Английская Википедия:Icosahedron
Шаблон:Pp-semi-indef Шаблон:Short description
In geometry, an icosahedron (Шаблон:IPAc-en or Шаблон:IPAc-enШаблон:Refn) is a polyhedron with 20 faces. The name comes Шаблон:Ety. The plural can be either "icosahedra" (Шаблон:IPAc-en) or "icosahedrons".
There are infinitely many non-similar shapes of icosahedra, some of them being more symmetrical than others. The best known is the (convex, non-stellated) regular icosahedron—one of the Platonic solids—whose faces are 20 equilateral triangles.
Regular icosahedra
Шаблон:Multiple image There are two objects, one convex and one nonconvex, that can both be called regular icosahedra. Each has 30 edges and 20 equilateral triangle faces with five meeting at each of its twelve vertices. Both have icosahedral symmetry. The term "regular icosahedron" generally refers to the convex variety, while the nonconvex form is called a great icosahedron.
Convex regular icosahedron
The convex regular icosahedron is usually referred to simply as the regular icosahedron, one of the five regular Platonic solids, and is represented by its Schläfli symbol {3, 5}, containing 20 triangular faces, with 5 faces meeting around each vertex.
Its dual polyhedron is the regular dodecahedron {5, 3} having three regular pentagonal faces around each vertex.
Great icosahedron
The great icosahedron is one of the four regular star Kepler-Poinsot polyhedra. Its Schläfli symbol is {3, Шаблон:Sfrac}. Like the convex form, it also has 20 equilateral triangle faces, but its vertex figure is a pentagram rather than a pentagon, leading to geometrically intersecting faces. The intersections of the triangles do not represent new edges.
Its dual polyhedron is the great stellated dodecahedron {Шаблон:Sfrac, 3}, having three regular star pentagonal faces around each vertex.
Stellated icosahedra
Stellation is the process of extending the faces or edges of a polyhedron until they meet to form a new polyhedron. It is done symmetrically so that the resulting figure retains the overall symmetry of the parent figure.
In their book The Fifty-Nine Icosahedra, Coxeter et al. enumerated 58 such stellations of the regular icosahedron.
Of these, many have a single face in each of the 20 face planes and so are also icosahedra. The great icosahedron is among them.
Other stellations have more than one face in each plane or form compounds of simpler polyhedra. These are not strictly icosahedra, although they are often referred to as such. Шаблон:Icosahedron stellations
Pyritohedral symmetry
| Pyritohedral and tetrahedral symmetries | |||||
|---|---|---|---|---|---|
| Coxeter diagrams | Шаблон:CDD (pyritohedral) Файл:Uniform polyhedron-43-h01.svg Шаблон:CDD (tetrahedral) Файл:Uniform polyhedron-33-s012.svg | ||||
| Schläfli symbol | s{3,4} sr{3,3} or <math>s\begin{Bmatrix} 3 \\ 3 \end{Bmatrix}</math> | ||||
| Faces | 20 triangles: 8 equilateral 12 isosceles | ||||
| Edges | 30 (6 short + 24 long) | ||||
| Vertices | 12 | ||||
| Symmetry group | Th, [4,3+], (3*2), order 24 | ||||
| Rotation group | Td, [3,3]+, (332), order 12 | ||||
| Dual polyhedron | Pyritohedron | ||||
| Properties | convex | ||||
| Файл:Pseudoicosahedron flat.png Net | |||||
| |||||
A regular icosahedron can be distorted or marked up as a lower pyritohedral symmetry,[1][2] and is called a snub octahedron, snub tetratetrahedron, snub tetrahedron, and pseudo-icosahedron.[3] This can be seen as an alternated truncated octahedron. If all the triangles are equilateral, the symmetry can also be distinguished by colouring the 8 and 12 triangle sets differently.
Pyritohedral symmetry has the symbol (3*2), [3+,4], with order 24. Tetrahedral symmetry has the symbol (332), [3,3]+, with order 12. These lower symmetries allow geometric distortions from 20 equilateral triangular faces, instead having 8 equilateral triangles and 12 congruent isosceles triangles.
These symmetries offer Coxeter diagrams: Шаблон:CDD and Шаблон:CDD respectively, each representing the lower symmetry to the regular icosahedron Шаблон:CDD, (*532), [5,3] icosahedral symmetry of order 120.
Cartesian coordinates
The coordinates of the 12 vertices can be defined by the vectors defined by all the possible cyclic permutations and sign-flips of coordinates of the form (2, 1, 0). These coordinates represent the truncated octahedron with alternated vertices deleted.
This construction is called a snub tetrahedron in its regular icosahedron form, generated by the same operations carried out starting with the vector (ϕ, 1, 0), where ϕ is the golden ratio.[2]
Jessen's icosahedron
Шаблон:Main In Jessen's icosahedron, sometimes called Jessen's orthogonal icosahedron, the 12 isosceles faces are arranged differently so that the figure is non-convex and has right dihedral angles.
It is scissors congruent to a cube, meaning that it can be sliced into smaller polyhedral pieces that can be rearranged to form a solid cube.
Cuboctahedron
Шаблон:Main A regular icosahedron is topologically identical to a cuboctahedron with its 6 square faces bisected on diagonals with pyritohedral symmetry. The icosahedra with pyritohedral symmetry constitute an infinite family of polyhedra which include the cuboctahedron, regular icosahedron, Jessen's icosahedron, and double cover octahedron. Cyclical kinematic transformations among the members of this family exist.
Other icosahedra
Rhombic icosahedron
Шаблон:Main The rhombic icosahedron is a zonohedron made up of 20 congruent rhombs. It can be derived from the rhombic triacontahedron by removing 10 middle faces. Even though all the faces are congruent, the rhombic icosahedron is not face-transitive.
Pyramid and prism symmetries
Common icosahedra with pyramid and prism symmetries include:
- 19-sided pyramid (plus 1 base = 20).
- 18-sided prism (plus 2 ends = 20).
- 9-sided antiprism (2 sets of 9 sides + 2 ends = 20).
- 10-sided bipyramid (2 sets of 10 sides = 20).
- 10-sided trapezohedron (2 sets of 10 sides = 20).
Johnson solids
Several Johnson solids are icosahedra:[4]
| J22 | J35 | J36 | J59 | J60 | J92 |
|---|---|---|---|---|---|
| Файл:Gyroelongated triangular cupola.png Gyroelongated triangular cupola |
Файл:Elongated triangular orthobicupola.png Elongated triangular orthobicupola |
Файл:Elongated triangular gyrobicupola.png Elongated triangular gyrobicupola |
Файл:Parabiaugmented dodecahedron.png Parabiaugmented dodecahedron |
Файл:Metabiaugmented dodecahedron.png Metabiaugmented dodecahedron |
Файл:Triangular hebesphenorotunda.png Triangular hebesphenorotunda |
| Файл:Johnson solid 22 net.png | Файл:Johnson solid 35 net.png | Файл:Johnson solid 36 net.png | Файл:Johnson solid 59 net.png | Файл:Johnson solid 60 net.png | Файл:Johnson solid 92 net.png |
| 16 triangles 3 squares 1 hexagon |
8 triangles 12 squares |
8 triangles 12 squares |
10 triangles 10 pentagons |
10 triangles 10 pentagons |
13 triangles 3 squares 3 pentagons 1 hexagon |
See also
References
Шаблон:Commons category Шаблон:Reflist
Шаблон:Polyhedra Шаблон:Polyhedron navigator Шаблон:Authority control
- ↑ Шаблон:Cite journal
- ↑ 2,0 2,1 Шаблон:Cite web
- ↑ Шаблон:Cite book
- ↑ Icosahedron on Mathworld.
