Icosahedral symmetry

Short description: 3D symmetry group

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Point groups in three dimensions
Involutional symmetry
Cs, (*)
[ ] =  |
Cyclic symmetry
Cnv, (*nn)
[n] =  |
Dihedral symmetry
Dnh, (*n22)
[n,2] =

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Polyhedral group, [n,3], (*n32)

Tetrahedral symmetry
Td, (*332)
[3,3] =  |
Octahedral symmetry
Oh, (*432)
[4,3] =  |
Icosahedral symmetry
Ih, (*532)
[5,3] =
Icosahedral symmetry fundamental domains A soccer ball, a common example of a spherical truncated icosahedron, has full icosahedral symmetry. Rotations and reflections form the symmetry group of a great icosahedron.

In mathematics, and especially in geometry, an object has icosahedral symmetry if it has the same symmetries as a regular icosahedron. Examples of other polyhedra with icosahedral symmetry include the regular dodecahedron (the dual of the icosahedron) and the rhombic triacontahedron.

Every polyhedron with icosahedral symmetry has 60 rotational (or orientation-preserving) symmetries and 60 orientation-reversing symmetries (that combine a rotation and a reflection), for a total symmetry order of 120. The full symmetry group is the Coxeter group of type H3. It may be represented by Coxeter notation [5,3] and Coxeter diagram . The set of rotational symmetries forms a subgroup that is isomorphic to the alternating group A5 on 5 letters.

## Contents

* 1 As point group
* 1.1 Visualizations
* 2 Group structure
* 2.1 Isomorphism of I with A5
* 2.2 Commonly confused groups
* 2.3 Conjugacy classes
* 2.4 Subgroups of the full icosahedral symmetry group
* 2.4.1 Vertex stabilizers
* 2.4.2 Edge stabilizers
* 2.4.3 Face stabilizers
* 2.4.4 Polyhedron stabilizers
* 2.4.5 Coxeter group generators
* 3 Fundamental domain
* 4 Polyhedra with icosahedral symmetry
* 4.1 Chiral polyhedra
* 4.2 Full icosahedral symmetry
* 5 Other objects with icosahedral symmetry
* 5.1 Liquid crystals with icosahedral symmetry
* 5.2 Icosahedral nanoparticles
* 6 Related geometries
* 7 See also
* 8 References
* 9 External links

## As point group

Apart from the two infinite series of prismatic and antiprismatic symmetry, rotational icosahedral symmetry or chiral icosahedral symmetry of chiral objects and full icosahedral symmetry or achiral icosahedral symmetry are the discrete point symmetries (or equivalently, symmetries on the sphere) with the largest symmetry groups.

Icosahedral symmetry is not compatible with translational symmetry, so there are no associated crystallographic point groups or space groups.

Schö.  | Coxeter | Orb. | Abstract
structure  | Order

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I | [5,3]+ |  | 532 | A5 | 60
Ih | [5,3] |  | *532 | A5×2 | 120

Presentations corresponding to the above are:

I:⟨s,t∣s2,t3,(st)5⟩
Ih:⟨s,t∣s3(st)−2,t5(st)−2⟩.

These correspond to the icosahedral groups (rotational and full) being the (2,3,5) triangle groups.

The first presentation was given by William Rowan Hamilton in 1856, in his paper on icosian calculus.[1]

Note that other presentations are possible, for instance as an alternating group (for I).

### Visualizations

The full symmetry group is the Coxeter group of type H3. It may be represented by Coxeter notation [5,3] and Coxeter diagram . The set of rotational symmetries forms a subgroup that is isomorphic to the alternating group A5 on 5 letters.

Schoe.
(Orb.)  | Coxeter
notation | Elements  | Mirror diagrams

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Orthogonal  | Stereographic projection

(*532)  |

[5,3] | Mirror
lines:
15  | 120px  | 150px  | 150px  | 150px

(532)  |
File:Coxeter diagram chiral icosahedral group.png
[5,3]+ | Gyration
points:
12512px
20312px
30212px  | 120px  | 150px
12px  | 150px
12px  | 150px
12px

## Group structure

Every polyhedron with icosahedral symmetry has 60 rotational (or orientation-preserving) symmetries and 60 orientation-reversing symmetries (that combine a rotation and a reflection), for a total symmetry order of 120.

160px  | 200px

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The edges of a spherical compound of five octahedra represent the 15 mirror planes as colored great circles. Each octahedron can represent 3 orthogonal mirror planes by its edges.
160px  | 200px
The pyritohedral symmetry is an index 5 subgroup of icosahedral symmetry, with 3 orthogonal green reflection lines and 8 red order-3 gyration points. There are 5 different orientations of pyritohedral symmetry.

The icosahedral rotation group I is of order 60. The group I is isomorphic to A5, the alternating group of even permutations of five objects. This isomorphism can be realized by I acting on various compounds, notably the compound of five cubes (which inscribe in the dodecahedron), the compound of five octahedra, or either of the two compounds of five tetrahedra (which are enantiomorphs, and inscribe in the dodecahedron). The group contains 5 versions of Th with 20 versions of D3 (10 axes, 2 per axis), and 6 versions of D5.

The full icosahedral group Ih has order 120. It has I as normal subgroup of index 2\. The group Ih is isomorphic to I × Z2, or A5 × Z2, with the inversion in the center corresponding to element (identity,-1), where Z2 is written multiplicatively.

Ih acts on the compound of five cubes and the compound of five octahedra, but −1 acts as the identity (as cubes and octahedra are centrally symmetric). It acts on the compound of ten tetrahedra: I acts on the two chiral halves (compounds of five tetrahedra), and −1 interchanges the two halves. Notably, it does not act as S5, and these groups are not isomorphic; see below for details.

The group contains 10 versions of D3d and 6 versions of D5d (symmetries like antiprisms).

I is also isomorphic to PSL2(5), but Ih is not isomorphic to SL2(5).

### Isomorphism of I with A5

It is useful to describe explicitly what the isomorphism between I and A5 looks like. In the following table, permutations Pi and Qi act on 5 and 12 elements respectively, while the rotation matrices Mi are the elements of I. If Pk is the product of taking the permutation Pi and applying Pj to it, then for the same values of i, j and k, it is also true that Qk is the product of taking Qi and applying Qj, and also that premultiplying a vector by Mk is the same as premultiplying that vector by Mi and then premultiplying that result with Mj, that is Mk = Mj × Mi. Since the permutations Pi are all the 60 even permutations of 12345, the one-to-one correspondence is made explicit, therefore the isomorphism too.

Rotation matrix  | Permutation of 5
on 1 2 3 4 5  | Permutation of 12
on 1 2 3 4 5 6 7 8 9 10 11 12

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M1=[100010001] | P1 = ()  | Q1 = ()
M2=[−1212ϕϕ2−12ϕϕ2−12−ϕ2−12−12ϕ] | P2 = (3 4 5)  | Q2 = (1 11 8)(2 9 6)(3 5 12)(4 7 10)
M3=[−12−12ϕ−ϕ212ϕϕ2−12ϕ2−12−12ϕ] | P3 = (3 5 4)  | Q3 = (1 8 11)(2 6 9)(3 12 5)(4 10 7)
M4=[−1212ϕ−ϕ212ϕ−ϕ2−12−ϕ2−1212ϕ] | P4 = (2 3)(4 5)  | Q4 = (1 12)(2 8)(3 6)(4 9)(5 10)(7 11)
M5=[ϕ21212ϕ12−12ϕ−ϕ2−12ϕϕ2−12] | P5 = (2 3 4)  | Q5 = (1 2 3)(4 5 6)(7 9 8)(10 11 12)
M6=[−12ϕ−ϕ212ϕ2−12−12ϕ1212ϕϕ2] | P6 = (2 3 5)  | Q6 = (1 7 5)(2 4 11)(3 10 9)(6 8 12)
M7=[ϕ212−12ϕ12−12ϕϕ212ϕ−ϕ2−12] | P7 = (2 4 3)  | Q7 = (1 3 2)(4 6 5)(7 8 9)(10 12 11)
M8=[0−10001−100] | P8 = (2 4 5)  | Q8 = (1 10 6)(2 7 12)(3 4 8)(5 11 9)
M9=[−ϕ21212ϕ1212ϕϕ212ϕϕ2−12] | P9 = (2 4)(3 5)  | Q9 = (1 9)(2 5)(3 11)(4 12)(6 7)(8 10)
M10=[−12ϕϕ212−ϕ2−1212ϕ12−12ϕϕ2] | P10 = (2 5 3)  | Q10 = (1 5 7)(2 11 4)(3 9 10)(6 12 8)
M11=[00−1−100010] | P11 = (2 5 4)  | Q11 = (1 6 10)(2 12 7)(3 8 4)(5 9 11)
M12=[12ϕ−ϕ212−ϕ2−12−12ϕ12−12ϕ−ϕ2] | P12 = (2 5)(3 4)  | Q12 = (1 4)(2 10)(3 7)(5 8)(6 11)(9 12)
M13=[1000−1000−1] | P13 = (1 2)(4 5)  | Q13 = (1 3)(2 4)(5 8)(6 7)(9 10)(11 12)
M14=[−1212ϕϕ212ϕ−ϕ212ϕ21212ϕ] | P14 = (1 2)(3 4)  | Q14 = (1 5)(2 7)(3 11)(4 9)(6 10)(8 12)
M15=[−12−12ϕ−ϕ2−12ϕ−ϕ212−ϕ21212ϕ] | P15 = (1 2)(3 5)  | Q15 = (1 12)(2 10)(3 8)(4 6)(5 11)(7 9)
M16=[−12−12ϕϕ212ϕϕ212−ϕ212−12ϕ] | P16 = (1 2 3)  | Q16 = (1 11 6)(2 5 9)(3 7 12)(4 10 8)
M17=[−12ϕϕ2−12ϕ21212ϕ12−12ϕ−ϕ2] | P17 = (1 2 3 4 5)  | Q17 = (1 6 5 3 9)(4 12 7 8 11)
M18=[ϕ2−12−12ϕ1212ϕϕ2−12ϕ−ϕ212] | P18 = (1 2 3 5 4)  | Q18 = (1 4 8 6 2)(5 7 10 12 9)
M19=[−12ϕ−ϕ2−12−ϕ212−12ϕ1212ϕ−ϕ2] | P19 = (1 2 4 5 3)  | Q19 = (1 8 7 3 10)(2 12 5 6 11)
M20=[001−1000−10] | P20 = (1 2 4)  | Q20 = (1 7 4)(2 11 8)(3 5 10)(6 9 12)
M21=[12ϕϕ2−12−ϕ21212ϕ1212ϕϕ2] | P21 = (1 2 4 3 5)  | Q21 = (1 2 9 11 7)(3 6 12 10 4)
M22=[ϕ2−1212ϕ1212ϕ−ϕ212ϕϕ212] | P22 = (1 2 5 4 3)  | Q22 = (2 3 4 7 5)(6 8 10 11 9)
M23=[01000−1−100] | P23 = (1 2 5)  | Q23 = (1 9 8)(2 6 3)(4 5 12)(7 11 10)
M24=[−ϕ2−12−12ϕ12−12ϕ−ϕ212ϕ−ϕ212] | P24 = (1 2 5 3 4)  | Q24 = (1 10 5 4 11)(2 8 9 3 12)
M25=[−1212ϕ−ϕ2−12ϕϕ212ϕ212−12ϕ] | P25 = (1 3 2)  | Q25 = (1 6 11)(2 9 5)(3 12 7)(4 8 10)
M26=[ϕ21212ϕ−1212ϕϕ212ϕ−ϕ212] | P26 = (1 3 4 5 2)  | Q26 = (2 5 7 4 3)(6 9 11 10 8)
M27=[−12ϕ−ϕ212−ϕ21212ϕ−12−12ϕ−ϕ2] | P27 = (1 3 5 4 2)  | Q27 = (1 10 3 7 8)(2 11 6 5 12)
M28=[−12−12ϕϕ2−12ϕ−ϕ2−12ϕ2−1212ϕ] | P28 = (1 3)(4 5)  | Q28 = (1 7)(2 10)(3 11)(4 5)(6 12)(8 9)
M29=[−12ϕϕ2−12−ϕ2−12−12ϕ−1212ϕϕ2] | P29 = (1 3 4)  | Q29 = (1 9 10)(2 12 4)(3 6 8)(5 11 7)
M30=[ϕ2−12−12ϕ−12−12ϕ−ϕ212ϕϕ2−12] | P30 = (1 3 5)  | Q30 = (1 3 4)(2 8 7)(5 6 10)(9 12 11)
M31=[−ϕ212−12ϕ1212ϕ−ϕ2−12ϕ−ϕ2−12] | P31 = (1 3)(2 4)  | Q31 = (1 12)(2 6)(3 9)(4 11)(5 8)(7 10)
M32=[12ϕ−ϕ2−12ϕ212−12ϕ12−12ϕϕ2] | P32 = (1 3 2 4 5)  | Q32 = (1 4 10 11 5)(2 3 8 12 9)
M33=[1212ϕϕ212ϕϕ2−12−ϕ21212ϕ] | P33 = (1 3 5 2 4)  | Q33 = (1 5 9 6 3)(4 7 11 12 8)
M34=[12ϕϕ212ϕ2−1212ϕ1212ϕ−ϕ2] | P34 = (1 3)(2 5)  | Q34 = (1 2)(3 5)(4 9)(6 7)(8 11)(10 12)
M35=[−ϕ2−1212ϕ12−12ϕϕ2−12ϕϕ212] | P35 = (1 3 2 5 4)  | Q35 = (1 11 2 7 9)(3 10 6 4 12)
M36=[12−12ϕ−ϕ212ϕ−ϕ212−ϕ2−12−12ϕ] | P36 = (1 3 4 2 5)  | Q36 = (1 8 2 4 6)(5 10 9 7 12)
M37=[ϕ212−12ϕ−1212ϕ−ϕ2−12ϕϕ212] | P37 = (1 4 5 3 2)  | Q37 = (1 2 6 8 4)(5 9 12 10 7)
M38=[0−1000−1100] | P38 = (1 4 2)  | Q38 = (1 4 7)(2 8 11)(3 10 5)(6 12 9)
M39=[−ϕ21212ϕ−12−12ϕ−ϕ2−12ϕ−ϕ212] | P39 = (1 4 3 5 2)  | Q39 = (1 11 4 5 10)(2 12 3 9 8)
M40=[−12ϕ−ϕ2−12ϕ2−1212ϕ−12−12ϕϕ2] | P40 = (1 4 3)  | Q40 = (1 10 9)(2 4 12)(3 8 6)(5 7 11)
M41=[001100010] | P41 = (1 4 5)  | Q41 = (1 5 2)(3 7 9)(4 11 6)(8 10 12)
M42=[12ϕϕ2−12ϕ2−12−12ϕ−12−12ϕ−ϕ2] | P42 = (1 4)(3 5)  | Q42 = (1 6)(2 3)(4 9)(5 8)(7 12)(10 11)
M43=[−ϕ212−12ϕ−12−12ϕϕ212ϕϕ212] | P43 = (1 4 5 2 3)  | Q43 = (1 9 7 2 11)(3 12 4 6 10)
M44=[12ϕ−ϕ2−12−ϕ2−1212ϕ−1212ϕ−ϕ2] | P44 = (1 4)(2 3)  | Q44 = (1 8)(2 10)(3 4)(5 12)(6 7)(9 11)
M45=[1212ϕϕ2−12ϕ−ϕ212ϕ2−12−12ϕ] | P45 = (1 4 2 3 5)  | Q45 = (2 7 3 5 4)(6 11 8 9 10)
M46=[1212ϕ−ϕ212ϕϕ212ϕ2−1212ϕ] | P46 = (1 4 2 5 3)  | Q46 = (1 3 6 9 5)(4 8 12 11 7)
M47=[12−12ϕϕ2−12ϕϕ212−ϕ2−1212ϕ] | P47 = (1 4 3 2 5)  | Q47 = (1 7 10 8 3)(2 5 11 12 6)
M48=[−10001000−1] | P48 = (1 4)(2 5)  | Q48 = (1 12)(2 9)(3 11)(4 10)(5 6)(7 8)
M49=[−12ϕϕ212ϕ212−12ϕ−1212ϕ−ϕ2] | P49 = (1 5 4 3 2)  | Q49 = (1 9 3 5 6)(4 11 8 7 12)
M50=[00−11000−10] | P50 = (1 5 2)  | Q50 = (1 8 9)(2 3 6)(4 12 5)(7 10 11)
M51=[12ϕ−ϕ212ϕ21212ϕ−1212ϕϕ2] | P51 = (1 5 3 4 2)  | Q51 = (1 7 11 9 2)(3 4 10 12 6)
M52=[ϕ2−1212ϕ−12−12ϕϕ2−12ϕ−ϕ2−12] | P52 = (1 5 3)  | Q52 = (1 4 3)(2 7 8)(5 10 6)(9 11 12)
M53=[010001100] | P53 = (1 5 4)  | Q53 = (1 2 5)(3 9 7)(4 6 11)(8 12 10)
M54=[−ϕ2−12−12ϕ−1212ϕϕ2−12ϕϕ2−12] | P54 = (1 5)(3 4)  | Q54 = (1 12)(2 11)(3 10)(4 8)(5 9)(6 7)
M55=[12ϕϕ212−ϕ212−12ϕ−12−12ϕϕ2] | P55 = (1 5 4 2 3)  | Q55 = (1 5 11 10 4)(2 9 12 8 3)
M56=[−ϕ2−1212ϕ−1212ϕ−ϕ212ϕ−ϕ2−12] | P56 = (1 5)(2 3)  | Q56 = (1 10)(2 12)(3 11)(4 7)(5 8)(6 9)
M57=[12−12ϕ−ϕ2−12ϕϕ2−12ϕ21212ϕ] | P57 = (1 5 2 3 4)  | Q57 = (1 3 8 10 7)(2 6 12 11 5)
M58=[1212ϕ−ϕ2−12ϕ−ϕ2−12−ϕ212−12ϕ] | P58 = (1 5 2 4 3)  | Q58 = (1 6 4 2 8)(5 12 7 9 10)
M59=[12−12ϕϕ212ϕ−ϕ2−12ϕ212−12ϕ] | P59 = (1 5 3 2 4)  | Q59 = (2 4 5 3 7)(6 10 9 8 11)
M60=[−1000−10001] | P60 = (1 5)(2 4)  | Q60 = (1 11)(2 10)(3 12)(4 9)(5 7)(6 8)

This non-abelian simple group is the only non-trivial normal subgroup of the symmetric group on five letters.[2] Since the Galois group of the general quintic equation is isomorphic to the symmetric group on five letters, and this normal subgroup is simple and non-abelian, the general quintic equation does not have a solution in radicals. The proof of the Abel–Ruffini theorem uses this simple fact,[3] and Felix Klein wrote a book that made use of the theory of icosahedral symmetries to derive an analytical solution to the general quintic equation.[4] A modern exposition is given in (Tóth 2002).[5]

### Commonly confused groups

The following groups all have order 120, but are not isomorphic:

* S5, the symmetric group on 5 elements
* Ih, the full icosahedral group (subject of this article, also known as H3)
* 2I, the binary icosahedral group

They correspond to the following short exact sequences (the latter of which does not split) and product

1->A5->S5->Z2->1
Ih=A5×Z2
1->Z2->2I->A5->1

In words,

* A5 is a normal subgroup of S5
* A5 is a factor of Ih, which is a direct product
* A5 is a quotient group of 2I

Note that A5 has an exceptional irreducible 3-dimensional representation (as the icosahedral rotation group), but S5 does not have an irreducible 3-dimensional representation, corresponding to the full icosahedral group not being the symmetric group.

These can also be related to linear groups over the finite field with five elements, which exhibit the subgroups and covering groups directly; none of these are the full icosahedral group:

* A5≅PSL⁡(2,5), the projective special linear group, see here for a proof;
* S5≅PGL⁡(2,5), the projective general linear group;
* 2I≅SL⁡(2,5), the special linear group.

### Conjugacy classes

The 120 symmetries fall into 10 conjugacy classes.

conjugacy classes I | additional classes of Ih

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* identity, order 1
* 12 × rotation by ±72°, order 5, around the 6 axes through the face centers of the dodecahedron
* 12 × rotation by ±144°, order 5, around the 6 axes through the face centers of the dodecahedron
* 20 × rotation by ±120°, order 3, around the 10 axes through vertices of the dodecahedron
* 15 × rotation by 180°, order 2, around the 15 axes through midpoints of edges of the dodecahedron

* central inversion, order 2
* 12 × rotoreflection by ±36°, order 10, around the 6 axes through the face centers of the dodecahedron
* 12 × rotoreflection by ±108°, order 10, around the 6 axes through the face centers of the dodecahedron
* 20 × rotoreflection by ±60°, order 6, around the 10 axes through the vertices of the dodecahedron
* 15 × reflection, order 2, at 15 planes through edges of the dodecahedron

### Subgroups of the full icosahedral symmetry group

Subgroup relations Chiral subgroup relations

Each line in the following table represents one class of conjugate (i.e., geometrically equivalent) subgroups. The column "Mult." (multiplicity) gives the number of different subgroups in the conjugacy class.

Explanation of colors: green = the groups that are generated by reflections, red = the chiral (orientation-preserving) groups, which contain only rotations.

The groups are described geometrically in terms of the dodecahedron.

The abbreviation "h.t.s.(edge)" means "halfturn swapping this edge with its opposite edge", and similarly for "face" and "vertex".

Schön. | Coxeter | Orb. | H-M | Structure | Cyc. | Order | Index | Mult. | Description

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Ih | [5,3] |  | *532 | 532/m | A5×Z2 |  | 120 | 1 | 1 | full group
D2h | [2,2] |  | *222 | mmm | D4×D2=D23 | 25px | 8 | 15 | 5 | fixing two opposite edges, possibly swapping them
C5v | [5] |  | *55 | 5m | D10 | 25px | 10 | 12 | 6 | fixing a face
C3v | [3] |  | *33 | 3m | D6=S3 | 25px | 6 | 20 | 10 | fixing a vertex
C2v | [2] |  | *22 | 2mm | D4=D22 | 25px | 4 | 30 | 15 | fixing an edge
Cs | [ ] |  | * | 2 or m | D2 | 25px | 2 | 60 | 15 | reflection swapping two endpoints of an edge
Th | [3+,4] |  | 3*2 | m3 | A4×Z2 | 25px | 24 | 5 | 5 | pyritohedral group
D5d | [2+,10] |  | 2*5 | 10m2 | D20=Z2×D10 | 25px | 20 | 6 | 6 | fixing two opposite faces, possibly swapping them
D3d | [2+,6] |  | 2*3 | 3m | D12=Z2×D6 | 25px | 12 | 10 | 10 | fixing two opposite vertices, possibly swapping them
D1d = C2h | [2+,2] |  | 2* | 2/m | D4=Z2×D2 | 25px | 4 | 30 | 15 | halfturn around edge midpoint, plus central inversion
S10 | [2+,10+] |  | 5× | 5 | Z10=Z2×Z5 | 25px | 10 | 12 | 6 | rotations of a face, plus central inversion
S6 | [2+,6+] |  | 3× | 3 | Z6=Z2×Z3 | 25px | 6 | 20 | 10 | rotations about a vertex, plus central inversion
S2 | [2+,2+] |  | × | 1 | Z2 | 25px | 2 | 60 | 1 | central inversion
I | [5,3]+ |  | 532 | 532 | A5 |  | 60 | 2 | 1 | all rotations
T | [3,3]+ |  | 332 | 332 | A4 | 25px | 12 | 10 | 5 | rotations of a contained tetrahedron
D5 | [2,5]+ |  | 522 | 522 | D10 | 25px | 10 | 12 | 6 | rotations around the center of a face, and h.t.s.(face)
D3 | [2,3]+ |  | 322 | 322 | D6=S3 | 25px | 6 | 20 | 10 | rotations around a vertex, and h.t.s.(vertex)
D2 | [2,2]+ |  | 222 | 222 | D4=Z22 | 25px | 4 | 30 | 5 | halfturn around edge midpoint, and h.t.s.(edge)
C5 | [5]+ |  | 55 | 5 | Z5 | 25px | 5 | 24 | 6 | rotations around a face center
C3 | [3]+ |  | 33 | 3 | Z3=A3 | 25px | 3 | 40 | 10 | rotations around a vertex
C2 | [2]+ |  | 22 | 2 | Z2 | 25px | 2 | 60 | 15 | half-turn around edge midpoint
C1 | [ ]+ |  | 11 | 1 | Z1 | 25px | 1 | 120 | 1 | trivial group

#### Vertex stabilizers

Stabilizers of an opposite pair of vertices can be interpreted as stabilizers of the axis they generate.

* vertex stabilizers in I give cyclic groups C3
* vertex stabilizers in Ih give dihedral groups D3
* stabilizers of an opposite pair of vertices in I give dihedral groups D3
* stabilizers of an opposite pair of vertices in Ih give D3×±1

#### Edge stabilizers

Stabilizers of an opposite pair of edges can be interpreted as stabilizers of the rectangle they generate.

* edges stabilizers in I give cyclic groups Z2
* edges stabilizers in Ih give Klein four-groups Z2×Z2
* stabilizers of a pair of edges in I give Klein four-groups Z2×Z2; there are 5 of these, given by rotation by 180° in 3 perpendicular axes.
* stabilizers of a pair of edges in Ih give Z2×Z2×Z2; there are 5 of these, given by reflections in 3 perpendicular axes.

#### Face stabilizers

Stabilizers of an opposite pair of faces can be interpreted as stabilizers of the antiprism they generate.

* face stabilizers in I give cyclic groups C5
* face stabilizers in Ih give dihedral groups D5
* stabilizers of an opposite pair of faces in I give dihedral groups D5
* stabilizers of an opposite pair of faces in Ih give D5×±1

#### Polyhedron stabilizers

For each of these, there are 5 conjugate copies, and the conjugation action gives a map, indeed an isomorphism, I->∼A5<S5.

* stabilizers of the inscribed tetrahedra in I are a copy of T
* stabilizers of the inscribed tetrahedra in Ih are a copy of T
* stabilizers of the inscribed cubes (or opposite pair of tetrahedra, or octahedra) in I are a copy of T
* stabilizers of the inscribed cubes (or opposite pair of tetrahedra, or octahedra) in Ih are a copy of Th

#### Coxeter group generators

The full icosahedral symmetry group [5,3] () of order 120 has generators represented by the reflection matrices R0, R1, R2 below, with relations R02 = R12 = R22 = (R0×R1)5 = (R1×R2)3 = (R0×R2)2 = Identity. The group [5,3]+ () of order 60 is generated by any two of the rotations S0,1, S1,2, S0,2. A rotoreflection of order 10 is generated by V0,1,2, the product of all 3 reflections. Here ϕ=5+12 denotes the golden ratio.

[5,3],  | Reflections  | Rotations  | Rotoreflection

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Name  | R0 | R1 | R2 | S0,1 | S1,2 | S0,2 | V0,1,2
Group  |  |  |  |  |  |  |
Order  | 2 | 2 | 2 | 5 | 3 | 2 | 10
Matrix  | [−100010001] | [1−ϕ2−ϕ2−12−ϕ2121−ϕ2−121−ϕ2ϕ2] | [1000−10001] | [ϕ−12ϕ212−ϕ2121−ϕ2−121−ϕ2ϕ2] | [1−ϕ2ϕ2−12−ϕ2−121−ϕ2−12ϕ−12ϕ2] | [−1000−10001] | [ϕ−12−ϕ212−ϕ2−121−ϕ2−12ϕ−12ϕ2]
| (1,0,0)n | (ϕ2,12,ϕ−12)n | (0,1,0)n | (0,−1,ϕ)axis | (1−ϕ,0,ϕ)axis | (0,0,1)axis |

## Fundamental domain

Fundamental domains for the icosahedral rotation group and the full icosahedral group are given by:

200px
Icosahedral rotation group
I | 200px
Full icosahedral group
Ih | 180px
Faces of disdyakis triacontahedron are the fundamental domain

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In the disdyakis triacontahedron one full face is a fundamental domain; other solids with the same symmetry can be obtained by adjusting the orientation of the faces, e.g. flattening selected subsets of faces to combine each subset into one face, or replacing each face by multiple faces, or a curved surface.

## Polyhedra with icosahedral symmetry

Examples of other polyhedra with icosahedral symmetry include the regular dodecahedron (the dual of the icosahedron) and the rhombic triacontahedron.

### Chiral polyhedra

Class  | Symbols  | Picture

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Archimedean | sr{5,3}
| 50px
Catalan | V3.3.3.3.5
| 50px

### Full icosahedral symmetry

Platonic solid | Kepler–Poinsot polyhedra | Archimedean solids

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50px
{5,3}
| 50px
{5/2,5}
| 50px
{5/2,3}
| 50px
t{5,3}
| 50px
t{3,5}
| 50px
r{3,5}
| 50px
rr{3,5}
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tr{3,5}

Platonic solid | Kepler–Poinsot polyhedra  | Catalan solids
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{3,5}
=  | 50px
{5,5/2}
=  | 50px
{3,5/2}
=  | 50px
V3.10.10
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V5.6.6
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V3.5.3.5
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V3.4.5.4
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V4.6.10

## Other objects with icosahedral symmetry

Examples of icosahedral symmetry

Circogonia icosahedra, a Radiolarian

Capsid of an Adenovirus

The dodecaborate ion [B12H12]2−

* Barth surfaces
* Virus structure, and Capsid
* In chemistry, the dodecaborate ion ([B12H12]2−) and the dodecahedrane molecule (C20H20)

### Liquid crystals with icosahedral symmetry

For the intermediate material phase called liquid crystals the existence of icosahedral symmetry was proposed by H. Kleinert and K. Maki[6] and its structure was first analyzed in detail in that paper. See the review article here. In aluminum, the icosahedral structure was discovered experimentally three years after this by Dan Shechtman, which earned him the Nobel Prize in 2011.

### Icosahedral nanoparticles

At small sizes, many elements form icosahedral nanoparticles, which are often lower in energy than single crystals.[7][8]

## Related geometries

Icosahedral symmetry is equivalently the projective special linear group PSL(2,5), and is the symmetry group of the modular curve X(5), and more generally PSL(2,p) is the symmetry group of the modular curve X(p). The modular curve X(5) is geometrically a dodecahedron with a cusp at the center of each polygonal face, which demonstrates the symmetry group.

This geometry, and associated symmetry group, was studied by Felix Klein as the monodromy groups of a Belyi surface – a Riemann surface with a holomorphic map to the Riemann sphere, ramified only at 0, 1, and infinity (a Belyi function) – the cusps are the points lying over infinity, while the vertices and the centers of each edge lie over 0 and 1; the degree of the covering (number of sheets) equals 5.

Klein's investigations continued with his discovery of order 7 and order 11 symmetries in (Klein 1878) and (Klein 1879) (and associated coverings of degree 7 and 11) and dessins d'enfants, the first yielding the Klein quartic, whose associated geometry has a tiling by 24 heptagons (with a cusp at the center of each).

Similar geometries occur for PSL(2,n) and more general groups for other modular curves.

More exotically, there are special connections between the groups PSL(2,5) (order 60), PSL(2,7) (order 168) and PSL(2,11) (order 660), which also admit geometric interpretations – PSL(2,5) is the symmetries of the icosahedron (genus 0), PSL(2,7) of the Klein quartic (genus 3), and PSL(2,11) the buckyball surface (genus 70). These groups form a "trinity" in the sense of Vladimir Arnold, which gives a framework for the various relationships; see trinities for details.

There is a close relationship to other Platonic solids.

Poincare used the rotational symmetry group I⊂SO3 to construct the Poincare homology sphere as the quotient manifold X=SO3/I, an important example of a space whose homology is the same as that of the sphere S3, but which is not homotopic to it.

## See also

* Tetrahedral symmetry
* Octahedral symmetry
* Binary icosahedral group
* Icosian calculus

## References

1. ↑ Sir William Rowan Hamilton (1856), "Memorandum respecting a new System of Roots of Unity", Philosophical Magazine 12: 446, http://www.maths.tcd.ie/pub/HistMath/People/Hamilton/Icosian/NewSys.pdf
2. ↑ Gray, Hermann (2018), A History of Abstract Algebra: From Algebraic Equations to Modern Algebra, Springer, p. 371, doi:10.1007/978-3-319-94772-3, ISBN 978-3-319-94772-3, https://books.google.com/books?id=gl1oDwAAQBAJ&pg=PA371
3. ↑ Rotman 1998, pp. 74–75. sfn error: no target: CITEREFRotman1998 (help)
4. ↑ Lectures on the ikosahedron and the solution of equations of the fifth degree, Courier Corporation, 1888, ISBN 978-0-486-49528-6, https://digital.library.cornell.edu/cgi/t/text/text-idx?c=math;cc=math;view=toc;subview=short;idno=03070001, Dover edition, translated from Klein, Felix (1884), Vorlesungen über das Ikosaeder und die Auflösung der Gleichungen vom fünften Grade, Teubner, https://archive.org/details/vorlesungenberd00kleigoog
5. ↑ Toth, Gabor (2002), "Section 1.6, Additional Topic", Klein's Theory of the Icosahedron, Springer, p. 66, https://books.google.com/books?id=i76mmyvDHYUC&pg=PA66
6. ↑ Kleinert, H.; Maki, K. (1981). "Lattice Textures in Cholesteric Liquid Crystals". Fortschritte der Physik 29 (5): 219–259. doi:10.1002/prop.19810290503. Bibcode: 1981ForPh..29..219K. http://www.physik.fu-berlin.de/~kleinert/75/75.pdf. Retrieved 2011-10-06.
7. ↑ Ino, Shozo (1969-10-15). "Stability of Multiply-Twinned Particles". Journal of the Physical Society of Japan 27 (4): 941–953. doi:10.1143/JPSJ.27.941. ISSN 0031-9015. Bibcode: 1969JPSJ...27..941I. https://journals.jps.jp/doi/10.1143/JPSJ.27.941.
8. ↑ Marks, L D; Peng, L (2016-01-21). "Nanoparticle shape, thermodynamics and kinetics". Journal of Physics: Condensed Matter 28 (5). doi:10.1088/0953-8984/28/5/053001. ISSN 0953-8984. PMID 26792459. Bibcode: 2016JPCM...28e3001M. https://doi.org/10.1088/0953-8984/28/5/053001.

* Klein, F. (1878). "Ueber die Transformation siebenter Ordnung der elliptischen Functionen". Mathematische Annalen 14 (3): 428–471. doi:10.1007/BF01677143. https://zenodo.org/record/1635511. Translated in Levy, Silvio, ed (1999). The Eightfold Way. Cambridge University Press. ISBN 978-0-521-66066-2. http://www.msri.org/communications/books/Book35/index.html.
* Klein, F. (1879), "Ueber die Transformation elfter Ordnung der elliptischen Functionen (On the eleventh order transformation of elliptic functions)", Mathematische Annalen 15 (3–4): 533–555, doi:10.1007/BF02086276, https://zenodo.org/record/1642598 , collected as pp. 140–165 in Oeuvres, Tome 3
* Klein, Felix (1888), Lectures on the Icosahedron and the Solution of Equations of the Fifth Degree, Trübner & Co., ISBN 0-486-49528-0trans. George Gavin Morrice
* Tóth, Gábor (2002), Finite Möbius groups, minimal immersions of spheres, and moduli
* Peter R. Cromwell, Polyhedra (1997), p. 296
* The Symmetries of Things 2008, John H. Conway, Heidi Burgiel, Chaim Goodman-Strauss, ISBN 978-1-56881-220-5
* Kaleidoscopes: Selected Writings of H.S.M. Coxeter, edited by F. Arthur Sherk, Peter McMullen, Anthony C. Thompson, Asia Ivic Weiss, Wiley-Interscience Publication, 1995, ISBN 978-0-471-01003-6 [1]
* N.W. Johnson: Geometries and Transformations, (2018) ISBN 978-1-107-10340-5 Chapter 11: Finite symmetry groups, 11.5 Spherical Coxeter groups

## External links

* Weisstein, Eric W.. "Icosahedral group". http://mathworld.wolfram.com/IcosahedralGroup.html.
* THE SUBGROUPS OF W(H3) (Subgroups of other Coxeter groups ) Gotz Pfeiffer

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