# Coxeter Groups As Matrix Groups¶

This implements a general Coxeter group as a matrix group by using the reflection representation.

AUTHORS:

• Travis Scrimshaw (2013-08-28): Initial version
class sage.groups.matrix_gps.coxeter_group.CoxeterMatrixGroup(coxeter_matrix, base_ring, index_set)

A Coxeter group represented as a matrix group.

Let $$(W, S)$$ be a Coxeter system. We construct a vector space $$V$$ over $$\RR$$ with a basis of $$\{ \alpha_s \}_{s \in S}$$ and inner product

$B(\alpha_s, \alpha_t) = -\cos\left( \frac{\pi}{m_{st}} \right)$

where we have $$B(\alpha_s, \alpha_t) = -1$$ if $$m_{st} = \infty$$. Next we define a representation $$\sigma_s : V \to V$$ by

$\sigma_s \lambda = \lambda - 2 B(\alpha_s, \lambda) \alpha_s.$

This representation is faithful so we can represent the Coxeter group $$W$$ by the set of matrices $$\sigma_s$$ acting on $$V$$.

INPUT:

• data – a Coxeter matrix or graph or a Cartan type
• base_ring – (default: the universal cyclotomic field or a number field) the base ring which contains all values $$\cos(\pi/m_{ij})$$ where $$(m_{ij})_{ij}$$ is the Coxeter matrix
• index_set – (optional) an indexing set for the generators

For finite Coxeter groups, the default base ring is taken to be $$\QQ$$ or a quadratic number field when possible.

For more on creating Coxeter groups, see CoxeterGroup().

Todo

Currently the label $$\infty$$ is implemented as $$-1$$ in the Coxeter matrix.

EXAMPLES:

We can create Coxeter groups from Coxeter matrices:

sage: W = CoxeterGroup([[1, 6, 3], [6, 1, 10], [3, 10, 1]])
sage: W
Coxeter group over Universal Cyclotomic Field with Coxeter matrix:
[ 1  6  3]
[ 6  1 10]
[ 3 10  1]
sage: W.gens()
(
[                 -1 -E(12)^7 + E(12)^11                   1]
[                  0                   1                   0]
[                  0                   0                   1],

[                  1                   0                   0]
[-E(12)^7 + E(12)^11                  -1     E(20) - E(20)^9]
[                  0                   0                   1],

[              1               0               0]
[              0               1               0]
[              1 E(20) - E(20)^9              -1]
)
sage: m = matrix([[1,3,3,3], [3,1,3,2], [3,3,1,2], [3,2,2,1]])
sage: W = CoxeterGroup(m)
sage: W.gens()
(
[-1  1  1  1]  [ 1  0  0  0]  [ 1  0  0  0]  [ 1  0  0  0]
[ 0  1  0  0]  [ 1 -1  1  0]  [ 0  1  0  0]  [ 0  1  0  0]
[ 0  0  1  0]  [ 0  0  1  0]  [ 1  1 -1  0]  [ 0  0  1  0]
[ 0  0  0  1], [ 0  0  0  1], [ 0  0  0  1], [ 1  0  0 -1]
)
sage: a,b,c,d = W.gens()
sage: (a*b*c)^3
[ 5  1 -5  7]
[ 5  0 -4  5]
[ 4  1 -4  4]
[ 0  0  0  1]
sage: (a*b)^3
[1 0 0 0]
[0 1 0 0]
[0 0 1 0]
[0 0 0 1]
sage: b*d == d*b
True
sage: a*c*a == c*a*c
True


We can create the matrix representation over different base rings and with different index sets. Note that the base ring must contain all $$2*\cos(\pi/m_{ij})$$ where $$(m_{ij})_{ij}$$ is the Coxeter matrix:

sage: W = CoxeterGroup(m, base_ring=RR, index_set=['a','b','c','d'])
sage: W.base_ring()
Real Field with 53 bits of precision
sage: W.index_set()
('a', 'b', 'c', 'd')

sage: CoxeterGroup(m, base_ring=ZZ)
Coxeter group over Integer Ring with Coxeter matrix:
[1 3 3 3]
[3 1 3 2]
[3 3 1 2]
[3 2 2 1]
sage: CoxeterGroup([[1,4],[4,1]], base_ring=QQ)
Traceback (most recent call last):
...
TypeError: unable to convert sqrt(2) to a rational


Using the well-known conversion between Coxeter matrices and Coxeter graphs, we can input a Coxeter graph. Following the standard convention, edges with no label (i.e. labelled by None) are treated as 3:

sage: G = Graph([(0,3,None), (1,3,15), (2,3,7), (0,1,3)])
sage: W = CoxeterGroup(G); W
Coxeter group over Universal Cyclotomic Field with Coxeter matrix:
[ 1  3  2  3]
[ 3  1  2 15]
[ 2  2  1  7]
[ 3 15  7  1]
sage: G2 = W.coxeter_diagram()
sage: CoxeterGroup(G2) is W
True


Because there currently is no class for $$\ZZ \cup \{ \infty \}$$, labels of $$\infty$$ are given by $$-1$$ in the Coxeter matrix:

sage: G = Graph([(0,1,None), (1,2,4), (0,2,oo)])
sage: W = CoxeterGroup(G)
sage: W.coxeter_matrix()
[ 1  3 -1]
[ 3  1  4]
[-1  4  1]


We can also create Coxeter groups from Cartan types using the implementation keyword:

sage: W = CoxeterGroup(['D',5], implementation="reflection")
sage: W
Finite Coxeter group over Integer Ring with Coxeter matrix:
[1 3 2 2 2]
[3 1 3 2 2]
[2 3 1 3 3]
[2 2 3 1 2]
[2 2 3 2 1]
sage: W = CoxeterGroup(['H',3], implementation="reflection")
sage: W
Finite Coxeter group over Number Field in a with defining polynomial
x^2 - 5 with Coxeter matrix:
[1 3 2]
[3 1 5]
[2 5 1]

class Element

A Coxeter group element.

action_on_root_indices(i, side='left')

Return the action on the set of roots.

The roots are ordered as in the output of the method $$roots$$.

EXAMPLES:

sage: W = CoxeterGroup(['A',3], implementation="reflection")
sage: w = W.w0
sage: w.action_on_root_indices(0)
11

canonical_matrix()

Return the matrix of self in the canonical faithful representation, which is self as a matrix.

EXAMPLES:

sage: W = CoxeterGroup(['A',3], implementation="reflection")
sage: a,b,c = W.gens()
sage: elt = a*b*c
sage: elt.canonical_matrix()
[ 0  0 -1]
[ 1  0 -1]
[ 0  1 -1]

descents(side='right', index_set=None, positive=False)

Return the descents of self, as a list of elements of the index_set.

INPUT:

• index_set – (default: all of them) a subset (as a list or iterable) of the nodes of the Dynkin diagram
• side – (default: 'right') 'left' or 'right'
• positive – (default: False) boolean

EXAMPLES:

sage: W = CoxeterGroup(['A',3], implementation="reflection")
sage: a,b,c = W.gens()
sage: elt = b*a*c
sage: elt.descents()
[1, 3]
sage: elt.descents(positive=True)
[2]
sage: elt.descents(index_set=[1,2])
[1]
sage: elt.descents(side='left')
[2]

first_descent(side='right', index_set=None, positive=False)

Return the first left (resp. right) descent of self, as ane element of index_set, or None if there is none.

See descents() for a description of the options.

EXAMPLES:

sage: W = CoxeterGroup(['A',3], implementation="reflection")
sage: a,b,c = W.gens()
sage: elt = b*a*c
sage: elt.first_descent()
1
sage: elt.first_descent(side='left')
2

has_right_descent(i)

Return whether i is a right descent of self.

A Coxeter system $$(W, S)$$ has a root system defined as $$\{ w(\alpha_s) \}_{w \in W}$$ and we define the positive (resp. negative) roots $$\alpha = \sum_{s \in S} c_s \alpha_s$$ by all $$c_s \geq 0$$ (resp. $$c_s \leq 0$$). In particular, we note that if $$\ell(w s) > \ell(w)$$ then $$w(\alpha_s) > 0$$ and if $$\ell(ws) < \ell(w)$$ then $$w(\alpha_s) < 0$$. Thus $$i \in I$$ is a right descent if $$w(\alpha_{s_i}) < 0$$ or equivalently if the matrix representing $$w$$ has all entries of the $$i$$-th column being non-positive.

INPUT:

• i – an element in the index set

EXAMPLES:

sage: W = CoxeterGroup(['A',3], implementation="reflection")
sage: a,b,c = W.gens()
sage: elt = b*a*c
sage: [elt.has_right_descent(i) for i in [1, 2, 3]]
[True, False, True]

bilinear_form()

Return the bilinear form associated to self.

Given a Coxeter group $$G$$ with Coxeter matrix $$M = (m_{ij})_{ij}$$, the associated bilinear form $$A = (a_{ij})_{ij}$$ is given by

$a_{ij} = -\cos\left( \frac{\pi}{m_{ij}} \right).$

If $$A$$ is positive definite, then $$G$$ is of finite type (and so the associated Coxeter group is a finite group). If $$A$$ is positive semidefinite, then $$G$$ is affine type.

EXAMPLES:

sage: W = CoxeterGroup(['D',4])
sage: W.bilinear_form()
[   1 -1/2    0    0]
[-1/2    1 -1/2 -1/2]
[   0 -1/2    1    0]
[   0 -1/2    0    1]

canonical_representation()

Return the canonical faithful representation of self, which is self.

EXAMPLES:

sage: W = CoxeterGroup([[1,3],[3,1]])
sage: W.canonical_representation() is W
True

coxeter_matrix()

Return the Coxeter matrix of self.

EXAMPLES:

sage: W = CoxeterGroup([[1,3],[3,1]])
sage: W.coxeter_matrix()
[1 3]
[3 1]
sage: W = CoxeterGroup(['H',3])
sage: W.coxeter_matrix()
[1 3 2]
[3 1 5]
[2 5 1]

fundamental_weight(i)

Return the fundamental weight with index i.

EXAMPLES:

sage: W = CoxeterGroup(['A',3], implementation='reflection')
sage: W.fundamental_weight(1)
(3/2, 1, 1/2)

fundamental_weights()

Return the fundamental weights for self.

This is the dual basis to the basis of simple roots.

The base ring must be a field.

EXAMPLES:

sage: W = CoxeterGroup(['A',3], implementation='reflection')
sage: W.fundamental_weights()
Finite family {1: (3/2, 1, 1/2), 2: (1, 2, 1), 3: (1/2, 1, 3/2)}

is_commutative()

Return whether self is commutative.

EXAMPLES:

sage: CoxeterGroup(['A', 2]).is_commutative()
False
sage: W = CoxeterGroup(['I',2])
sage: W.is_commutative()
True

is_finite()

Return True if this group is finite.

EXAMPLES:

sage: [l for l in range(2, 9) if
....:  CoxeterGroup([[1,3,2],[3,1,l],[2,l,1]]).is_finite()]
....:
[2, 3, 4, 5]
sage: [l for l in range(2, 9) if
....:  CoxeterGroup([[1,3,2,2],[3,1,l,2],[2,l,1,3],[2,2,3,1]]).is_finite()]
....:
[2, 3, 4]
sage: [l for l in range(2, 9) if
....:  CoxeterGroup([[1,3,2,2,2], [3,1,3,3,2], [2,3,1,2,2],
....:                [2,3,2,1,l], [2,2,2,l,1]]).is_finite()]
....:
[2, 3]
sage: [l for l in range(2, 9) if
....:  CoxeterGroup([[1,3,2,2,2], [3,1,2,3,3], [2,2,1,l,2],
....:                [2,3,l,1,2], [2,3,2,2,1]]).is_finite()]
....:
[2, 3]
sage: [l for l in range(2, 9) if
....:  CoxeterGroup([[1,3,2,2,2,2], [3,1,l,2,2,2], [2,l,1,3,l,2],
....:                [2,2,3,1,2,2], [2,2,l,2,1,3], [2,2,2,2,3,1]]).is_finite()]
....:
[2, 3]

order()

Return the order of self.

If the Coxeter group is finite, this uses an iterator.

EXAMPLES:

sage: W = CoxeterGroup([[1,3],[3,1]])
sage: W.order()
6
sage: W = CoxeterGroup([[1,-1],[-1,1]])
sage: W.order()
+Infinity

positive_roots(as_reflections=None)

Return the positive roots.

These are roots in the Coxeter sense, that all have the same norm. They are given by their coefficients in the base of simple roots, also taken to have all the same norm.

EXAMPLES:

sage: W = CoxeterGroup(['A',3], implementation='reflection')
sage: W.positive_roots()
((1, 0, 0), (1, 1, 0), (0, 1, 0), (1, 1, 1), (0, 1, 1), (0, 0, 1))
sage: W = CoxeterGroup(['I',5], implementation='reflection')
sage: W.positive_roots()
((1, 0),
(-E(5)^2 - E(5)^3, 1),
(-E(5)^2 - E(5)^3, -E(5)^2 - E(5)^3),
(1, -E(5)^2 - E(5)^3),
(0, 1))

reflections()

Return the set of reflections.

The order is the one given by positive_roots().

EXAMPLES:

sage: W = CoxeterGroup(['A',2], implementation='reflection')
sage: list(W.reflections())
[
[-1  1]  [ 0 -1]  [ 1  0]
[ 0  1], [-1  0], [ 1 -1]
]

roots()

Return the roots.

These are roots in the Coxeter sense, that all have the same norm. They are given by their coefficients in the base of simple roots, also taken to have all the same norm.

The positive roots are listed first, then the negative roots in the same order. The order is the one given by roots().

EXAMPLES:

sage: W = CoxeterGroup(['A',3], implementation='reflection')
sage: W.roots()
((1, 0, 0),
(1, 1, 0),
(0, 1, 0),
(1, 1, 1),
(0, 1, 1),
(0, 0, 1),
(-1, 0, 0),
(-1, -1, 0),
(0, -1, 0),
(-1, -1, -1),
(0, -1, -1),
(0, 0, -1))
sage: W = CoxeterGroup(['I',5], implementation='reflection')
sage: len(W.roots())
10

simple_reflection(i)

Return the simple reflection $$s_i$$.

INPUT:

• i – an element from the index set

EXAMPLES:

sage: W = CoxeterGroup(['A',3], implementation="reflection")
sage: W.simple_reflection(1)
[-1  1  0]
[ 0  1  0]
[ 0  0  1]
sage: W.simple_reflection(2)
[ 1  0  0]
[ 1 -1  1]
[ 0  0  1]
sage: W.simple_reflection(3)
[ 1  0  0]
[ 0  1  0]
[ 0  1 -1]

simple_root_index(i)

Return the index of the simple root $$\alpha_i$$.

This is the position of $$\alpha_i$$ in the list of all roots as given be roots().

EXAMPLES:

sage: W = CoxeterGroup(['A',3], implementation='reflection')
sage: [W.simple_root_index(i) for i in W.index_set()]
[0, 2, 5]