Uniform star-polyhedron


In geometry, a uniform star polyhedron is a self-intersecting uniform polyhedron. They are also sometimes called nonconvex polyhedra to imply self-intersecting. Each polyhedron can contain either star polygon faces, star polygon vertex figures or both.

The complete set of 57 nonprismatic uniform star polyhedra includes the 4 regular ones, called the Kepler–Poinsot polyhedra, 5 quasiregular ones, and 48 semiregular ones.

There are also two infinite sets of uniform star prisms and uniform star antiprisms.

Just as (nondegenerate) star polygons (which have Polygon density greater than 1) correspond to circular polygons with overlapping tiles, star polyhedra that do not pass through the center have polytope density greater than 1, and correspond to spherical polyhedra with overlapping tiles; there are 48 nonprismatic such uniform star polyhedra. The remaining 9 nonprismatic uniform star polyhedra, those that pass through the center, are the hemipolyhedra, and do not correspond to spherical polyhedra, as the center cannot be projected uniquely onto the sphere.

The nonconvex forms are constructed from Schwarz triangles.

All the uniform polyhedra are listed below by their symmetry groups and subgrouped by their vertex arrangements.

Regular polyhedra are labeled by their Schläfli symbol. Other nonregular uniform polyhedra are listed with their vertex configuration or their Uniform polyhedron index U(1-80).

Note: For nonconvex forms below an additional descriptor Nonuniform is used when the convex hull vertex arrangement has same topology as one of these, but has nonregular faces. For example an nonuniform cantellated form may have rectangles created in place of the edges rather than squares.

Dihedral symmetry

See Prismatic uniform polyhedron.

Tetrahedral symmetry

There are two nonconvex forms, the tetrahemihexahedron and octahemioctahedron which have tetrahedral symmetry (with fundamental domain Möbius triangle (3 3 2)).

There are two Schwarz triangles that generate unique nonconvex uniform polyhedra: one right triangle (3/2 3 2), and one general triangle (3/2 3 3).

Vertex arrangement
(Convex hull)
Nonconvex forms
Tetrahedron  
Octahedron (4.3/2.4.3)
3/2 3 | 2
Truncated tetrahedron  
Cuboctahedron) (6.3/2.6.3)
3/2 3 | 3
Truncated octahedron)  
Icosahedron)  

Octahedral symmetry

There are 8 convex forms, and 10 nonconvex forms with octahedral symmetry (with fundamental domain Möbius triangle (4 3 2)).

There are four Schwarz_triangles that generate nonconvex forms, two right triangles (3/2 4 2), and (4/3 3 2), and two general triangles: (4/3 4 3), (3/2 4 4).

Vertex arrangement
(Convex hull)
Nonconvex forms
Cube  
Octahedron  
Cuboctahedron (6.4/3.6.4)
4/3 4 | 3
Truncated cube (4.8/3.4/3.8/5)
2 4/3 (3/2 4/2) |
(8/3.3.8/3.4)
3 4 | 4/3
(4.3/2.4.4)
3/2 4 | 2
Truncated octahedron  
Rhombicuboctahedron (4.8.4/3.8)
2 4 (3/2 4/2) |
(8.3/2.8.4)
3/2 4 | 4
(8/3.8/3.3)
2 3 | 4/3
truncated cuboctahedron (4.6.8/3)
2 3 4/3 |
truncated cuboctahedron (8/3.6.8)
3 4 4/3 |
Snub cube  

Icosahedral symmetry

There are 8 convex forms and 46 nonconvex forms with icosahedral symmetry (with fundamental domain Möbius triangle (5 3 2)). (or 47 nonconvex forms if Skilling's figure is included). Some of the nonconvex snub forms have reflective vertex symmetry.

Vertex arrangement
(Convex hull)
Nonconvex forms
Icosahedron {5,5/2} {5/2,5} {3,5/2}
truncated icosahedron
2 5 |3
U37
2 5/2 | 5
U61
5/2 3 | 5/3
U67
5/3 3 | 2
U73
2 5/3 (3/2 5/4) |
truncated icosahedron
2 5 |3
U38
5/2 5 | 2
U44
5/3 5 | 3
U56
2 3 (5/4 5/2) |
truncated icosahedron
2 5 |3
U32
| 5/2 3 3
Icosidodecahedron
2 | 3 5
U49
3/2 3 | 5
U51
5/4 5 | 5
U54
2 | 3 5/2
U70
5/3 5/2 | 5/3
U71
3 3 | 5/3
U36
2 | 5 5/2
U62
5/3 5/2 | 3
U65
5/4 5 | 3
Truncated dodecahedron
2 3 | 5
U42 U48 U63

Nonuniform
truncated dodecahedron
U72
Dodecahedron {5/2,3} U30 U41 U47
Rhombicosidodecahedron U33 U39 U58

Beveled
Dodecahedron
U55

Nonuniform
rhombicosidodecahedron
U31 U43 U50 U66

Nonuniform
rhombicosidodecahedron
U75 U64 Skilling's figure
(see below)
truncated icosidodecahedron U45
truncated icosidodecahedron U59
truncated icosidodecahedron U68
snub dodecahedron U40 U46 U57 U69 U60 U74

and the simetric excavated dodecahedro Ef1g1:

Skilling's figure

One further nonconvex polyhedron is the Great disnub dirhombidodecahedron, also known as Skilling's figure, which is vertex-uniform, but has pairs of edges which coincide in space such that four faces meet at some edges. It is sometimes but not always counted as a uniform polyhedron. It has Ih symmetry.

Degenerate cases

Coxeter identified a number of degenerate star polyhedra by the Wythoff construction method, which contain overlapping edges or vertices. These degenerate forms include:

See also

Notes

References

  • Brückner, M. Vielecke und vielflache. Theorie und geschichte.. Leipzig, Germany: Teubner, 1900. [1]
  • Har'El, Z. dual images
  • [2]
  • Messer, Peter W. , Discrete & Computational Geometry 27:353-375 (2002).

External links

  • MathWorld.
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