# Polytope compound

(Redirected from Polyhedral 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.

Components Picture Spherical Convex hull Core Symmetry Subgroup
restricting
to one
constituent
Dual
Two tetrahedra
(stella octangula)
Cube Octahedron *432
[4,3]
Oh
*332
[3,3]
Td
Self-dual
Five tetrahedra Dodecahedron Icosahedron 532
[5,3]+
I
332
[3,3]+
T
enantiomorph
chiral twin
Ten tetrahedra Dodecahedron Icosahedron *532
[5,3]
Ih
332
[3,3]
T
Self-dual
Five cubes Dodecahedron Rhombic triacontahedron *532
[5,3]
Ih
3*2
[3,3]
Th
Five octahedra
Five octahedra 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 it 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, and the compound of 5 cubes is dual to the compound of 5 octahedra.

## 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 such compounds of the regular polyhedra.

Components Picture Convex hull Core Symmetry
two tetrahedra
(stella octangula)
Cube Octahedron *432
[4,3]
Oh
cube
and octahedron
Rhombic dodecahedron Cuboctahedron *432
[4,3]
Oh
dodecahedron
and icosahedron
Rhombic triacontahedron Icosidodecahedron *532
[5,3]
Ih
great icosahedron
and great stellated dodecahedron
Dodecahedron Icosidodecahedron *532
[5,3]
Ih
small stellated dodecahedron
and great dodecahedron
Icosahedron Dodecahedron *532
[5,3]
Ih

The tetrahedron is self-dual, so the dual compound of a tetrahedron with its dual polyhedron is also the regular Stella octangula.

The cube-octahedron and dodecahedron-icosahedron dual compounds are the first stellations of the cuboctahedron and icosidodecahedron, respectively.

The compound of the small stellated dodecahedron and great dodecahedron looks outwardly the same as the small stellated dodecahedron, because the great dodecahedron is completely contained inside. For this reason, the image shown above shows the small stellated dodecahedron in wireframe.

## 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. [1]

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 them in his book Regular Polytopes:[1]

Self-duals:

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

Dual pairs:

Compound 1 Compound 2 Symmetry
3 16-cells[2] 3 tesseracts [3,4,3], order 1152
15 16-cells 15 tesseracts [5,3,3], order 14400
75 16-cells 75 tesseracts [5,3,3], order 14400
300 16-cells 300 tesseracts [5,3,3]+, order 7200
600 16-cells 600 tesseracts [5,3,3], order 14400
25 24-cells 25 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-cells[3] 2 tesseracts [4,3,3], order 384
100 24-cell 100 24-cell [5,3,3]+, order 7200
200 24-cell 200 24-cell [5,3,3], order 14400
5 600-cell 5 120-cell [5,3,3]+, order 7200
10 600-cell 10 120-cell [5,3,3], order 14400

Dual positions:

Compound Symmetry
2 5-cell
{{3,3,3}}
[[3,3,3]], order 240
2 24-cell[4]
{{3,4,3}}
[[3,4,3]], order 2304

### 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

## 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. The Euclidean compounds of two hypercubic honeycombs are both regular and dual-regular.

## Footnotes

1. ^ Regular polytopes, Table VII, p. 305
2. ^ Richard Klitzing, Uniform compound, stellated icositetrachoron
3. ^ Richard Klitzing, Uniform compound, demidistesseract
4. ^ Richard Klitzing, Uniform compound, Dual positioned 24-cells