Icosahedron

The icosahedron, or ike, is one of the five Platonic solids. It has 20 triangles as faces, joining 5 to a vertex.

An alternate, lower symmetry construction as a snub tetrahedron, furthermore relates the icosahedron to the snub polytopes, most notably to the snub disicositetrachoron, of which it is a cell.

It is the only Platonic solid that does not appear as a cell in one of the convex regular polychora, because the dual dodecahedron's circumradius is greater than 1. It does, however, appear as the vertex figure of the hexacosichoron, and as the cell of the non-convex faceted hexacosichoron.

Vertex coordinates
The vertices of an icosahedron of edge length 1, centered at the origin, are all cyclic permutations of:


 * (0, ±1/2, ±($\sqrt{(5+√5)/8}$+1)/4).

In vertex figures
The icosahedron is seen as a vertex figure in two regular polychora: the hexacosichoron, where it has an edge length of 1, and the great stellated hecatonicosachoron, where it has an edge length of ($\sqrt{3}$–1)/2.

Snub tetrahedron
The icosahedron can also be considered to be a kind of snub tetrahedron, by analogy with the snub cube and snub dodecahedron. It is the result of alternating the vertices of a truncated octahedron and then adjusting edge lengths to be equal. It can be represented as s3s3s.

Related polyhedra
The icosahedron is the colonel of a two-member regiment that also includes the great dodecahedron.

The icosahedron is related to many Johnson solids. Most obviously, it can be constructed by joining two pentagonal pyramids to a pentagonal antiprism. This means the icosahedron could also be called a gyroelongated pentagonal bipyramid. Joining a single pentagonal pyramid, or diminishing one vertex from the icosahedron, yields the gyroelongated pentagonal pyramid, and replacing the antiprism by a pentagonal prism yields the elongated pentagonal pyramid and the elongated pentagonal bipyramid. Cutting off two pyramids from two non-parallel, non-adjacent vertices yields the metabidiminished icosahedron, and cutting off a further non-adjacent pyramid yields the tridiminished icosahedron.

A much less obvious connection is with the hebesphenomegacorona, which may be derived from the icosahedron by expanding a single edge into a square, thus turning the two adjacent faces into squares as well. Similarly, if we take two opposite edges of the icosahedron and "stretch" them into squares via a partial Stott expansion, we obtain the bilunabirotunda.

The icosahedron can be considered to be a snub triangular antiprism, by analogy with the snub disphenoid and snub square antiprism. This can be seen since the icosahedron can be constructed from the octahedron, that is a triangular antiprism, but cutting it into two halves and inserting a set of 12 triangles between the halves.