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weyl_groups.py
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weyl_groups.py
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r"""
Weyl Groups
"""
#*****************************************************************************
# Copyright (C) 2009 Nicolas M. Thiery <nthiery at users.sf.net>
#
# Distributed under the terms of the GNU General Public License (GPL)
# http://www.gnu.org/licenses/
#******************************************************************************
from sage.misc.cachefunc import cached_method, cached_in_parent_method
from sage.misc.lazy_import import LazyImport
from sage.categories.category_singleton import Category_singleton
from sage.categories.coxeter_groups import CoxeterGroups
from sage.rings.infinity import infinity
from sage.rings.rational_field import QQ
class WeylGroups(Category_singleton):
r"""
The category of Weyl groups
See the :wikipedia:`Wikipedia page of Weyl Groups <Weyl_group>`.
EXAMPLES::
sage: WeylGroups()
Category of weyl groups
sage: WeylGroups().super_categories()
[Category of coxeter groups]
Here are some examples::
sage: WeylGroups().example() # todo: not implemented
sage: FiniteWeylGroups().example()
The symmetric group on {0, ..., 3}
sage: AffineWeylGroups().example() # todo: not implemented
sage: WeylGroup(["B", 3])
Weyl Group of type ['B', 3] (as a matrix group acting on the ambient space)
This one will eventually be also in this category::
sage: SymmetricGroup(4)
Symmetric group of order 4! as a permutation group
TESTS::
sage: C = WeylGroups()
sage: TestSuite(C).run()
"""
def super_categories(self):
r"""
EXAMPLES::
sage: WeylGroups().super_categories()
[Category of coxeter groups]
"""
return [CoxeterGroups()]
def additional_structure(self):
r"""
Return ``None``.
Indeed, the category of Weyl groups defines no additional
structure: Weyl groups are a special class of Coxeter groups.
.. SEEALSO:: :meth:`Category.additional_structure`
.. TODO:: Should this category be a :class:`CategoryWithAxiom`?
EXAMPLES::
sage: WeylGroups().additional_structure()
"""
return None
Finite = LazyImport('sage.categories.finite_weyl_groups', 'FiniteWeylGroups')
class ParentMethods:
def pieri_factors(self, *args, **keywords):
r"""
Returns the set of Pieri factors in this Weyl group.
For any type, the set of Pieri factors forms a lower ideal
in Bruhat order, generated by all the conjugates of some
special element of the Weyl group. In type `A_n`, this
special element is `s_n\cdots s_1`, and the conjugates are
obtained by rotating around this reduced word.
These are used to compute Stanley symmetric functions.
.. SEEALSO::
* :meth:`WeylGroups.ElementMethods.stanley_symmetric_function`
* :mod:`sage.combinat.root_system.pieri_factors`
EXAMPLES::
sage: W = WeylGroup(['A',5,1])
sage: PF = W.pieri_factors()
sage: PF.cardinality()
63
sage: W = WeylGroup(['B',3])
sage: PF = W.pieri_factors()
sage: [w.reduced_word() for w in PF]
[[1, 2, 3, 2, 1], [1, 2, 3, 2], [2, 3, 2], [2, 3], [3, 1, 2, 1], [1, 2, 1], [2], [1, 2], [1], [], [2, 1], [3, 2, 1], [3, 1], [2, 3, 2, 1], [3], [3, 2], [1, 2, 3], [1, 2, 3, 1], [3, 1, 2], [2, 3, 1]]
sage: W = WeylGroup(['C',4,1])
sage: PF = W.pieri_factors()
sage: W.from_reduced_word([3,2,0]) in PF
True
"""
# Do not remove this line which makes sure the pieri factor
# code is properly inserted inside the Cartan Types
import sage.combinat.root_system.pieri_factors
ct = self.cartan_type()
if hasattr(ct, "PieriFactors"):
return ct.PieriFactors(self, *args, **keywords)
raise NotImplementedError("Pieri factors for type {}".format(ct))
@cached_method
def quantum_bruhat_graph(self, index_set=()):
r"""
Return the quantum Bruhat graph of the quotient of the Weyl
group by a parabolic subgroup `W_J`.
INPUT:
- ``index_set`` -- (default: ()) a tuple `J` of nodes of
the Dynkin diagram
By default, the value for ``index_set`` indicates that the
subgroup is trivial and the quotient is the full Weyl group.
EXAMPLES::
sage: W = WeylGroup(['A',3], prefix="s")
sage: g = W.quantum_bruhat_graph((1,3))
sage: g
Parabolic Quantum Bruhat Graph of Weyl Group of type ['A', 3] (as a matrix group acting on the ambient space) for nodes (1, 3): Digraph on 6 vertices
sage: g.vertices()
[s2*s3*s1*s2, s3*s1*s2, s1*s2, s3*s2, s2, 1]
sage: g.edges()
[(s2*s3*s1*s2, s2, alpha[2]),
(s3*s1*s2, s2*s3*s1*s2, alpha[1] + alpha[2] + alpha[3]),
(s3*s1*s2, 1, alpha[2]),
(s1*s2, s3*s1*s2, alpha[2] + alpha[3]),
(s3*s2, s3*s1*s2, alpha[1] + alpha[2]),
(s2, s1*s2, alpha[1] + alpha[2]),
(s2, s3*s2, alpha[2] + alpha[3]),
(1, s2, alpha[2])]
sage: W = WeylGroup(['A',3,1], prefix="s")
sage: g = W.quantum_bruhat_graph()
Traceback (most recent call last):
...
ValueError: the Cartan type ['A', 3, 1] is not finite
"""
if not self.cartan_type().is_finite():
raise ValueError("the Cartan type {} is not finite".format(self.cartan_type()))
# Find all the minimal length coset representatives
WP = [x for x in self if all(not x.has_descent(i) for i in index_set)]
# This is a modified form of quantum_bruhat_successors.
# It does not do any error checking and also is more efficient
# with how it handles memory and checks by using data stored
# at this function level rather than recomputing everything.
lattice = self.cartan_type().root_system().root_lattice()
NPR = lattice.nonparabolic_positive_roots(index_set)
NPR_sum = sum(NPR)
NPR_data = {}
full_NPR_sum = lattice.nonparabolic_positive_root_sum(())
for alpha in NPR:
ref = alpha.associated_reflection()
alphacheck = alpha.associated_coroot()
NPR_data[alpha] = [self.from_reduced_word(ref), # the element
len(ref) == full_NPR_sum.scalar(alphacheck) - 1, # is_quantum
NPR_sum.scalar(alphacheck)] # the scalar
# We also create a temporary cache of lengths as they are
# relatively expensive to compute and needed frequently
len_cache = {}
def length(x):
# It is sufficient and faster to use the matrices as the keys
m = x.matrix()
if m in len_cache:
return len_cache[m]
len_cache[m] = x.length()
return len_cache[m]
def succ(x):
w_length_plus_one = length(x) + 1
successors = []
for alpha in NPR:
elt, is_quantum, scalar = NPR_data[alpha]
wr = x * elt
wrc = wr.coset_representative(index_set)
if wrc == wr and length(wr) == w_length_plus_one:
successors.append((wr, alpha))
elif is_quantum and length(wrc) == w_length_plus_one - scalar:
successors.append((wrc, alpha))
return successors
from sage.graphs.digraph import DiGraph
return DiGraph([[x,i[0],i[1]] for x in WP for i in succ(x)],
name="Parabolic Quantum Bruhat Graph of %s for nodes %s"%(self, index_set))
class ElementMethods:
def is_pieri_factor(self):
r"""
Returns whether ``self`` is a Pieri factor, as used for
computing Stanley symmetric functions.
.. SEEALSO::
* :meth:`stanley_symmetric_function`
* :meth:`WeylGroups.ParentMethods.pieri_factors`
EXAMPLES::
sage: W = WeylGroup(['A',5,1])
sage: W.from_reduced_word([3,2,5]).is_pieri_factor()
True
sage: W.from_reduced_word([3,2,4,5]).is_pieri_factor()
False
sage: W = WeylGroup(['C',4,1])
sage: W.from_reduced_word([0,2,1]).is_pieri_factor()
True
sage: W.from_reduced_word([0,2,1,0]).is_pieri_factor()
False
sage: W = WeylGroup(['B',3])
sage: W.from_reduced_word([3,2,3]).is_pieri_factor()
False
sage: W.from_reduced_word([2,1,2]).is_pieri_factor()
True
"""
return self in self.parent().pieri_factors()
def left_pieri_factorizations(self, max_length = infinity):
r"""
Returns all factorizations of ``self`` as `uv`, where `u`
is a Pieri factor and `v` is an element of the Weyl group.
.. SEEALSO::
* :meth:`WeylGroups.ParentMethods.pieri_factors`
* :mod:`sage.combinat.root_system.pieri_factors`
EXAMPLES:
If we take `w = w_0` the maximal element of a strict parabolic
subgroup of type `A_{n_1} \times \cdots \times A_{n_k}`, then the Pieri
factorizations are in correspondence with all Pieri factors, and
there are `\prod 2^{n_i}` of them::
sage: W = WeylGroup(['A', 4, 1])
sage: W.from_reduced_word([]).left_pieri_factorizations().cardinality()
1
sage: W.from_reduced_word([1]).left_pieri_factorizations().cardinality()
2
sage: W.from_reduced_word([1,2,1]).left_pieri_factorizations().cardinality()
4
sage: W.from_reduced_word([1,2,3,1,2,1]).left_pieri_factorizations().cardinality()
8
sage: W.from_reduced_word([1,3]).left_pieri_factorizations().cardinality()
4
sage: W.from_reduced_word([1,3,4,3]).left_pieri_factorizations().cardinality()
8
sage: W.from_reduced_word([2,1]).left_pieri_factorizations().cardinality()
3
sage: W.from_reduced_word([1,2]).left_pieri_factorizations().cardinality()
2
sage: [W.from_reduced_word([1,2]).left_pieri_factorizations(max_length=i).cardinality() for i in [-1, 0, 1, 2]]
[0, 1, 2, 2]
sage: W = WeylGroup(['C',4,1])
sage: w = W.from_reduced_word([0,3,2,1,0])
sage: w.left_pieri_factorizations().cardinality()
7
sage: [(u.reduced_word(),v.reduced_word()) for (u,v) in w.left_pieri_factorizations()]
[([], [3, 2, 0, 1, 0]),
([0], [3, 2, 1, 0]),
([3], [2, 0, 1, 0]),
([3, 0], [2, 1, 0]),
([3, 2], [0, 1, 0]),
([3, 2, 0], [1, 0]),
([3, 2, 0, 1], [0])]
sage: W = WeylGroup(['B',4,1])
sage: W.from_reduced_word([0,2,1,0]).left_pieri_factorizations().cardinality()
6
"""
pieri_factors = self.parent().pieri_factors()
def predicate(u):
return u in pieri_factors and u.length() <= max_length
return self.binary_factorizations(predicate)
@cached_in_parent_method
def stanley_symmetric_function_as_polynomial(self, max_length = infinity):
r"""
Returns a multivariate generating function for the number
of factorizations of a Weyl group element into Pieri
factors of decreasing length, weighted by a statistic on
Pieri factors.
.. SEEALSO::
* :meth:`stanley_symmetric_function`
* :meth:`WeylGroups.ParentMethods.pieri_factors`
* :mod:`sage.combinat.root_system.pieri_factors`
INPUT:
- ``self`` -- an element `w` of a Weyl group `W`
- ``max_length`` -- a non negative integer or infinity (default: infinity)
Returns the generating series for the Pieri factorizations
`w = u_1 \cdots u_k`, where `u_i` is a Pieri factor for
all `i`, `l(w) = \sum_{i=1}^k l(u_i)` and
``max_length`` `\geq l(u_1) \geq \cdots \geq l(u_k)`.
A factorization `u_1 \cdots u_k` contributes a monomial of
the form `\prod_i x_{l(u_i)}`, with coefficient given by
`\prod_i 2^{c(u_i)}`, where `c` is a type-dependent
statistic on Pieri factors, as returned by the method
``u[i].stanley_symm_poly_weight()``.
EXAMPLES::
sage: W = WeylGroup(['A', 3, 1])
sage: W.from_reduced_word([]).stanley_symmetric_function_as_polynomial()
1
sage: W.from_reduced_word([1]).stanley_symmetric_function_as_polynomial()
x1
sage: W.from_reduced_word([1,2]).stanley_symmetric_function_as_polynomial()
x1^2
sage: W.from_reduced_word([2,1]).stanley_symmetric_function_as_polynomial()
x1^2 + x2
sage: W.from_reduced_word([1,2,1]).stanley_symmetric_function_as_polynomial()
2*x1^3 + x1*x2
sage: W.from_reduced_word([1,2,1,0]).stanley_symmetric_function_as_polynomial()
3*x1^4 + 2*x1^2*x2 + x2^2 + x1*x3
sage: W.from_reduced_word([1,2,3,1,2,1,0]).stanley_symmetric_function_as_polynomial() # long time
22*x1^7 + 11*x1^5*x2 + 5*x1^3*x2^2 + 3*x1^4*x3 + 2*x1*x2^3 + x1^2*x2*x3
sage: W.from_reduced_word([3,1,2,0,3,1,0]).stanley_symmetric_function_as_polynomial() # long time
8*x1^7 + 4*x1^5*x2 + 2*x1^3*x2^2 + x1*x2^3
sage: W = WeylGroup(['C',3,1])
sage: W.from_reduced_word([0,2,1,0]).stanley_symmetric_function_as_polynomial()
32*x1^4 + 16*x1^2*x2 + 8*x2^2 + 4*x1*x3
sage: W = WeylGroup(['B',3,1])
sage: W.from_reduced_word([3,2,1]).stanley_symmetric_function_as_polynomial()
2*x1^3 + x1*x2 + 1/2*x3
Algorithm: Induction on the left Pieri factors. Note that
this induction preserves subsets of `W` which are stable
by taking right factors, and in particular Grassmanian
elements.
"""
W = self.parent()
pieri_factors = W.pieri_factors()
R = QQ[','.join('x%s'%l for l in range(1,pieri_factors.max_length()+1))]
x = R.gens()
if self.is_one():
return R.one()
return R(sum(2**(pieri_factors.stanley_symm_poly_weight(u))*x[u.length()-1] * v.stanley_symmetric_function_as_polynomial(max_length = u.length())
for (u,v) in self.left_pieri_factorizations(max_length)
if u != W.one()))
def stanley_symmetric_function(self):
r"""
Return the affine Stanley symmetric function indexed by ``self``.
INPUT:
- ``self`` -- an element `w` of a Weyl group
Returns the affine Stanley symmetric function indexed by
`w`. Stanley symmetric functions are defined as generating
series of the factorizations of `w` into Pieri factors and
weighted by a statistic on Pieri factors.
.. SEEALSO::
* :meth:`stanley_symmetric_function_as_polynomial`
* :meth:`WeylGroups.ParentMethods.pieri_factors`
* :mod:`sage.combinat.root_system.pieri_factors`
EXAMPLES::
sage: W = WeylGroup(['A', 3, 1])
sage: W.from_reduced_word([3,1,2,0,3,1,0]).stanley_symmetric_function()
8*m[1, 1, 1, 1, 1, 1, 1] + 4*m[2, 1, 1, 1, 1, 1] + 2*m[2, 2, 1, 1, 1] + m[2, 2, 2, 1]
sage: A = AffinePermutationGroup(['A',3,1])
sage: A.from_reduced_word([3,1,2,0,3,1,0]).stanley_symmetric_function()
8*m[1, 1, 1, 1, 1, 1, 1] + 4*m[2, 1, 1, 1, 1, 1] + 2*m[2, 2, 1, 1, 1] + m[2, 2, 2, 1]
sage: W = WeylGroup(['C',3,1])
sage: W.from_reduced_word([0,2,1,0]).stanley_symmetric_function()
32*m[1, 1, 1, 1] + 16*m[2, 1, 1] + 8*m[2, 2] + 4*m[3, 1]
sage: W = WeylGroup(['B',3,1])
sage: W.from_reduced_word([3,2,1]).stanley_symmetric_function()
2*m[1, 1, 1] + m[2, 1] + 1/2*m[3]
sage: W = WeylGroup(['B',4])
sage: w = W.from_reduced_word([3,2,3,1])
sage: w.stanley_symmetric_function() # long time (6s on sage.math, 2011)
48*m[1, 1, 1, 1] + 24*m[2, 1, 1] + 12*m[2, 2] + 8*m[3, 1] + 2*m[4]
sage: A = AffinePermutationGroup(['A',4,1])
sage: a = A([-2,0,1,4,12])
sage: a.stanley_symmetric_function()
6*m[1, 1, 1, 1, 1, 1, 1, 1] + 5*m[2, 1, 1, 1, 1, 1, 1] + 4*m[2, 2, 1, 1, 1, 1]
+ 3*m[2, 2, 2, 1, 1] + 2*m[2, 2, 2, 2] + 4*m[3, 1, 1, 1, 1, 1] + 3*m[3, 2, 1, 1, 1]
+ 2*m[3, 2, 2, 1] + 2*m[3, 3, 1, 1] + m[3, 3, 2] + 3*m[4, 1, 1, 1, 1] + 2*m[4, 2, 1, 1]
+ m[4, 2, 2] + m[4, 3, 1]
One more example (:trac:`14095`)::
sage: G = SymmetricGroup(4)
sage: w = G.from_reduced_word([3,2,3,1])
sage: w.stanley_symmetric_function()
3*m[1, 1, 1, 1] + 2*m[2, 1, 1] + m[2, 2] + m[3, 1]
REFERENCES:
.. [BH1994] S. Billey, M. Haiman. *Schubert polynomials for the
classical groups*. J. Amer. Math. Soc., 1994.
.. [Lam2008] T. Lam. *Schubert polynomials for the affine
Grassmannian*. J. Amer. Math. Soc., 2008.
.. [LSS2009] T. Lam, A. Schilling, M. Shimozono. *Schubert
polynomials for the affine Grassmannian of the symplectic
group*. Mathematische Zeitschrift 264(4) (2010) 765-811
(:arxiv:`0710.2720`)
.. [Pon2010] S. Pon. *Types B and D affine Stanley symmetric
functions*, unpublished PhD Thesis, UC Davis, 2010.
"""
import sage.combinat.sf
m = sage.combinat.sf.sf.SymmetricFunctions(QQ).monomial()
return m.from_polynomial_exp(self.stanley_symmetric_function_as_polynomial())
@cached_in_parent_method
def reflection_to_root(self):
r"""
Returns the root associated with the reflection ``self``.
EXAMPLES::
sage: W=WeylGroup(['C',2],prefix="s")
sage: W.from_reduced_word([1,2,1]).reflection_to_root()
2*alpha[1] + alpha[2]
sage: W.from_reduced_word([1,2]).reflection_to_root()
Traceback (most recent call last):
...
ValueError: s1*s2 is not a reflection
sage: W.long_element().reflection_to_root()
Traceback (most recent call last):
...
ValueError: s2*s1*s2*s1 is not a reflection
"""
i = self.first_descent()
if i is None:
raise ValueError("{} is not a reflection".format(self))
if self == self.parent().simple_reflection(i):
return self.parent().cartan_type().root_system().root_lattice().simple_root(i)
rsi = self.apply_simple_reflection(i)
if not rsi.has_descent(i, side='left'):
raise ValueError("{} is not a reflection".format(self))
return rsi.apply_simple_reflection(i, side='left').reflection_to_root().simple_reflection(i)
@cached_in_parent_method
def reflection_to_coroot(self):
r"""
Returns the coroot associated with the reflection ``self``.
EXAMPLES::
sage: W=WeylGroup(['C',2],prefix="s")
sage: W.from_reduced_word([1,2,1]).reflection_to_coroot()
alphacheck[1] + alphacheck[2]
sage: W.from_reduced_word([1,2]).reflection_to_coroot()
Traceback (most recent call last):
...
ValueError: s1*s2 is not a reflection
sage: W.long_element().reflection_to_coroot()
Traceback (most recent call last):
...
ValueError: s2*s1*s2*s1 is not a reflection
"""
i = self.first_descent()
if i is None:
raise ValueError("{} is not a reflection".format(self))
if self == self.parent().simple_reflection(i):
return self.parent().cartan_type().root_system().root_lattice().simple_coroot(i)
rsi = self.apply_simple_reflection(i)
if not rsi.has_descent(i, side='left'):
raise ValueError("{} is not a reflection".format(self))
return rsi.apply_simple_reflection(i, side='left').reflection_to_coroot().simple_reflection(i)
def inversions(self, side = 'right', inversion_type = 'reflections'):
"""
Returns the set of inversions of ``self``.
INPUT:
- ``side`` -- 'right' (default) or 'left'
- ``inversion_type`` -- 'reflections' (default), 'roots', or 'coroots'.
OUTPUT:
For reflections, the set of reflections r in the Weyl group such that
``self`` ``r`` < ``self``. For (co)roots, the set of positive (co)roots that are sent
by ``self`` to negative (co)roots; their associated reflections are described above.
If ``side`` is 'left', the inverse Weyl group element is used.
EXAMPLES::
sage: W=WeylGroup(['C',2], prefix="s")
sage: w=W.from_reduced_word([1,2])
sage: w.inversions()
[s2, s2*s1*s2]
sage: w.inversions(inversion_type = 'reflections')
[s2, s2*s1*s2]
sage: w.inversions(inversion_type = 'roots')
[alpha[2], alpha[1] + alpha[2]]
sage: w.inversions(inversion_type = 'coroots')
[alphacheck[2], alphacheck[1] + 2*alphacheck[2]]
sage: w.inversions(side = 'left')
[s1, s1*s2*s1]
sage: w.inversions(side = 'left', inversion_type = 'roots')
[alpha[1], 2*alpha[1] + alpha[2]]
sage: w.inversions(side = 'left', inversion_type = 'coroots')
[alphacheck[1], alphacheck[1] + alphacheck[2]]
"""
if side == 'left':
self = self.inverse()
reflections = self.inversions_as_reflections()
if inversion_type == 'reflections':
return reflections
if inversion_type == 'roots':
return [r.reflection_to_root() for r in reflections]
if inversion_type == 'coroots':
return [r.reflection_to_coroot() for r in reflections]
raise ValueError("inversion_type {} is invalid".format(inversion_type))
def inversion_arrangement(self, side='right'):
r"""
Return the inversion hyperplane arrangement of ``self``.
INPUT:
- ``side`` -- ``'right'`` (default) or ``'left'``
OUTPUT:
A (central) hyperplane arrangement whose hyperplanes correspond
to the inversions of ``self`` given as roots.
The ``side`` parameter determines on which side
to compute the inversions.
EXAMPLES::
sage: W = WeylGroup(['A',3])
sage: w = W.from_reduced_word([1, 2, 3, 1, 2])
sage: A = w.inversion_arrangement(); A
Arrangement of 5 hyperplanes of dimension 3 and rank 3
sage: A.hyperplanes()
(Hyperplane 0*a1 + 0*a2 + a3 + 0,
Hyperplane 0*a1 + a2 + 0*a3 + 0,
Hyperplane 0*a1 + a2 + a3 + 0,
Hyperplane a1 + a2 + 0*a3 + 0,
Hyperplane a1 + a2 + a3 + 0)
The identity element gives the empty arrangement::
sage: W = WeylGroup(['A',3])
sage: W.one().inversion_arrangement()
Empty hyperplane arrangement of dimension 3
"""
inv = self.inversions(side=side, inversion_type='roots')
from sage.geometry.hyperplane_arrangement.arrangement import HyperplaneArrangements
I = self.parent().cartan_type().index_set()
H = HyperplaneArrangements(QQ, tuple(['a{}'.format(i) for i in I]))
gens = H.gens()
if not inv:
return H()
return H([sum(c * gens[I.index(i)] for (i, c) in root)
for root in inv])
def bruhat_lower_covers_coroots(self):
r"""
Return all 2-tuples (``v``, `\alpha`) where ``v`` is covered
by ``self`` and `\alpha` is the positive coroot such that
``self`` = ``v`` `s_\alpha` where `s_\alpha` is
the reflection orthogonal to `\alpha`.
ALGORITHM:
See :meth:`.bruhat_lower_covers` and
:meth:`.bruhat_lower_covers_reflections` for Coxeter groups.
EXAMPLES::
sage: W = WeylGroup(['A',3], prefix="s")
sage: w = W.from_reduced_word([3,1,2,1])
sage: w.bruhat_lower_covers_coroots()
[(s1*s2*s1, alphacheck[1] + alphacheck[2] + alphacheck[3]),
(s3*s2*s1, alphacheck[2]), (s3*s1*s2, alphacheck[1])]
"""
return [(x[0],x[1].reflection_to_coroot())
for x in self.bruhat_lower_covers_reflections()]
def bruhat_upper_covers_coroots(self):
r"""
Returns all 2-tuples (``v``, `\alpha`) where ``v`` is covers ``self`` and `\alpha`
is the positive coroot such that ``self`` = ``v`` `s_\alpha` where `s_\alpha` is
the reflection orthogonal to `\alpha`.
ALGORITHM:
See :meth:`~CoxeterGroups.ElementMethods.bruhat_upper_covers` and :meth:`.bruhat_upper_covers_reflections` for Coxeter groups.
EXAMPLES::
sage: W = WeylGroup(['A',4], prefix="s")
sage: w = W.from_reduced_word([3,1,2,1])
sage: w.bruhat_upper_covers_coroots()
[(s1*s2*s3*s2*s1, alphacheck[3]),
(s2*s3*s1*s2*s1, alphacheck[2] + alphacheck[3]),
(s3*s4*s1*s2*s1, alphacheck[4]),
(s4*s3*s1*s2*s1, alphacheck[1] + alphacheck[2] + alphacheck[3] + alphacheck[4])]
"""
return [(x[0],x[1].reflection_to_coroot())
for x in self.bruhat_upper_covers_reflections()]
def quantum_bruhat_successors(self, index_set=None, roots=False, quantum_only=False):
r"""
Return the successors of ``self`` in the quantum Bruhat graph
on the parabolic quotient of the Weyl group determined by the
subset of Dynkin nodes ``index_set``.
INPUT:
- ``self`` -- a Weyl group element, which is assumed to
be of minimum length in its coset with respect to the
parabolic subgroup
- ``index_set`` -- (default: ``None``) indicates the set of
simple reflections used to generate the parabolic subgroup;
the default value indicates that the subgroup is the identity
- ``roots`` -- (default: ``False``) if ``True``, returns the
list of 2-tuples (``w``, `\alpha`) where ``w`` is a successor
and `\alpha` is the positive root associated with the
successor relation
- ``quantum_only`` -- (default: ``False``) if ``True``, returns
only the quantum successors
EXAMPLES::
sage: W = WeylGroup(['A',3], prefix="s")
sage: w = W.from_reduced_word([3,1,2])
sage: w.quantum_bruhat_successors([1], roots = True)
[(s3, alpha[2]), (s1*s2*s3*s2, alpha[3]),
(s2*s3*s1*s2, alpha[1] + alpha[2] + alpha[3])]
sage: w.quantum_bruhat_successors([1,3])
[1, s2*s3*s1*s2]
sage: w.quantum_bruhat_successors(roots = True)
[(s3*s1*s2*s1, alpha[1]),
(s3*s1, alpha[2]),
(s1*s2*s3*s2, alpha[3]),
(s2*s3*s1*s2, alpha[1] + alpha[2] + alpha[3])]
sage: w.quantum_bruhat_successors()
[s3*s1*s2*s1, s3*s1, s1*s2*s3*s2, s2*s3*s1*s2]
sage: w.quantum_bruhat_successors(quantum_only = True)
[s3*s1]
sage: w = W.from_reduced_word([2,3])
sage: w.quantum_bruhat_successors([1,3])
Traceback (most recent call last):
...
ValueError: s2*s3 is not of minimum length in its coset of the parabolic subgroup generated by the reflections (1, 3)
"""
W = self.parent()
if not W.cartan_type().is_finite():
raise ValueError("the Cartan type {} is not finite".format(W.cartan_type()))
if index_set is None:
index_set = []
else:
index_set = [x for x in index_set]
index_set = tuple(index_set)
if self != self.coset_representative(index_set):
raise ValueError("{} is not of minimum length in its coset of the parabolic subgroup generated by the reflections {}".format(self, index_set))
lattice = W.cartan_type().root_system().root_lattice()
w_length_plus_one = self.length() + 1
successors = []
for alpha in lattice.nonparabolic_positive_roots(index_set):
wr = self * W.from_reduced_word(alpha.associated_reflection())
wrc = wr.coset_representative(index_set)
if wrc == wr and wr.length() == w_length_plus_one and not quantum_only:
if roots:
successors.append((wr,alpha))
else:
successors.append(wr)
elif alpha.quantum_root() and wrc.length() == w_length_plus_one - lattice.nonparabolic_positive_root_sum(index_set).scalar(alpha.associated_coroot()):
if roots:
successors.append((wrc,alpha))
else:
successors.append(wrc)
return successors