# Coxeter Groups¶

class sage.categories.coxeter_groups.CoxeterGroups(s=None)

The category of Coxeter groups.

A Coxeter group is a group $$W$$ with a distinguished (finite) family of involutions $$(s_i)_{i\in I}$$, called the simple reflections, subject to relations of the form $$(s_is_j)^{m_{i,j}} = 1$$.

$$I$$ is the index set of $$W$$ and $$|I|$$ is the rank of $$W$$.

See Wikipedia article Coxeter_group for details.

EXAMPLES:

sage: C = CoxeterGroups(); C
Category of coxeter groups
sage: C.super_categories()
[Category of generalized coxeter groups]

sage: W = C.example(); W
The symmetric group on {0, ..., 3}

sage: W.simple_reflections()
Finite family {0: (1, 0, 2, 3), 1: (0, 2, 1, 3), 2: (0, 1, 3, 2)}


Here are some further examples:

sage: FiniteCoxeterGroups().example()
The 5-th dihedral group of order 10
sage: FiniteWeylGroups().example()
The symmetric group on {0, ..., 3}
sage: WeylGroup(["B", 3])
Weyl Group of type ['B', 3] (as a matrix group acting on the ambient space)

sage: S4 = SymmetricGroup(4); S4
Symmetric group of order 4! as a permutation group
sage: S4 in CoxeterGroups().Finite()
True


Those will eventually be also in this category:

sage: DihedralGroup(5)
Dihedral group of order 10 as a permutation group


Todo

add a demo of usual computations on Coxeter groups.

Warning

It is assumed that morphisms in this category preserve the distinguished choice of simple reflections. In particular, subobjects in this category are parabolic subgroups. In this sense, this category might be better named Coxeter Systems. In the long run we might want to have two distinct categories, one for Coxeter groups (with morphisms being just group morphisms) and one for Coxeter systems:

sage: CoxeterGroups().is_full_subcategory(Groups())
False
sage: from sage.categories.generalized_coxeter_groups import GeneralizedCoxeterGroups
sage: CoxeterGroups().is_full_subcategory(GeneralizedCoxeterGroups())
True

Algebras
class ElementMethods

Bases: object

absolute_covers()

Return the list of covers of self in absolute order.

EXAMPLES:

sage: W = WeylGroup(["A", 3])
sage: s = W.simple_reflections()
sage: w0 = s[1]
sage: w1 = s[1]*s[2]*s[3]
sage: w0.absolute_covers()
[
[0 0 1 0]  [0 1 0 0]  [0 1 0 0]  [0 0 0 1]  [0 1 0 0]
[1 0 0 0]  [1 0 0 0]  [0 0 1 0]  [1 0 0 0]  [0 0 0 1]
[0 1 0 0]  [0 0 0 1]  [1 0 0 0]  [0 0 1 0]  [0 0 1 0]
[0 0 0 1], [0 0 1 0], [0 0 0 1], [0 1 0 0], [1 0 0 0]
]

absolute_le(other)

Return whether self is smaller than other in the absolute order.

A general reflection is an element of the form $$w s_i w^{-1}$$, where $$s_i$$ is a simple reflection. The absolute order is defined analogously to the weak order but using general reflections rather than just simple reflections.

This partial order can be used to define noncrossing partitions associated with this Coxeter group.

EXAMPLES:

sage: W = WeylGroup(["A", 3])
sage: s = W.simple_reflections()
sage: w0 = s[1]
sage: w1 = s[1]*s[2]*s[3]
sage: w0.absolute_le(w1)
True
sage: w1.absolute_le(w0)
False
sage: w1.absolute_le(w1)
True

absolute_length()

Return the absolute length of self.

The absolute length is the length of the shortest expression of the element as a product of reflections.

For permutations in the symmetric groups, the absolute length is the size minus the number of its disjoint cycles.

EXAMPLES:

sage: W = WeylGroup(["A", 3])
sage: s = W.simple_reflections()
sage: (s[1]*s[2]*s[3]).absolute_length()
3

sage: W = SymmetricGroup(4)
sage: s = W.simple_reflections()
sage: (s[3]*s[2]*s[1]).absolute_length()
3

apply_demazure_product(element, side='right', length_increasing=True)

Returns the Demazure or 0-Hecke product of self with another Coxeter group element.

INPUT:

• element – either an element of the same Coxeter

group as self or a tuple or a list (such as a reduced word) of elements from the index set of the Coxeter group.

• side – ‘left’ or ‘right’ (default: ‘right’); the

side of self on which the element should be applied. If side is ‘left’ then the operation is applied on the left.

• length_increasing – a boolean (default True)

whether to act length increasingly or decreasingly

EXAMPLES:

sage: W = WeylGroup(['C',4],prefix="s")
sage: v = W.from_reduced_word([1,2,3,4,3,1])
sage: v.apply_demazure_product([1,3,4,3,3])
s4*s1*s2*s3*s4*s3*s1
sage: v.apply_demazure_product([1,3,4,3],side='left')
s3*s4*s1*s2*s3*s4*s2*s3*s1
sage: v.apply_demazure_product((1,3,4,3),side='left')
s3*s4*s1*s2*s3*s4*s2*s3*s1
sage: v.apply_demazure_product(v)
s2*s3*s4*s1*s2*s3*s4*s2*s3*s2*s1

apply_simple_projection(i, side='right', length_increasing=True)

INPUT:

• i - an element of the index set of the Coxeter group

• side - ‘left’ or ‘right’ (default: ‘right’)

• length_increasing - a boolean (default: True) specifying the direction of the projection

Returns the result of the application of the simple projection $$\pi_i$$ (resp. $$\overline\pi_i$$) on self.

See CoxeterGroups.ParentMethods.simple_projections() for the definition of the simple projections.

EXAMPLES:

sage: W = CoxeterGroups().example()
sage: w = W.an_element()
sage: w
(1, 2, 3, 0)
sage: w.apply_simple_projection(2)
(1, 2, 3, 0)
sage: w.apply_simple_projection(2, length_increasing=False)
(1, 2, 0, 3)
sage: W = WeylGroup(['C',4],prefix="s")
sage: v = W.from_reduced_word([1,2,3,4,3,1])
sage: v
s1*s2*s3*s4*s3*s1
sage: v.apply_simple_projection(2)
s1*s2*s3*s4*s3*s1*s2
sage: v.apply_simple_projection(2, side='left')
s1*s2*s3*s4*s3*s1
sage: v.apply_simple_projection(1, length_increasing = False)
s1*s2*s3*s4*s3

binary_factorizations(predicate=The constant function (...) -> True)

Return the set of all the factorizations $$self = u v$$ such that $$l(self) = l(u) + l(v)$$.

Iterating through this set is Constant Amortized Time (counting arithmetic operations in the Coxeter group as constant time) complexity, and memory linear in the length of $$self$$.

One can pass as optional argument a predicate p such that $$p(u)$$ implies $$p(u')$$ for any $$u$$ left factor of $$self$$ and $$u'$$ left factor of $$u$$. Then this returns only the factorizations $$self = uv$$ such $$p(u)$$ holds.

EXAMPLES:

We construct the set of all factorizations of the maximal element of the group:

sage: W = WeylGroup(['A',3])
sage: s = W.simple_reflections()
sage: w0 = W.from_reduced_word([1,2,3,1,2,1])
sage: w0.binary_factorizations().cardinality()
24


The same number of factorizations, by bounded length:

sage: [w0.binary_factorizations(lambda u: u.length() <= l).cardinality() for l in [-1,0,1,2,3,4,5,6]]
[0, 1, 4, 9, 15, 20, 23, 24]


The number of factorizations of the elements just below the maximal element:

sage: [(s[i]*w0).binary_factorizations().cardinality() for i in [1,2,3]]
[12, 12, 12]
sage: w0.binary_factorizations(lambda u: False).cardinality()
0

bruhat_le(other)

Bruhat comparison

INPUT:

• other – an element of the same Coxeter group

OUTPUT: a boolean

Returns whether self <= other in the Bruhat order.

EXAMPLES:

sage: W = WeylGroup(["A",3])
sage: u = W.from_reduced_word([1,2,1])
sage: v = W.from_reduced_word([1,2,3,2,1])
sage: u.bruhat_le(u)
True
sage: u.bruhat_le(v)
True
sage: v.bruhat_le(u)
False
sage: v.bruhat_le(v)
True
sage: s = W.simple_reflections()
sage: s[1].bruhat_le(W.one())
False


The implementation uses the equivalent condition that any reduced word for other contains a reduced word for self as subword. See Stembridge, A short derivation of the Möbius function for the Bruhat order. J. Algebraic Combin. 25 (2007), no. 2, 141–148, Proposition 1.1.

Complexity: $$O(l * c)$$, where $$l$$ is the minimum of the lengths of $$u$$ and of $$v$$, and $$c$$ is the cost of the low level methods first_descent(), has_descent(), apply_simple_reflection()), etc. Those are typically $$O(n)$$, where $$n$$ is the rank of the Coxeter group.

bruhat_lower_covers()

Returns all elements that self covers in (strong) Bruhat order.

If w = self has a descent at $$i$$, then the elements that $$w$$ covers are exactly $$\{ws_i, u_1s_i, u_2s_i,..., u_js_i\}$$, where the $$u_k$$ are elements that $$ws_i$$ covers that also do not have a descent at $$i$$.

EXAMPLES:

sage: W = WeylGroup(["A",3])
sage: w = W.from_reduced_word([3,2,3])
sage: print([v.reduced_word() for v in w.bruhat_lower_covers()])
[[3, 2], [2, 3]]

sage: W = WeylGroup(["A",3])
sage: print([v.reduced_word() for v in W.simple_reflection(1).bruhat_lower_covers()])
[[]]
sage: print([v.reduced_word() for v in W.one().bruhat_lower_covers()])
[]
sage: W = WeylGroup(["B",4,1])
sage: w = W.from_reduced_word([0,2])
sage: print([v.reduced_word() for v in w.bruhat_lower_covers()])
[[2], [0]]
sage: W = WeylGroup("A3",prefix="s",implementation="permutation")
sage: [s1,s2,s3]=W.simple_reflections()
sage: (s1*s2*s3*s1).bruhat_lower_covers()
[s2*s1*s3, s1*s2*s1, s1*s2*s3]


We now show how to construct the Bruhat poset:

sage: W = WeylGroup(["A",3])
sage: covers = tuple([u, v] for v in W for u in v.bruhat_lower_covers() )
sage: P = Poset((W, covers), cover_relations = True)
sage: P.show()


Alternatively, one can just use:

sage: P = W.bruhat_poset()


The algorithm is taken from Stembridge’s ‘coxeter/weyl’ package for Maple.

bruhat_lower_covers_reflections()

Returns all 2-tuples of lower_covers and reflections (v, r) where v is covered by self and r is the reflection such that self = v r.

ALGORITHM:

EXAMPLES:

sage: W = WeylGroup(['A',3], prefix="s")
sage: w = W.from_reduced_word([3,1,2,1])
sage: w.bruhat_lower_covers_reflections()
[(s1*s2*s1, s1*s2*s3*s2*s1), (s3*s2*s1, s2), (s3*s1*s2, s1)]

bruhat_upper_covers()

Returns all elements that cover self in (strong) Bruhat order.

The algorithm works recursively, using the ‘inverse’ of the method described for lower covers bruhat_lower_covers(). Namely, it runs through all $$i$$ in the index set. Let $$w$$ equal self. If $$w$$ has no right descent $$i$$, then $$w s_i$$ is a cover; if $$w$$ has a decent at $$i$$, then $$u_j s_i$$ is a cover of $$w$$ where $$u_j$$ is a cover of $$w s_i$$.

EXAMPLES:

sage: W = WeylGroup(['A',3,1], prefix="s")
sage: w = W.from_reduced_word([1,2,1])
sage: w.bruhat_upper_covers()
[s1*s2*s1*s0, s1*s2*s0*s1, s0*s1*s2*s1, s3*s1*s2*s1, s2*s3*s1*s2, s1*s2*s3*s1]

sage: W = WeylGroup(['A',3])
sage: w = W.long_element()
sage: w.bruhat_upper_covers()
[]

sage: W = WeylGroup(['A',3])
sage: w = W.from_reduced_word([1,2,1])
sage: S = [v for v in W if w in v.bruhat_lower_covers()]
sage: C = w.bruhat_upper_covers()
sage: set(S) == set(C)
True

bruhat_upper_covers_reflections()

Returns all 2-tuples of covers and reflections (v, r) where v covers self and r is the reflection such that self = v r.

ALGORITHM:

EXAMPLES:

sage: W = WeylGroup(['A',4], prefix="s")
sage: w = W.from_reduced_word([3,1,2,1])
sage: w.bruhat_upper_covers_reflections()
[(s1*s2*s3*s2*s1, s3), (s2*s3*s1*s2*s1, s2*s3*s2), (s3*s4*s1*s2*s1, s4), (s4*s3*s1*s2*s1, s1*s2*s3*s4*s3*s2*s1)]

canonical_matrix()

Return the matrix of self in the canonical faithful representation.

This is an $$n$$-dimension real faithful essential representation, where $$n$$ is the number of generators of the Coxeter group. Note that this is not always the most natural matrix representation, for instance in type $$A_n$$.

EXAMPLES:

sage: W = WeylGroup(["A", 3])
sage: s = W.simple_reflections()
sage: (s[1]*s[2]*s[3]).canonical_matrix()
[ 0  0 -1]
[ 1  0 -1]
[ 0  1 -1]

coset_representative(index_set, side='right')

INPUT:

• index_set - a subset (or iterable) of the nodes of the Dynkin diagram

• side - ‘left’ or ‘right’

Returns the unique shortest element of the Coxeter group $$W$$ which is in the same left (resp. right) coset as self, with respect to the parabolic subgroup $$W_I$$.

EXAMPLES:

sage: W = CoxeterGroups().example(5)
sage: s = W.simple_reflections()
sage: w = s[2]*s[1]*s[3]
sage: w.coset_representative([]).reduced_word()
[2, 3, 1]
sage: w.coset_representative([1]).reduced_word()
[2, 3]
sage: w.coset_representative([1,2]).reduced_word()
[2, 3]
sage: w.coset_representative([1,3]                 ).reduced_word()
[2]
sage: w.coset_representative([2,3]                 ).reduced_word()
[2, 1]
sage: w.coset_representative([1,2,3]               ).reduced_word()
[]
sage: w.coset_representative([],      side='left').reduced_word()
[2, 3, 1]
sage: w.coset_representative([1],     side='left').reduced_word()
[2, 3, 1]
sage: w.coset_representative([1,2],   side='left').reduced_word()
[3]
sage: w.coset_representative([1,3],   side='left').reduced_word()
[2, 3, 1]
sage: w.coset_representative([2,3],   side='left').reduced_word()
[1]
sage: w.coset_representative([1,2,3], side='left').reduced_word()
[]

cover_reflections(side='right')

Return the set of reflections t such that self t covers self.

If side is ‘left’, t self covers self.

EXAMPLES:

sage: W = WeylGroup(['A',4], prefix="s")
sage: w = W.from_reduced_word([3,1,2,1])
sage: w.cover_reflections()
[s3, s2*s3*s2, s4, s1*s2*s3*s4*s3*s2*s1]
sage: w.cover_reflections(side='left')
[s4, s2, s1*s2*s1, s3*s4*s3]

coxeter_sorting_word(c)

Return the c-sorting word of self.

For a Coxeter element $$c$$ and an element $$w$$, the $$c$$-sorting word of $$w$$ is the lexicographic minimal reduced expression of $$w$$ in the infinite word $$c^\infty$$.

INPUT:

• c– a Coxeter element.

OUTPUT:

the c-sorting word of self as a list of integers.

EXAMPLES:

sage: W = CoxeterGroups().example()
sage: c = W.from_reduced_word([0,2,1])
sage: w = W.from_reduced_word([1,2,1,0,1])
sage: w.coxeter_sorting_word(c)
[2, 1, 2, 0, 1]

deodhar_factor_element(w, index_set)

Returns Deodhar’s Bruhat order factoring element.

INPUT:

• w is an element of the same Coxeter group W as self

• index_set is a subset of Dynkin nodes defining a parabolic subgroup W' of W

It is assumed that v = self and w are minimum length coset representatives for W/W' such that v $$\le$$ w in Bruhat order.

OUTPUT:

Deodhar’s element f(v,w) is the unique element of W' such that, for all v' and w' in W', vv' $$\le$$ ww' in W if and only if v' $$\le$$ f(v,w) * w' in W' where * is the Demazure product.

EXAMPLES:

sage: W = WeylGroup(['A',5],prefix="s")
sage: v = W.from_reduced_word([5])
sage: w = W.from_reduced_word([4,5,2,3,1,2])
sage: v.deodhar_factor_element(w,[1,3,4])
s3*s1
sage: W = WeylGroup(['C',2])
sage: w = W.from_reduced_word([2,1])
sage: w.deodhar_factor_element(W.from_reduced_word([2]),[1])
Traceback (most recent call last):
...
ValueError: [2, 1] is not of minimum length in its coset for the parabolic subgroup with index set [1]


REFERENCES:

deodhar_lift_down(w, index_set)

Letting v = self, given a Bruhat relation v W' $$\ge$$ w W' among cosets with respect to the subgroup W' given by the Dynkin node subset index_set, returns the Bruhat-maximum lift x of wW' such that v $$\ge$$ x.

INPUT:

• w is an element of the same Coxeter group W as self.

• index_set is a subset of Dynkin nodes defining a parabolic subgroup W'.

OUTPUT:

The unique Bruhat-maximum element x in W such that x W' = w W' and v \ge x.

EXAMPLES:

sage: W = WeylGroup(['A',3],prefix="s")
sage: v = W.from_reduced_word([1,2,3,2])
sage: w = W.from_reduced_word([3,2])
sage: v.deodhar_lift_down(w, [3])
s2*s3*s2

deodhar_lift_up(w, index_set)

Letting v = self, given a Bruhat relation v W' $$\le$$ w W' among cosets with respect to the subgroup W' given by the Dynkin node subset index_set, returns the Bruhat-minimum lift x of wW' such that v $$\le$$ x.

INPUT:

• w is an element of the same Coxeter group W as self.

• index_set is a subset of Dynkin nodes defining a parabolic subgroup W'.

OUTPUT:

The unique Bruhat-minimum element x in W such that x W' = w W' and v $$\le$$ x.

EXAMPLES:

sage: W = WeylGroup(['A',3],prefix="s")
sage: v = W.from_reduced_word([1,2,3])
sage: w = W.from_reduced_word([1,3,2])
sage: v.deodhar_lift_up(w, [3])
s1*s2*s3*s2

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

INPUT:

• index_set - a subset (as a list or iterable) of the nodes of the Dynkin diagram; (default: all of them)

• side - ‘left’ or ‘right’ (default: ‘right’)

• positive - a boolean (default: False)

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

The index_set option can be used to restrict to the parabolic subgroup indexed by index_set.

If positive is True, then returns the non-descents instead

Todo

find a better name for positive: complement? non_descent?

Caveat: the return type may change to some other iterable (tuple, …) in the future. Please use keyword arguments also, as the order of the arguments may change as well.

EXAMPLES:

sage: W = CoxeterGroups().example()
sage: s = W.simple_reflections()
sage: w = s[0]*s[1]
sage: w.descents()
[1]
sage: w = s[0]*s[2]
sage: w.descents()
[0, 2]


Todo

side, index_set, positive

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

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

See descents() for a description of the options.

EXAMPLES:

sage: W = CoxeterGroups().example()
sage: s = W.simple_reflections()
sage: w = s[2]*s[0]
sage: w.first_descent()
0
sage: w = s[0]*s[2]
sage: w.first_descent()
0
sage: w = s[0]*s[1]
sage: w.first_descent()
1

has_descent(i, side='right', positive=False)

Returns whether i is a (left/right) descent of self.

See descents() for a description of the options.

EXAMPLES:

sage: W = CoxeterGroups().example()
sage: s = W.simple_reflections()
sage: w = s[0] * s[1] * s[2]
sage: w.has_descent(2)
True
sage: [ w.has_descent(i)                  for i in [0,1,2] ]
[False, False, True]
sage: [ w.has_descent(i, side='left')   for i in [0,1,2] ]
[True, False, False]
sage: [ w.has_descent(i, positive=True) for i in [0,1,2] ]
[True, True, False]


This default implementation delegates the work to has_left_descent() and has_right_descent().

has_full_support()

Return whether self has full support.

An element is said to have full support if its support contains all simple reflections.

EXAMPLES:

sage: W = CoxeterGroups().example()
sage: w = W.from_reduced_word([1,2,1])
sage: w.has_full_support()
False
sage: w = W.from_reduced_word([1,2,1,0,1])
sage: w.has_full_support()
True

has_left_descent(i)

Returns whether $$i$$ is a left descent of self.

This default implementation uses that a left descent of $$w$$ is a right descent of $$w^{-1}$$.

EXAMPLES:

sage: W = CoxeterGroups().example(); W
The symmetric group on {0, ..., 3}
sage: w = W.an_element(); w
(1, 2, 3, 0)
sage: w.has_left_descent(0)
True
sage: w.has_left_descent(1)
False
sage: w.has_left_descent(2)
False

has_right_descent(i)

Returns whether i is a right descent of self.

EXAMPLES:

sage: W = CoxeterGroups().example(); W
The symmetric group on {0, ..., 3}
sage: w = W.an_element(); w
(1, 2, 3, 0)
sage: w.has_right_descent(0)
False
sage: w.has_right_descent(1)
False
sage: w.has_right_descent(2)
True

inversions_as_reflections()

Returns the set of reflections r such that self r < self.

EXAMPLES:

sage: W = WeylGroup(['A',3], prefix="s")
sage: w = W.from_reduced_word([3,1,2,1])
sage: w.inversions_as_reflections()
[s1, s1*s2*s1, s2, s1*s2*s3*s2*s1]

is_coxeter_sortable(c, sorting_word=None)

Return whether self is c-sortable.

Given a Coxeter element $$c$$, an element $$w$$ is $$c$$-sortable if its $$c$$-sorting word decomposes into a sequence of weakly decreasing subwords of $$c$$.

INPUT:

• c – a Coxeter element.

• sorting_word – sorting word (default: None) used to not recompute the c-sorting word if already computed.

OUTPUT:

is self c-sortable

EXAMPLES:

sage: W = CoxeterGroups().example()
sage: c = W.from_reduced_word([0,2,1])
sage: w = W.from_reduced_word([1,2,1,0,1])
sage: w.coxeter_sorting_word(c)
[2, 1, 2, 0, 1]
sage: w.is_coxeter_sortable(c)
False
sage: w = W.from_reduced_word([0,2,1,0,2])
sage: w.coxeter_sorting_word(c)
[2, 0, 1, 2, 0]
sage: w.is_coxeter_sortable(c)
True
sage: W = CoxeterGroup(['A',3])
sage: c = W.from_reduced_word([1,2,3])
sage: len([w for w in W if w.is_coxeter_sortable(c)]) # number of c-sortable elements in A_3 (Catalan number)
14

is_grassmannian(side='right')

Return whether self is Grassmannian.

INPUT:

• side – “left” or “right” (default: “right”)

An element is Grassmannian if it has at most one descent on the right (resp. on the left).

EXAMPLES:

sage: W = CoxeterGroups().example(); W
The symmetric group on {0, ..., 3}
sage: s = W.simple_reflections()
sage: W.one().is_grassmannian()
True
sage: s[1].is_grassmannian()
True
sage: (s[1]*s[2]).is_grassmannian()
True
sage: (s[0]*s[1]).is_grassmannian()
True
sage: (s[1]*s[2]*s[1]).is_grassmannian()
False

sage: (s[0]*s[2]*s[1]).is_grassmannian(side="left")
False
sage: (s[0]*s[2]*s[1]).is_grassmannian(side="right")
True
sage: (s[0]*s[2]*s[1]).is_grassmannian()
True

left_inversions_as_reflections()

Returns the set of reflections r such that r self < self.

EXAMPLES:

sage: W = WeylGroup(['A',3], prefix="s")
sage: w = W.from_reduced_word([3,1,2,1])
sage: w.left_inversions_as_reflections()
[s1, s3, s1*s2*s3*s2*s1, s2*s3*s2]

length()

Return the length of self.

This is the minimal length of a product of simple reflections giving self.

EXAMPLES:

sage: W = CoxeterGroups().example()
sage: s1 = W.simple_reflection(1)
sage: s2 = W.simple_reflection(2)
sage: s1.length()
1
sage: (s1*s2).length()
2
sage: W = CoxeterGroups().example()
sage: s = W.simple_reflections()
sage: w = s[0]*s[1]*s[0]
sage: w.length()
3
sage: W = CoxeterGroups().example()
sage: sum((x^w.length()) for w in W) - expand(prod(sum(x^i for i in range(j+1)) for j in range(4))) # This is scandalously slow!!!
0


Todo

Should use reduced_word_iterator (or reverse_iterator)

lower_cover_reflections(side='right')

Returns the reflections t such that self covers self t.

If side is ‘left’, self covers t self.

EXAMPLES:

sage: W = WeylGroup(['A',3],prefix="s")
sage: w = W.from_reduced_word([3,1,2,1])
sage: w.lower_cover_reflections()
[s1*s2*s3*s2*s1, s2, s1]
sage: w.lower_cover_reflections(side='left')
[s2*s3*s2, s3, s1]

lower_covers(side='right', index_set=None)

Return all elements that self covers in weak order.

INPUT:

• side – ‘left’ or ‘right’ (default: ‘right’)

• index_set – a list of indices or None

OUTPUT: a list

EXAMPLES:

sage: W = WeylGroup(['A',3])
sage: w = W.from_reduced_word([3,2,1])
sage: [x.reduced_word() for x in w.lower_covers()]
[[3, 2]]


To obtain covers for left weak order, set the option side to ‘left’:

sage: [x.reduced_word() for x in w.lower_covers(side='left')]
[[2, 1]]
sage: w = W.from_reduced_word([3,2,3,1])
sage: [x.reduced_word() for x in w.lower_covers()]
[[2, 3, 2], [3, 2, 1]]


Covers w.r.t. a parabolic subgroup are obtained with the option index_set:

sage: [x.reduced_word() for x in w.lower_covers(index_set = [1,2])]
[[2, 3, 2]]
sage: [x.reduced_word() for x in w.lower_covers(side='left')]
[[3, 2, 1], [2, 3, 1]]

min_demazure_product_greater(element)

Find the unique Bruhat-minimum element u such that v $$\le$$ w * u where v is self, w is element and * is the Demazure product.

INPUT:

• element is either an element of the same Coxeter group as self or a list (such as a reduced word) of elements from the index set of the Coxeter group.

EXAMPLES:

sage: W = WeylGroup(['A',4],prefix="s")
sage: v = W.from_reduced_word([2,3,4,1,2])
sage: u = W.from_reduced_word([2,3,2,1])
sage: v.min_demazure_product_greater(u)
s4*s2
sage: v.min_demazure_product_greater([2,3,2,1])
s4*s2
sage: v.min_demazure_product_greater((2,3,2,1))
s4*s2

reduced_word()

Return a reduced word for self.

This is a word $$[i_1,i_2,\ldots,i_k]$$ of minimal length such that $$s_{i_1} s_{i_2} \cdots s_{i_k} = \operatorname{self}$$, where the $$s_i$$ are the simple reflections.

EXAMPLES:

sage: W = CoxeterGroups().example()
sage: s = W.simple_reflections()
sage: w = s[0]*s[1]*s[2]
sage: w.reduced_word()
[0, 1, 2]
sage: w = s[0]*s[2]
sage: w.reduced_word()
[2, 0]

reduced_word_graph()

Return the reduced word graph of self.

The reduced word graph of an element $$w$$ in a Coxeter group is the graph whose vertices are the reduced words for $$w$$ (see reduced_word() for a definition of this term), and which has an $$m$$-colored edge between two reduced words $$x$$ and $$y$$ whenever $$x$$ and $$y$$ differ by exactly one length-$$m$$ braid move (with $$m \geq 2$$).

This graph is always connected (a theorem due to Tits) and has no multiple edges.

EXAMPLES:

sage: W = WeylGroup(['A',3], prefix='s')
sage: w0 = W.long_element()
sage: G = w0.reduced_word_graph()
sage: G.num_verts()
16
sage: len(w0.reduced_words())
16
sage: G.num_edges()
18
sage: len([e for e in G.edges() if e[2] == 2])
10
sage: len([e for e in G.edges() if e[2] == 3])
8

reduced_word_reverse_iterator()

Return a reverse iterator on a reduced word for self.

EXAMPLES:

sage: W = CoxeterGroups().example()
sage: s = W.simple_reflections()
sage: sigma = s[0]*s[1]*s[2]
sage: rI=sigma.reduced_word_reverse_iterator()
sage: [i for i in rI]
[2, 1, 0]
sage: s[0]*s[1]*s[2]==sigma
True
sage: sigma.length()
3


Default implementation: recursively remove the first right descent until the identity is reached (see first_descent() and apply_simple_reflection()).

reduced_words()

Return all reduced words for self.

See reduced_word() for the definition of a reduced word.

The algorithm uses the Matsumoto property that any two reduced expressions are related by braid relations, see Theorem 3.3.1(ii) in [BB2005].

braid_orbit()

EXAMPLES:

sage: W = CoxeterGroups().example()
sage: s = W.simple_reflections()
sage: w = s[0] * s[2]
sage: sorted(w.reduced_words())
[[0, 2], [2, 0]]

sage: W = WeylGroup(['E',6])
sage: w = W.from_reduced_word([2,3,4,2])
sage: sorted(w.reduced_words())
[[2, 3, 4, 2], [3, 2, 4, 2], [3, 4, 2, 4]]

sage: W = ReflectionGroup(['A',3], index_set=["AA","BB",5])  # optional - gap3
sage: w = W.long_element()                                   # optional - gap3
sage: w.reduced_words()                                      # optional - gap3
[['AA', 5, 'BB', 5, 'AA', 'BB'],
['AA', 'BB', 5, 'BB', 'AA', 'BB'],
[5, 'BB', 'AA', 5, 'BB', 5],
['BB', 5, 'AA', 'BB', 5, 'AA'],
[5, 'BB', 5, 'AA', 'BB', 5],
['BB', 5, 'AA', 'BB', 'AA', 5],
[5, 'AA', 'BB', 'AA', 5, 'BB'],
['BB', 'AA', 5, 'BB', 5, 'AA'],
['AA', 'BB', 'AA', 5, 'BB', 'AA'],
[5, 'BB', 'AA', 'BB', 5, 'BB'],
['BB', 'AA', 5, 'BB', 'AA', 5],
[5, 'AA', 'BB', 5, 'AA', 'BB'],
['AA', 'BB', 5, 'AA', 'BB', 'AA'],
['BB', 5, 'BB', 'AA', 'BB', 5],
['AA', 5, 'BB', 'AA', 5, 'BB'],
['BB', 'AA', 'BB', 5, 'BB', 'AA']]


Todo

The result should be full featured finite enumerated set (e.g., counting can be done much faster than iterating).

reflection_length()

Return the reflection length of self.

The reflection length is the length of the shortest expression of the element as a product of reflections.

EXAMPLES:

sage: W = WeylGroup(['A',3])
sage: s = W.simple_reflections()
sage: (s[1]*s[2]*s[3]).reflection_length()
3

sage: W = SymmetricGroup(4)
sage: s = W.simple_reflections()
sage: (s[3]*s[2]*s[3]).reflection_length()
1

support()

Return the support of self, that is the simple reflections that appear in the reduced expressions of self.

OUTPUT:

The support of self as a set of integers

EXAMPLES:

sage: W = CoxeterGroups().example()
sage: w = W.from_reduced_word([1,2,1])
sage: w.support()
{1, 2}

upper_covers(side='right', index_set=None)

Return all elements that cover self in weak order.

INPUT:

• side – ‘left’ or ‘right’ (default: ‘right’)

• index_set – a list of indices or None

OUTPUT: a list

EXAMPLES:

sage: W = WeylGroup(['A',3])
sage: w = W.from_reduced_word([2,3])
sage: [x.reduced_word() for x in w.upper_covers()]
[[2, 3, 1], [2, 3, 2]]


To obtain covers for left weak order, set the option side to ‘left’:

sage: [x.reduced_word() for x in w.upper_covers(side='left')]
[[1, 2, 3], [2, 3, 2]]


Covers w.r.t. a parabolic subgroup are obtained with the option index_set:

sage: [x.reduced_word() for x in w.upper_covers(index_set = [1])]
[[2, 3, 1]]
sage: [x.reduced_word() for x in w.upper_covers(side='left', index_set = [1])]
[[1, 2, 3]]

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

Return all elements that self covers in weak order.

INPUT:

• side – ‘left’ or ‘right’ (default: ‘right’)

• positive – a boolean (default: False)

• index_set – a list of indices or None

OUTPUT: a list

EXAMPLES:

sage: W = WeylGroup(['A',3])
sage: w = W.from_reduced_word([3,2,1])
sage: [x.reduced_word() for x in w.weak_covers()]
[[3, 2]]


To obtain instead elements that cover self, set positive=True:

sage: [x.reduced_word() for x in w.weak_covers(positive=True)]
[[3, 1, 2, 1], [2, 3, 2, 1]]


To obtain covers for left weak order, set the option side to ‘left’:

sage: [x.reduced_word() for x in w.weak_covers(side='left')]
[[2, 1]]
sage: w = W.from_reduced_word([3,2,3,1])
sage: [x.reduced_word() for x in w.weak_covers()]
[[2, 3, 2], [3, 2, 1]]
sage: [x.reduced_word() for x in w.weak_covers(side='left')]
[[3, 2, 1], [2, 3, 1]]


Covers w.r.t. a parabolic subgroup are obtained with the option index_set:

sage: [x.reduced_word() for x in w.weak_covers(index_set = [1,2])]
[[2, 3, 2]]

weak_le(other, side='right')

comparison in weak order

INPUT:

• other – an element of the same Coxeter group

• side – ‘left’ or ‘right’ (default: ‘right’)

OUTPUT: a boolean

Returns whether self <= other in left (resp. right) weak order, that is if ‘v’ can be obtained from ‘v’ by length increasing multiplication by simple reflections on the left (resp. right).

EXAMPLES:

sage: W = WeylGroup(["A",3])
sage: u = W.from_reduced_word([1,2])
sage: v = W.from_reduced_word([1,2,3,2])
sage: u.weak_le(u)
True
sage: u.weak_le(v)
True
sage: v.weak_le(u)
False
sage: v.weak_le(v)
True


Comparison for left weak order is achieved with the option side:

sage: u.weak_le(v, side='left')
False


The implementation uses the equivalent condition that any reduced word for $$u$$ is a right (resp. left) prefix of some reduced word for $$v$$.

Complexity: $$O(l * c)$$, where $$l$$ is the minimum of the lengths of $$u$$ and of $$v$$, and $$c$$ is the cost of the low level methods first_descent(), has_descent(), apply_simple_reflection()), etc. Those are typically $$O(n)$$, where $$n$$ is the rank of the Coxeter group.

We now run consistency tests with permutations:

sage: W = WeylGroup(["A",3])
sage: P4 = Permutations(4)
sage: def P4toW(w): return W.from_reduced_word(w.reduced_word())
sage: for u in P4:  # long time (5s on sage.math, 2011)
....:     for v in P4:
....:         assert u.permutohedron_lequal(v) == P4toW(u).weak_le(P4toW(v))
....:         assert u.permutohedron_lequal(v, side='left') == P4toW(u).weak_le(P4toW(v), side='left')

Finite
class ParentMethods

Bases: object

braid_group_as_finitely_presented_group()

Return the associated braid group.

EXAMPLES:

sage: W = CoxeterGroup(['A',2])
sage: W.braid_group_as_finitely_presented_group()
Finitely presented group < S1, S2 | S1*S2*S1*S2^-1*S1^-1*S2^-1 >

sage: W = WeylGroup(['B',2])
sage: W.braid_group_as_finitely_presented_group()
Finitely presented group < S1, S2 | (S1*S2)^2*(S1^-1*S2^-1)^2 >

sage: W = ReflectionGroup(['B',3], index_set=["AA","BB",5])  # optional - gap3
sage: W.braid_group_as_finitely_presented_group()            # optional - gap3
Finitely presented group < SAA, SBB, S5 |
SAA*SBB*SAA*SBB^-1*SAA^-1*SBB^-1, SAA*S5*SAA^-1*S5^-1,
(SBB*S5)^2*(SBB^-1*S5^-1)^2 >

braid_orbit(word)

Return the braid orbit of a word word of indices.

The input word does not need to be a reduced expression of an element.

INPUT:

• word: a list (or iterable) of indices in self.index_set()

OUTPUT: a list of all lists that can be obtained from

word by replacements of braid relations

See braid_relations() for the definition of braid relations.

EXAMPLES:

sage: W = CoxeterGroups().example()
sage: s = W.simple_reflections()
sage: w = s[0] * s[1] * s[2] * s[1]
sage: word = w.reduced_word(); word
[0, 1, 2, 1]

sage: sorted(W.braid_orbit(word))
[[0, 1, 2, 1], [0, 2, 1, 2], [2, 0, 1, 2]]

sage: sorted(W.braid_orbit([2,1,1,2,1]))
[[1, 2, 1, 1, 2], [2, 1, 1, 2, 1], [2, 1, 2, 1, 2], [2, 2, 1, 2, 2]]

sage: W = ReflectionGroup(['A',3], index_set=["AA","BB",5])  # optional - gap3
sage: w = W.long_element()                                   # optional - gap3
sage: W.braid_orbit(w.reduced_word())                        # optional - gap3
[['AA', 5, 'BB', 5, 'AA', 'BB'],
['AA', 'BB', 5, 'BB', 'AA', 'BB'],
[5, 'BB', 'AA', 5, 'BB', 5],
['BB', 5, 'AA', 'BB', 5, 'AA'],
[5, 'BB', 5, 'AA', 'BB', 5],
['BB', 5, 'AA', 'BB', 'AA', 5],
[5, 'AA', 'BB', 'AA', 5, 'BB'],
['BB', 'AA', 5, 'BB', 5, 'AA'],
['AA', 'BB', 'AA', 5, 'BB', 'AA'],
[5, 'BB', 'AA', 'BB', 5, 'BB'],
['BB', 'AA', 5, 'BB', 'AA', 5],
[5, 'AA', 'BB', 5, 'AA', 'BB'],
['AA', 'BB', 5, 'AA', 'BB', 'AA'],
['BB', 5, 'BB', 'AA', 'BB', 5],
['AA', 5, 'BB', 'AA', 5, 'BB'],
['BB', 'AA', 'BB', 5, 'BB', 'AA']]


Todo

The result should be full featured finite enumerated set (e.g., counting can be done much faster than iterating).

braid_relations()

Return the braid relations of self as a list of reduced words of the braid relations.

EXAMPLES:

sage: W = WeylGroup(["A",2])
sage: W.braid_relations()
[[[1, 2, 1], [2, 1, 2]]]

sage: W = WeylGroup(["B",3])
sage: W.braid_relations()
[[[1, 2, 1], [2, 1, 2]], [[1, 3], [3, 1]], [[2, 3, 2, 3], [3, 2, 3, 2]]]

bruhat_graph(x=None, y=None, edge_labels=False)

Return the Bruhat graph as a directed graph, with an edge $$u \to v$$ if and only if $$u < v$$ in the Bruhat order, and $$u = r \cdot v$$.

The Bruhat graph $$\Gamma(x,y)$$, defined if $$x \leq y$$ in the Bruhat order, has as its vertices the Bruhat interval $$\{ t | x \leq t \leq y \}$$, and as its edges are the pairs $$(u, v)$$ such that $$u = r \cdot v$$ where $$r$$ is a reflection, that is, a conjugate of a simple reflection.

REFERENCES:

Carrell, The Bruhat graph of a Coxeter group, a conjecture of Deodhar, and rational smoothness of Schubert varieties. Algebraic groups and their generalizations: classical methods (University Park, PA, 1991), 53–61, Proc. Sympos. Pure Math., 56, Part 1, Amer. Math. Soc., Providence, RI, 1994.

EXAMPLES:

sage: W = CoxeterGroup(['H',3])
sage: G = W.bruhat_graph(); G
Digraph on 120 vertices

sage: W = CoxeterGroup(['A',2,1])
sage: s1, s2, s3 = W.simple_reflections()
sage: W.bruhat_graph(s1, s1*s3*s2*s3)
Digraph on 6 vertices

sage: W.bruhat_graph(s1, s3*s2*s3)
Digraph on 0 vertices

sage: W = WeylGroup("A3", prefix="s")
sage: s1, s2, s3 = W.simple_reflections()
sage: G = W.bruhat_graph(s1*s3, s1*s2*s3*s2*s1); G
Digraph on 10 vertices


Check that the graph has the correct number of edges (see trac ticket #17744):

sage: len(G.edges())
16

bruhat_interval(x, y)

Return the list of t such that x <= t <= y.

EXAMPLES:

sage: W = WeylGroup("A3", prefix="s")
sage: [s1,s2,s3] = W.simple_reflections()
sage: W.bruhat_interval(s2,s1*s3*s2*s1*s3)
[s1*s2*s3*s2*s1, s2*s3*s2*s1, s3*s1*s2*s1, s1*s2*s3*s1,
s1*s2*s3*s2, s3*s2*s1, s2*s3*s1, s2*s3*s2, s1*s2*s1,
s3*s1*s2, s1*s2*s3, s2*s1, s3*s2, s2*s3, s1*s2, s2]

sage: W = WeylGroup(['A',2,1], prefix="s")
sage: [s0,s1,s2] = W.simple_reflections()
sage: W.bruhat_interval(1,s0*s1*s2)
[s0*s1*s2, s1*s2, s0*s2, s0*s1, s2, s1, s0, 1]


Return the poset of the Bruhat interval between x and y in Bruhat order.

EXAMPLES:

sage: W = WeylGroup("A3", prefix="s")
sage: s1,s2,s3 = W.simple_reflections()
sage: W.bruhat_interval_poset(s2, s1*s3*s2*s1*s3)
Finite poset containing 16 elements

sage: W = WeylGroup(['A',2,1], prefix="s")
sage: s0,s1,s2 = W.simple_reflections()
sage: W.bruhat_interval_poset(1, s0*s1*s2)
Finite poset containing 8 elements

canonical_representation()

Return the canonical faithful representation of self.

EXAMPLES:

sage: W = WeylGroup("A3")
sage: W.canonical_representation()
Finite Coxeter group over Integer Ring with Coxeter matrix:
[1 3 2]
[3 1 3]
[2 3 1]

coxeter_diagram()

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

coxeter_element()

Return a Coxeter element.

The result is the product of the simple reflections, in some order.

Note

This implementation is shared with well generated complex reflection groups. It would be nicer to put it in some joint super category; however, in the current state of the art, there is none where it is clear that this is the right construction for obtaining a Coxeter element.

In this context, this is an element having a regular eigenvector (a vector not contained in any reflection hyperplane of self).

EXAMPLES:

sage: CoxeterGroup(['A', 4]).coxeter_element().reduced_word()
[1, 2, 3, 4]
sage: CoxeterGroup(['B', 4]).coxeter_element().reduced_word()
[1, 2, 3, 4]
sage: CoxeterGroup(['D', 4]).coxeter_element().reduced_word()
[1, 2, 4, 3]
sage: CoxeterGroup(['F', 4]).coxeter_element().reduced_word()
[1, 2, 3, 4]
sage: CoxeterGroup(['E', 8]).coxeter_element().reduced_word()
[1, 3, 2, 4, 5, 6, 7, 8]
sage: CoxeterGroup(['H', 3]).coxeter_element().reduced_word()
[1, 2, 3]


This method is also used for well generated finite complex reflection groups:

sage: W = ReflectionGroup((1,1,4))          # optional - gap3
sage: W.coxeter_element().reduced_word()    # optional - gap3
[1, 2, 3]

sage: W = ReflectionGroup((2,1,4))          # optional - gap3
sage: W.coxeter_element().reduced_word()    # optional - gap3
[1, 2, 3, 4]

sage: W = ReflectionGroup((4,1,4))          # optional - gap3
sage: W.coxeter_element().reduced_word()    # optional - gap3
[1, 2, 3, 4]

sage: W = ReflectionGroup((4,4,4))          # optional - gap3
sage: W.coxeter_element().reduced_word()    # optional - gap3
[1, 2, 3, 4]

coxeter_matrix()

Return the Coxeter matrix associated to self.

EXAMPLES:

sage: G = WeylGroup(['A',3])
sage: G.coxeter_matrix()
[1 3 2]
[3 1 3]
[2 3 1]

coxeter_type()

Return the Coxeter type of self.

EXAMPLES:

sage: W = CoxeterGroup(['H',3])
sage: W.coxeter_type()
Coxeter type of ['H', 3]

demazure_product(Q)

Return the Demazure product of the list Q in self.

INPUT:

• Q is a list of elements from the index set of self.

This returns the Coxeter group element that represents the composition of 0-Hecke or Demazure operators.

EXAMPLES:

sage: W = WeylGroup(['A',2])
sage: w = W.demazure_product([2,2,1])
sage: w.reduced_word()
[2, 1]

sage: w = W.demazure_product([2,1,2,1,2])
sage: w.reduced_word()
[1, 2, 1]

sage: W = WeylGroup(['B',2])
sage: w = W.demazure_product([2,1,2,1,2])
sage: w.reduced_word()
[2, 1, 2, 1]

elements_of_length(n)

Return all elements of length $$n$$.

EXAMPLES:

sage: A = AffinePermutationGroup(['A',2,1])
sage: [len(list(A.elements_of_length(i))) for i in [0..5]]
[1, 3, 6, 9, 12, 15]

sage: W = CoxeterGroup(['H',3])
sage: [len(list(W.elements_of_length(i))) for i in range(4)]
[1, 3, 5, 7]

sage: W = CoxeterGroup(['A',2])
sage: [len(list(W.elements_of_length(i))) for i in range(6)]
[1, 2, 2, 1, 0, 0]

fully_commutative_elements()

Return the set of fully commutative elements in this Coxeter group.

EXAMPLES:

sage: CoxeterGroup(['A', 3]).fully_commutative_elements()
Fully commutative elements of Finite Coxeter group over Integer Ring with Coxeter matrix:
[1 3 2]
[3 1 3]
[2 3 1]

grassmannian_elements(side='right')

Return the left or right Grassmannian elements of self as an enumerated set.

INPUT:

• side – (default: "right") "left" or "right"

EXAMPLES:

sage: S = CoxeterGroups().example()
sage: G = S.grassmannian_elements()
sage: G.cardinality()
12
sage: G.list()
[(0, 1, 2, 3), (1, 0, 2, 3), (0, 2, 1, 3), (0, 1, 3, 2),
(2, 0, 1, 3), (1, 2, 0, 3), (0, 3, 1, 2), (0, 2, 3, 1),
(3, 0, 1, 2), (1, 3, 0, 2), (1, 2, 3, 0), (2, 3, 0, 1)]
sage: sorted(tuple(w.descents()) for w in G)
[(), (0,), (0,), (0,), (1,), (1,), (1,), (1,), (1,), (2,), (2,), (2,)]
sage: G = S.grassmannian_elements(side = "left")
sage: G.cardinality()
12
sage: sorted(tuple(w.descents(side = "left")) for w in G)
[(), (0,), (0,), (0,), (1,), (1,), (1,), (1,), (1,), (2,), (2,), (2,)]

index_set()

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)

random_element_of_length(n)

Return a random element of length n in self.

Starts at the identity, then chooses an upper cover at random.

Not very uniform: actually constructs a uniformly random reduced word of length $$n$$. Thus we most likely get elements with lots of reduced words!

EXAMPLES:

sage: A = AffinePermutationGroup(['A', 7, 1])
sage: p = A.random_element_of_length(10)
sage: p in A
True
sage: p.length() == 10
True

sage: W = CoxeterGroup(['A', 4])
sage: p = W.random_element_of_length(5)
sage: p in W
True
sage: p.length() == 5
True

sign_representation(base_ring=None, side='twosided')

Return the sign representation of self over base_ring.

INPUT:

• base_ring – (optional) the base ring; the default is $$\ZZ$$

• side – ignored

EXAMPLES:

sage: W = WeylGroup(["A", 1, 1])
sage: W.sign_representation()
Sign representation of Weyl Group of type ['A', 1, 1] (as a matrix group acting on the root space) over Integer Ring

simple_projection(i, side='right', length_increasing=True)

Return the simple projection $$\pi_i$$ (or $$\overline\pi_i$$ if $$length_increasing$$ is False).

INPUT:

• i - an element of the index set of self

See simple_projections() for the options and for the definition of the simple projections.

EXAMPLES:

sage: W = CoxeterGroups().example()
sage: W
The symmetric group on {0, ..., 3}
sage: s = W.simple_reflections()
sage: sigma = W.an_element()
sage: sigma
(1, 2, 3, 0)
sage: u0 = W.simple_projection(0)
sage: d0 = W.simple_projection(0,length_increasing=False)
sage: sigma.length()
3
sage: pi=sigma*s[0]
sage: pi.length()
4
sage: u0(sigma)
(2, 1, 3, 0)
sage: pi
(2, 1, 3, 0)
sage: u0(pi)
(2, 1, 3, 0)
sage: d0(sigma)
(1, 2, 3, 0)
sage: d0(pi)
(1, 2, 3, 0)

simple_projections(side='right', length_increasing=True)

Return the family of simple projections, also known as 0-Hecke or Demazure operators.

INPUT:

• self – a Coxeter group $$W$$

• side – ‘left’ or ‘right’ (default: ‘right’)

• length_increasing – a boolean (default: True) specifying whether the operator increases or decreases length

Returns the simple projections of $$W$$, as a family.

To each simple reflection $$s_i$$ of $$W$$, corresponds a simple projection $$\pi_i$$ from $$W$$ to $$W$$ defined by:

$$\pi_i(w) = w s_i$$ if $$i$$ is not a descent of $$w$$ $$\pi_i(w) = w$$ otherwise.

The simple projections $$(\pi_i)_{i\in I}$$ move elements down the right permutohedron, toward the maximal element. They satisfy the same braid relations as the simple reflections, but are idempotents $$\pi_i^2=\pi$$ not involutions $$s_i^2 = 1$$. As such, the simple projections generate the $$0$$-Hecke monoid.

By symmetry, one can also define the projections $$(\overline\pi_i)_{i\in I}$$ (when the option length_increasing is False):

$$\overline\pi_i(w) = w s_i$$ if $$i$$ is a descent of $$w$$ $$\overline\pi_i(w) = w$$ otherwise.

as well as the analogues acting on the left (when the option side is ‘left’).

EXAMPLES:

sage: W = CoxeterGroups().example(); W
The symmetric group on {0, ..., 3}
sage: s = W.simple_reflections()
sage: sigma = W.an_element(); sigma
(1, 2, 3, 0)
sage: pi = W.simple_projections(); pi
Finite family {0: <function ...<lambda> at ...>, 1: <function ...<lambda> at ...>, 2: <function ...<lambda> ...>}
sage: pi[1](sigma)
(1, 3, 2, 0)
sage: W.simple_projection(1)(sigma)
(1, 3, 2, 0)

standard_coxeter_elements()

Return all standard Coxeter elements in self.

This is the set of all elements in self obtained from any product of the simple reflections in self.

Note

• self is assumed to be well-generated.

• This works even beyond real reflection groups, but the conjugacy class is not unique and we only obtain one such class.

EXAMPLES:

sage: W = ReflectionGroup(4)                 # optional - gap3
sage: sorted(W.standard_coxeter_elements())  # optional - gap3
[(1,7,6,12,23,20)(2,8,17,24,9,5)(3,16,10,19,15,21)(4,14,11,22,18,13),
(1,10,4,12,21,22)(2,11,19,24,13,3)(5,15,7,17,16,23)(6,18,8,20,14,9)]

weak_order_ideal(predicate, side='right', category=None)

Return a weak order ideal defined by a predicate

INPUT:

• predicate: a predicate on the elements of self defining an weak order ideal in self

• side: “left” or “right” (default: “right”)

OUTPUT: an enumerated set

EXAMPLES:

sage: D6 = FiniteCoxeterGroups().example(5)
sage: I = D6.weak_order_ideal(predicate = lambda w: w.length() <= 3)
sage: I.cardinality()
7
sage: list(I)
[(), (1,), (2,), (1, 2), (2, 1), (1, 2, 1), (2, 1, 2)]


We now consider an infinite Coxeter group:

sage: W = WeylGroup(["A",1,1])
sage: I = W.weak_order_ideal(predicate = lambda w: w.length() <= 2)
sage: list(iter(I))
[
[1 0]  [-1  2]  [ 1  0]  [ 3 -2]  [-1  2]
[0 1], [ 0  1], [ 2 -1], [ 2 -1], [-2  3]
]


Even when the result is finite, some features of FiniteEnumeratedSets are not available:

sage: I.cardinality() # todo: not implemented
5
sage: list(I)         # todo: not implemented


unless this finiteness is explicitly specified:

sage: I = W.weak_order_ideal(predicate = lambda w: w.length() <= 2,
....:                        category = FiniteEnumeratedSets())
sage: I.cardinality()
5
sage: list(I)
[
[1 0]  [-1  2]  [ 1  0]  [ 3 -2]  [-1  2]
[0 1], [ 0  1], [ 2 -1], [ 2 -1], [-2  3]
]


Background

The weak order is returned as a RecursivelyEnumeratedSet_forest. This is achieved by assigning to each element $$u1$$ of the ideal a single ancestor $$u=u1 s_i$$, where $$i$$ is the smallest descent of $$u$$.

This allows for iterating through the elements in roughly Constant Amortized Time and constant memory (taking the operations and size of the generated objects as constants).

Return None.

Indeed, all the structure Coxeter groups have in addition to groups (simple reflections, …) is already defined in the super category.

EXAMPLES:

sage: CoxeterGroups().additional_structure()

super_categories()

EXAMPLES:

sage: CoxeterGroups().super_categories()
[Category of generalized coxeter groups]