16-cell

In four dimensional geometry, a **16-cell**, is a regular convex polychoron, or polytope existing in four dimensions. It is also known as the **hexadecachoron**. It is one of the six regular convex polychora first described by the Swiss mathematician Ludwig Schläfli in the mid-19th century. 

Conway calls it an **orthoplex** for *orthant complex*, as well as the entire class of cross-polytopes.

## Geometry 
The hexadecachoron is a member of the family of polytopes called the cross-polytopes, which exist in all dimensions. As such, its dual polychoron is the tesseract (the 4-dimensional hypercube).

It is bounded by 16 cells, all of which are regular tetrahedra. It has 32 triangular faces, 24 edges, and 8 vertices. The 24 edges bound 6 squares lying in the 6 coordinate planes.

The eight vertices of the hexadecachoron are (±1, 0, 0, 0), (0, ±1, 0, 0), (0, 0, ±1, 0), (0, 0, 0, ±1). All vertices are connected by edges except opposite pairs.

The Schläfli symbol of the hexadecachoron is {3,3,4}. Its vertex figure is a regular octahedron. There are 8 tetrahedra, 12 triangles, and 6 edges meeting at every vertex. Its edge figure is a square. There are 4 tetrahedra and 4 triangles meeting at every edge.

There is a lower symmetry form of the *16-cell*, called a **demitesseract** or **4-demicube**, a member of the demihypercube family, and represented by h{4,3,3}, and can be drawn bicolored with alternating tetrahedral cells.

## Images 

File:stereographic_polytope_16cell_colour.png|Stereographic projection
File:Cell16-4dpolytope.svg|Four orthographic projections
File:Cross graph 4.svg|A skew orthogonal projection inside its regular octagonal Petrie polygon, connecting all vertices except opposite ones.
File:16-cell nets.png|The 16-cell has two Wythoff constructions, a regular form and alternated form, shown here as nets, the second being represented by alternately two colors of tetrahedral cells.
File:16-cell.gif|A 3D projection of a 16-cell performing a double rotation about two orthogonal planes.

## Tessellations 
One can tessellate 4-dimensional Euclidean space by regular 16-cells. This is called the hexadecachoric honeycomb and has Schläfli symbol {3,3,4,3}. The dual tessellation, icositetrachoric honeycomb, {3,4,3,3}, is made of by regular 24-cells. Together with the tesseractic honeycomb {4,3,3,4}, these are the only three regular tessellations of **R**4. Each 16-cell has 16 neighbors with which it shares an octahedron, 24 neighbors with which it shares only an edge, and 72 neighbors with which it shares only a single point. Twenty-four 16-cells meet at any given vertex in this tessellation.

## Projections 
The cell-first parallel projection of the 16-cell into 3-space has a cubical envelope. The closest and farthest cells are projected to inscribed tetrahedra within the cube, corresponding with the two possible ways to inscribe a regular tetrahedron in a cube. Surrounding each of these tetrahedra are 4 other (non-regular) tetrahedral volumes that are the images of the 4 surrounding tetrahedral cells, filling up the space between the inscribed tetrahedron and the cube. The remaining 6 cells are projected onto the square faces of the cube. In this projection of the 16-cell, all its edges lie on the faces of the cubical envelope.

The cell-first perspective projection of the 16-cell into 3-space has a triakis tetrahedral envelope. The layout of the cells within this envelope are analogous to that of the cell-first parallel projection.

The vertex-first parallel projection of the 16-cell into 3-space has an octahedral envelope. This octahedron can be divided into 8 tetrahedral volumes, by cutting along the coordinate planes. Each of these volumes is the image of a pair of cells in the 16-cell. The closest vertex of the 16-cell to the viewer projects onto the center of the octahedron.

Finally the edge-first parallel projection has a shortened octahedral envelope, and the face-first parallel projection has a hexagonal bipyramidal envelope.

## Related pages
* 24-cell
* Polychoron

## References 
* H. S. M. Coxeter, *Regular Polytopes*, 3rd. ed., Dover Publications, 1973. .

## Other websites 
* 
* Olshevsky, George, *Hexadecachoron* at *Glossary for Hyperspace*.
** 2. Convex uniform polychora based on the tesseract (8-cell) and hexadecachoron (16-cell) - Model 12, George Olshevsky.
* Der 16-Zeller (16-cell) Marco Möller's Regular polytopes in R4 (German)
* Description and diagrams of 16-cell projections