# Uniform polyhedron

A uniform polyhedron is a polyhedron which has regular polygons as faces and is vertex-transitive (transitive on its vertices, isogonal, i.e. there is an isometry mapping any vertex onto any other). It follows that all vertices are congruent, and the polyhedron has a high degree of reflectional and rotational symmetry.

Uniform polyhedra may be regular (if also face and edge transitive), quasi-regular (if edge transitive but not face transitive) or semi-regular (if neither edge nor face transitive). The faces and vertices need not be convex, so many of the uniform polyhedra are also star polyhedra.

Excluding the infinite sets, there are 75 uniform polyhedra (or 76 if edges are allowed to coincide).

There are also two infinite sets of uniform prisms and antiprisms, including convex and star forms.

Dual polyhedra to uniform polyhedra are face-transitive (isohedral) and have regular vertex figures, and are generally classified in parallel with their dual (uniform) polyhedron. The dual of a regular polyhedron is regular, while the dual of an Archimedean solid is a Catalan solid.

The concept of uniform polyhedron is a special case of the concept of uniform polytope, which also applies to shapes in higher-dimensional (or lower-dimensional) space.

## History

Regular star polyhedra:

Other 53 nonregular star polyhedra:

• Of the remaining 53, Albert Badoureau (1881) discovered 36. Edmund Hess (1878) discovered 2 more and Pitsch (1881) independently discovered 18, of which 15 had not previously been discovered.
• The geometer H.S.M. Coxeter discovered the remaining twelve in collaboration with J. C. P. Miller (1930–1932) but did not publish. M.S. Longuet-Higgins and H.C. Longuet-Higgins and independently discovered 11 of these.
• Template:Harvtxt published the list of uniform polyhedra.
• Template:Harvtxt proved their conjecture that the list was complete.
• In 1974, Magnus Wenninger published his book Polyhedron models, which lists all 75 nonprismatic uniform polyhedra, with many previously unpublished names given to them by Norman Johnson.
• Template:Harvtxt independently proved the completeness, and showed that if the definition of uniform polyhedron is relaxed to allow edges to coincide then there is just one extra possibility.
• In 1987, Edmond Bonan draw all the uniform polyhedra and their duals in 3D, with a Turbo Pascal program called Polyca : almost of them were shown during the International Stereoscopic Union Congress held at the Congress Theatre, Eastbourne, United Kingdom.
• In 1993, Zvi Har'El produced a complete kaleidoscopic construction of the uniform polyhedra and duals with a computer program called Kaleido, and summarized in a paper Uniform Solution for Uniform Polyhedra, counting figures 1-80.
• Also in 1993, R. Mäder ported this Kaleido solution to Mathematica with a slightly different indexing system.
• In 2002 Peter W. Messer discovered a minimal set of closed-form expressions for determining the main combinatorial and metrical quantities of any uniform polyhedron (and its dual) given only its Wythoff symbol.

## Uniform star polyhedra

The 57 nonprismatic nonconvex forms are compiled by Wythoff constructions within Schwarz triangles.

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## Convex forms by Wythoff construction

The convex uniform polyhedra can be named by Wythoff construction operations and can be named in relation to the regular form.

In more detail the convex uniform polyhedron are given below by their Wythoff construction within each symmetry group.

Within the Wythoff construction, there are repetitions created by lower symmetry forms. The cube is a regular polyhedron, and a square prism. The octahedron is a regular polyhedron, and a triangular antiprism. The octahedron is also a rectified tetrahedron. Many polyhedra are repeated from different construction sources and are colored differently.

The Wythoff construction applies equally to uniform polyhedra and uniform tilings on the surface of a sphere, so images of both are given. The spherical tilings including the set of hosohedrons and dihedrons which are degenerate polyhedra.

These symmetry groups are formed from the reflectional point groups in three dimensions, each represented by a fundamental triangle (p q r), where p>1, q>1, r>1 and 1/p+1/q+1/r<1.

The remaining nonreflective forms are constructed by alternation operations applied to the polyhedra with an even number of sides.

Along with the prisms and their dihedral symmetry, the spherical Wythoff construction process adds two regular classes which become degenerate as polyhedra - the dihedra and hosohedra, the first having only two faces, and the second only two vertices. The truncation of the regular hosohedra creates the prisms.

Below the convex uniform polyhedra are indexed 1-18 for the nonprismatic forms as they are presented in the tables by symmetry form. Repeated forms are in brackets.

For the infinite set of prismatic forms, they are indexed in four families:

1. Hosohedra H2... (Only as spherical tilings)
2. Dihedra D2... (Only as spherical tilings)
3. Prisms P3... (Truncated hosohedra)
4. Antiprisms A3... (Snub prisms)

### Summary tables

And a sampling of Dihedral symmetries:

### Wythoff construction operators

Operation Symbol Coxeter
diagram
Description
Parent {p,q}
t0{p,q}
Template:CDD Any regular polyhedron or tiling
Rectified (r) r{p,q}
t1{p,q}
Template:CDD The edges are fully truncated into single points. The polyhedron now has the combined faces of the parent and dual.
Birectified (2r)
(also dual)
2r{p,q}
t2{p,q}
Template:CDD The birectified (dual) is a further truncation so that the original faces are reduced to points. New faces are formed under each parent vertex. The number of edges is unchanged and are rotated 90 degrees. The dual of the regular polyhedron {p, q} is also a regular polyhedron {q, p}.
Truncated (t) t{p,q}
t0,1{p,q}
Template:CDD Each original vertex is cut off, with a new face filling the gap. Truncation has a degree of freedom, which has one solution that creates a uniform truncated polyhedron. The polyhedron has its original faces doubled in sides, and contains the faces of the dual. Bitruncated (2t)
(also truncated dual)
2t{p,q}
t1,2{p,q}
Template:CDD Same as truncated dual.
Cantellated (rr)
(Also expanded)
rr{p,q} Template:CDD In addition to vertex truncation, each original edge is beveled with new rectangular faces appearing in their place. A uniform cantellation is half way between both the parent and dual forms. Cantitruncated (tr)
(Also omnitruncated)
tr{p,q}
t0,1,2{p,q}
Template:CDD The truncation and cantellation operations are applied together to create an omnitruncated form which has the parent's faces doubled in sides, the dual's faces doubled in sides, and squares where the original edges existed.
Alternation operations
Operation Symbol Coxeter
diagram
Description
Snub rectified (sr) sr{p,q} Template:CDD The alternated cantitruncated. All the original faces end up with half as many sides, and the squares degenerate into edges. Since the omnitruncated forms have 3 faces/vertex, new triangles are formed. Usually these alternated faceting forms are slightly deformed thereafter in order to end again as uniform polyhedra. The possibility of the latter variation depends on the degree of freedom. Snub (s) s{p,2q} Template:CDD Alternated truncation
Cantic snub (s2) s2{p,2q} Template:CDD
Alternated cantellation (hrr) hrr{2p,2q} Template:CDD Only possible in uniform tilings (infinite polyhedra), alternation of Template:CDD
For example, Template:CDD
Half (h) h{2p,q} Template:CDD Alternation of Template:CDD, same as Template:CDD
Cantic (h2) h2{2p,q} Template:CDD Same as Template:CDD
Half rectified (hr) hr{2p,2q} Template:CDD Only possible in uniform tilings (infinite polyhedra), alternation of Template:CDD, same as Template:CDD or Template:CDD
For example, Template:CDD = Template:CDD or Template:CDD
Quarter (q) q{2p,2q} Template:CDD Only possible in uniform tilings (infinite polyhedra), same as Template:CDD
For example, Template:CDD = Template:CDD or Template:CDD

### (3 3 2) Td Tetrahedral symmetry

The tetrahedral symmetry of the sphere generates 5 uniform polyhedra, and a 6th form by a snub operation.

The tetrahedral symmetry is represented by a fundamental triangle with one vertex with two mirrors, and two vertices with three mirrors, represented by the symbol (3 3 2). It can also be represented by the Coxeter group A2 or [3,3], as well as a Coxeter-Dynkin diagram: Template:CDD.

There are 24 triangles, visible in the faces of the tetrakis hexahedron and alternately colored triangles on a sphere:   ### (4 3 2) Oh Octahedral symmetry

The octahedral symmetry of the sphere generates 7 uniform polyhedra, and a 3 more by alternation. Four of these forms are repeated from the tetrahedral symmetry table above.

The octahedral symmetry is represented by a fundamental triangle (4 3 2) counting the mirrors at each vertex. It can also be represented by the Coxeter group B2 or [4,3], as well as a Coxeter-Dynkin diagram: Template:CDD.

There are 48 triangles, visible in the faces of the disdyakis dodecahedron and alternately colored triangles on a sphere:   ### (5 3 2) Ih Icosahedral symmetry

The icosahedral symmetry of the sphere generates 7 uniform polyhedra, and a 1 more by alternation. Only one is repeated from the tetrahedral and octahedral symmetry table above.

The icosahedral symmetry is represented by a fundamental triangle (5 3 2) counting the mirrors at each vertex. It can also be represented by the Coxeter group G2 or [5,3], as well as a Coxeter-Dynkin diagram: Template:CDD.

There are 120 triangles, visible in the faces of the disdyakis triacontahedron and alternately colored triangles on a sphere:   ### (p 2 2) Prismatic [p,2], I2(p) family (Dph Dihedral symmetry)

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The dihedral symmetry of the sphere generates two infinite sets of uniform polyhedra, prisms and antiprisms, and two more infinite set of degenerate polygons, the hosohedrons and dihedrons which exists as tilings on the sphere.

The dihedral symmetry is represented by a fundamental triangle (p 2 2) counting the mirrors at each vertex. It can also be represented by the Coxeter group I2(p) or [n,2], as well as a prismatic Coxeter-Dynkin diagram: Template:CDD.

Below are the first five dihedral symmetries: D2 ... D6. The dihedral symmetry Dp has order 4n, represented the faces of a bipyramid, and on the sphere as an equator line on the longitude, and n equally-spaced lines of longitude.

#### (2 2 2) dihedral symmetry

There are 8 fundamental triangles, visible in the faces of the square bipyramid (Octahedron) and alternately colored triangles on a sphere:  #### (3 2 2) D3hdihedral symmetry

There are 12 fundamental triangles, visible in the faces of the hexagonal bipyramid and alternately colored triangles on a sphere:  #### (4 2 2) D4hdihedral symmetry

There are 16 fundamental triangles, visible in the faces of the octagonal bipyramid and alternately colored triangles on a sphere: #### (5 2 2) D5h dihedral symmetry

There are 20 fundamental triangles, visible in the faces of the decagonal bipyramid and alternately colored triangles on a sphere: #### (6 2 2) D6hdihedral symmetry

There are 24 fundamental triangles, visible in the faces of the dodecagonal bipyramid and alternately colored triangles on a sphere.