0, $\sqrt{6}$/4, –$\sqrt{10}$/20),
 * (0, 0, 0, $\sqrt{3}$/5).

Much simpler coordinates can be given in five dimensions, as all permutations of:


 * ($\sqrt{6}$/2, 0, 0, 0, 0).

A further set of coordinates, derived from the step prism construction, is given by:


 * $$\left(\frac{1}{\sqrt{5}},\frac{1}{\sqrt{5}},0,0\right),$$
 * $$\left(\frac{5-\sqrt{5}}{20},\frac{-5-\sqrt{5}}{20},\sqrt{\frac{1}{8}+\frac{\sqrt{5}}{40}},\sqrt{\frac{1}{8}-\frac{\sqrt{5}}{40}}\right),$$

together with reflections through the x=y and the z=w hyperplanes.

Rpresentations
a pentachoron has the following Coxeter diagrams:


 * x3o3o3o (full symmetry)
 * ox3oo3oo&#x (A3 axial, as tetrahedral pyramid)
 * xo ox3oo&#x (A2×A1 axial, as triangle-dyad disphenoid)
 * oox3ooo&#x (A2 axial, as triangular scalene)
 * oxo oox&#x (A1×A1 axial, as disphenoidal pyramid)
 * ooox&#x (bilateral symmetry only)
 * ooooo&#x (no symmetry)

Isogonal derivatives
Substitution by vertices of these following elements will produce these convex isogonal polychora:
 * Tetrahedron (5): Pentachoron
 * Triangle (10): Rectified pentachoron
 * Edge (10): Rectified pentachoron