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Four dimensional analog of the octahedron

In geometry, the 16 cell is the regular convex 4 polytope (four dimensional analogue of a Platonic solid) with Schläfli symbol {3,3,4}. 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
It is the 4 dimesional member of an infinite family of polytopes called cross polytopes, orthoplexes, or hyperoctahedrons which are analogous to the octahedron in three dimensions. It is Coxeter's polytope. The dual polytope is the tesseract (4 cube), which it can be combined with to form a compound figure. The cells of the 16 cell are dual to the 16 vertices of the tesseract.

The 16 cell is the second in the sequence of 6 convex regular 4 polytopes (in order of size and complexity). Each of its 4 successor convex regular 4 polytopes can be constructed as the convex hull of a polytope compound of multiple 16 cells: the 16 vertex tesseract as a compound of two 16 cells, the 24 vertex 24 cell as a compound of three 16 cells, the 120 vertex 600 cell as a compound of fifteen 16 cells, and the 600 vertex 120 cell as a compound of seventy five 16 cells.

Disjoint squaresxy plane( 0, 1, 0, 0)( 0, 0, 1, 0)( 0, 0, 1, 0)( 0, 1, 0, 0)wz plane( 1, 0, 0, 0)( 0, 0, 0, 1)( 0, 0, 0, 1)( 1, 0, 0, 0)The 16 cell is the 4 dimensional cross polytope (4 orthoplex), which means its vertices lie in opposite pairs on the 4 axes of a (w, x, y, z) Cartesian coordinate system. The eight vertices 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 edge length is
The vertex coordinates form 6 orthogonal central squares lying in the 6 coordinate planes. Squares in opposite planes that do not share an axis (e. g. in the xy and wz planes) are completely disjoint (they do not intersect at any vertices). The 16 cell constitutes an orthonormal basis for the choice of a 4 dimensional reference frame, because its vertices exactly define the four orthogonal axes.

The Schläfli symbol of the 16 cell is {3,3,4}, indicating that its cells are regular tetrahedra {3,3} and its vertex figure is a regular octahedron {3,4}. 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 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 orthogonal central squares lying on great circles in the 6 coordinate planes (3 pairs of completely orthogonal great squares). At each vertex, 3 great squares cross perpendicularly. The 6 edges meet at the vertex the way 6 edges meet at the apex of a canonical octahedral pyramid. The 6 orthogonal central planes of the 16 cell can be divided into 4 orthogonal central hyperplanes (3 spaces) each forming an octahedron with 3 orthogonal great squares.

A 16 cell can be constructed (three different ways) from two Boerdijk–Coxeter helixes of eight chained tetrahedra, each bent in the fourth dimension into a ring. The two circular helixes spiral around each other, nest into each other and pass through each other forming a Hopf link. 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. The eight cell ring of tetrahedra contains three octagrams of different colors, eight edge circular paths that wind twice around the 16 cell on every third vertex of the octagram. The orange and yellow edges are two four edge halves of one octagram, which join their ends to form a Möbius strip. Thus the 16 cell can be decomposed into two cell disjoint circular chains of eight tetrahedrons each, four edges long, one spiraling to the right (clockwise) and the other spiraling to the left (counterclockwise). The left handed and right handed cell rings fit together, nesting into each other and entirely filling the 16 cell, even though they are of opposite chirality. This decomposition can be seen in a 4 4 duoantiprism construction of the 16 cell: or , Schläfli symbol {2}⨂{2} or s{2}s{2}, symmetry [4,2+,4], order 64. Three eight edge paths (of different colors) spiral along each eight cell ring, making 90° angles at each vertex. (In the Boerdijk–Coxeter helix before it is bent into a ring, the angles in different paths vary, but are not 90°.) Three paths (with three different colors and apparent angles) pass through each vertex. When the helix is bent into a ring, the segments of each eight edge path (of various lengths) join their ends, forming a Möbius strip eight edges long along its single sided circumference of 4𝝅, and one edge wide. The six four edge halves of the three eight edge paths each make four 90° angles, but they are not the six orthogonal great squares: they are open ended squares, four edge 360° helices whose open ends are antipodal vertices. The four edges come from four different great squares, and are mutually orthogonal. Combined end to end in pairs of the same chirality, the six four edge paths make three eight edge Möbius loops, helical octagrams. Each octagram is both a Petrie polygon of the 16 cell, and the helical track along which all eight vertices rotate together, in one of the 16 cell's distinct isoclinic rotations. Five ways of looking at the same skew octagramEdge pathPetrie polygon16 cellDiscrete fibrationDiameter chordsOctagram{8/3}Octagram{8/1}Coxeter plane B4Octagram{8/2}=2{4}Octagram{8/4}=4{2}The eight chords of the edge path of an isocline. Skew octagon of eight edges. The 16 cell has 3 of these 8 vertex circuits. All 24 edges and the four orthogonal axes. Two completely orthogonal (disjoint) great squares of edges. The four chords of an isocline. Every fourth isocline vertex is joined to its antipodal vertex by a 16 cell axis. Each eight edge helix is a skew octagram{8/3} that winds three times around the 16 cell and visits every vertex before closing into a loop. Its eight edges are chords of an isocline, a helical arc on which the 8 vertices circle during an isoclinic rotation. All eight 16 cell vertices are apart except for opposite (antipodal) vertices, which are apart. A vertex moving on the isocline visits three other vertices that are apart before reaching the fourth vertex that is away. The eight cell ring is chiral: there is a right handed form which spirals clockwise, and a left handed form which spirals counterclockwise. The 16 cell contains one of each, so it also contains a left and a right isocline; the isocline is the circular axis around which the eight cell ring twists. Each isocline visits all eight vertices of the 16 cell. Each eight cell ring contains half of the 16 cells, but all 8 vertices; the two rings share the vertices, as they nest into each other and fit together. They also share the 24 edges, though left and right octagram helices are different eight edge paths. Because there are three pairs of completely orthogonal great squares, there are three congruent ways to compose a 16 cell from two eight cell rings. The 16 cell contains three left right pairs of eight cell rings in different orientations, with each cell ring containing its axial isocline. Each left right pair of isoclines is the track of a left right pair of distinct isoclinic rotations: the rotations in one pair of completely orthogonal invariant planes of rotation. At each vertex, there are three great squares and six octagram isoclines that cross at the vertex and share a 16 cell axis chord.

This configuration matrix represents the 16 cell. The rows and columns correspond to vertices, edges, faces, and cells. The diagonal numbers say how many of each element occur in the whole 16 cell. The nondiagonal numbers say how many of the column's element occur in or at the row's element.

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°. 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. The dual tessellation, the 24 cell honeycomb, {3,4,3,3}, is made of regular 24 cells. Together with the tesseractic honeycomb {4,3,3,4} these are the only three regular tessellations of R4.

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.

A 3 dimensional projection of the 16 cell and 4 intersecting spheres (a Venn diagram of 4 sets) are topologically equivalent.

The Möbius–Kantor polygon is a regular complex polygon 3{3}3, , in shares the same vertices as the 16 cell. It has 8 vertices, and 8 3 edges. The regular complex polygon, 2{4}4, , 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. + Orthographic projections of 2{4}4 polygonIn B4 Coxeter plane, 2{4}4 has 8 vertices and 16 2 edges, shown here with 4 sets of colors. 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, K4,4.

The regular 16 cell and tesseract are the regular members of a set of 15 uniform 4 polytopes with the same B4 symmetry. The 16 cell is also one of the uniform polytopes of D4 symmetry. The 16 cell is also related to the cubic honeycomb, order 4 dodecahedral honeycomb, and order 4 hexagonal tiling honeycomb which all have octahedral vertex figures. It belongs to the sequence of {3,3,p} 4 polytopes which have tetrahedral cells. The sequence includes three regular 4 polytopes of Euclidean 4 space, the 5 cell {3,3,3}, 16 cell {3,3,4}, and 600 cell {3,3,5}), and the order 6 tetrahedral honeycomb {3,3,6} of hyperbolic space. 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}.