16-cell

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Regular hexadecachoron
(16-cell)
(4-orthoplex)
Schlegel wireframe 16-cell.png
Schlegel diagram
(vertices and edges)
Type Convex regular 4-polytope
4-orthoplex
4-demicube
Schläfli symbol {3,3,4}
Coxeter diagram CDel node 1.pngCDel 3.pngCDel node.pngCDel 3.pngCDel node.pngCDel 4.pngCDel node.png
Cells 16 {3,3} 3-simplex t0.svg
Faces 32 {3} 2-simplex t0.svg
Edges 24
Vertices 8
Vertex figure 16-cell verf.png
Octahedron
Petrie polygon octagon
Coxeter group BC4, [3,3,4], order 384
D4, order 192
Dual Tesseract
Properties convex, isogonal, isotoxal, isohedral, quasiregular
Uniform index 12

In four-dimensional geometry, a 16-cell is a regular convex 4-polytope. It is one of the six regular convex 4-polytopes first described by the Swiss mathematician Ludwig Schläfli in the mid-19th century. It is also called C16, hexadecachoron, or hexdecahedroid.[1]

It is a part of an infinite family of polytopes, called cross-polytopes or orthoplexes. The dual polytope is the tesseract (4-cube). Conway's name for a cross-polytope is orthoplex, for orthant complex. The 16-cell has 16 cells as the tesseract has 16 vertices.

Geometry[edit]

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 16-cell 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 16-cell 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.

The 16-cell can be decomposed into two similar disjoint circular chains of eight tetrahedrons each, four edges long. Each chain, when stretched out straight, forms a Boerdijk–Coxeter helix. This decomposition can be seen in a 4-4 duoantiprism construction of the 16-cell: CDel node h.pngCDel 2x.pngCDel node h.pngCDel 2x.pngCDel node h.pngCDel 2x.pngCDel node h.png or CDel node.pngCDel 4.pngCDel node h.pngCDel 2x.pngCDel node h.pngCDel 4.pngCDel node.png, Schläfli symbol {2}⨂{2} or s{2}s{2}, symmetry [[4,2+,4]], order 64.

The 16-cell can be dissected into two octahedral pyramids, which share a new octahedron base through the 16-cell center.

Images[edit]

Stereographic polytope 16cell.png
Stereographic projection
16-cell.gif
A 3D projection of a 16-cell performing a simple rotation.
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.

Orthogonal projections[edit]

orthographic projections
Coxeter plane B4 B3 / D4 / A2 B2 / D3
Graph 4-cube t3.svg 4-demicube t0 D4.svg 4-cube t3 B2.svg
Dihedral symmetry [8] [6] [4]
Coxeter plane F4 A3
Graph 4-cube t3 F4.svg 4-cube t3 A3.svg
Dihedral symmetry [12/3] [4]

Tessellations[edit]

One can tessellate 4-dimensional Euclidean space by regular 16-cells. This is called the 16-cell honeycomb and has Schläfli symbol {3,3,4,3}. Hence, the 16-cell has a dihedral angle of 120°.[2] The dual tessellation, 24-cell 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 R4. Each 16-cell has 16 neighbors with which it shares a tetrahedron, 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.

Boerdijk–Coxeter helix[edit]

A 16-cell can constructed from two Boerdijk–Coxeter helixes of eight chained tetrahedra, each folded into a 4-dimensional ring. The 16 triangle faces can be seen in a 2D net within a triangular tiling, with 6 triangles around every vertex. The purple edges represent the Petrie polygon of the 16-cell.

16-cell 8-ring net4.png

Projections[edit]

Projection envelopes of the 16-cell. (Each cell is drawn with different color faces, inverted cells are undrawn)

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.

4 sphere Venn Diagram[edit]

The usual projection of the 16-cell Stereographic polytope 16cell.png and 4 intersecting spheres (a Venn diagram of 4 sets) form topologically the same object in 3D-space:

Venn 1000 0000 0000 0000.png Venn 0110 1000 1000 0000.png

Venn 0100 0000 0000 0000.pngVenn 0010 0000 0000 0000.pngVenn 0000 1000 0000 0000.pngVenn 0000 0000 1000 0000.png

Venn 0001 0110 0110 1000.png

Venn 0001 0000 0000 0000.pngVenn 0000 0100 0000 0000.pngVenn 0000 0010 0000 0000.pngVenn 0000 0000 0100 0000.pngVenn 0000 0000 0010 0000.pngVenn 0000 0000 0000 1000.png

Venn 0000 0001 0001 0110.png

Venn 0000 0001 0000 0000.pngVenn 0000 0000 0001 0000.pngVenn 0000 0000 0000 0100.pngVenn 0000 0000 0000 0010.png

Venn 0000 0000 0000 0001.png

Symmetry constructions[edit]

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 Coxeter diagrams CDel node h1.pngCDel 4.pngCDel node.pngCDel 3.pngCDel node.pngCDel 3.pngCDel node.png or CDel nodes 10ru.pngCDel split2.pngCDel node.pngCDel 3.pngCDel node.png. It can be drawn bicolored with alternating tetrahedral cells.

It can also be seen in lower symmetry form as a tetrahedral antiprism, constructed by 2 parallel tetrahedra in dual configurations, connected by 8 (possibly elongated) tetrahedra. It is represented by s{2,4,3}, and Coxeter diagram: CDel node h.pngCDel 2x.pngCDel node h.pngCDel 4.pngCDel node.pngCDel 3.pngCDel node.png.

It can also be seen as a snub 4-orthotope, represented by s{21,1,1}, and Coxeter diagram: CDel node h.pngCDel 2x.pngCDel node h.pngCDel 2x.pngCDel node h.pngCDel 2x.pngCDel node h.png or CDel node h.pngCDel 2x.pngCDel node h.pngCDel split1-22.pngCDel nodes hh.png.

With the tesseract constructed as a 4-4 duoprism, the 16-cell can be seen as its dual, a 4-4 duopyramid.

Name Coxeter diagram Schläfli symbol Coxeter notation Order Vertex figure
Regular 16-cell CDel node 1.pngCDel 3.pngCDel node.pngCDel 3.pngCDel node.pngCDel 4.pngCDel node.png {3,3,4} [3,3,4] 384 CDel node 1.pngCDel 3.pngCDel node.pngCDel 4.pngCDel node.png
Demitesseract CDel nodes 10ru.pngCDel split2.pngCDel node.pngCDel 3.pngCDel node.png = CDel node h1.pngCDel 4.pngCDel node.pngCDel 3.pngCDel node.pngCDel 3.pngCDel node.png
CDel node 1.pngCDel 3.pngCDel node.pngCDel split1.pngCDel nodes.png = CDel node 1.pngCDel 3.pngCDel node.pngCDel 3.pngCDel node.pngCDel 4.pngCDel node h0.png
h{4,3,3}
{3,31,1}
[31,1,1] = [1+,4,3,3] 192 CDel node.pngCDel 3.pngCDel node 1.pngCDel 3.pngCDel node.png
Alternated 4-4 duoprism CDel label2.pngCDel branch hh.pngCDel 4a4b.pngCDel nodes.png 2s{4,2,4} [[4,2+,4]] 64
Tetrahedral antiprism CDel node h.pngCDel 2x.pngCDel node h.pngCDel 4.pngCDel node.pngCDel 3.pngCDel node.png s{2,4,3} [2+,4,3] 48
Alternated square prism prism CDel node h.pngCDel 2x.pngCDel node h.pngCDel 2x.pngCDel node h.pngCDel 4.pngCDel node.png sr{2,2,4} [(2,2)+,4] 16
Snub 4-orthotope CDel node h.pngCDel 2x.pngCDel node h.pngCDel 2x.pngCDel node h.pngCDel 2x.pngCDel node h.png = CDel node h.pngCDel 2x.pngCDel node h.pngCDel split1-22.pngCDel nodes hh.png s{21,1,1} [2,2,2]+ = [21,1,1]+ 8 CDel node h.pngCDel 2x.pngCDel node h.pngCDel 2x.pngCDel node h.png
4-fusil
CDel node f1.pngCDel 4.pngCDel node.pngCDel 3.pngCDel node.pngCDel 3.pngCDel node.png {3,3,4} [3,3,4] 384 CDel node f1.pngCDel 4.pngCDel node.pngCDel 3.pngCDel node.png
CDel node f1.pngCDel 4.pngCDel node.pngCDel 2x.pngCDel node f1.pngCDel 4.pngCDel node.png {4}+{4} or 2{4} [[4,2,4]] = [8,2+,8] 128 CDel node f1.pngCDel 4.pngCDel node.pngCDel 2x.pngCDel node f1.png
CDel node f1.pngCDel 4.pngCDel node.pngCDel 3.pngCDel node.pngCDel 2x.pngCDel node f1.png {3,4}+{ } [4,3,2] 96 CDel node f1.pngCDel 4.pngCDel node.pngCDel 3.pngCDel node.png
CDel node f1.pngCDel 4.pngCDel node.pngCDel 2x.pngCDel node f1.png
CDel node f1.pngCDel 4.pngCDel node.pngCDel 2x.pngCDel node f1.pngCDel 2x.pngCDel node f1.png {4}+2{ } [4,2,2] 32 CDel node f1.pngCDel 4.pngCDel node.pngCDel 2x.pngCDel node f1.png
CDel node f1.pngCDel 2x.pngCDel node f1.pngCDel 2x.pngCDel node f1.png
CDel node f1.pngCDel 2x.pngCDel node f1.pngCDel 2x.pngCDel node f1.pngCDel 2x.pngCDel node f1.png { }+{ }+{ }+{ } or 4{ } [2,2,2] 16 CDel node f1.pngCDel 2x.pngCDel node f1.pngCDel 2x.pngCDel node f1.png

Related complex polygons[edit]

The regular complex polygon, 2{4}4, CDel node 1.pngCDel 4.pngCDel 4node.png, in has a real representation as a 16-cell in 4-dimensional space with 8 vertices, 16 2-edges, only half of the edges of the 16-cell. Its symmetry is 4[4]2, order 32. [3]

Orthographic projections of 2{4}4 polygon
Complex polygon 2-4-4.png
In B4 Coxeter plane, 2{4}4 has 8 vertices and 16 2-edges, shown here with 4 sets of colors.
Complex polygon 2-4-4 bipartite graph.png
The 8 vertices are grouped in 2 sets (shown red and blue), each only connected with edges to vertices in the other set, making this polygon a complete bipartite graph.[4]

Related uniform polytopes and honeycombs[edit]

The regular 16-cell along with the tesseract exist in a set of 15 uniform 4-polytopes with the same symmetry. It is also a part of the uniform polytopes of D4 symmery.

This 4-polytope is also related to the cubic honeycomb, order-4 dodecahedral honeycomb, and order-4 hexagonal tiling honeycomb which all have octahedral vertex figures.

It is in a sequence to three regular 4-polytopes: the 5-cell {3,3,3}, 600-cell {3,3,5} of Euclidean 4-space, and the order-6 tetrahedral honeycomb {3,3,6} of hyperbolic space. All of these have a tetrahedral cells.

It is first in a sequence of quasiregular polytopes and honeycombs h{4,p,q}, and a half symmetry sequence, for regular forms {p,3,4}.

See also[edit]

Fundamental convex regular and uniform polytopes in dimensions 2–10
Family An Bn I2(p) / Dn E6 / E7 / E8 / F4 / G2 Hn
Regular polygon Triangle Square p-gon Hexagon Pentagon
Uniform polyhedron Tetrahedron OctahedronCube Demicube DodecahedronIcosahedron
Uniform 4-polytope 5-cell 16-cellTesseract Demitesseract 24-cell 120-cell600-cell
Uniform 5-polytope 5-simplex 5-orthoplex5-cube 5-demicube
Uniform 6-polytope 6-simplex 6-orthoplex6-cube 6-demicube 122221
Uniform 7-polytope 7-simplex 7-orthoplex7-cube 7-demicube 132231321
Uniform 8-polytope 8-simplex 8-orthoplex8-cube 8-demicube 142241421
Uniform 9-polytope 9-simplex 9-orthoplex9-cube 9-demicube
Uniform 10-polytope 10-simplex 10-orthoplex10-cube 10-demicube
Uniform n-polytope n-simplex n-orthoplexn-cube n-demicube 1k22k1k21 n-pentagonal polytope
Topics: Polytope familiesRegular polytopeList of regular polytopes and compounds

References[edit]

  1. ^ Matila Ghyka, The Geometry of Art and Life (1977), p.68
  2. ^ Coxeter, Regular polygons, p.293
  3. ^ Regular Complex Polytopes, p. 108
  4. ^ Regular Complex Polytopes, p.114
  • T. Gosset: On the Regular and Semi-Regular Figures in Space of n Dimensions, Messenger of Mathematics, Macmillan, 1900
  • H.S.M. Coxeter:
    • Coxeter, Regular Polytopes, (3rd edition, 1973), Dover edition, ISBN 0-486-61480-8, p. 296, Table I (iii): Regular Polytopes, three regular polytopes in n-dimensions (n≥5)
    • H.S.M. Coxeter, Regular Polytopes, 3rd Edition, Dover New York, 1973, p. 296, Table I (iii): Regular Polytopes, three regular polytopes in n-dimensions (n≥5)
    • 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]
  • John H. Conway, Heidi Burgiel, Chaim Goodman-Strass, The Symmetries of Things 2008, ISBN 978-1-56881-220-5 (Chapter 26. pp. 409: Hemicubes: 1n1)
  • Norman Johnson Uniform Polytopes, Manuscript (1991)
    • N.W. Johnson: The Theory of Uniform Polytopes and Honeycombs, Ph.D. (1966)

External links[edit]