Icosahedron
In geometry, an icosahedron (/ˌaɪkɒsəˈhiːdrən, kə, koʊ/ or /aɪˌkɒsəˈhiːdrən/^{[1]}) is a polyhedron with 20 faces. The name comes from Ancient Greek εἴκοσι (eíkosi) 'twenty' and from Ancient Greek ἕδρα (hédra) ' seat'. The plural can be either "icosahedra" (/drə/) or "icosahedrons".
There are infinitely many nonsimilar shapes of icosahedra, some of them being more symmetrical than others. The best known is the (convex, nonstellated) regular icosahedron—one of the Platonic solids—whose faces are 20 equilateral triangles.
Regular icosahedra
Convex regular icosahedron 
Great icosahedron 
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 KeplerPoinsot polyhedra. Its Schläfli symbol is {3, 5/2}. 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 {5/2, 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 FiftyNine 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.
Notable stellations of the icosahedron  
Regular  Uniform duals  Regular compounds  Regular star  Others  
(Convex) icosahedron  Small triambic icosahedron  Medial triambic icosahedron  Great triambic icosahedron  Compound of five octahedra  Compound of five tetrahedra  Compound of ten tetrahedra  Great icosahedron  Excavated dodecahedron  Final stellation 

The stellation process on the icosahedron creates a number of related polyhedra and compounds with icosahedral symmetry. 
Pyritohedral symmetry
Pyritohedral and tetrahedral symmetries  

Coxeter diagrams  (pyritohedral) (tetrahedral)  
Schläfli symbol  s{3,4} sr{3,3} or  
Faces  20 triangles: 8 equilateral 12 isosceles  
Edges  30 (6 short + 24 long)  
Vertices  12  
Symmetry group  T_{h}, [4,3^{+}], (3*2), order 24  
Rotation group  T_{d}, [3,3]^{+}, (332), order 12  
Dual polyhedron  Pyritohedron  
Properties  convex  
Net  

A regular icosahedron can be distorted or marked up as a lower pyritohedral symmetry,^{[2]} and is called a snub octahedron, snub tetratetrahedron, snub tetrahedron, and pseudoicosahedron. 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: and respectively, each representing the lower symmetry to the regular icosahedron , (*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 signflips 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
In Jessen's icosahedron, sometimes called Jessen's orthogonal icosahedron, the 12 isosceles faces are arranged differently so that the figure is nonconvex 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.
Other icosahedra
Rhombic icosahedron
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 facetransitive.
Pyramid and prism symmetries
Common icosahedra with pyramid and prism symmetries include:
 19sided pyramid (plus 1 base = 20).
 18sided prism (plus 2 ends = 20).
 9sided antiprism (2 sets of 9 sides + 2 ends = 20).
 10sided bipyramid (2 sets of 10 sides = 20).
 10sided trapezohedron (2 sets of 10 sides = 20).
Johnson solids
Several Johnson solids are icosahedra:^{[3]}
J22  J35  J36  J59  J60  J92 

Gyroelongated triangular cupola 
Elongated triangular orthobicupola 
Elongated triangular gyrobicupola 
Parabiaugmented dodecahedron 
Metabiaugmented dodecahedron 
Triangular hebesphenorotunda 
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
 ^ Jones, Daniel (2003) [1917], Peter Roach; James Hartmann; Jane Setter (eds.), English Pronouncing Dictionary, Cambridge: Cambridge University Press, ISBN 3125396832
 ^ ^{a} ^{b} John Baez (September 11, 2011). "Fool's Gold".
 ^ Icosahedron on Mathworld.
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