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coxeter_group.py
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coxeter_group.py
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"""
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
"""
##############################################################################
# Copyright (C) 2013 Travis Scrimshaw <tscrim at ucdavis.edu>
#
# Distributed under the terms of the GNU General Public License (GPL)
#
# The full text of the GPL is available at:
#
# http://www.gnu.org/licenses/
##############################################################################
from sage.structure.unique_representation import UniqueRepresentation
from sage.categories.coxeter_groups import CoxeterGroups
from sage.combinat.root_system.cartan_type import CartanType, CartanType_abstract
from sage.combinat.root_system.coxeter_matrix import CoxeterMatrix
from sage.groups.matrix_gps.finitely_generated import FinitelyGeneratedMatrixGroup_generic
from sage.groups.matrix_gps.group_element import MatrixGroupElement_generic
from sage.graphs.graph import Graph
from sage.matrix.constructor import matrix
from sage.matrix.matrix_space import MatrixSpace
from sage.rings.all import ZZ
from sage.rings.infinity import infinity
from sage.rings.universal_cyclotomic_field import UniversalCyclotomicField
from sage.misc.cachefunc import cached_method
from sage.misc.superseded import deprecated_function_alias
class CoxeterMatrixGroup(FinitelyGeneratedMatrixGroup_generic, UniqueRepresentation):
r"""
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
.. MATH::
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
.. MATH::
\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) 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 more on creating Coxeter groups, see
:meth:`~sage.combinat.root_system.coxeter_group.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],
<BLANKLINE>
[ 1 0 0]
[-E(12)^7 + E(12)^11 -1 E(20) - E(20)^9]
[ 0 0 1],
<BLANKLINE>
[ 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 Universal Cyclotomic Field 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 Universal Cyclotomic Field with Coxeter matrix:
[1 3 2]
[3 1 5]
[2 5 1]
"""
@staticmethod
def __classcall_private__(cls, data, base_ring=None, index_set=None):
"""
Normalize arguments to ensure a unique representation.
EXAMPLES::
sage: W1 = CoxeterGroup(['A',2], implementation="reflection", base_ring=UniversalCyclotomicField())
sage: W2 = CoxeterGroup([[1,3],[3,1]], index_set=(1,2))
sage: W1 is W2
True
sage: G1 = Graph([(1,2)])
sage: W3 = CoxeterGroup(G1)
sage: W1 is W3
True
sage: G2 = Graph([(1,2,3)])
sage: W4 = CoxeterGroup(G2)
sage: W1 is W4
True
"""
data = CoxeterMatrix(data, index_set=index_set)
if base_ring is None:
base_ring = UniversalCyclotomicField()
return super(CoxeterMatrixGroup, cls).__classcall__(cls,
data, base_ring, data.index_set())
def __init__(self, coxeter_matrix, base_ring, index_set):
"""
Initialize ``self``.
EXAMPLES::
sage: W = CoxeterGroup([[1,3,2],[3,1,3],[2,3,1]])
sage: TestSuite(W).run() # long time
sage: W = CoxeterGroup([[1,3,2],[3,1,4],[2,4,1]], base_ring=QQbar)
sage: TestSuite(W).run() # long time
sage: W = CoxeterGroup([[1,3,2],[3,1,6],[2,6,1]])
sage: TestSuite(W).run(max_runs=30) # long time
sage: W = CoxeterGroup([[1,3,2],[3,1,-1],[2,-1,1]])
sage: TestSuite(W).run(max_runs=30) # long time
We check that :trac:`16630` is fixed::
sage: CoxeterGroup(['D',4], base_ring=QQ).category()
Category of finite coxeter groups
sage: CoxeterGroup(['H',4], base_ring=QQbar).category()
Category of finite coxeter groups
sage: F = CoxeterGroups().Finite()
sage: all(CoxeterGroup([letter,i]) in F
....: for i in range(2,5) for letter in ['A','B','D'])
True
sage: all(CoxeterGroup(['E',i]) in F for i in range(6,9))
True
sage: CoxeterGroup(['F',4]).category()
Category of finite coxeter groups
sage: CoxeterGroup(['G',2]).category()
Category of finite coxeter groups
sage: all(CoxeterGroup(['H',i]) in F for i in range(3,5))
True
sage: all(CoxeterGroup(['I',i]) in F for i in range(2,5))
True
"""
self._matrix = coxeter_matrix
n = coxeter_matrix.rank()
# Compute the matrix with entries `2 \cos( \pi / m_{ij} )`.
MS = MatrixSpace(base_ring, n, sparse=True)
MC = MS._get_matrix_class()
# FIXME: Hack because there is no ZZ \cup \{ \infty \}: -1 represents \infty
if base_ring is UniversalCyclotomicField():
val = lambda x: base_ring.gen(2*x) + ~base_ring.gen(2*x) if x != -1 else base_ring(2)
else:
from sage.functions.trig import cos
from sage.symbolic.constants import pi
val = lambda x: base_ring(2*cos(pi / x)) if x != -1 else base_ring(2)
gens = [MS.one() + MC(MS, entries={(i, j): val(coxeter_matrix[index_set[i], index_set[j]])
for j in range(n)},
coerce=True, copy=True)
for i in range(n)]
category = CoxeterGroups()
# Now we shall see if the group is finite, and, if so, refine
# the category to ``category.Finite()``. Otherwise the group is
# infinite and we refine the category to ``category.Infinite()``.
if self._matrix.is_finite():
category = category.Finite()
else:
category = category.Infinite()
FinitelyGeneratedMatrixGroup_generic.__init__(self, ZZ(n), base_ring,
gens, category=category)
def _repr_(self):
"""
Return a string representation of ``self``.
EXAMPLES::
sage: CoxeterGroup([[1,3,2],[3,1,4],[2,4,1]])
Finite Coxeter group over Universal Cyclotomic Field with Coxeter matrix:
[1 3 2]
[3 1 4]
[2 4 1]
"""
rep = "Finite " if self.is_finite() else ""
rep += "Coxeter group over {} with Coxeter matrix:\n{}".format(self.base_ring(), self._matrix)
return rep
def index_set(self):
"""
Return the index set of ``self``.
EXAMPLES::
sage: W = CoxeterGroup([[1,3],[3,1]])
sage: W.index_set()
(1, 2)
sage: W = CoxeterGroup([[1,3],[3,1]], index_set=['x', 'y'])
sage: W.index_set()
('x', 'y')
sage: W = CoxeterGroup(['H',3])
sage: W.index_set()
(1, 2, 3)
"""
return self._matrix.index_set()
def coxeter_matrix(self):
"""
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]
"""
return self._matrix
def coxeter_diagram(self):
"""
Return the Coxeter diagram of ``self``.
EXAMPLES::
sage: W = CoxeterGroup(['H',3], implementation="reflection")
sage: G = W.coxeter_diagram(); G
Graph on 3 vertices
sage: G.edges()
[(1, 2, 3), (2, 3, 5)]
sage: CoxeterGroup(G) is W
True
sage: G = Graph([(0, 1, 3), (1, 2, oo)])
sage: W = CoxeterGroup(G)
sage: W.coxeter_diagram() == G
True
sage: CoxeterGroup(W.coxeter_diagram()) is W
True
"""
return self._matrix.coxeter_graph()
coxeter_graph = deprecated_function_alias(17798, coxeter_diagram)
def bilinear_form(self):
r"""
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
.. MATH::
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]
"""
return self._matrix.bilinear_form()
def is_finite(self):
"""
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]
"""
# Finite Coxeter groups are marked as finite in
# their ``__init__`` method, so we can just check
# the category of ``self``.
return "Finite" in self.category().axioms()
@cached_method
def order(self):
"""
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
"""
if self.is_finite():
try:
return ZZ(len(self._list))
except AttributeError:
return self._cardinality_from_iterator()
return infinity
def canonical_representation(self):
r"""
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
"""
return self
def simple_reflection(self, 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]
"""
if not i in self.index_set():
raise ValueError("%s is not in the index set %s" % (i, self.index_set()))
return self.gen(self.index_set().index(i))
class Element(MatrixGroupElement_generic):
"""
A Coxeter group element.
"""
def has_right_descent(self, i):
r"""
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]
"""
i = self.parent().index_set().index(i)
n = len(self.parent().index_set())
M = self.matrix()
zero = M.base_ring().zero()
# When working over the UCF, this is the bottleneck because it has
# to convert the entries to QQbar and do the comparison there.
return all(M[j,i] <= zero for j in range(n))
def canonical_matrix(self):
r"""
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]
"""
return self.matrix()