# Coxeter-Dynkin diagram

Coxeter groups in the plane with equivalent diagrams. Domain mirrors are labeled as edge m1, m2, etc. Vertices are colored by their reflection order. The prismatic group [I~1xI~2] is shown as a doubling of the C~2, but can also be created as rectangular domains from doubling the H~2 triangles. The A~2 is a doubling of the H~2 triangle.
Coxeter groups in the sphere with equivalent diagrams. One fundamental domain is outlined in yellow. Vertices are colored by their reflection order.
Coxeter groups in 3-space with diagrams. Mirrors (triangle faces) are labeled by opposite vertex 0..3. Edges are colored by their reflection order.
C~3 fills 1/24 of the cube. B~3 fills 1/12 of the cube. A~3 fills 1/6 of the cube.

In geometry, a Coxeter-Dynkin diagram is a graph with labelled edges. It represents the spatial relations between a collection of mirrors (or reflecting hyperplanes), and describes a kaleidoscopic construction.

The diagram represents a Coxeter group. Each graph node represents a mirror (domain facet) and the label attached to a graph edge encodes the dihedral angle order between two mirrors (on a domain ridge).

In addition, when used to represent a specific uniform polytope, the diagram has rings (circles) around nodes for active mirrors and hollow nodes (holes) to represent alternation.

Dynkin diagrams are used to classify root systems and therefore Lie algebras.[1]

## Description

The diagram can also represent polytopes by adding rings (circles) around nodes. Every diagram needs at least one active node to represent a polytope.

The rings express information on whether a generating point is on or off the mirror. Specifically a mirror is active (creates reflections) only when points are off the mirror, so adding a ring means a point is off the mirror and creates a reflection.

Hollow rings (holes) are also used. A polytope with an alternation operator applied has all the ringed nodes replaced by holes. If all the nodes are holes, the figure is considered a snub.

Edges are labeled with an integer n (or sometimes more generally a rational number p/q) representing a dihedral angle of 180/n. If an edge is unlabeled, it is assumed to be 3. If n=2 the angle is 90 degrees and the mirrors have no interaction, and the edge can be omitted. Two parallel mirrors can be marked with "∞".

In principle, n mirrors can be represented by a complete graph in which all n*(n-1)/2 edges are drawn. In practice interesting configurations of mirrors will include a number of right angles, and the corresponding edges can be omitted.

Polytopes and tessellations can be generated using these mirrors and a single generator point. Mirror images create new points as reflections. Edges can be created between points and a mirror image. Faces can be constructed by cycles of edges created, etc.

## Examples

• A single node represents a single mirror. This is called group A1. If ringed this creates a line segment perpendicular to the mirror, represented as {}.
• Two unattached nodes represent two perpendicular mirrors. If both nodes are ringed, a rectangle can be created, or a square if the point is equal distance from both mirrors.
• Two nodes attached by an order-n edge can creates an n-gon if the point is on one mirror, and a 2n-gon if the point is off both mirrors. This forms the I1(n) group.
• Two parallel mirrors can represent an infinite polygon I1(∞) group, also called I~1.
• Three mirrors in a triangle form images seen in a traditional kaleidoscope and be represented by 3 nodes connected in a triangle. Repeating examples will have edges labeled as (3 3 3), (2 4 4), (2 3 6), although the last two can be drawn in a line with the 2 edge ignored. These will generate uniform tilings.
• Three mirrors can generate uniform polyhedrons, including rational numbers is the set of Schwarz triangles.
• Three mirrors with one perpendicular to the other two can form the uniform prisms.

In general all regular n-polytopes, represented by Schläfli symbol symbol {p,q,r,...} can have their fundamental domains represented by a set of n mirrors and a related in a Coxeter-Dynkin diagram in a line of nodes and edges labeled by p,q,r...

## Finite Coxeter groups

Families of convex uniform polytopes are defined by Coxeter groups.

Notes:

• Three different symbols are given for the same groups - as a letter/number, as a bracketed set of numbers, and as the Coxeter diagram.
• The bifurcated Dn groups are also given an h[] notation representing the fact it is half or alternated version of the regular Cn groups.
• The bifurcated Dn and En groups are also labeled by a superscript form [3a,b,c] where a,b,c are the number of segments in each of the 3 branches.
n A1+ C2+ D3+ E4-8 F4 H2-4 I2(p)
1 A1=[]

2 A2=[3]
C2=[4]
H2=[5]
I2(p)=[p]

3 A3=[32]

C3=[4,3]
D3=A3=[30,1,1]

H3=[5,3]

4 A4=[33]

C4=[4,32]

D4=h[4,3,3]=[31,1,1]

E4=A4=[30,2,1]

F4=[3,4,3]
H4=[5,3,3]

5 A5=[34]

C5=[4,33]

D5=h[4,33]=[32,1,1]

E5=B5=[31,2,1]

6 A6=[35]

C6=[4,34]

D6=h[4,34]=[33,1,1]

E6=[32,2,1]

7 A7=[36]

C7=[4,35]

D7=h[4,35]=[34,1,1]

E7=[33,2,1]

8 A8=[37]

C8=[4,36]

D8=h[4,36]=[35,1,1]

E8=[34,2,1]

9 A9=[38]

C9=[4,37]

D9=h[4,37]=[36,1,1]

10+ .. .. ..
1. An forms the simplex polytope family.
2. Dn is the family of demihypercubes, beginning at n=4 with the 16-cell, and n=5 with the demipenteract. (Also named Bn)
3. Bn or Cn forms the hypercube/orthoplex polytope family.
4. I2n forms the regular polygons. (Also named D2n)
5. E6,E7,E8 are the generators of the Gosset Semiregular polytopes
6. F4 is the 24-cell polychoron family.
7. H3 is the dodecahedron/icosahedron polyhedron family. (Also named G3)
8. H4 is the 120-cell/600-cell polychoron family. (Also named G4)

## Infinite Coxeter groups

Families of convex uniform tessellations are defined by Coxeter groups.

Note:

1. A~n-1 is a cyclic group. (Also named Pn)
2. B~n-1 forms the alternated hypercubic tessellation family. (Also named Sn) Also labeled by a h[] notation as a half of the regular one.
3. C~n-1 forms the hypercube regular tessellation family {4,3,....,4} family. (Also named Rn)
4. D~n-1 (Also named Qn) Also labeled by a q[] notation as a quarter of the regular one.
5. E~6,E~7,E~8,E~9 are Gosset tessellations. (Also named T7,T8,T9,T10) T10 exists in hyperbolic space. Also labeled by a superscript form [3a,b,c] where a,b,c are the number of segments in each of the 3 branches.
6. F~4 is the 24-cell {3,4,3,3} regular tessellation. (Also named U5)
7. H~2 is the hexagonal tiling. (Also named V3)
8. I~1 is two parallel mirrors. (Also named W2)
n A~2+ B~3+ C~2+ D~4+ E~6-9 F~4 H~2 I~1
1               I~1=[∞]

2 A~2=h[6,3]

C~2=[4,4]

H~2=[6,3]

3 A~3=q[4,3,4]

B~3=h[4,3,4]

C~3=[4,3,4]

4 A~4

B~4=h[4,32,4]

C~4=[4,32,4]

D~4=q[4,32,4]

F~4=[3,4,3,3]

5 A~5

B~5=h[4,33,4]

C~5=[4,33,4]

D~5=q[4,33,4]

6 A~6

B~6=h[4,34,4]

C~6=[4,34,4]

D~6=q[4,34,4]

E~6=[32,2,2]

7 A~7

B~7=h[4,35,4]

C~7=[4,35,4]

D~7=q[4,35,4]

E~7=[33,3,1]

8 A~8

B~8=h[4,36,4]

C~8=[4,36,4]

D~8=q[4,36,4]

E~8=[35,2,1]

9 A~9

B~9=h[4,37,4]

C~9=[4,37,4]

D~9=q[4,37,4]

E~9=[36,2,1]

10 ... ... ... ...

### Hyperbolic infinite Coxeter groups

There are many infinite Coxeter groups whose symmetry can tessellate hyperbolic space. There are infinitely many linear groups for order 2, in the form [p,q], and no compact groups beyond order 5.

The two bifurcating groups have doubled fundamental domains as linear ones. [5,3,4] --> [5,31,1], and [5,3,3,4] --> [5,3,31,1].

n Linear Bifurcating Cyclic
3 ∞: , 2(p+q)<pq

...

...

...

0: ∞: , p+q+r≥10

...

...

...

...

4 3:

1:
5:

5 3:

1:
1:

## Notes

1. ^ Hall, Brian C. (2003), Lie Groups, Lie Algebras, and Representations: An Elementary Introduction, Springer, ISBN 0-387-40122-9 .

## References

• James E. Humphreys, Reflection Groups and Coxeter Groups, Cambridge studies in advanced mathematics, 29 (1990)
• 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 17) Coxeter, The Evolution of Coxeter-Dynkin diagrams, [Nieuw Archief voor Wiskunde 9 (1991) 233-248]
• Coxeter, The Beauty of Geometry: Twelve Essays, Dover Publications, 1999, ISBN 978-0-486-40919-1 (Chapter 3: Wythoff's Construction for Uniform Polytopes)
• Coxeter, Regular Polytopes (1963), Macmillian Company
• Regular Polytopes, Third edition, (1973), Dover edition, ISBN 0-486-61480-8 (Chapter 5: The Kaleidoscope, and Section 11.3 Representation by graphs)