Coxeter group
In mathematics, a Coxeter group, named after H. S. M. Coxeter, is an abstract group that admits a formal description in terms of reflections (or kaleidoscopic mirrors). Indeed, the finite Coxeter groups are precisely the finite Euclidean reflection groups; the symmetry groups of regular polyhedra are an example. However, not all Coxeter groups are finite, and not all can be described in terms of symmetries and Euclidean reflections. Coxeter groups were introduced (Coxeter 1934) as abstractions of reflection groups, and finite Coxeter groups were classified in 1935 (Coxeter 1935).
Coxeter groups find applications in many areas of mathematics. Examples of finite Coxeter groups include the symmetry groups of regular polytopes, and the Weyl groups of simple Lie algebras. Examples of infinite Coxeter groups include the triangle groups corresponding to regular tessellations of the Euclidean plane and the hyperbolic plane, and the Weyl groups of infinitedimensional Kac–Moody algebras.
Standard references include (Humphreys 1992) and (Davis 2007).
Definition
Formally, a Coxeter group can be defined as a group with the presentation
where and for . The condition means no relation of the form should be imposed.
The pair (W,S) where W is a Coxeter group with generators S={r_{1},...,r_{n}} is called Coxeter system. Note that in general S is not uniquely determined by W. For example, the Coxeter groups of type B_{3} and A_{1}xA_{3} are isomorphic but the Coxeter systems are not equivalent (see below for an explanation of this notation).
A number of conclusions can be drawn immediately from the above definition.
 The relation m_{i i} = 1 means that (r_{i}r_{i} )^{1} = (r_{i} )^{2} = 1 for all i ; the generators are involutions.
 If m_{i j} = 2, then the generators r_{i} and r_{j} commute. This follows by observing that
 xx = yy = 1,
 together with
 xyxy = 1
 implies that
 xy = x(xyxy)y = (xx)yx(yy) = yx.
 Alternatively, since the generators are involutions, , so , and thus is equal to the commutator.
 In order to avoid redundancy among the relations, it is necessary to assume that m_{i j} = m_{j i}. This follows by observing that
 yy = 1,
 together with
 (xy)^{m} = 1
 implies that
 (yx)^{m} = (yx)^{m}yy = y(xy)^{m}y = yy = 1.
 Alternatively, and are conjugate elements, as .
Coxeter matrix and Schläfli matrix
The Coxeter matrix is the n×n, symmetric matrix with entries m_{i j}. Indeed, every symmetric matrix with positive integer and ∞ entries and with 1's on the diagonal such that all nondiagonal entries are greater than 1 serves to define a Coxeter group.
The Coxeter matrix can be conveniently encoded by a Coxeter diagram, as per the following rules.
 The vertices of the graph are labelled by generator subscripts.
 Vertices i and j are connected if and only if m_{i j} ≥ 3.
 An edge is labelled with the value of m_{i j} whenever it is 4 or greater.
In particular, two generators commute if and only if they are not connected by an edge. Furthermore, if a Coxeter graph has two or more connected components, the associated group is the direct product of the groups associated to the individual components. Thus the disjoint union of Coxeter graphs yields a direct product of Coxeter groups.
The Coxeter matrix, M_{i,j}, is related to the Schläfli matrix, C_{i,j}, but the elements are modified, being proportional to the dot product of the pairwise generators: Schläfli matrix C_{i,j}=2cos(π/M_{i,j}). The Schläfli matrix is useful because its eigenvalues determine whether the Coxeter group is of finite type (all positive), affine type (all nonnegative, at least one zero), or indefinite type (otherwise). The indefinite type is sometimes further subdivided, e.g. into hyperbolic and other Coxeter groups. However, there are multiple nonequivalent definitions for hyperbolic Coxeter groups.
Coxeter group  A_{1}×A_{1}  A_{2}  A_{3}  B_{3}  D_{4}  

Coxeter diagram  
Coxeter matrix  
Schläfli matrix 
An example
The graph in which vertices 1 through n are placed in a row with each vertex connected by an unlabelled edge to its immediate neighbors gives rise to the symmetric group S_{n+1}; the generators correspond to the transpositions (1 2), (2 3), ... (n n+1). Two nonconsecutive transpositions always commute, while (k k+1) (k+1 k+2) gives the 3cycle (k k+2 k+1). Of course this only shows that S_{n+1} is a quotient group of the Coxeter group described by the graph, but it is not too difficult to check that equality holds.
Connection with reflection groups
Coxeter groups are deeply connected with reflection groups. Simply put, Coxeter groups are abstract groups (given via a presentation), while reflection groups are concrete groups (given as subgroups of linear groups or various generalizations). Coxeter groups grew out of the study of reflection groups — they are an abstraction: a reflection group is a subgroup of a linear group generated by reflections (which have order 2), while a Coxeter group is an abstract group generated by involutions (elements of order 2, abstracting from reflections), and whose relations have a certain form (, corresponding to hyperplanes meeting at an angle of , with being of order k abstracting from a rotation by ).
The abstract group of a reflection group is a Coxeter group, while conversely a reflection group can be seen as a linear representation of a Coxeter group. For finite reflection groups, this yields an exact correspondence: every finite Coxeter group admits a faithful representation as a finite reflection group of some Euclidean space. For infinite Coxeter groups, however, a Coxeter group may not admit a representation as a reflection group.
Historically, (Coxeter 1934) proved that every reflection group is a Coxeter group (i.e., has a presentation where all relations are of the form or ), and indeed this paper introduced the notion of a Coxeter group, while (Coxeter 1935) proved that every finite Coxeter group had a representation as a reflection group, and classified finite Coxeter groups.
Finite Coxeter groups
Classification
The finite Coxeter groups were classified in (Coxeter 1935), in terms of Coxeter–Dynkin diagrams; they are all represented by reflection groups of finitedimensional Euclidean spaces.
The finite Coxeter groups consist of three oneparameter families of increasing rank one oneparameter family of dimension two, and six exceptional groups: and
Weyl groups
Many, but not all of these, are Weyl groups, and every Weyl group can be realized as a Coxeter group. The Weyl groups are the families and and the exceptions and denoted in Weyl group notation as The nonWeyl groups are the exceptions and and the family except where this coincides with one of the Weyl groups (namely and ).
This can be proven by comparing the restrictions on (undirected) Dynkin diagrams with the restrictions on Coxeter diagrams of finite groups: formally, the Coxeter graph can be obtained from the Dynkin diagram by discarding the direction of the edges, and replacing every double edge with an edge labelled 4 and every triple edge by an edge labelled 6. Also note that every finitely generated Coxeter group is an Automatic group.^{[1]} Dynkin diagrams have the additional restriction that the only permitted edge labels are 2, 3, 4, and 6, which yields the above. Geometrically, this corresponds to the crystallographic restriction theorem, and the fact that excluded polytopes do not fill space or tile the plane – for the dodecahedron (dually, icosahedron) does not fill space; for the 120cell (dually, 600cell) does not fill space; for a pgon does not tile the plane except for or (the triangular, square, and hexagonal tilings, respectively).
Note further that the (directed) Dynkin diagrams B_{n} and C_{n} give rise to the same Weyl group (hence Coxeter group), because they differ as directed graphs, but agree as undirected graphs – direction matters for root systems but not for the Weyl group; this corresponds to the hypercube and crosspolytope being different regular polytopes but having the same symmetry group.
Properties
Some properties of the finite Coxeter groups are given in the following table:
Group symbol  Alternate symbol  Bracket notation  Rank  Order  Related polytopes  CoxeterDynkin diagram 

A_{n}  A_{n}  [3^{n1}]  n  (n + 1)!  nsimplex  .. 
B_{n}  C_{n}  [4,3^{n2}]  n  2^{n} n!  nhypercube / ncrosspolytope  ... 
D_{n}  B_{n}  [3^{n3,1,1}]  n  2^{n−1} n!  ndemihypercube  ... 
E_{6}  E_{6}  [3^{2,2,1}]  6  72x6! = 51840  2_{21}, 1_{22}  or 
E_{7}  E_{7}  [3^{3,2,1}]  7  72x8! = 2903040  3_{21}, 2_{31}, 1_{32}  
E_{8}  E_{8}  [3^{4,2,1}]  8  192x10! = 696729600  4_{21}, 2_{41}, 1_{42}  
F_{4}  F_{4}  [3,4,3]  4  1152  24cell  
G_{2}    [6]  2  12  hexagon  
H_{2}  G_{2}  [5]  2  10  pentagon  
H_{3}  G_{3}  [3,5]  3  120  icosahedron / dodecahedron  
H_{4}  G_{4}  [3,3,5]  4  14400  120cell / 600cell  
I_{2}(p)  D_{2}^{p}  [p]  2  2p  pgon 
Symmetry groups of regular polytopes
All symmetry groups of regular polytopes are finite Coxeter groups. Note that dual polytopes have the same symmetry group.
There are three series of regular polytopes in all dimensions. The symmetry group of a regular nsimplex is the symmetric group S_{n+1}, also known as the Coxeter group of type A_{n}. The symmetry group of the ncube and its dual, the ncrosspolytope, is B_{n}, and is known as the hyperoctahedral group.
The exceptional regular polytopes in dimensions two, three, and four, correspond to other Coxeter groups. In two dimensions, the dihedral groups, which are the symmetry groups of regular polygons, form the series I_{2}(p). In three dimensions, the symmetry group of the regular dodecahedron and its dual, the regular icosahedron, is H_{3}, known as the full icosahedral group. In four dimensions, there are three special regular polytopes, the 24cell, the 120cell, and the 600cell. The first has symmetry group F_{4}, while the other two are dual and have symmetry group H_{4}.
The Coxeter groups of type D_{n}, E_{6}, E_{7}, and E_{8} are the symmetry groups of certain semiregular polytopes.
Table of irreducible polytope families  

Family n 
nsimplex  nhypercube  northoplex  ndemicube  1_{k2}  2_{k1}  k_{21}  pentagonal polytope  
Group  A_{n}  BC_{n} 


H_{n}  
2  pgon (example: p=7) 
Hexagon 
Pentagon  
3  Tetrahedron 
Cube 
Octahedron 
Tetrahedron 
Dodecahedron 
Icosahedron  
4  5cell 
16cell 
24cell 
120cell 
600cell  
5  5simplex 
5cube 
5orthoplex 
5demicube 

6  6simplex 
6cube 
6orthoplex 
6demicube 
1_{22} 
2_{21} 

7  7simplex 
7cube 
7orthoplex 
7demicube 
1_{32} 
2_{31} 
3_{21} 

8  8simplex 
8cube 
8orthoplex 
8demicube 
1_{42} 
2_{41} 
4_{21} 

9  9simplex 
9cube 
9orthoplex 
9demicube 

10  10simplex 
10cube 
10orthoplex 
10demicube 
Affine Coxeter groups
The affine Coxeter groups form a second important series of Coxeter groups. These are not finite themselves, but each contains a normal abelian subgroup such that the corresponding quotient group is finite. In each case, the quotient group is itself a Coxeter group, and the Coxeter graph is obtained from the Coxeter graph of the Coxeter group by adding another vertex and one or two additional edges. For example, for n ≥ 2, the graph consisting of n+1 vertices in a circle is obtained from A_{n} in this way, and the corresponding Coxeter group is the affine Weyl group of A_{n}. For n = 2, this can be pictured as the symmetry group of the standard tiling of the plane by equilateral triangles.
A list of the affine Coxeter groups follows:
Group symbol  Witt symbol  Bracket notation  Related uniform tessellation(s)  CoxeterDynkin diagram 

P_{n+1}  [3^{[n]}]  Simplectic honeycomb  ... or ...  
S_{n+1}  [4,3^{n3},3^{1,1}]  Demihypercubic honeycomb  ...  
R_{n+1}  [4,3^{n2},4]  Hypercubic honeycomb  ...  
Q_{n+1}  [ 3^{1,1},3^{n4},3^{1,1}]  Demihypercubic honeycomb  ...  
T_{7}  [3^{2,2,2}]  2_{22}  or  
T_{8}  [3^{3,3,1}]  3_{31}, 1_{33}  or  
T_{9}  [3^{5,2,1}]  5_{21}, 2_{51}, 1_{52}  
U_{5}  [3,4,3,3]  16cell honeycomb 24cell honeycomb  
V_{3}  [6,3]  Hexagonal tiling and Triangular tiling  
W_{2}  [∞]  apeirogon 
The group symbol 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.
Hyperbolic Coxeter groups
There are infinitely many hyperbolic Coxeter groups describing reflection groups in hyperbolic space, notably including the hyperbolic triangle groups.
Partial orders
A choice of reflection generators gives rise to a length function l on a Coxeter group, namely the minimum number of uses of generators required to express a group element; this is precisely the length in the word metric in the Cayley graph. An expression for v using l(v) generators is a reduced word. For example, the permutation (13) in S_{3} has two reduced words, (12)(23)(12) and (23)(12)(23). The function defines a map generalizing the sign map for the symmetric group.
Using reduced words one may define three partial orders on the Coxeter group, the (right) weak order, the absolute order and the Bruhat order (named for François Bruhat). An element v exceeds an element u in the Bruhat order if some (or equivalently, any) reduced word for v contains a reduced word for u as a substring, where some letters (in any position) are dropped. In the weak order, v ≥ u if some reduced word for v contains a reduced word for u as an initial segment. Indeed, the word length makes this into a graded poset. The Hasse diagrams corresponding to these orders are objects of study, and are related to the Cayley graph determined by the generators. The absolute order is defined analogously to the weak order, but with generating set/alphabet consisting of all conjugates of the Coxeter generators.
For example, the permutation (1 2 3) in S_{3} has only one reduced word, (12)(23), so covers (12) and (23) in the Bruhat order but only covers (12) in the weak order.
Homology
Since a Coxeter group W is generated by finitely many elements of order 2, its abelianization is an elementary abelian 2group, i.e. it is isomorphic to the direct sum of several copies of the cyclic group Z_{2}. This may be restated in terms of the first homology group of W.
The Schur multiplier M(W) (related to the second homology) was computed in (Ihara & Yokonuma 1965) for finite reflection groups and in (Yokonuma 1965) for affine reflection groups, with a more unified account given in (Howlett 1988). In all cases, the Schur multiplier is also an elementary abelian 2group. For each infinite family {W_{n}} of finite or affine Weyl groups, the rank of M(W) stabilizes as n goes to infinity.
See also
 Artin group
 Triangle group
 Coxeter element
 Coxeter number
 Complex reflection group
 Chevalley–Shephard–Todd theorem
 CoxeterDynkin diagram
 Hecke algebra, a quantum deformation of the group algebra
 Kazhdan–Lusztig polynomial
 Longest element of a Coxeter group
 Supersoluble arrangement
References
 ↑ Brink, Brigitte; Howlett, RobertB. (1993), "A finiteness property and an automatic structure for Coxeter groups", Mathematische Annalen, 296 (1): 179–190, doi:10.1007/BF01445101, Zbl 0793.20036.
Further reading
 Coxeter, H. S. M. (1934), "Discrete groups generated by reflections", Ann. of Math., 35 (3): 588–621, JSTOR 1968753
 Coxeter, H. S. M. (1935), "The complete enumeration of finite groups of the form ", J. London Math. Soc., 1, 10 (1): 21–25, doi:10.1112/jlms/s110.37.21
 Davis, Michael W. (2007), The Geometry and Topology of Coxeter Groups (PDF), ISBN 9780691131382, Zbl 1142.20020
 Grove, Larry C.; Benson, Clark T. (1985), Finite Reflection Groups, Graduate texts in mathematics, 99, Springer, ISBN 9780387960821
 Humphreys, James E. (1992) [1990], Reflection Groups and Coxeter Groups, Cambridge Studies in Advanced Mathematics, 29, Cambridge University Press, ISBN 9780521436137, Zbl 0725.20028
 Kane, Richard (2001), Reflection Groups and Invariant Theory, CMS Books in Mathematics, Springer, ISBN 9780387989792, Zbl 0986.20038
 Björner, Anders; Brenti, Francesco (2005), Combinatorics of Coxeter Groups, Graduate Texts in Mathematics, 231, Springer, ISBN 9783540275961, Zbl 1110.05001
 Hiller, Howard (1982), Geometry of Coxeter groups, Research Notes in Mathematics, 54, Pitman, ISBN 9780273085171, Zbl 0483.57002
 Bourbaki, Nicolas (2002), Lie Groups and Lie Algebras: Chapters 46, Elements of Mathematics, Springer, ISBN 9783540426509, Zbl 0983.17001
 Howlett, Robert B. (1988), "On the Schur Multipliers of Coxeter Groups", J. London Math. Soc., 2, 38 (2): 263–276, doi:10.1112/jlms/s238.2.263, Zbl 0627.20019
 Vinberg, E. B. (1984), "Absence of crystallographic groups of reflections in Lobachevski spaces of large dimension", Trudy Moskov. Mat. Obshch., 47
 Ihara, S.; Yokonuma, Takeo (1965), "On the second cohomology groups (Schurmultipliers) of finite reflection groups" (PDF), Jour. Fac. Sci. Univ. Tokyo, Sect. 1, 11: 155–171, Zbl 0136.28802
 Yokonuma, Takeo (1965), "On the second cohomology groups (Schurmultipliers) of infinite discrete reflection groups", Jour. Fac. Sci. Univ. Tokyo, Sect. 1, 11: 173–186, Zbl 0136.28803
External links
 Hazewinkel, Michiel, ed. (2001), "Coxeter group", Encyclopedia of Mathematics, Springer, ISBN 9781556080104
 Weisstein, Eric W. "Coxeter group". MathWorld.