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coxeter_matrix.py
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coxeter_matrix.py
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"""
Coxeter Matrices
"""
#*****************************************************************************
# Copyright (C) 2007 Mike Hansen <mhansen@gmail.com>,
# 2015 Travis Scrimshaw <tscrim at ucdavis.edu>
# 2015 Jean-Philippe Labbe <labbe at math.huji.ac.il>
#
# Distributed under the terms of the GNU General Public License (GPL)
#
# This code is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
# General Public License for more details.
#
# The full text of the GPL is available at:
#
# http://www.gnu.org/licenses/
#*****************************************************************************
from six import add_metaclass
from sage.misc.cachefunc import cached_method
from sage.matrix.constructor import matrix
from sage.matrix.matrix_space import MatrixSpace
from sage.misc.classcall_metaclass import ClasscallMetaclass, typecall
from sage.matrix.matrix_generic_dense import Matrix_generic_dense
from sage.graphs.graph import Graph
from sage.rings.all import ZZ, QQ, RR
from sage.rings.infinity import infinity
from sage.combinat.root_system.cartan_type import CartanType
from sage.combinat.root_system.coxeter_type import CoxeterType
@add_metaclass(ClasscallMetaclass)
class CoxeterMatrix(CoxeterType):
r"""
A Coxeter matrix.
A Coxeter matrix `M = (m_{ij})_{i,j \in I}` is a matrix encoding
a Coxeter system `(W, S)`, where the relations are given by
`(s_i s_j)^{m_{ij}}`. Thus `M` is symmetric and has entries
in `\{1, 2, 3, \ldots, \infty\}` with `m_{ij} = 1` if and only
if `i = j`.
We represent `m_{ij} = \infty` by any number `m_{ij} \leq -1`. In
particular, we can construct a bilinear form `B = (b_{ij})_{i,j \in I}`
from `M` by
.. MATH::
b_{ij} = \begin{cases}
m_{ij} & m_{ij} < 0\ (\text{i.e., } m_{ij} = \infty), \\
-\cos\left( \frac{\pi}{m_{ij}} \right) & \text{otherwise}.
\end{cases}
EXAMPLES::
sage: CoxeterMatrix(['A', 4])
[1 3 2 2]
[3 1 3 2]
[2 3 1 3]
[2 2 3 1]
sage: CoxeterMatrix(['B', 4])
[1 3 2 2]
[3 1 3 2]
[2 3 1 4]
[2 2 4 1]
sage: CoxeterMatrix(['C', 4])
[1 3 2 2]
[3 1 3 2]
[2 3 1 4]
[2 2 4 1]
sage: CoxeterMatrix(['D', 4])
[1 3 2 2]
[3 1 3 3]
[2 3 1 2]
[2 3 2 1]
sage: CoxeterMatrix(['E', 6])
[1 2 3 2 2 2]
[2 1 2 3 2 2]
[3 2 1 3 2 2]
[2 3 3 1 3 2]
[2 2 2 3 1 3]
[2 2 2 2 3 1]
sage: CoxeterMatrix(['F', 4])
[1 3 2 2]
[3 1 4 2]
[2 4 1 3]
[2 2 3 1]
sage: CoxeterMatrix(['G', 2])
[1 6]
[6 1]
By default, entries representing `\infty` are given by `-1`
in the Coxeter matrix::
sage: G = Graph([(0,1,None), (1,2,4), (0,2,oo)])
sage: CoxeterMatrix(G)
[ 1 3 -1]
[ 3 1 4]
[-1 4 1]
It is possible to give a number `\leq -1` to represent an infinite label::
sage: CoxeterMatrix([[1,-1],[-1,1]])
[ 1 -1]
[-1 1]
sage: CoxeterMatrix([[1,-3/2],[-3/2,1]])
[ 1 -3/2]
[-3/2 1]
"""
@staticmethod
def __classcall_private__(cls, data=None, index_set=None, coxeter_type=None,
cartan_type=None, coxeter_type_check=True):
r"""
A Coxeter matrix can we created via a graph, a Coxeter type, or
a matrix.
.. NOTE::
To disable the Coxeter type check, use the optional argument
``coxeter_type_check = False``.
EXAMPLES::
sage: C = CoxeterMatrix(['A',1,1],['a','b'])
sage: C2 = CoxeterMatrix([[1, -1], [-1, 1]])
sage: C3 = CoxeterMatrix(matrix([[1, -1], [-1, 1]]), [0, 1])
sage: C == C2 and C == C3
True
Check with `\infty` because of the hack of using `-1` to represent
`\infty` in the Coxeter matrix::
sage: G = Graph([(0, 1, 3), (1, 2, oo)])
sage: W1 = CoxeterMatrix([[1, 3, 2], [3, 1, -1], [2, -1, 1]])
sage: W2 = CoxeterMatrix(G)
sage: W1 == W2
True
sage: CoxeterMatrix(W1.coxeter_graph()) == W1
True
The base ring of the matrix depends on the entries given::
sage: CoxeterMatrix([[1,-1],[-1,1]])._matrix.base_ring()
Integer Ring
sage: CoxeterMatrix([[1,-3/2],[-3/2,1]])._matrix.base_ring()
Rational Field
sage: CoxeterMatrix([[1,-1.5],[-1.5,1]])._matrix.base_ring()
Real Field with 53 bits of precision
"""
if not data:
if coxeter_type:
data = CoxeterType(coxeter_type)
elif cartan_type:
data = CoxeterType(CartanType(cartan_type))
# Special cases with no arguments passed
if not data:
data = []
n = 0
index_set = tuple()
coxeter_type = None
base_ring = ZZ
mat = typecall(cls, MatrixSpace(base_ring, n, sparse=False), data, coxeter_type, index_set)
mat._subdivisions = None
return mat
if isinstance(data, CoxeterMatrix): # Initiate from itself
return data
# Initiate from a graph:
# TODO: Check if a CoxeterDiagram once implemented
if isinstance(data, Graph):
return cls._from_graph(data, coxeter_type_check)
# Get the Coxeter type
coxeter_type = None
from sage.combinat.root_system.cartan_type import CartanType_abstract
if isinstance(data, CartanType_abstract):
coxeter_type = data.coxeter_type()
else:
try:
coxeter_type = CoxeterType(data)
except (TypeError, ValueError, NotImplementedError):
pass
# Initiate from a Coxeter type
if coxeter_type:
return cls._from_coxetertype(coxeter_type)
# TODO:: remove when oo is possible in matrices.
n = len(data[0])
data = [x if x != infinity else -1 for r in data for x in r]
data = matrix(n, n, data)
# until here
# Get the index set
if index_set:
index_set = tuple(index_set)
else:
index_set = tuple(range(1,n+1))
if len(set(index_set)) != n:
raise ValueError("the given index set is not valid")
return cls._from_matrix(data, coxeter_type, index_set, coxeter_type_check)
def __init__(self, parent, data, coxeter_type, index_set):
"""
Initialize ``self``.
TESTS::
sage: C = CoxeterMatrix(['A', 2, 1])
sage: TestSuite(C).run(skip=["_test_category", "_test_change_ring"])
"""
self._matrix = Matrix_generic_dense(parent, data, False, True)
self._matrix.set_immutable()
if self._matrix.base_ring() not in [ZZ, QQ]:
self._is_cyclotomic = False
else:
self._is_cyclotomic = True
self._coxeter_type = coxeter_type
if self._coxeter_type is not None:
if self._coxeter_type.is_finite():
self._is_finite = True
self._is_affine = False
elif self._coxeter_type.is_affine():
self._is_finite = False
self._is_affine = True
else:
self._is_finite = False
self._is_affine = False
else:
self._is_finite = False
self._is_affine = False
self._index_set = index_set
self._rank = self._matrix.nrows()
self._dict = {(self._index_set[i], self._index_set[j]): self._matrix[i, j]
for i in range(self._rank) for j in range(self._rank)}
for i,key in enumerate(self._index_set):
self._dict[key] = {key2: self._matrix[i,j]
for j,key2 in enumerate(self._index_set)}
@classmethod
def _from_matrix(cls, data, coxeter_type, index_set, coxeter_type_check):
"""
Initiate the Coxeter matrix from a matrix.
TESTS::
sage: CM = CoxeterMatrix([[1,2],[2,1]]); CM
[1 2]
[2 1]
sage: CM = CoxeterMatrix([[1,-1],[-1,1]]); CM
[ 1 -1]
[-1 1]
sage: CM = CoxeterMatrix([[1,-1.5],[-1.5,1]]); CM
[ 1.00000000000000 -1.50000000000000]
[-1.50000000000000 1.00000000000000]
sage: CM = CoxeterMatrix([[1,-3/2],[-3/2,1]]); CM
[ 1 -3/2]
[-3/2 1]
sage: CM = CoxeterMatrix([[1,-3/2,5],[-3/2,1,-1],[5,-1,1]]); CM
[ 1 -3/2 5]
[-3/2 1 -1]
[ 5 -1 1]
sage: CM = CoxeterMatrix([[1,-3/2,5],[-3/2,1,oo],[5,oo,1]]); CM
[ 1 -3/2 5]
[-3/2 1 -1]
[ 5 -1 1]
"""
# Check that the data is valid
check_coxeter_matrix(data)
M = matrix(data)
n = M.ncols()
base_ring = M.base_ring()
if not coxeter_type:
if n == 1:
coxeter_type = CoxeterType(['A', 1])
elif coxeter_type_check:
coxeter_type = recognize_coxeter_type_from_matrix(M, index_set)
else:
coxeter_type = None
raw_data = M.list()
mat = typecall(cls, MatrixSpace(base_ring, n, sparse=False), raw_data,
coxeter_type, index_set)
mat._subdivisions = M._subdivisions
return mat
@classmethod
def _from_graph(cls, graph, coxeter_type_check):
"""
Initiate the Coxeter matrix from a graph.
TESTS::
sage: CoxeterMatrix(CoxeterMatrix(['A',4,1]).coxeter_graph())
[1 3 2 2 3]
[3 1 3 2 2]
[2 3 1 3 2]
[2 2 3 1 3]
[3 2 2 3 1]
sage: CoxeterMatrix(CoxeterMatrix(['B',4,1]).coxeter_graph())
[1 2 3 2 2]
[2 1 3 2 2]
[3 3 1 3 2]
[2 2 3 1 4]
[2 2 2 4 1]
sage: CoxeterMatrix(CoxeterMatrix(['F',4]).coxeter_graph())
[1 3 2 2]
[3 1 4 2]
[2 4 1 3]
[2 2 3 1]
sage: G=Graph()
sage: G.add_edge([0,1,oo])
sage: CoxeterMatrix(G)
[ 1 -1]
[-1 1]
sage: H = Graph()
sage: H.add_edge([0,1,-1.5])
sage: CoxeterMatrix(H)
[ 1.00000000000000 -1.50000000000000]
[-1.50000000000000 1.00000000000000]
"""
verts = sorted(graph.vertices())
index_set = tuple(verts)
n = len(index_set)
# Setup the basis matrix as all 2 except 1 on the diagonal
data = []
for i in range(n):
data += [[]]
for j in range(n):
if i == j:
data[-1] += [ZZ.one()]
else:
data[-1] += [2]
for e in graph.edges():
label = e[2]
if label is None:
label = 3
elif label == infinity:
label = -1
elif label not in ZZ and label > -1:
raise ValueError("invalid Coxeter graph label")
elif label == 0 or label == 1:
raise ValueError("invalid Coxeter graph label")
i = verts.index(e[0])
j = verts.index(e[1])
data[j][i] = data[i][j] = label
return cls._from_matrix(data, None, index_set, coxeter_type_check)
@classmethod
def _from_coxetertype(cls, coxeter_type):
"""
Initiate the Coxeter matrix from a Coxeter type.
TESTS::
sage: CoxeterMatrix(['A',4]).coxeter_type()
Coxeter type of ['A', 4]
sage: CoxeterMatrix(['A',4,1]).coxeter_type()
Coxeter type of ['A', 4, 1]
sage: CoxeterMatrix(['D',4,1]).coxeter_type()
Coxeter type of ['D', 4, 1]
"""
index_set = coxeter_type.index_set()
n = len(index_set)
reverse = {index_set[i]: i for i in range(n)}
data = [[1 if i == j else 2 for j in range(n)] for i in range(n)]
for (i, j, l) in coxeter_type.coxeter_graph().edge_iterator():
if l == infinity:
l = -1
data[reverse[i]][reverse[j]] = l
data[reverse[j]][reverse[i]] = l
return cls._from_matrix(data, coxeter_type, index_set, False)
@classmethod
def samples(self, finite=None, affine=None, crystallographic=None, higher_rank=None):
"""
Return a sample of the available Coxeter types.
INPUT:
- ``finite`` -- (default: ``None``) a boolean or ``None``
- ``affine`` -- (default: ``None``) a boolean or ``None``
- ``crystallographic`` -- (default: ``None``) a boolean or ``None``
- ``higher_rank`` -- (default: ``None``) a boolean or ``None``
The sample contains all the exceptional finite and affine
Coxeter types, as well as typical representatives of the
infinite families.
Here the ``higher_rank`` term denotes non-finite, non-affine,
Coxeter groups (including hyperbolic types).
.. TODO:: Implement the hyperbolic and compact hyperbolic in the samples.
EXAMPLES::
sage: [CM.coxeter_type() for CM in CoxeterMatrix.samples()]
[
Coxeter type of ['A', 1], Coxeter type of ['A', 5],
<BLANKLINE>
Coxeter type of ['B', 5], Coxeter type of ['D', 4],
<BLANKLINE>
Coxeter type of ['D', 5], Coxeter type of ['E', 6],
<BLANKLINE>
Coxeter type of ['E', 7], Coxeter type of ['E', 8],
<BLANKLINE>
Coxeter type of ['F', 4], Coxeter type of ['H', 3],
<BLANKLINE>
Coxeter type of ['H', 4], Coxeter type of ['I', 10],
<BLANKLINE>
Coxeter type of ['A', 2, 1], Coxeter type of ['B', 5, 1],
<BLANKLINE>
Coxeter type of ['C', 5, 1], Coxeter type of ['D', 5, 1],
<BLANKLINE>
Coxeter type of ['E', 6, 1], Coxeter type of ['E', 7, 1],
<BLANKLINE>
Coxeter type of ['E', 8, 1], Coxeter type of ['F', 4, 1],
<BLANKLINE>
[ 1 -1 -1]
[-1 1 -1]
Coxeter type of ['G', 2, 1], Coxeter type of ['A', 1, 1], [-1 -1 1],
<BLANKLINE>
[ 1 -2 3 2]
[1 2 3] [-2 1 2 3]
[2 1 7] [ 3 2 1 -8]
[3 7 1], [ 2 3 -8 1]
]
The finite, affine and crystallographic options allow
respectively for restricting to (non) finite, (non) affine,
and (non) crystallographic Cartan types::
sage: [CM.coxeter_type() for CM in CoxeterMatrix.samples(finite=True)]
[Coxeter type of ['A', 1], Coxeter type of ['A', 5],
Coxeter type of ['B', 5], Coxeter type of ['D', 4],
Coxeter type of ['D', 5], Coxeter type of ['E', 6],
Coxeter type of ['E', 7], Coxeter type of ['E', 8],
Coxeter type of ['F', 4], Coxeter type of ['H', 3],
Coxeter type of ['H', 4], Coxeter type of ['I', 10]]
sage: [CM.coxeter_type() for CM in CoxeterMatrix.samples(affine=True)]
[Coxeter type of ['A', 2, 1], Coxeter type of ['B', 5, 1],
Coxeter type of ['C', 5, 1], Coxeter type of ['D', 5, 1],
Coxeter type of ['E', 6, 1], Coxeter type of ['E', 7, 1],
Coxeter type of ['E', 8, 1], Coxeter type of ['F', 4, 1],
Coxeter type of ['G', 2, 1], Coxeter type of ['A', 1, 1]]
sage: [CM.coxeter_type() for CM in CoxeterMatrix.samples(crystallographic=True)]
[Coxeter type of ['A', 1], Coxeter type of ['A', 5],
Coxeter type of ['B', 5], Coxeter type of ['D', 4],
Coxeter type of ['D', 5], Coxeter type of ['E', 6],
Coxeter type of ['E', 7], Coxeter type of ['E', 8],
Coxeter type of ['F', 4], Coxeter type of ['A', 2, 1],
Coxeter type of ['B', 5, 1], Coxeter type of ['C', 5, 1],
Coxeter type of ['D', 5, 1], Coxeter type of ['E', 6, 1],
Coxeter type of ['E', 7, 1], Coxeter type of ['E', 8, 1],
Coxeter type of ['F', 4, 1], Coxeter type of ['G', 2, 1]]
sage: CoxeterMatrix.samples(crystallographic=False)
[
[1 3 2 2]
[1 3 2] [3 1 3 2] [ 1 -1 -1] [1 2 3]
[3 1 5] [2 3 1 5] [ 1 10] [ 1 -1] [-1 1 -1] [2 1 7]
[2 5 1], [2 2 5 1], [10 1], [-1 1], [-1 -1 1], [3 7 1],
<BLANKLINE>
[ 1 -2 3 2]
[-2 1 2 3]
[ 3 2 1 -8]
[ 2 3 -8 1]
]
.. TODO:: add some reducible Coxeter types (suggestions?)
TESTS::
sage: for ct in CoxeterMatrix.samples(): TestSuite(ct).run()
"""
result = self._samples()
if crystallographic is not None:
result = [t for t in result if t.is_crystallographic() == crystallographic]
if finite is not None:
result = [t for t in result if t.is_finite() == finite]
if affine is not None:
result = [t for t in result if t.is_affine() == affine]
if higher_rank is not None:
result = [t for t in result if not t.is_affine() and not t.is_finite()]
return result
@cached_method
def _samples(self):
"""
Return a sample of all implemented Coxeter types.
.. NOTE:: This is intended to be used through :meth:`samples`.
EXAMPLES::
sage: [CM.coxeter_type() for CM in CoxeterMatrix._samples()]
[
Coxeter type of ['A', 1], Coxeter type of ['A', 5],
<BLANKLINE>
Coxeter type of ['B', 5], Coxeter type of ['D', 4],
<BLANKLINE>
Coxeter type of ['D', 5], Coxeter type of ['E', 6],
<BLANKLINE>
Coxeter type of ['E', 7], Coxeter type of ['E', 8],
<BLANKLINE>
Coxeter type of ['F', 4], Coxeter type of ['H', 3],
<BLANKLINE>
Coxeter type of ['H', 4], Coxeter type of ['I', 10],
<BLANKLINE>
Coxeter type of ['A', 2, 1], Coxeter type of ['B', 5, 1],
<BLANKLINE>
Coxeter type of ['C', 5, 1], Coxeter type of ['D', 5, 1],
<BLANKLINE>
Coxeter type of ['E', 6, 1], Coxeter type of ['E', 7, 1],
<BLANKLINE>
Coxeter type of ['E', 8, 1], Coxeter type of ['F', 4, 1],
<BLANKLINE>
[ 1 -1 -1]
[-1 1 -1]
Coxeter type of ['G', 2, 1], Coxeter type of ['A', 1, 1], [-1 -1 1],
<BLANKLINE>
[ 1 -2 3 2]
[1 2 3] [-2 1 2 3]
[2 1 7] [ 3 2 1 -8]
[3 7 1], [ 2 3 -8 1]
]
"""
finite = [CoxeterMatrix(t) for t in [['A', 1], ['A', 5], ['B', 5],
['D', 4], ['D', 5], ['E', 6], ['E', 7],
['E', 8], ['F', 4], ['H', 3], ['H', 4],
['I', 10]]]
affine = [CoxeterMatrix(t) for t in [['A', 2, 1], ['B', 5, 1],
['C', 5, 1], ['D', 5, 1], ['E', 6, 1],
['E', 7, 1], ['E', 8, 1], ['F', 4, 1],
['G', 2, 1], ['A', 1, 1]]]
higher_matrices = [[[1, -1, -1], [-1, 1, -1], [-1, -1, 1]],
[[1, 2, 3], [2, 1, 7], [3, 7, 1]],
[[1, -2, 3, 2], [-2, 1, 2, 3], [3, 2, 1, -8], [2, 3, -8, 1]]]
higher = [CoxeterMatrix(m) for m in higher_matrices]
return finite + affine + higher
def relabel(self, relabelling):
"""
Return a relabelled copy of this Coxeter matrix.
INPUT:
- ``relabelling`` -- a function (or dictionary)
OUTPUT:
an isomorphic Coxeter type obtained by relabelling the nodes of
the Coxeter graph. Namely, the node with label ``i`` is
relabelled ``f(i)`` (or, by ``f[i]`` if ``f`` is a dictionary).
EXAMPLES::
sage: CoxeterMatrix(['F',4]).relabel({ 1:2, 2:3, 3:4, 4:1})
[1 4 2 3]
[4 1 3 2]
[2 3 1 2]
[3 2 2 1]
sage: CoxeterMatrix(['F',4]).relabel(lambda x: x+1 if x<4 else 1)
[1 4 2 3]
[4 1 3 2]
[2 3 1 2]
[3 2 2 1]
"""
if isinstance(relabelling, dict):
data = [[self[relabelling[i]][relabelling[j]]
for j in self.index_set()] for i in self.index_set()]
else:
data = [[self[relabelling(i)][relabelling(j)]
for j in self.index_set()] for i in self.index_set()]
return CoxeterMatrix(data)
def __reduce__(self):
"""
Used for pickling.
TESTS::
sage: C = CoxeterMatrix(['A',4])
sage: M = loads(dumps(C))
sage: M._index_set
(1, 2, 3, 4)
"""
if self._coxeter_type:
return (CoxeterMatrix, (self._coxeter_type,))
return (CoxeterMatrix, (self._matrix, self._index_set))
def _repr_(self):
"""
String representation of the Coxeter matrix.
EXAMPLES::
sage: CM = CoxeterMatrix(['A',3]); CM
[1 3 2]
[3 1 3]
[2 3 1]
sage: CM = CoxeterMatrix([[1,-3/2],[-3/2,1]]); CM
[ 1 -3/2]
[-3/2 1]
"""
return repr(self._matrix)
def _repr_option(self, key):
"""
Metadata about the :meth:`_repr_` output.
See :meth:`sage.structure.parent._repr_option` for details.
EXAMPLES::
sage: CM = CoxeterMatrix(['A',3])
sage: CM._repr_option('ascii_art')
True
"""
if key == 'ascii_art' or key == 'element_ascii_art':
return self._matrix.nrows() > 1
return super(CoxeterMatrix, self)._repr_option(key)
def _latex_(self):
r"""
Latex representation of the Coxeter matrix.
EXAMPLES::
sage: CM = CoxeterMatrix(['A',3])
sage: latex(CM)
\left(\begin{array}{rrr}
1 & 3 & 2 \\
3 & 1 & 3 \\
2 & 3 & 1
\end{array}\right)
"""
return self._matrix._latex_()
def __iter__(self):
"""
Return an iterator for the rows of the Coxeter matrix.
EXAMPLES::
sage: CM = CoxeterMatrix([[1,8],[8,1]])
sage: next(CM.__iter__())
(1, 8)
"""
return iter(self._matrix)
def __getitem__(self, key):
"""
Return a dictionary of labels adjacent to a node or
the label of an edge in the Coxeter graph.
EXAMPLES::
sage: CM = CoxeterMatrix([[1,-2],[-2,1]])
sage: CM = CoxeterMatrix([[1,-2],[-2,1]], ['a','b'])
sage: CM['a']
{'a': 1, 'b': -2}
sage: CM['b']
{'a': -2, 'b': 1}
sage: CM['a','b']
-2
sage: CM['a','a']
1
"""
return self._dict[key]
def __hash__(self):
r"""
Return hash of the Coxeter matrix.
EXAMPLES::
sage: CM = CoxeterMatrix([[1,-2],[-2,1]],['a','b'])
sage: CM.__hash__()
1
sage: CM = CoxeterMatrix([[1,-3],[-3,1]],['1','2'])
sage: CM.__hash__()
4
"""
return hash(self._matrix)
def __eq__(self, other):
r"""
Return if ``self`` and ``other`` are equal.
EXAMPLES::
sage: CM = CoxeterMatrix([[1,-2],[-2,1]],['a','b'])
sage: CM2 = CoxeterMatrix([[1,-2],[-2,1]],['1','2'])
sage: CM == CM2
True
sage: CM == matrix(CM)
False
sage: CM3 = CoxeterMatrix([[1,-3],[-3,1]],['1','2'])
sage: CM == CM3
False
"""
return isinstance(other, CoxeterMatrix) and self._matrix == other._matrix
def __ne__(self, other):
"""
Return if ``self`` and ``other`` are not equal.
EXAMPLES::
sage: CM = CoxeterMatrix([[1,-2],[-2,1]],['a','b'])
sage: CM2 = CoxeterMatrix([[1,-2],[-2,1]],['1','2'])
sage: CM != CM2
False
sage: matrix(CM) != CM
True
sage: CM3 = CoxeterMatrix([[1,-3],[-3,1]],['1','2'])
sage: CM != CM3
True
"""
return not (self == other)
def _matrix_(self, R=None):
"""
Return ``self`` as a matrix over the ring ``R``.
EXAMPLES::
sage: CM = CoxeterMatrix([[1,-3],[-3,1]])
sage: matrix(CM)
[ 1 -3]
[-3 1]
sage: matrix(RR, CM)
[ 1.00000000000000 -3.00000000000000]
[-3.00000000000000 1.00000000000000]
"""
if R is not None:
return self._matrix.change_ring(R)
else:
return self._matrix
##########################################################################
# Coxeter type methods
def index_set(self):
"""
Return the index set of ``self``.
EXAMPLES::
sage: C = CoxeterMatrix(['A',1,1])
sage: C.index_set()
(0, 1)
sage: C = CoxeterMatrix(['E',6])
sage: C.index_set()
(1, 2, 3, 4, 5, 6)
"""
return self._index_set
def coxeter_type(self):
"""
Return the Coxeter type of ``self`` or ``self`` if unknown.
EXAMPLES::
sage: C = CoxeterMatrix(['A',4,1])
sage: C.coxeter_type()
Coxeter type of ['A', 4, 1]
If the Coxeter type is unknown::
sage: C = CoxeterMatrix([[1,3,4], [3,1,-1], [4,-1,1]])
sage: C.coxeter_type()
[ 1 3 4]
[ 3 1 -1]
[ 4 -1 1]
"""
if self._coxeter_type is None:
return self
return self._coxeter_type
def rank(self):
r"""
Return the rank of ``self``.
EXAMPLES::
sage: CoxeterMatrix(['C',3]).rank()
3
sage: CoxeterMatrix(["A2","B2","F4"]).rank()
8
"""
return self._rank
def coxeter_matrix(self):
r"""
Return the Coxeter matrix of ``self``.
EXAMPLES::
sage: CoxeterMatrix(['C',3]).coxeter_matrix()
[1 3 2]
[3 1 4]
[2 4 1]
"""
return self
def bilinear_form(self, R=None):
r"""
Return the bilinear form of ``self``.
EXAMPLES::
sage: CoxeterType(['A', 2, 1]).bilinear_form()
[ 1 -1/2 -1/2]
[-1/2 1 -1/2]
[-1/2 -1/2 1]
sage: CoxeterType(['H', 3]).bilinear_form()
[ 1 -1/2 0]
[ -1/2 1 1/2*E(5)^2 + 1/2*E(5)^3]
[ 0 1/2*E(5)^2 + 1/2*E(5)^3 1]
sage: C = CoxeterMatrix([[1,-1,-1],[-1,1,-1],[-1,-1,1]])
sage: C.bilinear_form()
[ 1 -1 -1]
[-1 1 -1]
[-1 -1 1]
"""
return CoxeterType.bilinear_form(self, R=R)
@cached_method
def coxeter_graph(self):
"""
Return the Coxeter graph of ``self``.
EXAMPLES::
sage: C = CoxeterMatrix(['A',3])
sage: C.coxeter_graph()
Graph on 3 vertices
sage: C = CoxeterMatrix([['A',3],['A',1]])
sage: C.coxeter_graph()
Graph on 4 vertices
"""
n = self.rank()
I = self.index_set()
val = lambda x: infinity if x == -1 else x
G = Graph([(I[i], I[j], val((self._matrix)[i, j]))
for i in range(n) for j in range(i)
if self._matrix[i, j] not in [1, 2]])
G.add_vertices(I)
return G.copy(immutable = True)
def is_simply_laced(self):
"""
Return if ``self`` is simply-laced.
A Coxeter matrix is simply-laced if all non-diagonal entries are
either 2 or 3.
EXAMPLES::
sage: cm = CoxeterMatrix([[1,3,3,3], [3,1,3,3], [3,3,1,3], [3,3,3,1]])
sage: cm.is_simply_laced()
True
"""
# We include 1 in this list to account for the diagonal
L = [1, 2, 3]
return all(x in L for row in self for x in row)
def is_crystallographic(self):
"""
Return whether ``self`` is crystallographic.
A Coxeter matrix is crystallographic if all non-diagonal entries
are either 2, 4, or 6.
EXAMPLES::
sage: CoxeterMatrix(['F',4]).is_crystallographic()
True
sage: CoxeterMatrix(['H',3]).is_crystallographic()
False
"""
# We include 1 in this list to account for the diagonal
L = [1, 2, 3, 4, 6]
return all(x in L for row in self for x in row)
def is_irreducible(self):
"""
Return whether ``self`` is irreducible.
A Coxeter matrix is irreducible if the Coxeter graph is connected.
EXAMPLES::
sage: CoxeterMatrix([['F',4],['A',1]]).is_irreducible()
False
sage: CoxeterMatrix(['H',3]).is_irreducible()
True
"""
return self.coxeter_graph().is_connected()
def is_finite(self):
"""
Return if ``self`` is a finite type or ``False`` if unknown.
EXAMPLES::
sage: M = CoxeterMatrix(['C',4])
sage: M.is_finite()
True
sage: M = CoxeterMatrix(['D',4,1])
sage: M.is_finite()
False
sage: M = CoxeterMatrix([[1, -1], [-1, 1]])
sage: M.is_finite()
False
"""
return self._is_finite
def is_affine(self):
"""
Return if ``self`` is an affine type or ``False`` if unknown.
EXAMPLES::
sage: M = CoxeterMatrix(['C',4])
sage: M.is_affine()
False
sage: M = CoxeterMatrix(['D',4,1])
sage: M.is_affine()
True
sage: M = CoxeterMatrix([[1, 3],[3,1]])
sage: M.is_affine()
False
sage: M = CoxeterMatrix([[1, -1, 7], [-1, 1, 3], [7, 3, 1]])
sage: M.is_affine()
False
"""
return self._is_affine
#####################################################################
## Type check functions
def recognize_coxeter_type_from_matrix(coxeter_matrix, index_set):
"""
Return the Coxeter type of ``coxeter_matrix`` if known,
otherwise return ``None``.
EXAMPLES:
Some infinite ones::
sage: C = CoxeterMatrix([[1,3,2],[3,1,-1],[2,-1,1]])
sage: C.is_finite() # indirect doctest
False
sage: C = CoxeterMatrix([[1,-1,-1],[-1,1,-1],[-1,-1,1]])
sage: C.is_finite() # indirect doctest
False
Some finite ones::
sage: m = matrix(CoxeterMatrix(['D', 4]))