Munn semigroup

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In geometry, Coxeter notation is a system of classifying symmetry groups, describing the angles between with fundamental reflections of a Coxeter group. It uses a bracketed notation, with modifiers to indicate certain subgroups. The notation is named after H. S. M. Coxeter, and has been more comprehensively defined by Norman Johnson.

Reflectional groups

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For Coxeter groups defined by pure reflections, there is a direct correspondence between the bracket notation and Coxeter-Dynkin diagram graphs. The numbers in the bracket notation represent the mirror reflection orders in the branches of the Coxeter graph.

The Coxeter notation is simplified with exponents to represent the number of branches in a row for linear graphs. So the An group is represented by [3n-1], to imply n nodes connected by n-1 order-3 branches.

Coxeter initially represented bifurcating diagrams with vertical positioning of numbers, but later abbreviated with an exponent notation, like [3p,q,r], starting with [31,1,1] as D4. Coxeter allowed for zeros as special cases to fit the rectified n-simplex polytopes into the same notation, and also allowed one -1 index for sequences that remove the common node to all the branches.

Coxeter groups formed by cyclic graphs are represented by parenthesese inside of brackets, like [(a,b,c)] for the triangle group (a b c). If they are equal, they can be grouped as an exponent as the length the cycle in brackets, like [(3,3,3,3)] = [3[4]].

Finite Coxeter groups
Rank Group
symbol
Bracket
notation
Coxeter
graph
2 A2 [3] Template:CDD
2 BC2 [4] Template:CDD
2 H2 [5] Template:CDD
2 G2 [6] Template:CDD
2 I2(p) [p] Template:CDD
3 H3 [5,3] Template:CDD
3 A3 [3,3] Template:CDD
3 BC3 [4,3] Template:CDD
4 A4 [3,3,3] Template:CDD
4 BC4 [4,3,3] Template:CDD
4 D4 [31,1,1] Template:CDD
4 F4 [3,4,3] Template:CDD
4 H4 [5,3,3] Template:CDD
n An [3n-1] Template:CDD..Template:CDD
n BCn [4,3n-2] Template:CDD...Template:CDD
n Dn [3n-3,1,1] Template:CDD...Template:CDD
6 E6 [32,2,1] Template:CDD
7 E7 [33,2,1] Template:CDD
8 E8 [34,2,1] Template:CDD
Affine Coxeter groups
Group
symbol
Bracket
notation
Coxeter
graph
[∞] Template:CDD
[3[3]] Template:CDD
[4,4] Template:CDD
[6,3] Template:CDD
[3[4]] Template:CDD
[4,31,1] Template:CDD
[4,3,4] Template:CDD
[3[5]] Template:CDD
[4,3,31,1] Template:CDD
[4,3,3,4] Template:CDD
[ 31,1,1,1] Template:CDD
[3,4,3,3] Template:CDD
[3[n+1]] Template:CDD...Template:CDD
or
Template:CDD...Template:CDD
[4,3n-2,31,1] Template:CDD...Template:CDD
[4,3n-1,4] Template:CDD...Template:CDD
[ 31,1,3n-3,31,1] Template:CDD...Template:CDD
[32,2,2] Template:CDD
[33,3,1] Template:CDD
[35,2,1] Template:CDD
Compact Hyperbolic Coxeter groups
Group
symbol
Bracket
notation
Coxeter
graph
[p,q]
with 2(p+q)<pq
Template:CDD
[(p,q,r)]
with p+q+r>9
Template:CDD
[4,3,5] Template:CDD
[5,3,5] Template:CDD
[3,5,3] Template:CDD
[5,31,1] Template:CDD
[(3,3,3,4)] Template:CDD 
[(3,3,3,5)] Template:CDD 
[(3,4,3,4)] Template:CDD
[(3,4,3,5)] Template:CDD
[(3,5,3,5)] Template:CDD
[3,3,3,5] Template:CDD
[4,3,3,5] Template:CDD
[5,3,3,5] Template:CDD
[5,3,31,1] Template:CDD
[(3,3,3,3,4)] Template:CDD

For the affine and hyperbolic groups, the subscript is one less than the number of nodes in each case, since each of these groups was obtained by adding a node to a finite group's graph.

The Coxeter graph usually leaves order-2 branches undrawn, but the bracket notation includes an explicit 2 to connect the subgraphs. So the Coxeter graph, Template:CDD, H3×A2 can be represented by [5,3]×[3] and [5,3,2,3].

Subsymmetry by even/odd alternation

Coxeter's notation represents rotational/translational symmetry by adding a + superscript operator outside the brackets which cuts the order of the group in half (called index 2 subgroup). Multiple + operators may exist if neighboring elements are all even order, and the subgroup index is 2n for n operators.

For Coxeter groups with even order branches, elements by parentheses inside of a Coxeter group can be give a + superscript operator, having the effect of dividing adjacent ordered branches into half order, thus is usually only applied with even numbers. Example [4,3+].

Groups without neighboring + elements can be seen in ringed Coxeter-Dynkin diagram for uniform polytopes and honeycomb are related to holes on the nodes around the + elements, empty circles with the alternated nodes removed. So Template:CDD, has symmetry in Coxeter notation of [4,3]+, and Template:CDD has group notation [4,3+], while Template:CDD has symmetry [1+,4,3].

Subsymmetry by mirror removal

Johnson extends the + operator to work with a placeholder 1 nodes. In general this operation only applies to mirrors bounded by all even-order branches.

The 1 represents a mirror so [2p] can be seen as [1,2p,1], like diagram Template:CDD, with 2 mirrors related by an order-2p dihedral angle. Each of these mirrors can be removed so [1+,2p,1] = [1,2p,1+] = [p], a reflective subgroup index 2. This can be shown in a Coxeter diagram by adding a + symbol above the node: Template:CDD = Template:CDD = Template:CDD.

If both mirrors are removed, the branch order becomes a gyration point of half the order: [1+,2p,1+] = [1+,2p]+ = [2p,1+]+ = [p]+, a rotational subgroup of index 4. Example (with p=1): [1+,2] = [ ], order 2. [1+,2,1+] = [ ]+, order 1.

Commutator subgroups

Simple groups with only odd-order branch elements have only a single rotational/translational subgroup of order 2, which is also the commutator subgroup, examples [3,3]+, [3,5]+, [3,3,3]+, [3,3,5]+. For other Coxeter groups with even-order branches, the commutator subgroup has index 2c, where c is the number of disconnected subgraphs when all the even-order branches are removed.[1] For example [4,4] has 3 independent nodes in the Coxeter graph when the 4s are removed, so its commutator subgroup is index 23, and can have different representions, all with three + operators: [4+,4+]+, [1+,4,1+,4,1+], [1+,4,4,1+]+, or [(4+,4+,2+)]. A general notation can be used with +c as a group exponent, like [4,4]+3.

Example subgroups

The [4,4] group has 15 small index subgroups. This table shows them all, with a yellow fundamental domain for pure reflective groups, and alternating white and blue domains which are paired up to make rotational domains.

Small index subgroups of [4,4]
Subgroup index 1 2 4
Diagram
Coxeter [4,4]
Template:CDD
[1+,4,4]
Template:CDD
[4,4,1+]
Template:CDD
[4,1+,4]
Template:CDD
[4,1+,4,1+]
Template:CDD
[1+,4,4,1+]
Template:CDD
Orbifold (*442) (*2222) (2*22) (*2222)
Diagram
Coxeter [4,4+]
Template:CDD
[4+,4]
Template:CDD
[(4,4,2+)]
Template:CDD
[1+,4,1+,4]
Template:CDD
[4+,4+]
Template:CDD
Orbifold (4*2) (2*22) (22×)
Rotational subgroups
Index 2 4 8
Diagram
Coxeter [4,4]+
Template:CDD
[1+,4,4+]
Template:CDD
[4+,4,1+]
Template:CDD
[(4,1+,4,2+)]
Template:CDD
[1+,4,1+,4,1+] = [(4+,4+,2+)]
Template:CDD
Orbifold (442) (2222)

The same set of 15 small subgroups exists on all triangle groups with even order elements, like [6,4] in the hyperbolic plane:

Small index subgroups of [6,4]
Subgroup index 1 2 4
Diagrams
Coxeter
(orbifold)
[6,4]
(*642)
Template:CDD
[1+,6,4]
Template:CDD
(*443)
[6,4,1+]
Template:CDD
(*662)
[6,1+,4]
Template:CDD
(*3222)
[6,1+,4,1+]
Template:CDD
(2*33)
[1+,6,4,1+]
Template:CDD
(*3232)
Diagrams
Coxeter
(orbifold)
[6,4+]
Template:CDD
(4*3)
[6+,4]
Template:CDD
(6*2)
[(6,4,2+)]
Template:CDD
(2*32)
[1+,6,1+,4]
Template:CDD
(3*22)
[6+,4+]
Template:CDD
(32×)
Rotational subgroups
Subgroup index 2 4 8
Diagrams
Coxeter
(orbifold)
[6,4]+
(642)
Template:CDD
[1+,6,4+]
Template:CDD
(443)
[6+,4,1+]
Template:CDD
(662)
[(6,1+,4,2+)]
Template:CDD
(3222)
[1+,6,1+,4,1+] = [(6+,4+,2+)]
Template:CDD
(3232)

Extended symmetry

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Coxeter's notation includes double square bracket notation, [[X]] to express isomorphic symmetry within a Coxeter diagram. Johnson added alternative of angled-bracket <[X]> option as equivalent to square brackets for doubling to distinguish diagram symmetry through the nodes versus through the branches. Johnson also added a prefix symmetry modifier [Y[X]], where Y is the symmetry of the fundamental domain of [X].

For example in these equivalent rectangle and rhombic geometry diagrams of : Template:CDD and Template:CDD, the first doubled with square brackets, [[3[4]]] or twice doubled as [2[3[4]]], with [2], order 4 higher symmetry. To differentiate the second, angled brackets are used for doubling, <[3[4]]> and twice doubled as <2[3[4]]>, also with a different [2], order 4 symmetry. Finally a full symmetry where all 4 nodes are equivalent can be represented by [4[3[4]]], with the order 8, [4] symmetry of the square.

Further symmetry exists in the cyclic and branching , , and diagrams. graph has order 2n symmetry of a regular n-gon, {n}, and is represented by [n[3[n]]]. and are represented by [3[31,1,1]] = [3,4,3] and [3[32,2,2]] respectively while by [(3,3)[31,1,1,1]] = [3,3,4,3], with the diagram containing the order 24 symmetry of the regular tetrahedron, {3,3}. The noncompact hyperbolic group graph = [31,1,1,1,1], Template:CDD, contains the symmetry of a 5-cell, {3,3,3}, and thus is represented by [(3,3,3)[31,1,1,1,1]] = [3,4,3,3,3].

A "*" superscript is effectively an inverse operation, creating radial subgroups removing connected of odd-ordered mirrors.[2]

Examples:

Extended groups Radial subgroups Coxeter diagrams Index
[3[2,2]] = [4,3] [4,3*] = [2,2] Template:CDD = Template:CDD 6
[(3,3)[2,2,2]] = [4,3,3] [4,(3,3)*] = [2,2,2] Template:CDD = Template:CDD 24
[1[31,1]] = [[3,3]] = [3,4] [3,4,1+] = [3,3] Template:CDD = Template:CDD 2
[3[31,1,1]] = [3,4,3] [3*,4,3] = [31,1,1] Template:CDD = Template:CDD 6
[2[31,1,1,1]] = [4,3,3,4] [1+,4,3,3,4,1+] = [31,1,1,1] Template:CDD = Template:CDD 4
[3[3,31,1,1]] = [3,3,4,3] [3*,4,3,3] = [31,1,1,1] Template:CDD = Template:CDD 6
[(3,3)[31,1,1,1]] = [3,4,3,3] [3,4,(3,3)*] = [31,1,1,1] Template:CDD = Template:CDD 24
[2[3,31,1,1,1]] = [3,(3,4)1,1] [3,(3,4,1+)1,1] = [3,31,1,1,1] Template:CDD = Template:CDD 4
[(2,3)[1,131,1,1]] = [4,3,3,4,3] [1+,4,3,3,4,3+] = [31,1,1,1,1] Template:CDD = Template:CDD 12
[(3,3)[3,31,1,1,1]] = [3,3,4,3,3] [3,3,4,(3,3)*] = [31,1,1,1,1] Template:CDD = Template:CDD 24
[(3,3,3)[31,1,1,1,1]] = [3,4,3,3,3] [3,4,(3,3,3)*] = [31,1,1,1,1] Template:CDD = Template:CDD 120
Extended groups Radial subgroups Coxeter diagrams Index
[1[3[3]]] = [3,6] [3,6,1+] = [3[3]] Template:CDD = Template:CDD 2
[3[3[3]]] = [6,3] [6,3*] = [3[3]] Template:CDD = Template:CDD 6
[1[3,3[3]]] = [3,3,6] [3,3,6,1+] = [3,3[3]] Template:CDD = Template:CDD 2
[(3,3)[3[3,3]]] = [6,3,3] [6,(3,3)*] = [3[3,3]] Template:CDD = Template:CDD 24
[1[∞]2] = [4,4] [4,1+,4] = [∞]2 = [∞]×[∞] = [∞,2,∞] Template:CDD = Template:CDD 2
[2[∞]2] = [4,4] [1+,4,4,1+] = [(4,4,2*)] = [∞]2 Template:CDD = Template:CDD 4
[4[∞]2] = [4,4] [4,4*] = [∞]2 Template:CDD = Template:CDD 8
[2[3[4]]] = [4,3,4] [1+,4,3,4,1+] = [(4,3,4,2*)] = [3[4]] Template:CDD = Template:CDD 4
[3[∞]3] = [4,3,4] [4,3*,4] = [∞]3 = [∞,2,∞,2,∞] Template:CDD = Template:CDD 6
[(3,3)[∞]3] = [4,31,1] [4,(31,1)*] = [∞]3 Template:CDD = Template:CDD 24
[(4,3)[∞]3] = [4,3,4] [4,(3,4)*] = [∞]3 Template:CDD = Template:CDD 48
[(3,3)[∞]4] = [4,3,3,4] [4,(3,3)*,4] = [∞]4 Template:CDD = Template:CDD 24
[(4,3,3)[∞]4] = [4,3,3,4] [4,(3,3,4)*] = [∞]4 Template:CDD = Template:CDD 384

Looking at generators, the double symmmetry is seen as adding a new operator that maps symmetric positions in the Coxeter diagram, making some original generators redundant. For 3D space groups, and 4D point groups, Coxeter defines an index two subgroup of [[X]], [[X]+], which he defines as the product of the original generators of [X] by the doubling generator. This looks similar to [[X]]+, which is the chiral subgroup of [[X]]. So for example the 3D space groups [[4,3,4]]+ (I432, 211) and [[4,3,4]+] (PmTemplate:Overlinen, 223) are distinct subgroups of [[4,3,4]] (ImTemplate:Overlinem, 229).

Computation with reflection matrices as symmetry generators

A Coxeter group, represented by Coxeter diagram Template:CDD, is given Coxeter notation [p,q] for the branch orders. Each node in the Coxeter diagram represents a mirror, by convention called ρi (and matrix Ri). The generators of this group [p,q] are reflections: ρ1, ρ2, and ρ3. Rotational subsymmetry is given as products of reflections: By convention, σ1,2 (and matrix S1,2) = ρ1ρ2 represents a rotation of angle π/p, and σ2,3 = ρ2ρ3 is a rotation of angle π/q, and σ1,3 = ρ1ρ3 represents a rotation of angle π/2.

[p,q]+ is an index 2 subgroup represented by two rotation generators, each a products of two reflections: σ1,2, σ2,3, and representing rotations of π/p, and π/q angles respectively.

If q is even, [p+,q] is another subgroup of index 2, represented by rotation generator σ1,2, and reflectional ρ3.

If both p and q are even, [p+,q+] is a subgroup of index 4 with a single generator type, constructed as a product of all three reflection matrices: By convention as: ψ1,2,3 (and matrix U1,2,3) = σ1,2ρ3 = ρ1σ2,3 = ρ1ρ2ρ3, which is an improper rotation or roto-reflection, representing a reflection and rotation.

In the case of affine Coxeter groups like Template:CDD, or Template:CDD, one mirror, usually the last, is translated off the origin. A translation generator τ1,2 (and matrix T1,2) is constructed as the product of two (or an even number of) reflections, including the affine reflection. A trans-reflection (reflection plus a translation) can be the product of an odd number of reflections φ1,2,3 (and matrix V1,2,3), like the index 4 subgroup Template:CDD: [4+,4+].

Another composite generator, by convention as ζ (and matrix Z), represents the inversion, mapping a point to its inverse. For [4,3] and [5,3], ζ = (ρ1ρ2ρ3)h/2, where h is 6 and 10 respectively, the Coxeter number for each family.

Example, in 2D, the Coxeter group [p] is represented by two reflection matrices R1 and R2, The cyclic symmetry [p]+ is represented by rotation generator of matrix S1,2.

R1 R2 S1,2=R1xR2

A simple example affine group is [4,4] (p4m), can be given by three reflection matrices, constructed as a reflection across the x axis (y=0), a diagonal (x=y), and the affine reflection across the line (x=1). [4,4]+ (p4) is generated by S1,2 S2,3, and S1,3. [4+,4+] (pgg) is generated by 2-fold rotation S1,3 and trans-reflection V1,2,3. [4+,4] (p4g) is generated by S1,2 and R3. The group [(4,4,2+)] (cmm), is generated by 2-fold rotation S1,3 and reflection R2.

R1 R2 R3 S1,2 S2,3 S1,3 V1,2,3

Coxeter groups are categorized by their rank, being the number of nodes in its Coxeter graph. The structure of the groups are also given with their abstract group types: In this article, the abstract dihedral groups are represented as Dihn, and cyclic groups are represented by Zn, with Dih1=Z2.

Rank one groups

In one dimension, the bilateral group [ ] represents a single mirror symmetry, abstract Dih1 or Z2, symmetry order 2. It is represented as a Coxeter graph with a single node, Template:CDD. The identity group is the direct subgroup [ ]+, Z1, symmetry order 1. The + superscript simply implies that alternate mirror reflections are ignored, leaving the identity group in this simplest case.

Group Coxeter Coxeter diagram Order Description
C1 [ ]+ 1 Identity
D1 [ ] Template:CDD 2 Reflection group

Rank two groups

Family correspondence: A mirror added between two corresponding mirror doubles the symmetry order
A regular hexagon, with markings on edges and vertices has 8 symmetries: [6], [3], [2], [1], [6]+, [3]+, [2]+, [1]+, with [3] and [2] existing in two forms, depending whether the mirrors are on the edges or vertices.

In two dimensions, the rectangular group [2], abstract Dih2, also can be represented as a direct product [ ]×[ ] or Z2×Z2, being the product of two bilateral groups, represents two orthogonal mirrors, with Coxeter graph, Template:CDD, with order 4. The 2 in [2] comes from linearization of the orthogonal subgraphs in the Coxeter graph, as Template:CDD, with explicit branch order 2. The rhombic group, [2]+, half of the rectangular group, the point reflection symmetry, Z2, order 2.

Coxeter notation allows a 1 place-holder for lower rank groups, so [1] is the same as [ ], and [1]+ is the same as [ ]+. This may be done to imply the group exists in 2-dimensions rather than 1 dimension.

The full p-gonal group [p], abstract dihedral group Dihp, (nonabelian for p>2), of order 2p, is generated by two mirrors at angle π/p, represented by Coxeter graph Template:CDD. The p-gonal subgroup [p]+, cyclic group Zp, of order p, generated by a rotation angle of π/p.

Coxeter notation uses double-bracking to represent an automorphic doubling of symmetry by adding a bisecting mirror to the fundamental domain. For example [[p]] adds a bisecting mirror to [p], and is isomorphic to [2p].

In the limit, going down to one dimensions, the full apeirogonal group is obtained when the angle goes to zero, so [∞], abstractly the infinite dihedral group Dih, represents two parallel mirrors and has a Coxeter graph Template:CDD. The apeirogonal group [∞]+, abstractly the infinite cyclic group Z, isomorphic to the additive group of the integers, is generated by a single nonzero translation.

In the hyperbolic plane, there's a full pseudogonal group [πi/λ], and pseudogonal subgroup [πi/λ]+. These groups exist in regular infinite sided polygons, with edge length λ. The mirrors are all orthogonal to a single line.

Group Intl Orbifold Coxeter Order Description
Finite
Zn n n• [n]+ n Cyclic: n-fold rotations. Abstract group Zn, the group of integers under addition modulo n.
Dn nm *n• [n] 2n Dihedral: cyclic with reflections. Abstract group Dihn, the dihedral group.
Affine
Z ∞• [∞]+ Cyclic: apeirogonal group. Abstract group Z, the group of integers under addition.
Dih m *∞• [∞] Dihedral: parallel reflections. Abstract infinite dihedral group Dih.
Hyperbolic
Z [πi/λ]+ pseudogonal group
Dih [πi/λ] full pseudogonal group

Rank three groups

Finite family correspondence
Affine isomorphism and correspondences

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In three dimensions, the full orthorhombic group [2,2], astracttly Z2×Dih2, order 8, represents three orthogonal mirrors, and also can be represented by Coxeter graph as three separate dots Template:CDD. It can also can be represented as a direct product [ ]×[ ]×[ ], but the [2,2] expression allows subgroups to be defined:

First there is a semidirect subgroup, the orthorhombic group, [2,2+], abstractly Dih1×Z2=Z2×Z2, of order 4. When the + superscript is given inside of the brackets, it means reflections generated only from the adjacent mirrors (as defined by the Coxeter graph, Template:CDD) are alternated. In general, the branch orders neighboring the + node must be even. In this case [2,2+] and [2+,2] represent two isomorphic subgroups that are geometrically distinct. The other subgroups are the pararhombic group [2,2]+, also order 4, and finally the central group [2+,2+] of order 2.

Next there is the full ortho-p-gonal group, [2,p], abstractly Dih1×Dihp=Z2×Dihp, of order 4p, representing two mirrors at a dihedral angle π/p, and both are orthogonal to a third mirror. It is also represented by Coxeter graph as Template:CDD.

The direct subgroup is called the para-p-gonal group, [2,p]+, abstractly Dihp, of order 2p, and another subgroup is [2,p+] abstractly Zp×Z2, also of order 2p.

The full gyro-p-gonal group, [2+,2p], abstractly Dih2p, of order 4p. The gyro-p-gonal group, [2+,2p+], abstractly Z2p, of order 2p is a subgroup of both [2+,2p] and [2,2p+].

The polyhedral groups are based on the symmetry of platonic solids, the tetrahedron, octahedron, cube, icosahedron, and dodecahedron, with Schläfli symbols {3,3}, {3,4}, {4,3}, {3,5}, and {5,3} respectively. The Coxeter groups for these are called in Coxeter's bracket notation [3,3], [3,4], [3,5] called full tetrahedral symmetry, octahedral symmetry, and icosahedral symmetry, with orders of 24, 48, and 120. The front-to-back order can be reversed in the Coxeter notation, unlike the Schläfli symbol.

The tetrahedral group, [3,3], has a doubling [[3,3]] which maps the first and last mirrors onto each other, and this produces the [3,4] group.

In all these symmetries, alternate reflections can be removed producing the rotational tetrahedral, octahedral, and icosahedral groups of order 12, 24, and 60. The octahedral group also has a unique subgroup called the pyritohedral symmetry group, [3+,4], of order 12, with a mixture of rotational and reflectional symmetry.

In the Euclidean plane there's 3 fundamental reflective groups generated by 3 mirrors, represented by Coxeter graphs Template:CDD, Template:CDD, and Template:CDD, and are given Coxeter notation as [4,4], [6,3], and [(3,3,3)]. The parentheses of the last group imply the graphic cyclic, and also has a shorthand notation [3[3]].

[[4,4]] as a doubling of the [4,4] group produced the same symmetry rotated π/4 from the original set of mirrors.

Direct subgroups of rotational symmetry are: [4,4]+, [6,3]+, and [(3,3,3)]+. [4+,4] and [6,3+] represent mixed reflectional and rotational symmetry.

Tetrahedral symmetry Octahedral symmetry
Icosahedral symmetry
Finite
Intl* Geo
[3]
Orbifold Schönflies Conway Coxeter Order
1 Template:Overline 1 C1 C1 [ ]+ 1
Template:Overline Template:Overline ×1 Ci = S2 CC2 [2+,2+] 2
Template:Overline = m 1 *1 Cs = C1v = C1h ±C1 = CD2 [ ] 2
2
3
4
5
6
n
Template:Overline
Template:Overline
Template:Overline
Template:Overline
Template:Overline
Template:Overline
22
33
44
55
66
nn
C2
C3
C4
C5
C6
Cn
C2
C3
C4
C5
C6
Cn
[2]+
[3]+
[4]+
[5]+
[6]+
[n]+
2
3
4
5
6
n
2mm
3m
4mm
5m
6mm
nmm
nm
2
3
4
5
6
n
*22
*33
*44
*55
*66
*nn
C2v
C3v
C4v
C5v
C6v
Cnv
CD4
CD6
CD8
CD10
CD12
CD2n
[2]
[3]
[4]
[5]
[6]
[n]
4
6
8
10
12
2n
2/m
3/m
4/m
5/m
6/m
n/m
Template:Overline 2
Template:Overline 2
Template:Overline 2
Template:Overline 2
Template:Overline 2
Template:Overline 2
2*
3*
4*
5*
6*
n*
C2h
C3h
C4h
C5h
C6h
Cnh
±C2
CC6
±C4
CC10
±C6
±Cn / CC2n
[2,2+]
[2,3+]
[2,4+]
[2,5+]
[2,6+]
[2,n+]
4
6
8
10
12
2n
Template:Overline
Template:Overline
Template:Overline
Template:Overline
Template:Overline
Template:Overline
Template:Overline
Template:Overline
Template:Overline
Template:Overline
Template:Overline
Template:Overline
Template:Overline





S4
S6
S8
S10
S12
S2n
CC4
±C3
CC8
±C5
CC12
CC2n / ±Cn
[2+,4+]
[2+,6+]
[2+,8+]
[2+,10+]
[2+,12+]
[2+,2n+]
4
6
8
10
12
2n
Intl Geo Orbifold Schönflies Conway Coxeter Order
222
32
422
52
622
n22
n2
Template:Overline Template:Overline
Template:Overline Template:Overline
Template:Overline Template:Overline
Template:Overline Template:Overline
Template:Overline Template:Overline
Template:Overline Template:Overline
222
223
224
225
226
22n
D2
D3
D4
D5
D6
Dn
D4
D6
D8
D10
D12
D2n
[2,2]+
[2,3]+
[2,4]+
[2,5]+
[2,6]+
[2,n]+
4
6
8
10
12
2n
mmm
Template:Overlinem2
4/mmm
Template:Overlinem2
6/mmm
n/mmm
Template:Overlinem2
2 2
3 2
4 2
5 2
6 2
n 2
*222
*223
*224
*225
*226
*22n
D2h
D3h
D4h
D5h
D6h
Dnh
±D4
DD12
±D8
DD20
±D12
±D2n / DD4n
[2,2]
[2,3]
[2,4]
[2,5]
[2,6]
[2,n]
8
12
16
20
24
4n
Template:Overline2m
Template:Overlinem
Template:Overline2m
Template:Overlinem
Template:Overline2m
Template:Overline2m
Template:Overlinem
4 Template:Overline
6 Template:Overline
8 Template:Overline
10 Template:Overline
12 Template:Overline
n Template:Overline
2*2
2*3
2*4
2*5
2*6
2*n
D2d
D3d
D4d
D5d
D6d
Dnd
±D4
±D6
DD16
±D10
DD24
DD4n / ±D2n
[2+,4]
[2+,6]
[2+,8]
[2+,10]
[2+,12]
[2+,2n]
8
12
16
20
24
4n
23 Template:Overline Template:Overline 332 T T [3,3]+ 12
mTemplate:Overline 4 Template:Overline 3*2 Th ±T [3+,4] 24
Template:Overline3m 3 3 *332 Td TO [3,3] 24
432 Template:Overline Template:Overline 432 O O [3,4]+ 24
mTemplate:Overlinem 4 3 *432 Oh ±O [3,4] 48
532 Template:Overline Template:Overline 532 I I [3,5]+ 60
Template:OverlineTemplate:Overlinem 5 3 *532 Ih ±I [3,5] 120
Semiaffine
Intl (orbifold) Geo
Schönflies Coxeter Order
n nn Template:Overline Cn [1,n]+ n
nm *nn n Dn [1,n] 2n
IUC (Orbifold) Geo Schönflies Coxeter
p1 ∞∞ pTemplate:Overline C [1,∞]+
p1m1 *∞∞ p1 C∞v [1,∞]
IUC (Orbifold) Geo Schönflies Coxeter
p11g ∞x p.g1 S2∞ [∞+,2+]
p11m ∞* p. 1 C∞h [∞+,2]
p2 22∞ pTemplate:Overline D [∞,2]+
p2mg 2*∞ p2g D∞d [∞,2+]
p2mm *22∞ p2 D∞h [∞,2]
Affine
IUC (Orbifold) Geometric Coxeter
p2 (2222) pTemplate:Overline [1+,4,4]+
p2gg (22x) pg2g [4+,4+]
p2mm (*2222) p2 [1+,4,4]
c2mm (2*22) c2 [[4+,4+]]
p4 (442) pTemplate:Overline [4,4]+
p4gm (4*2) pg4 [4+,4]
p4mm (*442) p4 [4,4]
IUC (Orbifold) Geometric Coxeter
p3 (333) pTemplate:Overline [1+,6,3+] = [3[3]]+
p3m1 (*333) p3 [1+,6,3] = [3[3]]
p31m (3*3) h3 [6,3+] = [3[3[3]]+]
p6 (632) pTemplate:Overline [6,3]+ = [3[3[3]]]+
p6mm (*632) p6 [6,3] = [3[3[3]]]

Subgroups

Given in Schönflies notation, Coxeter notation, (orbifold notation), some low index point subgroups are:

Reflection Reflection
subgroups
Rotation subgroup Mixed Improper rotation Commutator
subgroup
C1v, [ ], (*) C1, [ ]+, (11) S2, [2+,2+], (×) [ ]+
C2v, [2], (*22) [1+,2]=[ ] (*) C2, [2]+, (22) C2h, [2+,2], (2*) S4, [4+,2+], (2×)
Cnv, [n], (*nn) [1+,2n]=[n] (*nn) Cn, [n]+, (nn) Cnh, [n+,2], (n*) S2n, [2n+,2+], (n×) [n]+, n odd
[n/2]+, n even
Dnh, [2,n], (*22n) [1+,2,n]=[n] (*nn) Dn, [2,n]+, (22n) Dnd, [2+,2n], (2*n)
Td, [3,3], (*332) T, [3,3]+, (332) [3,3]+, (332)
Oh, [4,3], (*432) [1+,4,3]=[3,3] (*332) O, [4,3]+, (432) Th, [3+,4], (3*2)
Ih, [5,3], (*532) I, [5,3]+, (532) [5,3]+, (532)

Given in Coxeter notation, (orbifold notation), some low index affine subgroups are:

Reflective
group
Reflective
subgroup
Mixed
subgroup
Rotation
subgroup
Improper rotation/
translation
Commutator
subgroup
[∞,2,∞], (*2222) [1+,∞,2,∞], (*2222) [∞+,2,∞], (**) [∞,2,∞]+, (2222)
[∞+,2+,∞+]+, (2222)
[∞+,2,∞+], (°)
[∞+,2+,∞], (*×)
[(∞,2)+,∞+], (××)
[∞+,2+,∞+], (22×)
(2222)
[∞,2+,∞], (2*22)
[(∞,2)+,∞], (22*)
[4,4], (*442) [1+,4,4], (*442)
[4,1+,4], (*2222)
[1+,4,4,1+], (*2222)
[4+,4], (4*2)
[(4,4,2+)], (2*22)
[1+,4,1+,4], (2*22)
[4,4]+, (442)
[1+,4,4+], (442)
[1+,4,1+4,1+], (2222)
[4+,4+], (22×)
[3[3]], (*333) [3[3]]+, (333)   (333)
[6,3], (*632) [1+,6,3], (*333) [3+,6], (3*3) [6,3]+, (632)
[1+,6,3+], (333)

Rank four groups


Finite isomorphism and correspondences

Subgroup relations

Point groups

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Rank four groups defined the 4-dimensional point groups: Template:3-sphere symmetry groups2

Subgroups

1D-4D reflective point groups and subgroups
Order Reflection Mixed
subgroups
Improper rotation
subgroups
Rotation
subgroups
Commutator
subgroup
2 [ ] Template:CDD [ ]+ [ ]+1 [ ]+
4 [2] Template:CDD [2]+ [2]+2
8 [2,2] Template:CDD [2+,2] [2+,2+] [2,2]+ [2,2]+3
16 [2,2,2] Template:CDD [2+,2,2]
[(2,2)+,2]
[2+,2+,2]
[(2,2)+,2+]
[2+,2+,2+]
[2,2,2]+
[2+,2,2+]
[2,2,2]+4
[21,1,1] Template:CDD [(2+)1,1,1]
2n [n] Template:CDD [n]+ [n]+1 [n]+
4n [2n] Template:CDD [2n]+ [2n]+2
4n [2,n] Template:CDD [2,n+] [2,n]+ [2,n]+2
8n [2,2n] Template:CDD [2+,2n] [2+,2n+] [2,2n]+ [2,2n]+3
8n [2,2,n] Template:CDD [2+,2,n]
[2,2,n+]
[2+,(2,n)+] [2,2,n]+
[2+,2,n+]
[2,2,n]+3
16n [2,2,2n] Template:CDD [2,2+,2n] [2+,2+,2n]
[2,2+,2n+]
[(2,2)+,2n+]
[2+,2+,2n+]
[2,2,2n]+
[2+,2n,2+]
[2,2,2n]+4
[2,2n,2] Template:CDD [2+,2n+,2+]
[2n,21,1] Template:CDD [2n+,(2+)1,1]
24 [3,3] Template:CDD [3,3]+ [3,3]+1 [3,3]+
48 [3,3,2] Template:CDD [(3,3)+,2] [3,3,2]+ [3,3,2]+2
48 [4,3] Template:CDD [4,3+] [4,3]+ [4,3]+2
96 [4,3,2] Template:CDD [(4,3)+,2]
[4,(3,2)+]
[4,3,2]+ [4,3,2]+3
[3,4,2] Template:CDD [3,4,2+]
[3+,4,2]
[(3,4)+,2+] [3+,4,2+]
120 [5,3] Template:CDD [5,3]+ [5,3]+1 [5,3]+
240 [5,3,2] Template:CDD [(5,3)+,2] [5,3,2]+ [5,3,2]+2
4pq [p,2,q] Template:CDD [p+,2,q] [p,2,q]+
[p+,2,q+]
[p,2,q]+2 [p+,2,q+]
8pq [2p,2,q] Template:CDD [2p,(2,q)+] [2p+,(2,q)+] [2p,2,q]+ [2p,2,q]+3
16pq [2p,2,2q] Template:CDD [2p,2+,2q] [2p+,2+,2q]
[2p+,2+,2q+]
[2p,2,2q]+ [2p,2,2q]+4
120 [3,3,3] Template:CDD [3,3,3]+ [3,3,3]+1 [3,3,3]+
192 [31,1,1] Template:CDD [31,1,1]+ [31,1,1]+1 [31,1,1]+
384 [4,3,3] Template:CDD [4,(3,3)+] [4,3,3]+ [4,3,3]+2
1152 [3,4,3] Template:CDD [3+,4,3] [3,4,3]+
[3+,4,3+]
[3,4,3]+2 [3+,4,3+]
14400 [5,3,3] Template:CDD [5,3,3]+ [5,3,3]+1 [5,3,3]+

Space groups


Affine isomorphism and correspondences

35 cubic space groups in International, Fibrifold notation, and Coxeter notation

Template:List space groups Coxeter notation

Line groups

Rank four groups also defined the 3-dimensional line groups:

Semiaffine (3D)
Point group Line group
Hermann-Mauguin Schönflies Hermann-Mauguin Offset type Wallpaper Coxeter
[∞h,2,pv]
Even n Odd n Even n Odd n IUC Orbifold Diagram
n Cn Pnq Helical: q p1 o [∞+,2,n+]
Template:Overline Template:Overline S2n PTemplate:Overline PTemplate:Overline None p11g, pg(h) xx [(∞,2)+,2n+]
n/m Template:Overline Cnh Pn/m PTemplate:Overline None p11m, pm(h) ** [∞+,2,n]
2n/m C2nh P2nn/m Zigzag c11m, cm(h) *x [∞+,2+,2n]
nmm nm Cnv Pnmm Pnm None p1m1, pm(v) ** [∞,2,n+]
Pncc Pnc Planar reflection p1g1, pg(v) xx [∞+,(2,n)+]
2nmm C2nv P2nnmc Zigzag c1m1, cm(v) *x [∞,2+,2n+]
n22 n2 Dn Pnq22 Pnq2 Helical: q p2 2222 [∞,2,n]+
Template:Overline2m Template:Overlinem Dnd PTemplate:Overline2m PTemplate:Overlinem None p2mg, pmg(h) 22* [(∞,2)+,2n]
PTemplate:Overline2c PTemplate:Overlinec Planar reflection p2gg, pgg 22x [∞+,2+,2n+]
n/mmm Template:Overline2m Dnh Pn/mmm PTemplate:Overline2m None p2mm, pmm *2222 [∞,2,n]
Pn/mcc PTemplate:Overline2c Planar reflection p2mg, pmg(v) 22* [∞,(2,n)+]
2n/mmm D2nh P2nn/mcm Zigzag c2mm, cmm 2*22 [∞,2+,2n]

Wallpaper groups

Rank four groups also defined some of the 2-dimensional wallpaper groups:

Affine (2D plane)
IUC (Orbifold) Geo Coxeter
p1 (o) pTemplate:Overline [∞+,2,∞+]
p2 (2222) pTemplate:Overline [∞,2,∞]+
c2mm (2*22) c2 [∞,2+,∞]
p11g (xx) pg1 h: [∞+,(2,∞)+]
p1g1 (xx) pg1 v: [(∞,2)+,∞+]
p2gm (22*) pg2 h: [(∞,2)+,∞]
p2mg (22*) pg2 v: [∞,(2,∞)+]
IUC (Orbifold) Geo Coxeter
p11m (**) p1 h: [∞+,2,∞]
p1m1 (**) p1 v: [∞,2,∞+]
p2mm (*2222) p2 [∞,2,∞]
c11m (*x) c1 h: [∞+,2+,∞]
c1m1 (*x) c1 v: [∞,2+,∞+]
p2gg (22x) pg2g [∞+,2+,∞+]
c2mm (2*22) c2 [∞,2+,∞]

Notes

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References

  • H.S.M. Coxeter:
    • Kaleidoscopes: Selected Writings of H.S.M. Coxeter, editied by F. Arthur Sherk, Peter McMullen, Anthony C. Thompson, Asia Ivic Weiss, Wiley-Interscience Publication, 1995, ISBN 978-0-471-01003-6 [1]
      • (Paper 22) H.S.M. Coxeter, Regular and Semi Regular Polytopes I, [Math. Zeit. 46 (1940) 380–407, MR 2,10]
      • (Paper 23) H.S.M. Coxeter, Regular and Semi-Regular Polytopes II, [Math. Zeit. 188 (1985) 559–591]
      • (Paper 24) H.S.M. Coxeter, Regular and Semi-Regular Polytopes III, [Math. Zeit. 200 (1988) 3–45]
    • 20 year-old Real Estate Agent Rusty from Saint-Paul, has hobbies and interests which includes monopoly, property developers in singapore and poker. Will soon undertake a contiki trip that may include going to the Lower Valley of the Omo.

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  • Norman Johnson Uniform Polytopes, Manuscript (1991)
    • N.W. Johnson: The Theory of Uniform Polytopes and Honeycombs, Ph.D. (1966)
    • N.W. Johnson: Geometries and Transformations, Manuscript, (2011) Chapter 11: Finite symmetry groups
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  • The Symmetries of Things 2008, John H. Conway, Heidi Burgiel, Chaim Goodman-Strass, ISBN 978-1-56881-220-5 Ch.22 35 prime space groups, ch.25 184 composite space groups, ch.26 Higher still, 4D point groups
  1. Coxeter and Moser, 1980, Sec 9.5 Commutator subgroup, p. 124-126
  2. Norman W. Johnson, Asia Ivić Weiss, Quaternionic modular groups, Linear Algebra and its Applications, Volume 295, Issues 1–3, 1 July 1999, Pages 159–189 [2]
  3. The Crystallographic Space groups in Geometric algebra, D. Hestenes and J. Holt, Journal of Mathematical Physics. 48, 023514 (2007) (22 pages) PDF [3]