# Polytope compound

A polyhedral compound is a figure that is composed of several polyhedra sharing a common centre. They are the three-dimensional analogs of polygonal compounds such as the hexagram.

The outer vertices of a compound can be connected to form a convex polyhedron called the convex hull. The compound is a facetting of the convex hull.

Another convex polyhedron is formed by the small central space common to all members of the compound. This polyhedron can be used as the core for a set of stellations.

## Regular compounds

A regular polyhedron compound can be defined as a compound which, like a regular polyhedron, is vertex-transitive, edge-transitive, and face-transitive. There are five regular compounds of polyhedra.

Regular compound
(Coxeter symbol)
Picture Spherical Up: Convex hull
(Down: Core)
Symmetry group Subgroup
restricting
to one
constituent
Dual
Two tetrahedra
{4,3}[2{3,3}]{3,4}
Cube
(Octahedron)
*432
[4,3]
Oh
*332
[3,3]
Td
Self-dual
Five tetrahedra
{5,3}[5{3,3}]{3,5}
Dodecahedron
(Icosahedron)
532
[5,3]+
I
332
[3,3]+
T
Enantiomorph
chiral twin
Ten tetrahedra
2{5,3}[10{3,3}]2{3,5}
Dodecahedron
(Icosahedron)
*532
[5,3]
Ih
332
[3,3]
T
Self-dual
Five cubes
{5,3}[5{4,3}]
Dodecahedron
(Rhombic triacontahedron)
*532
[5,3]
Ih
3*2
[3,3]
Th
Five octahedra
Five octahedra
[5{3,4}]{3,5}
Icosidodecahedron
(Icosahedron)
*532
[5,3]
Ih
3*2
[3,3]
Th
Five cubes

Best known is the compound of two tetrahedra, often called the stella octangula, a name given to it by Kepler. The vertices of the two tetrahedra define a cube, and the intersection of the two: an octahedron, which shares the same face-planes as the compound. Thus the compound of two tetrahedra is a stellation of the octahedron, and in fact, the only finite stellation thereof.

The stella octangula can also be regarded as a dual-regular compound.

The compound of five tetrahedra comes in two enantiomorphic versions, which together make up the compound of 10 tetrahedra. Each of the tetrahedral compounds is self-dual or "chiral-twin-dual", and the compound of 5 cubes is dual to the compound of 5 octahedra.

The five octahedra defining any given icosahedron form a regular polyhedral compound.

Coxeter's notation for regular compounds is given in the table above, incorporating Schläfli symbols. The material inside the square brackets, [d{p,q}], denotes the components of the compound: d separate {p,q}'s. The material before the square brackets denotes the vertex arrangement of the compound: c{m,n}[d{p,q}] is a compound of d {p,q}'s sharing the vertices of {m,n} counted c times. The material after the square brackets denotes the facet arrangement of the compound: [d{p,q}]e{s,t} is a compound of d {p,q}'s sharing the faces of {s,t} counted e times. These may be combined: thus c{m,n}[d{p,q}]e{s,t} is a compound of d {p,q}'s sharing the vertices of {m,n} counted c times and the faces of {s,t} counted e times. This notation can be generalised to compounds in any number of dimensions.

## Dual compounds

A dual compound is composed of a polyhedron and its dual, arranged reciprocally about a common intersphere or midsphere, such that the edge of one polyhedron intersects the dual edge of the dual polyhedron. There are five dual compounds of the regular polyhedra.

The core is the rectification of both solids. The hull is the dual of this rectification, and its rhombic faces have the intersecting edges of the two solids as diagonals (and have their four alternate vertices). For the convex solids, this is the convex hull.

Dual compound Picture Hull Core Symmetry group
Two tetrahedra
(Compound of two tetrahedra, stellated octahedron)
Cube Octahedron *432
[4,3]
Oh
Cube-octahedron
(Compound of cube and octahedron)
Rhombic dodecahedron Cuboctahedron *432
[4,3]
Oh
Dodecahedron-icosahedron
(Compound of dodecahedron and icosahedron)
Rhombic triacontahedron Icosidodecahedron *532
[5,3]
Ih
Small stellated dodecahedron-great dodecahedron
(Compound of sD and gD)
Medial rhombic triacontahedron
(Convex: Icosahedron)
Dodecadodecahedron
(Convex: Dodecahedron)
*532
[5,3]
Ih
Great icosahedron-great stellated dodecahedron
(Compound of gI and gsD)
Great rhombic triacontahedron
(Convex: Dodecahedron)
Great icosidodecahedron
(Convex: Icosahedron)
*532
[5,3]
Ih

The tetrahedron is self-dual, so the dual compound of a tetrahedron with its dual is the regular stellated octahedron.

The octahedral and icosahedral dual compounds are the first stellations of the cuboctahedron and icosidodecahedron, respectively.

## Uniform compounds

In 1976 John Skilling published Uniform Compounds of Uniform Polyhedra which enumerated 75 compounds (including 6 as infinite prismatic sets of compounds, #20-#25) made from uniform polyhedra with rotational symmetry. (Every vertex is vertex-transitive and every vertex is transitive with every other vertex.) This list includes the five regular compounds above.

The 75 uniform compounds are listed in the Table below. Most are shown singularly colored by each polyhedron element. Some chiral pairs of face groups are colored by symmetry of the faces within each polyhedron.

• 1-19: Miscellaneous (4,5,6,9,17 are the 5 regular compounds)
• 46-67: Tetrahedral symmetry embedded in octahedral or icosahedral symmetry,

## Other compounds

 These compounds, of four cubes, and (dual) four octahedra, are neither regular compounds, nor dual compounds, nor uniform compounds.

Two polyhedra that are compounds but have their elements rigidly locked into place are the small complex icosidodecahedron (compound of icosahedron and great dodecahedron) and the great complex icosidodecahedron (compound of small stellated dodecahedron and great icosahedron). If the definition of a uniform polyhedron is generalised they are uniform.

The section for entianomorphic pairs in Skilling's list does not contain the compound of two great snub dodecicosidodecahedra, as the pentagram faces would coincide. Removing the coincident faces results in the compound of twenty octahedra.

## 4-polytope compounds

In 4-dimensions, there are a large number of regular compounds of regular polytopes. Coxeter lists a few of these in his book Regular Polytopes:

Self-duals:

Compound Constituent Symmetry
120 5-cell5-cell[5,3,3], order 14400
5 24-cell24-cell[5,3,3], order 14400

Dual pairs:

Compound 1 Compound 2 Symmetry
3 16-cells3 tesseracts[3,4,3], order 1152
15 16-cells15 tesseracts[5,3,3], order 14400
75 16-cells75 tesseracts[5,3,3], order 14400
300 16-cells300 tesseracts[5,3,3]+, order 7200
600 16-cells600 tesseracts[5,3,3], order 14400
25 24-cells25 24-cells[5,3,3], order 14400

Uniform compounds and duals with convex 4-polytopes:

Compound 1
Vertex-transitive
Compound 2
Cell-transitive
Symmetry
2 16-cells2 tesseracts[4,3,3], order 384
100 24-cell100 24-cell[5,3,3]+, order 7200
200 24-cell200 24-cell[5,3,3], order 14400
5 600-cell5 120-cell[5,3,3]+, order 7200
10 600-cell10 120-cell[5,3,3], order 14400

### Compounds with regular star 4-polytopes

Self-dual star compounds:

Compound Symmetry
5 {5,5/2,5}[5,3,3]+, order 7200
10 {5,5/2,5}[5,3,3], order 14400
5 {5/2,5,5/2}[5,3,3]+, order 7200
10 {5/2,5,5/2}[5,3,3], order 14400

Dual pairs of compound stars:

Compound 1 Compound 2 Symmetry
5 {3,5,5/2}5 {5/2,5,3}[5,3,3]+, order 7200
10 {3,5,5/2}10 {5/2,5,3}[5,3,3], order 14400
5 {5,5/2,3}5 {3,5/2,5}[5,3,3]+, order 7200
10 {5,5/2,3}10 {3,5/2,5}[5,3,3], order 14400
5 {5/2,3,5}5 {5,3,5/2}[5,3,3]+, order 7200
10 {5/2,3,5}10 {5,3,5/2}[5,3,3], order 14400

Uniform compound stars and duals:

Compound 1
Vertex-transitive
Compound 2
Cell-transitive
Symmetry
5 {3,3,5/2}5 {5/2,3,3}[5,3,3]+, order 7200
10 {3,3,5/2}10 {5/2,3,3}[5,3,3], order 14400

### Compounds with duals

Dual positions:

Compound Constituent Symmetry
2 5-cell5-cell[[3,3,3]], order 240
2 24-cell24-cell[[3,4,3]], order 2304
1 tesseract, 1 16-celltesseract, 16-cell
1 120-cell, 1 600-cell120-cell, 600-cell
2 great 120-cellgreat 120-cell
2 grand stellated 120-cellgrand stellated 120-cell
1 icosahedral 120-cell, 1 small stellated 120-cellicosahedral 120-cell, small stellated 120-cell
1 grand 120-cell, 1 great stellated 120-cellgrand 120-cell, great stellated 120-cell
1 great grand 120-cell, 1 great icosahedral 120-cellgreat grand 120-cell, great icosahedral 120-cell
1 great grand stellated 120-cell, 1 grand 600-cellgreat grand stellated 120-cell, grand 600-cell

Only the first two of these dual compounds are also regular.

## Group theory

In terms of group theory, if G is the symmetry group of a polyhedral compound, and the group acts transitively on the polyhedra (so that each polyhedron can be sent to any of the others, as in uniform compounds), then if H is the stabilizer of a single chosen polyhedron, the polyhedra can be identified with the orbit space G/H – the coset gH corresponds to which polyhedron g sends the chosen polyhedron to.

## Compounds of tilings

There are eighteen two-parameter families of regular compound tessellations of the Euclidean plane. In the hyperbolic plane, five one-parameter families and seventeen isolated cases are known, but the completeness of this listing has not been enumerated.

The Euclidean and hyperbolic compound families 2 {p,p} (4 ≤ p ≤ ∞, p an integer) are analogous to the spherical stella octangula, 2 {3,3}.

A few examples of Euclidean and hyperbolic regular compounds
Self-dual Duals Self-dual
2 {4,4} 2 {6,3} 2 {3,6} 2 {,}
3 {6,3} 3 {3,6} 3 {,}

A known family of regular Euclidean compound honeycombs in five or more dimensions is an infinite family of compounds of hypercubic honeycombs, all sharing vertices and faces with another hypercubic honeycomb. This compound can have any number of hypercubic honeycombs.

There are also dual-regular tiling compounds. A simple example is the E2 compound of a hexagonal tiling and its dual triangular tiling, which shares its edges with the deltoidal trihexagonal tiling. The Euclidean compounds of two hypercubic honeycombs are both regular and dual-regular.

## Footnotes

1. Coxeter, Harold Scott MacDonald (1973) . Regular Polytopes (Third ed.). Dover Publications. p. 48. ISBN 0-486-61480-8. OCLC 798003.
2. Regular polytopes, Table VII, p. 305
3. Klitzing, Richard. "Uniform compound stellated icositetrachoron".
4. Klitzing, Richard. "Uniform compound demidistesseract".
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