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r""" Symmetric Group Algebra """ #***************************************************************************** # Copyright (C) 2007 Mike Hansen <mhansen@gmail.com>, # # Distributed under the terms of the GNU General Public License (GPL) # http://www.gnu.org/licenses/ #***************************************************************************** from __future__ import print_function, absolute_import from six.moves import range
from sage.misc.cachefunc import cached_method from .combinatorial_algebra import CombinatorialAlgebra from .free_module import CombinatorialFreeModule from sage.algebras.group_algebra import GroupAlgebra_class from sage.categories.weyl_groups import WeylGroups from sage.combinat.permutation import Permutation, Permutations, from_permutation_group_element from . import partition from .tableau import Tableau, StandardTableaux_size, StandardTableaux_shape, StandardTableaux from sage.interfaces.all import gap from sage.rings.all import QQ, PolynomialRing from sage.arith.all import factorial from sage.matrix.all import matrix from sage.modules.all import vector from sage.groups.perm_gps.permgroup_element import PermutationGroupElement import itertools from sage.combinat.permutation_cython import (left_action_same_n, right_action_same_n) import six
# TODO: Remove this function and replace it with the class # TODO: Create parents for other bases (such as the seminormal basis) def SymmetricGroupAlgebra(R, W, category=None): """ Return the symmetric group algebra of order ``W`` over the ring ``R``.
INPUT:
- ``W`` -- a symmetric group; alternatively an integer `n` can be provided, as shorthand for ``Permutations(n)``. - ``R`` -- a base ring - ``category`` -- a category (default: the category of ``W``)
This supports several implementations of the symmetric group. At this point this has been tested with ``W=Permutations(n)`` and ``W=SymmetricGroup(n)``.
.. WARNING::
Some features are failing in the latter case, in particular if the domain of the symmetric group is not `1,\ldots,n`.
.. NOTE::
The brave can also try setting ``W=WeylGroup(['A',n-1])``, but little support for this currently exists.
EXAMPLES::
sage: QS3 = SymmetricGroupAlgebra(QQ, 3); QS3 Symmetric group algebra of order 3 over Rational Field sage: QS3(1) [1, 2, 3] sage: QS3(2) 2*[1, 2, 3] sage: basis = [QS3(p) for p in Permutations(3)] sage: a = sum(basis); a [1, 2, 3] + [1, 3, 2] + [2, 1, 3] + [2, 3, 1] + [3, 1, 2] + [3, 2, 1] sage: a^2 6*[1, 2, 3] + 6*[1, 3, 2] + 6*[2, 1, 3] + 6*[2, 3, 1] + 6*[3, 1, 2] + 6*[3, 2, 1] sage: a^2 == 6*a True sage: b = QS3([3, 1, 2]) sage: b [3, 1, 2] sage: b*a [1, 2, 3] + [1, 3, 2] + [2, 1, 3] + [2, 3, 1] + [3, 1, 2] + [3, 2, 1] sage: b*a == a True
We now construct the symmetric group algebra by providing explicitly the underlying group::
sage: SGA = SymmetricGroupAlgebra(QQ, Permutations(4)); SGA Symmetric group algebra of order 4 over Rational Field sage: SGA.group() Standard permutations of 4 sage: SGA.an_element() [1, 2, 3, 4] + 2*[1, 2, 4, 3] + 3*[1, 3, 2, 4] + [4, 1, 2, 3]
sage: SGA = SymmetricGroupAlgebra(QQ, SymmetricGroup(4)); SGA Symmetric group algebra of order 4 over Rational Field sage: SGA.group() Symmetric group of order 4! as a permutation group sage: SGA.an_element() () + 2*(1,2) + 4*(1,2,3,4)
sage: SGA = SymmetricGroupAlgebra(QQ, WeylGroup(["A",3], prefix='s')); SGA Symmetric group algebra of order 4 over Rational Field sage: SGA.group() Weyl Group of type ['A', 3] (as a matrix group acting on the ambient space) sage: SGA.an_element() 2*s1*s2*s3*s2*s1 + 3*s1*s2*s3*s1 + s1*s2*s3 + 1
The preferred way to construct the symmetric group algebra is to go through the usual ``algebra`` method::
sage: SGA = Permutations(3).algebra(QQ); SGA Symmetric group algebra of order 3 over Rational Field sage: SGA.group() Standard permutations of 3
sage: SGA = SymmetricGroup(3).algebra(QQ); SGA Symmetric group algebra of order 3 over Rational Field sage: SGA.group() Symmetric group of order 3! as a permutation group
The canonical embedding from the symmetric group algebra of order `n` to the symmetric group algebra of order `p > n` is available as a coercion::
sage: QS3 = SymmetricGroupAlgebra(QQ, 3) sage: QS4 = SymmetricGroupAlgebra(QQ, 4) sage: QS4.coerce_map_from(QS3) Generic morphism: From: Symmetric group algebra of order 3 over Rational Field To: Symmetric group algebra of order 4 over Rational Field
sage: x3 = QS3([3,1,2]) + 2 * QS3([2,3,1]); x3 2*[2, 3, 1] + [3, 1, 2] sage: QS4(x3) 2*[2, 3, 1, 4] + [3, 1, 2, 4]
This allows for mixed expressions::
sage: x4 = 3*QS4([3, 1, 4, 2]) sage: x3 + x4 2*[2, 3, 1, 4] + [3, 1, 2, 4] + 3*[3, 1, 4, 2]
sage: QS0 = SymmetricGroupAlgebra(QQ, 0) sage: QS1 = SymmetricGroupAlgebra(QQ, 1) sage: x0 = QS0([]) sage: x1 = QS1([1]) sage: x0 * x1 [1] sage: x3 - (2*x0 + x1) - x4 -3*[1, 2, 3, 4] + 2*[2, 3, 1, 4] + [3, 1, 2, 4] - 3*[3, 1, 4, 2]
Caveat: to achieve this, constructing ``SymmetricGroupAlgebra(QQ, 10)`` currently triggers the construction of all symmetric group algebras of smaller order. Is this a feature we really want to have?
.. WARNING::
The semantics of multiplication in symmetric group algebras with index set ``Permutations(n)`` is determined by the order in which permutations are multiplied, which currently defaults to "in such a way that multiplication is associative with permutations acting on integers from the right", but can be changed to the opposite order at runtime by setting the global variable ``Permutations.options['mult']`` (see :meth:`sage.combinat.permutation.Permutations.options` ). On the other hand, the semantics of multiplication in symmetric group algebras with index set ``SymmetricGroup(n)`` does not depend on this global variable. (This has the awkward consequence that the coercions between these two sorts of symmetric group algebras do not respect multiplication when this global variable is set to ``'r2l'``.) In view of this, it is recommended that code not rely on the usual multiplication function, but rather use the methods :meth:`left_action_product` and :meth:`right_action_product` for multiplying permutations (these methods don't depend on the setting). See :trac:`14885` for more information.
We conclude by constructing the algebra of the symmetric group as a monoid algebra::
sage: QS3 = SymmetricGroupAlgebra(QQ, 3, category=Monoids()) sage: QS3.category() Category of finite dimensional monoid algebras over Rational Field sage: TestSuite(QS3).run()
TESTS::
sage: QS3 = SymmetricGroupAlgebra(QQ, 3) sage: TestSuite(QS3).run()
sage: QS3.group() Standard permutations of 3
sage: QS3.one_basis() [1, 2, 3]
sage: p1 = Permutation([1,2,3]) sage: p2 = Permutation([2,1,3]) sage: QS3.product_on_basis(p1,p2) [2, 1, 3]
sage: W = WeylGroup(["A",3]) sage: SGA = SymmetricGroupAlgebra(QQ, W) sage: SGA.group() is W True sage: TestSuite(SGA).run()
sage: SG = SymmetricGroupAlgebra(ZZ, 3) sage: SG.group().conjugacy_classes_representatives() [[1, 2, 3], [2, 1, 3], [2, 3, 1]]
sage: SGg = SymmetricGroup(3).algebra(ZZ) sage: SGg.group().conjugacy_classes_representatives() [(), (1,2), (1,2,3)] """
class SymmetricGroupAlgebra_n(GroupAlgebra_class):
def __init__(self, R, W, category): """ TESTS::
sage: QS3 = SymmetricGroupAlgebra(QQ, 3) sage: TestSuite(QS3).run()
sage: QS3 in GroupAlgebras(QQ) True sage: QS3 in FiniteDimensionalAlgebrasWithBasis(QQ) True
Check that :trac:`16926` works::
sage: S = SymmetricGroup(4) sage: SGA = S.algebra(QQ) sage: TestSuite(SGA).run()
Checking that coercion works between equivalent indexing sets::
sage: G = SymmetricGroup(4).algebra(QQ) sage: S = SymmetricGroupAlgebra(QQ,4) sage: S(G.an_element()) [1, 2, 3, 4] + 2*[2, 1, 3, 4] + 4*[2, 3, 4, 1] sage: G(S.an_element()) () + 2*(3,4) + 3*(2,3) + (1,4,3,2)
Checking the recovery of `n`:
sage: SymmetricGroup(4).algebra(QQ).n 4 sage: SymmetricGroup(1).algebra(QQ).n 1 sage: SymmetricGroup(0).algebra(QQ).n 0 sage: Permutations(4).algebra(QQ).n 4 sage: Permutations(1).algebra(QQ).n 1 sage: Permutations(0).algebra(QQ).n 0 sage: SymmetricGroupAlgebra(QQ, WeylGroup(["A",3])).n 4 sage: SymmetricGroupAlgebra(QQ, WeylGroup(["A",1])).n 2 sage: SymmetricGroupAlgebra(QQ, WeylGroup(["A",0])).n # todo: not implemented 1 """ raise ValueError("W (=%s) should be a symmetric group or a nonnegative integer") # The following trick works for both SymmetricGroup(n) and # Permutations(n) and it's currently not possible to # construct the WeylGroup for n=0 else: latex_prefix='', category=category)
def _repr_(self): """ Return a string representation of ``self``.
EXAMPLES::
sage: SymmetricGroupAlgebra(QQ, 3) Symmetric group algebra of order 3 over Rational Field """
def _coerce_map_from_(self, S): """ Return ``True`` or a morphism if there exists a coercion from ``S`` into ``self`` or ``False`` otherwise.
EXAMPLES:
Symmetric group algebras::
sage: SGA4 = SymmetricGroupAlgebra(QQ, 4) sage: SGA2 = SymmetricGroupAlgebra(QQ, 2) sage: SGA4.has_coerce_map_from(SGA2) True sage: SGA2Z = SymmetricGroupAlgebra(ZZ, 2) sage: SGA4.has_coerce_map_from(SGA2Z) True sage: p = Permutation([2,1]) sage: SGA4(-3*SGA2Z.monomial(p)) -3*[2, 1, 3, 4]
Descent algebras::
sage: DA = DescentAlgebra(QQ, 4) sage: SGA4 = SymmetricGroupAlgebra(QQ, 4) sage: SGA4.has_coerce_map_from(DA.D()) True sage: SGA4.has_coerce_map_from(DA.B()) True sage: SGA4.has_coerce_map_from(DA.I()) True sage: x = DA.B()[4] sage: SGA4(x) [1, 2, 3, 4]
sage: DAB = DescentAlgebra(ZZ,2).B() sage: SGA4.has_coerce_map_from(DAB) True sage: SGA4(DAB[2]) [1, 2, 3, 4]
sage: QSG4 = SymmetricGroup(4).algebra(ZZ) sage: DAB = DescentAlgebra(ZZ,4).B() sage: QSG4(DAB[1,2,1]) () + (3,4) + (2,3,4) + (1,2) + (1,2)(3,4) + (1,2,3,4) + (1,3,2) + (1,3,4,2) + (1,3,4) + (1,4,3,2) + (1,4,2) + (1,4) """ # Symmetric group algebras of smaller rank and self.base_ring().has_coerce_map_from(S.base_ring())):
# Descent algebras # TODO: A better way to handle all of the bases # Same rank and base ring, just the natural morphism and self._indices == Permutations(self.n)): # Otherwise compose with the canonical embedding in order to ensure # that the right base ring and the right index set are being used. # Slightly hacky! self.base_ring().has_coerce_map_from(S.base_ring())):
def _element_constructor_(self, x): """ Construct an element of ``self``.
EXAMPLES::
sage: S = SymmetricGroupAlgebra(QQ, 4) sage: G = SymmetricGroup(3) sage: p = Permutation((1,2)) sage: S(p) [2, 1, 3, 4] sage: S(G(p)) [2, 1, 3, 4] sage: S(p) == S(G(p)) True """ from_permutation_group_element(x))
def _sibling(self, n): r""" Return the sibling group algebra of order `n`.
EXAMPLES::
sage: SGA = SymmetricGroupAlgebra(QQ, Permutations(3))._sibling(4); SGA Symmetric group algebra of order 4 over Rational Field sage: SGA.group() Standard permutations of 4
sage: SGA = SymmetricGroupAlgebra(QQ, SymmetricGroup(3))._sibling(4); SGA Symmetric group algebra of order 4 over Rational Field sage: SGA.group() Symmetric group of order 4! as a permutation group
sage: SGA = SymmetricGroupAlgebra(QQ, WeylGroup(["A",2]))._sibling(4); SGA Traceback (most recent call last): ... NotImplementedError: Constructing the sibling algebra of a different order only implemented for PermutationGroup and SymmetricGroup """ "only implemented for PermutationGroup and SymmetricGroup")
# _repr_ customization: output the basis element indexed by [1,2,3] as [1,2,3] _repr_option_bracket = False
def left_action_product(self, left, right): """ Return the product of two elements ``left`` and ``right`` of ``self``, where multiplication is defined in such a way that for two permutations `p` and `q`, the product `pq` is the permutation obtained by first applying `q` and then applying `p`. This definition of multiplication is tailored to make multiplication of permutations associative with their action on numbers if permutations are to act on numbers from the left.
EXAMPLES::
sage: QS3 = SymmetricGroupAlgebra(QQ, 3) sage: p1 = Permutation([2, 1, 3]) sage: p2 = Permutation([3, 1, 2]) sage: QS3.left_action_product(QS3(p1), QS3(p2)) [3, 2, 1] sage: x = QS3([1, 2, 3]) - 2*QS3([1, 3, 2]) sage: y = 1/2 * QS3([3, 1, 2]) + 3*QS3([1, 2, 3]) sage: QS3.left_action_product(x, y) 3*[1, 2, 3] - 6*[1, 3, 2] - [2, 1, 3] + 1/2*[3, 1, 2] sage: QS3.left_action_product(0, x) 0
The method coerces its input into the algebra ``self``::
sage: QS4 = SymmetricGroupAlgebra(QQ, 4) sage: QS4.left_action_product(QS3([1, 2, 3]), QS3([2, 1, 3])) [2, 1, 3, 4] sage: QS4.left_action_product(1, Permutation([4, 1, 2, 3])) [4, 1, 2, 3]
TESTS::
sage: QS4 = SymmetricGroup(4).algebra(QQ) sage: QS4.left_action_product(QS4((1,2)), QS4((2,3))) (1,2,3) sage: QS4.left_action_product(1, QS4((1,2))) (1,2)
.. WARNING::
Note that coercion presently works from permutations of ``n`` into the ``n``-th symmetric group algebra, and also from all smaller symmetric group algebras into the ``n``-th symmetric group algebra, but not from permutations of integers smaller than ``n`` into the ``n``-th symmetric group algebra. """ for (p, x) in a for (q, y) in b]) # Why did we use left_action_same_n instead of # left_action_product? # Because having cast a and b into self, we already know that # p and q are permutations of the same number of elements, # and thus we don't need to waste our time on the input # sanitizing of left_action_product.
def right_action_product(self, left, right): """ Return the product of two elements ``left`` and ``right`` of ``self``, where multiplication is defined in such a way that for two permutations `p` and `q`, the product `pq` is the permutation obtained by first applying `p` and then applying `q`. This definition of multiplication is tailored to make multiplication of permutations associative with their action on numbers if permutations are to act on numbers from the right.
EXAMPLES::
sage: QS3 = SymmetricGroupAlgebra(QQ, 3) sage: p1 = Permutation([2, 1, 3]) sage: p2 = Permutation([3, 1, 2]) sage: QS3.right_action_product(QS3(p1), QS3(p2)) [1, 3, 2] sage: x = QS3([1, 2, 3]) - 2*QS3([1, 3, 2]) sage: y = 1/2 * QS3([3, 1, 2]) + 3*QS3([1, 2, 3]) sage: QS3.right_action_product(x, y) 3*[1, 2, 3] - 6*[1, 3, 2] + 1/2*[3, 1, 2] - [3, 2, 1] sage: QS3.right_action_product(0, x) 0
The method coerces its input into the algebra ``self``::
sage: QS4 = SymmetricGroupAlgebra(QQ, 4) sage: QS4.right_action_product(QS3([1, 2, 3]), QS3([2, 1, 3])) [2, 1, 3, 4] sage: QS4.right_action_product(1, Permutation([4, 1, 2, 3])) [4, 1, 2, 3]
TESTS::
sage: QS4 = SymmetricGroup(4).algebra(QQ) sage: QS4.right_action_product(QS4((1,2)), QS4((2,3))) (1,3,2) sage: QS4.right_action_product(1, QS4((1,2))) (1,2)
.. WARNING::
Note that coercion presently works from permutations of ``n`` into the ``n``-th symmetric group algebra, and also from all smaller symmetric group algebras into the ``n``-th symmetric group algebra, but not from permutations of integers smaller than ``n`` into the ``n``-th symmetric group algebra. """ for (p, x) in a for (q, y) in b]) # Why did we use right_action_same_n instead of # right_action_product? # Because having cast a and b into self, we already know that # p and q are permutations of the same number of elements, # and thus we don't need to waste our time on the input # sanitizing of right_action_product.
def canonical_embedding(self, other): """ Return the canonical coercion of ``self`` into a symmetric group algebra ``other``.
INPUT:
- ``other`` -- a symmetric group algebra with order `p` satisfying `p \geq n`, where `n` is the order of ``self``, over a ground ring into which the ground ring of ``self`` coerces.
EXAMPLES::
sage: QS2 = SymmetricGroupAlgebra(QQ, 2) sage: QS4 = SymmetricGroupAlgebra(QQ, 4) sage: phi = QS2.canonical_embedding(QS4); phi Generic morphism: From: Symmetric group algebra of order 2 over Rational Field To: Symmetric group algebra of order 4 over Rational Field
sage: x = QS2([2,1]) + 2 * QS2([1,2]) sage: phi(x) 2*[1, 2, 3, 4] + [2, 1, 3, 4]
sage: loads(dumps(phi)) Generic morphism: From: Symmetric group algebra of order 2 over Rational Field To: Symmetric group algebra of order 4 over Rational Field
sage: ZS2 = SymmetricGroupAlgebra(ZZ, 2) sage: phi = ZS2.canonical_embedding(QS4); phi Generic morphism: From: Symmetric group algebra of order 2 over Integer Ring To: Symmetric group algebra of order 4 over Rational Field
sage: phi = ZS2.canonical_embedding(QS2); phi Generic morphism: From: Symmetric group algebra of order 2 over Integer Ring To: Symmetric group algebra of order 2 over Rational Field
sage: QS4.canonical_embedding(QS2) Traceback (most recent call last): ... ValueError: There is no canonical embedding from Symmetric group algebra of order 2 over Rational Field to Symmetric group algebra of order 4 over Rational Field
sage: QS4g = SymmetricGroup(4).algebra(QQ) sage: QS4.canonical_embedding(QS4g)(QS4([1,3,2,4])) (2,3) sage: QS4g.canonical_embedding(QS4)(QS4g((2,3))) [1, 3, 2, 4] sage: ZS2.canonical_embedding(QS4g)(ZS2([2,1])) (1,2) sage: ZS2g = SymmetricGroup(2).algebra(ZZ) sage: ZS2g.canonical_embedding(QS4)(ZS2g((1,2))) [2, 1, 3, 4] """
def monomial_from_smaller_permutation(self, permutation): """ Convert ``permutation`` into a permutation, possibly extending it to the appropriate size, and return the corresponding basis element of ``self``.
EXAMPLES::
sage: QS5 = SymmetricGroupAlgebra(QQ, 5) sage: QS5.monomial_from_smaller_permutation([]) [1, 2, 3, 4, 5] sage: QS5.monomial_from_smaller_permutation(Permutation([3,1,2])) [3, 1, 2, 4, 5] sage: QS5.monomial_from_smaller_permutation([5,3,4,1,2]) [5, 3, 4, 1, 2] sage: QS5.monomial_from_smaller_permutation(SymmetricGroup(2)((1,2))) [2, 1, 3, 4, 5]
sage: QS5g = SymmetricGroup(5).algebra(QQ) sage: QS5g.monomial_from_smaller_permutation([2,1]) (1,2)
TESTS::
sage: QS5.monomial_from_smaller_permutation([5,3,4,1,2]).parent() Symmetric group algebra of order 5 over Rational Field """
def antipode(self, x): r""" Return the image of the element ``x`` of ``self`` under the antipode of the Hopf algebra ``self`` (where the comultiplication is the usual one on a group algebra).
Explicitly, this is obtained by replacing each permutation `\sigma` by `\sigma^{-1}` in ``x`` while keeping all coefficients as they are.
EXAMPLES::
sage: QS4 = SymmetricGroupAlgebra(QQ, 4) sage: QS4.antipode(2 * QS4([1, 3, 4, 2]) - 1/2 * QS4([1, 4, 2, 3])) -1/2*[1, 3, 4, 2] + 2*[1, 4, 2, 3] sage: all( QS4.antipode(QS4(p)) == QS4(p.inverse()) ....: for p in Permutations(4) ) True
sage: ZS3 = SymmetricGroupAlgebra(ZZ, 3) sage: ZS3.antipode(ZS3.zero()) 0 sage: ZS3.antipode(-ZS3(Permutation([2, 3, 1]))) -[3, 1, 2] """ (p, coeff) in self(x)], distinct=True)
def retract_plain(self, f, m): r""" Return the plain retract of the element `f \in R S_n` to `R S_m`, where `m \leq n` (and where `R S_n` is ``self``).
If `m` is a nonnegative integer less or equal to `n`, then the plain retract from `S_n` to `S_m` is defined as an `R`-linear map `S_n \to S_m` which sends every permutation `p \in S_n` to
.. MATH::
\begin{cases} \mbox{pret}(p) &\mbox{if } \mbox{pret}(p)\mbox{ is defined;} \\ 0 & \mbox{otherwise} \end{cases}.
Here `\mbox{pret}(p)` denotes the plain retract of the permutation `p` to `S_m`, which is defined in :meth:`~sage.combinat.permutation.Permutation.retract_plain`.
EXAMPLES::
sage: SGA3 = SymmetricGroupAlgebra(QQ, 3) sage: SGA3.retract_plain(2*SGA3([1,2,3]) - 4*SGA3([2,1,3]) + 7*SGA3([1,3,2]), 2) 2*[1, 2] - 4*[2, 1] sage: SGA3.retract_plain(2*SGA3([1,3,2]) - 5*SGA3([2,3,1]), 2) 0
sage: SGA5 = SymmetricGroupAlgebra(QQ, 5) sage: SGA5.retract_plain(8*SGA5([1,4,2,5,3]) - 6*SGA5([1,3,2,5,4]) + 11*SGA5([3,2,1,4,5]), 4) 11*[3, 2, 1, 4] sage: SGA5.retract_plain(8*SGA5([1,4,2,5,3]) - 6*SGA5([1,3,2,5,4]) + 11*SGA5([3,2,1,4,5]), 3) 11*[3, 2, 1] sage: SGA5.retract_plain(8*SGA5([1,4,2,5,3]) - 6*SGA5([1,3,2,5,4]) + 11*SGA5([3,2,1,4,5]), 2) 0 sage: SGA5.retract_plain(8*SGA5([1,4,2,5,3]) - 6*SGA5([1,3,2,5,4]) + 11*SGA5([3,2,1,4,5]), 1) 0
sage: SGA5.retract_plain(8*SGA5([1,2,3,4,5]) - 6*SGA5([1,3,2,4,5]), 3) 8*[1, 2, 3] - 6*[1, 3, 2] sage: SGA5.retract_plain(8*SGA5([1,2,3,4,5]) - 6*SGA5([1,3,2,4,5]), 1) 8*[1] sage: SGA5.retract_plain(8*SGA5([1,2,3,4,5]) - 6*SGA5([1,3,2,4,5]), 0) 8*[]
TESTS:
Check this works with other indexing sets::
sage: G = SymmetricGroup(4).algebra(QQ) sage: G.retract_plain(G.an_element(), 3) () + 2*(1,2)
.. SEEALSO::
:meth:`retract_direct_product`, :meth:`retract_okounkov_vershik` """
def retract_direct_product(self, f, m): r""" Return the direct-product retract of the element `f \in R S_n` to `R S_m`, where `m \leq n` (and where `R S_n` is ``self``).
If `m` is a nonnegative integer less or equal to `n`, then the direct-product retract from `S_n` to `S_m` is defined as an `R`-linear map `S_n \to S_m` which sends every permutation `p \in S_n` to
.. MATH::
\begin{cases} \mbox{dret}(p) &\mbox{if } \mbox{dret}(p)\mbox{ is defined;} \\ 0 & \mbox{otherwise} \end{cases}.
Here `\mbox{dret}(p)` denotes the direct-product retract of the permutation `p` to `S_m`, which is defined in :meth:`~sage.combinat.permutation.Permutation.retract_direct_product`.
EXAMPLES::
sage: SGA3 = SymmetricGroupAlgebra(QQ, 3) sage: SGA3.retract_direct_product(2*SGA3([1,2,3]) - 4*SGA3([2,1,3]) + 7*SGA3([1,3,2]), 2) 2*[1, 2] - 4*[2, 1] sage: SGA3.retract_direct_product(2*SGA3([1,3,2]) - 5*SGA3([2,3,1]), 2) 0
sage: SGA5 = SymmetricGroupAlgebra(QQ, 5) sage: SGA5.retract_direct_product(8*SGA5([1,4,2,5,3]) - 6*SGA5([1,3,2,5,4]) + 11*SGA5([3,2,1,4,5]), 4) 11*[3, 2, 1, 4] sage: SGA5.retract_direct_product(8*SGA5([1,4,2,5,3]) - 6*SGA5([1,3,2,5,4]) + 11*SGA5([3,2,1,4,5]), 3) -6*[1, 3, 2] + 11*[3, 2, 1] sage: SGA5.retract_direct_product(8*SGA5([1,4,2,5,3]) - 6*SGA5([1,3,2,5,4]) + 11*SGA5([3,2,1,4,5]), 2) 0 sage: SGA5.retract_direct_product(8*SGA5([1,4,2,5,3]) - 6*SGA5([1,3,2,5,4]) + 11*SGA5([3,2,1,4,5]), 1) 2*[1]
sage: SGA5.retract_direct_product(8*SGA5([1,2,3,4,5]) - 6*SGA5([1,3,2,4,5]), 3) 8*[1, 2, 3] - 6*[1, 3, 2] sage: SGA5.retract_direct_product(8*SGA5([1,2,3,4,5]) - 6*SGA5([1,3,2,4,5]), 1) 2*[1] sage: SGA5.retract_direct_product(8*SGA5([1,2,3,4,5]) - 6*SGA5([1,3,2,4,5]), 0) 2*[]
TESTS:
Check this works with other indexing sets::
sage: G = SymmetricGroup(4).algebra(QQ) sage: G.retract_direct_product(G.an_element(), 3) () + 2*(1,2)
.. SEEALSO::
:meth:`retract_plain`, :meth:`retract_okounkov_vershik` """ else:
def retract_okounkov_vershik(self, f, m): r""" Return the Okounkov-Vershik retract of the element `f \in R S_n` to `R S_m`, where `m \leq n` (and where `R S_n` is ``self``).
If `m` is a nonnegative integer less or equal to `n`, then the Okounkov-Vershik retract from `S_n` to `S_m` is defined as an `R`-linear map `S_n \to S_m` which sends every permutation `p \in S_n` to the Okounkov-Vershik retract of the permutation `p` to `S_m`, which is defined in :meth:`~sage.combinat.permutation.Permutation.retract_okounkov_vershik`.
EXAMPLES::
sage: SGA3 = SymmetricGroupAlgebra(QQ, 3) sage: SGA3.retract_okounkov_vershik(2*SGA3([1,2,3]) - 4*SGA3([2,1,3]) + 7*SGA3([1,3,2]), 2) 9*[1, 2] - 4*[2, 1] sage: SGA3.retract_okounkov_vershik(2*SGA3([1,3,2]) - 5*SGA3([2,3,1]), 2) 2*[1, 2] - 5*[2, 1]
sage: SGA5 = SymmetricGroupAlgebra(QQ, 5) sage: SGA5.retract_okounkov_vershik(8*SGA5([1,4,2,5,3]) - 6*SGA5([1,3,2,5,4]) + 11*SGA5([3,2,1,4,5]), 4) -6*[1, 3, 2, 4] + 8*[1, 4, 2, 3] + 11*[3, 2, 1, 4] sage: SGA5.retract_okounkov_vershik(8*SGA5([1,4,2,5,3]) - 6*SGA5([1,3,2,5,4]) + 11*SGA5([3,2,1,4,5]), 3) 2*[1, 3, 2] + 11*[3, 2, 1] sage: SGA5.retract_okounkov_vershik(8*SGA5([1,4,2,5,3]) - 6*SGA5([1,3,2,5,4]) + 11*SGA5([3,2,1,4,5]), 2) 13*[1, 2] sage: SGA5.retract_okounkov_vershik(8*SGA5([1,4,2,5,3]) - 6*SGA5([1,3,2,5,4]) + 11*SGA5([3,2,1,4,5]), 1) 13*[1]
sage: SGA5.retract_okounkov_vershik(8*SGA5([1,2,3,4,5]) - 6*SGA5([1,3,2,4,5]), 3) 8*[1, 2, 3] - 6*[1, 3, 2] sage: SGA5.retract_okounkov_vershik(8*SGA5([1,2,3,4,5]) - 6*SGA5([1,3,2,4,5]), 1) 2*[1] sage: SGA5.retract_okounkov_vershik(8*SGA5([1,2,3,4,5]) - 6*SGA5([1,3,2,4,5]), 0) 2*[]
TESTS:
Check this works with other indexing sets::
sage: G = SymmetricGroup(4).algebra(QQ) sage: G.retract_okounkov_vershik(G.an_element(), 3) () + 2*(1,2) + 4*(1,2,3)
.. SEEALSO::
:meth:`retract_plain`, :meth:`retract_direct_product` """ else:
def cpis(self): """ Return a list of the centrally primitive idempotents of ``self``.
EXAMPLES::
sage: QS3 = SymmetricGroupAlgebra(QQ,3) sage: a = QS3.cpis() sage: a[0] # [3] 1/6*[1, 2, 3] + 1/6*[1, 3, 2] + 1/6*[2, 1, 3] + 1/6*[2, 3, 1] + 1/6*[3, 1, 2] + 1/6*[3, 2, 1] sage: a[1] # [2, 1] 2/3*[1, 2, 3] - 1/3*[2, 3, 1] - 1/3*[3, 1, 2]
TESTS:
Check this works with other indexing sets::
sage: G = SymmetricGroup(3).algebra(QQ) sage: a = G.cpis() sage: a[0] 1/6*() + 1/6*(2,3) + 1/6*(1,2) + 1/6*(1,2,3) + 1/6*(1,3,2) + 1/6*(1,3) sage: a[1] 2/3*() - 1/3*(1,2,3) - 1/3*(1,3,2) """
def cpi(self, p): """ Return the centrally primitive idempotent for the symmetric group of order `n` corresponding to the irreducible representation indexed by the partition ``p``.
EXAMPLES::
sage: QS3 = SymmetricGroupAlgebra(QQ,3) sage: QS3.cpi([2,1]) 2/3*[1, 2, 3] - 1/3*[2, 3, 1] - 1/3*[3, 1, 2] sage: QS3.cpi([3]) 1/6*[1, 2, 3] + 1/6*[1, 3, 2] + 1/6*[2, 1, 3] + 1/6*[2, 3, 1] + 1/6*[3, 1, 2] + 1/6*[3, 2, 1] sage: QS3.cpi([1,1,1]) 1/6*[1, 2, 3] - 1/6*[1, 3, 2] - 1/6*[2, 1, 3] + 1/6*[2, 3, 1] + 1/6*[3, 1, 2] - 1/6*[3, 2, 1]
sage: QS0 = SymmetricGroupAlgebra(QQ, 0) sage: QS0.cpi(Partition([])) []
TESTS::
sage: QS3.cpi([2,2]) Traceback (most recent call last): ... TypeError: p (= [2, 2]) must be a partition of n (= 3) """
for g in P }
@cached_method def algebra_generators(self): r""" Return generators of this group algebra (as algebra) as a list of permutations.
The generators used for the group algebra of `S_n` are the transposition `(2, 1)` and the `n`-cycle `(1, 2, \ldots, n)`, unless `n \leq 1` (in which case no generators are needed).
EXAMPLES::
sage: SymmetricGroupAlgebra(ZZ,5).algebra_generators() Family ([2, 1, 3, 4, 5], [2, 3, 4, 5, 1])
sage: SymmetricGroupAlgebra(QQ,0).algebra_generators() Family ()
sage: SymmetricGroupAlgebra(QQ,1).algebra_generators() Family ()
TESTS:
Check that :trac:`15309` is fixed::
sage: S3 = SymmetricGroupAlgebra(QQ, 3) sage: S3.algebra_generators() Family ([2, 1, 3], [2, 3, 1]) sage: C = CombinatorialFreeModule(ZZ, ZZ) sage: M = C.module_morphism(lambda x: S3.zero(), codomain=S3) sage: M.register_as_coercion() """
def _conjugacy_classes_representatives_underlying_group(self): r""" Return a complete list of representatives of conjugacy classes of the underlying symmetric group.
.. WARNING::
This currently is only implemented when ``self`` is built using the index set ``Permutations(n)``.
EXAMPLES::
sage: SG=SymmetricGroupAlgebra(ZZ,3) sage: SG._conjugacy_classes_representatives_underlying_group() [[2, 3, 1], [2, 1, 3], [1, 2, 3]]
sage: SGg = SymmetricGroup(3).algebra(ZZ) sage: SGg._conjugacy_classes_representatives_underlying_group() # not tested [(1,2,3), (1,2), ()] """
def rsw_shuffling_element(self, k): r""" Return the `k`-th Reiner-Saliola-Welker shuffling element in the group algebra ``self``.
The `k`-th Reiner-Saliola-Welker shuffling element in the symmetric group algebra `R S_n` over a ring `R` is defined as the sum `\sum_{\sigma \in S_n} \mathrm{noninv}_k(\sigma) \cdot \sigma`, where for every permutation `\sigma`, the number `\mathrm{noninv}_k(\sigma)` is the number of all `k`-noninversions of `\sigma` (that is, the number of all `k`-element subsets of `\{ 1, 2, \ldots, n \}` on which `\sigma` restricts to a strictly increasing map). See :meth:`sage.combinat.permutation.number_of_noninversions` for the `\mathrm{noninv}` map.
This element is more or less the operator `\nu_{k, 1^{n-k}}` introduced in [RSW2011]_; more precisely, `\nu_{k, 1^{n-k}}` is the left multiplication by this element.
It is a nontrivial theorem (Theorem 1.1 in [RSW2011]_) that the operators `\nu_{k, 1^{n-k}}` (for fixed `n` and varying `k`) pairwise commute. It is a conjecture (Conjecture 1.2 in [RSW2011]_) that all their eigenvalues are integers (which, in light of their commutativity and easily established symmetry, yields that they can be simultaneously diagonalized over `\QQ` with only integer eigenvalues).
EXAMPLES:
The Reiner-Saliola-Welker shuffling elements on `\QQ S_3`::
sage: QS3 = SymmetricGroupAlgebra(QQ, 3) sage: QS3.rsw_shuffling_element(0) [1, 2, 3] + [1, 3, 2] + [2, 1, 3] + [2, 3, 1] + [3, 1, 2] + [3, 2, 1] sage: QS3.rsw_shuffling_element(1) 3*[1, 2, 3] + 3*[1, 3, 2] + 3*[2, 1, 3] + 3*[2, 3, 1] + 3*[3, 1, 2] + 3*[3, 2, 1] sage: QS3.rsw_shuffling_element(2) 3*[1, 2, 3] + 2*[1, 3, 2] + 2*[2, 1, 3] + [2, 3, 1] + [3, 1, 2] sage: QS3.rsw_shuffling_element(3) [1, 2, 3] sage: QS3.rsw_shuffling_element(4) 0
Checking the commutativity of Reiner-Saliola-Welker shuffling elements (we leave out the ones for which it is trivial)::
sage: def test_rsw_comm(n): ....: QSn = SymmetricGroupAlgebra(QQ, n) ....: rsws = [QSn.rsw_shuffling_element(k) for k in range(2, n)] ....: return all( all( rsws[i] * rsws[j] == rsws[j] * rsws[i] ....: for j in range(i) ) ....: for i in range(len(rsws)) ) sage: test_rsw_comm(3) True sage: test_rsw_comm(4) True sage: test_rsw_comm(5) # long time True
.. NOTE::
For large ``k`` (relative to ``n``), it might be faster to call ``QSn.left_action_product(QSn.semi_rsw_element(k), QSn.antipode(binary_unshuffle_sum(k)))`` than ``QSn.rsw_shuffling_element(n)``.
.. SEEALSO::
:meth:`semi_rsw_element`, :meth:`binary_unshuffle_sum` """ distinct=True)
def semi_rsw_element(self, k): r""" Return the `k`-th semi-RSW element in the group algebra ``self``.
The `k`-th semi-RSW element in the symmetric group algebra `R S_n` over a ring `R` is defined as the sum of all permutations `\sigma \in S_n` satisfying `\sigma(1) < \sigma(2) < \cdots < \sigma(k)`.
This element has the property that, if it is denoted by `s_k`, then `s_k S(s_k)` is `(n-k)!` times the `k`-th Reiner-Saliola-Welker shuffling element of `R S_n` (see :meth:`rsw_shuffling_element`). Here, `S` denotes the antipode of the group algebra `R S_n`.
The `k`-th semi-RSW element is the image of the complete non-commutative symmetric function `S^{(k, 1^{n-k})}` in the ring of non-commutative symmetric functions under the canonical projection on the symmetric group algebra (through the descent algebra).
EXAMPLES:
The semi-RSW elements on `\QQ S_3`::
sage: QS3 = SymmetricGroupAlgebra(QQ, 3) sage: QS3.semi_rsw_element(0) [1, 2, 3] + [1, 3, 2] + [2, 1, 3] + [2, 3, 1] + [3, 1, 2] + [3, 2, 1] sage: QS3.semi_rsw_element(1) [1, 2, 3] + [1, 3, 2] + [2, 1, 3] + [2, 3, 1] + [3, 1, 2] + [3, 2, 1] sage: QS3.semi_rsw_element(2) [1, 2, 3] + [1, 3, 2] + [2, 3, 1] sage: QS3.semi_rsw_element(3) [1, 2, 3] sage: QS3.semi_rsw_element(4) 0
Let us check the relation with the `k`-th Reiner-Saliola-Welker shuffling element stated in the docstring::
sage: def test_rsw(n): ....: ZSn = SymmetricGroupAlgebra(ZZ, n) ....: for k in range(1, n): ....: a = ZSn.semi_rsw_element(k) ....: b = ZSn.left_action_product(a, ZSn.antipode(a)) ....: if factorial(n-k) * ZSn.rsw_shuffling_element(k) != b: ....: return False ....: return True sage: test_rsw(3) True sage: test_rsw(4) True sage: test_rsw(5) # long time True
Let us also check the statement about the complete non-commutative symmetric function::
sage: def test_rsw_ncsf(n): ....: ZSn = SymmetricGroupAlgebra(ZZ, n) ....: NSym = NonCommutativeSymmetricFunctions(ZZ) ....: S = NSym.S() ....: for k in range(1, n): ....: a = S(Composition([k] + [1]*(n-k))).to_symmetric_group_algebra() ....: if a != ZSn.semi_rsw_element(k): ....: return False ....: return True sage: test_rsw_ncsf(3) True sage: test_rsw_ncsf(4) True sage: test_rsw_ncsf(5) # long time True """ for q in itertools.permutations(range(1, n+1), n-k)])
def binary_unshuffle_sum(self, k): r""" Return the `k`-th binary unshuffle sum in the group algebra ``self``.
The `k`-th binary unshuffle sum in the symmetric group algebra `R S_n` over a ring `R` is defined as the sum of all permutations `\sigma \in S_n` satisfying `\sigma(1) < \sigma(2) < \cdots < \sigma(k)` and `\sigma(k+1) < \sigma(k+2) < \cdots < \sigma(n)`.
This element has the property that, if it is denoted by `t_k`, and if the `k`-th semi-RSW element (see :meth:`semi_rsw_element`) is denoted by `s_k`, then `s_k S(t_k)` and `t_k S(s_k)` both equal the `k`-th Reiner-Saliola-Welker shuffling element of `R S_n` (see :meth:`rsw_shuffling_element`).
The `k`-th binary unshuffle sum is the image of the complete non-commutative symmetric function `S^{(k, n-k)}` in the ring of non-commutative symmetric functions under the canonical projection on the symmetric group algebra (through the descent algebra).
EXAMPLES:
The binary unshuffle sums on `\QQ S_3`::
sage: QS3 = SymmetricGroupAlgebra(QQ, 3) sage: QS3.binary_unshuffle_sum(0) [1, 2, 3] sage: QS3.binary_unshuffle_sum(1) [1, 2, 3] + [2, 1, 3] + [3, 1, 2] sage: QS3.binary_unshuffle_sum(2) [1, 2, 3] + [1, 3, 2] + [2, 3, 1] sage: QS3.binary_unshuffle_sum(3) [1, 2, 3] sage: QS3.binary_unshuffle_sum(4) 0
Let us check the relation with the `k`-th Reiner-Saliola-Welker shuffling element stated in the docstring::
sage: def test_rsw(n): ....: ZSn = SymmetricGroupAlgebra(ZZ, n) ....: for k in range(1, n): ....: a = ZSn.semi_rsw_element(k) ....: b = ZSn.binary_unshuffle_sum(k) ....: c = ZSn.left_action_product(a, ZSn.antipode(b)) ....: d = ZSn.left_action_product(b, ZSn.antipode(a)) ....: e = ZSn.rsw_shuffling_element(k) ....: if c != e or d != e: ....: return False ....: return True sage: test_rsw(3) True sage: test_rsw(4) # long time True sage: test_rsw(5) # long time True
Let us also check the statement about the complete non-commutative symmetric function::
sage: def test_rsw_ncsf(n): ....: ZSn = SymmetricGroupAlgebra(ZZ, n) ....: NSym = NonCommutativeSymmetricFunctions(ZZ) ....: S = NSym.S() ....: for k in range(1, n): ....: a = S(Composition([k, n-k])).to_symmetric_group_algebra() ....: if a != ZSn.binary_unshuffle_sum(k): ....: return False ....: return True sage: test_rsw_ncsf(3) True sage: test_rsw_ncsf(4) True sage: test_rsw_ncsf(5) # long time True """ for q in itertools.combinations(range(1, n+1), k)])
def jucys_murphy(self, k): r""" Return the Jucys-Murphy element `J_k` (also known as a Young-Jucys-Murphy element) for the symmetric group algebra ``self``.
The Jucys-Murphy element `J_k` in the symmetric group algebra `R S_n` is defined for every `k \in \{ 1, 2, \ldots, n \}` by
.. MATH::
J_k = (1, k) + (2, k) + \cdots + (k-1, k) \in R S_n,
where the addends are transpositions in `S_n` (regarded as elements of `R S_n`). We note that there is not a dependence on `n`, so it is often surpressed in the notation.
EXAMPLES::
sage: QS3 = SymmetricGroupAlgebra(QQ, 3) sage: QS3.jucys_murphy(1) 0 sage: QS3.jucys_murphy(2) [2, 1, 3] sage: QS3.jucys_murphy(3) [1, 3, 2] + [3, 2, 1]
sage: QS4 = SymmetricGroupAlgebra(QQ, 4) sage: j3 = QS4.jucys_murphy(3); j3 [1, 3, 2, 4] + [3, 2, 1, 4] sage: j4 = QS4.jucys_murphy(4); j4 [1, 2, 4, 3] + [1, 4, 3, 2] + [4, 2, 3, 1] sage: j3*j4 == j4*j3 True
sage: QS5 = SymmetricGroupAlgebra(QQ, 5) sage: QS5.jucys_murphy(4) [1, 2, 4, 3, 5] + [1, 4, 3, 2, 5] + [4, 2, 3, 1, 5]
TESTS::
sage: QS3.jucys_murphy(4) Traceback (most recent call last): ... ValueError: k (= 4) must be between 1 and n (= 3) (inclusive) """
def seminormal_basis(self, mult='l2r'): r""" Return a list of the seminormal basis elements of ``self``.
The seminormal basis of a symmetric group algebra is defined as follows:
Let `n` be a nonnegative integer. Let `R` be a `\QQ`-algebra. In the following, we will use the "left action" convention for multiplying permutations. This means that for all permutations `p` and `q` in `S_n`, the product `pq` is defined in such a way that `(pq)(i) = p(q(i))` for each `i \in \{ 1, 2, \ldots, n \}` (this is the same convention as in :meth:`left_action_product`, but not the default semantics of the `*` operator on permutations in Sage). Thus, for instance, `s_2 s_1` is the permutation obtained by first transposing `1` with `2` and then transposing `2` with `3` (where `s_i = (i, i+1)`).
For every partition `\lambda` of `n`, let
.. MATH::
\kappa_{\lambda} = \frac{n!}{f^{\lambda}}
where `f^{\lambda}` is the number of standard Young tableaux of shape `\lambda`. Note that `\kappa_{\lambda}` is an integer, namely the product of all hook lengths of `\lambda` (by the hook length formula). In Sage, this integer can be computed by using :func:`sage.combinat.symmetric_group_algebra.kappa()`.
Let `T` be a standard tableau of size `n`.
Let `a(T)` denote the formal sum (in `R S_n`) of all permutations in `S_n` which stabilize the rows of `T` (as sets), i. e., which map each entry `i` of `T` to an entry in the same row as `i`. (See :func:`sage.combinat.symmetric_group_algebra.a()` for an implementation of this.)
Let `b(T)` denote the signed formal sum (in `R S_n`) of all permutations in `S_n` which stabilize the columns of `T` (as sets). Here, "signed" means that each permutation is multiplied with its sign. (This is implemented in :func:`sage.combinat.symmetric_group_algebra.b()`.)
Define an element `e(T)` of `R S_n` to be `a(T) b(T)`. (This is implemented in :func:`sage.combinat.symmetric_group_algebra.e()` for `R = \QQ`.)
Let `\mathrm{sh}(T)` denote the shape of `T`. (See :meth:`~sage.combinat.tableau.Tableau.shape`.)
Let `\overline{T}` denote the standard tableau of size `n-1` obtained by removing the letter `n` (along with its cell) from `T` (if `n \geq 1`).
Now, we define an element `\epsilon(T)` of `R S_n`. We define it by induction on the size `n` of `T`, so we set `\epsilon(\emptyset) = 1` and only need to define `\epsilon(T)` for `n \geq 1`, assuming that `\epsilon(\overline{T})` is already defined. We do this by setting
.. MATH::
\epsilon(T) = \frac{1}{\kappa_{\mathrm{sh}(T)}} \epsilon(\overline{T}) e(T) \epsilon(\overline{T}).
This element `\epsilon(T)` is implemented as :func:`sage.combinat.symmetric_group_algebra.epsilon` for `R = \QQ`, but it is also a particular case of the elements `\epsilon(T, S)` defined below.
Now let `S` be a further tableau of the same shape as `T` (possibly equal to `T`). Let `\pi_{T, S}` denote the permutation in `S_n` such that applying this permutation to the entries of `T` yields the tableau `S`. Define an element `\epsilon(T, S)` of `R S_n` by
.. MATH::
\epsilon(T, S) = \frac{1}{\kappa_{\mathrm{sh}(T)}} \epsilon(\overline S) \pi_{T, S} e(T) \epsilon(\overline T) = \frac{1}{\kappa_{\mathrm{sh}(T)}} \epsilon(\overline S) a(S) \pi_{T, S} b(T) \epsilon(\overline T).
This element `\epsilon(T, S)` is called *Young's seminormal unit corresponding to the bitableau `(T, S)`*, and is the return value of :meth:`epsilon_ik` applied to ``T`` and ``S``. Note that `\epsilon(T, T) = \epsilon(T)`.
If we let `\lambda` run through all partitions of `n`, and `(T, S)` run through all pairs of tableaux of shape `\lambda`, then the elements `\epsilon(T, S)` form a basis of `R S_n`. This basis is called *Young's seminormal basis* and has the properties that
.. MATH::
\epsilon(T, S) \epsilon(U, V) = \delta_{T, V} \epsilon(U, S)
(where `\delta` stands for the Kronecker delta).
.. WARNING::
Because of our convention, we are multiplying our elements in reverse of those given in some papers, for example [Ram1997]_. Using the other convention of multiplying permutations, we would instead have `\epsilon(U, V) \epsilon(T, S) = \delta_{T, V} \epsilon(U, S)`.
In other words, Young's seminormal basis consists of the matrix units in a (particular) Artin-Wedderburn decomposition of `R S_n` into a direct product of matrix algebras over `\QQ`.
The output of :meth:`seminormal_basis` is a list of all elements of the seminormal basis of ``self``.
INPUT:
- ``mult`` -- string (default: ``'l2r'``). If set to ``'r2l'``, this causes the method to return the list of the antipodes (:meth:`antipode`) of all `\epsilon(T, S)` instead of the `\epsilon(T, S)` themselves.
EXAMPLES::
sage: QS3 = SymmetricGroupAlgebra(QQ,3) sage: QS3.seminormal_basis() [1/6*[1, 2, 3] + 1/6*[1, 3, 2] + 1/6*[2, 1, 3] + 1/6*[2, 3, 1] + 1/6*[3, 1, 2] + 1/6*[3, 2, 1], 1/3*[1, 2, 3] + 1/6*[1, 3, 2] - 1/3*[2, 1, 3] - 1/6*[2, 3, 1] - 1/6*[3, 1, 2] + 1/6*[3, 2, 1], 1/3*[1, 3, 2] + 1/3*[2, 3, 1] - 1/3*[3, 1, 2] - 1/3*[3, 2, 1], 1/4*[1, 3, 2] - 1/4*[2, 3, 1] + 1/4*[3, 1, 2] - 1/4*[3, 2, 1], 1/3*[1, 2, 3] - 1/6*[1, 3, 2] + 1/3*[2, 1, 3] - 1/6*[2, 3, 1] - 1/6*[3, 1, 2] - 1/6*[3, 2, 1], 1/6*[1, 2, 3] - 1/6*[1, 3, 2] - 1/6*[2, 1, 3] + 1/6*[2, 3, 1] + 1/6*[3, 1, 2] - 1/6*[3, 2, 1]]
TESTS::
sage: QS3g = SymmetricGroup(3).algebra(QQ) sage: QS3g.seminormal_basis() [1/6*() + 1/6*(2,3) + 1/6*(1,2) + 1/6*(1,2,3) + 1/6*(1,3,2) + 1/6*(1,3), 1/3*() + 1/6*(2,3) - 1/3*(1,2) - 1/6*(1,2,3) - 1/6*(1,3,2) + 1/6*(1,3), 1/3*(2,3) + 1/3*(1,2,3) - 1/3*(1,3,2) - 1/3*(1,3), 1/4*(2,3) - 1/4*(1,2,3) + 1/4*(1,3,2) - 1/4*(1,3), 1/3*() - 1/6*(2,3) + 1/3*(1,2) - 1/6*(1,2,3) - 1/6*(1,3,2) - 1/6*(1,3), 1/6*() - 1/6*(2,3) - 1/6*(1,2) + 1/6*(1,2,3) + 1/6*(1,3,2) - 1/6*(1,3)]
REFERENCES:
.. [Ram1997] Arun Ram. *Seminormal representations of Weyl groups and Iwahori-Hecke algebras*. Proc. London Math. Soc. (3) **75** (1997). 99-133. :arxiv:`math/9511223v1`. http://www.ms.unimelb.edu.au/~ram/Publications/1997PLMSv75p99.pdf """
def dft(self, form="seminormal", mult='l2r'): """ Return the discrete Fourier transform for ``self``.
INPUT:
- ``mult`` -- string (default: `l2r`). If set to `r2l`, this causes the method to use the antipodes (:meth:`antipode`) of the seminormal basis instead of the seminormal basis.
EXAMPLES::
sage: QS3 = SymmetricGroupAlgebra(QQ, 3) sage: QS3.dft() [ 1 1 1 1 1 1] [ 1 1/2 -1 -1/2 -1/2 1/2] [ 0 3/4 0 3/4 -3/4 -3/4] [ 0 1 0 -1 1 -1] [ 1 -1/2 1 -1/2 -1/2 -1/2] [ 1 -1 -1 1 1 -1] """ else: raise ValueError("invalid form (= %s)"%form)
def _dft_seminormal(self, mult='l2r'): """ Return the seminormal form of the discrete Fourier for ``self``.
INPUT:
- ``mult`` -- string (default: `l2r`). If set to `r2l`, this causes the method to use the antipodes (:meth:`antipode`) of the seminormal basis instead of the seminormal basis.
EXAMPLES::
sage: QS3 = SymmetricGroupAlgebra(QQ, 3) sage: QS3._dft_seminormal() [ 1 1 1 1 1 1] [ 1 1/2 -1 -1/2 -1/2 1/2] [ 0 3/4 0 3/4 -3/4 -3/4] [ 0 1 0 -1 1 -1] [ 1 -1/2 1 -1/2 -1/2 -1/2] [ 1 -1 -1 1 1 -1]
.. SEEALSO::
:meth:`seminormal_basis` """
def epsilon_ik(self, itab, ktab, star=0, mult='l2r'): r""" Return the seminormal basis element of ``self`` corresponding to the pair of tableaux ``itab`` and ``ktab`` (or restrictions of these tableaux, if the optional variable ``star`` is set).
INPUT:
- ``itab``, ``ktab`` -- two standard tableaux of size `n`.
- ``star`` -- integer (default: `0`).
- ``mult`` -- string (default: `l2r`). If set to `r2l`, this causes the method to return the antipode (:meth:`antipode`) of `\epsilon(I, K)` instead of `\epsilon(I, K)` itself.
OUTPUT:
The element `\epsilon(I, K)`, where `I` and `K` are the tableaux obtained by removing all entries higher than `n - \mathrm{star}` from ``itab`` and ``ktab``, respectively. Here, we are using the notations from :meth:`seminormal_basis`.
EXAMPLES::
sage: QS3 = SymmetricGroupAlgebra(QQ, 3) sage: a = QS3.epsilon_ik([[1,2,3]], [[1,2,3]]); a 1/6*[1, 2, 3] + 1/6*[1, 3, 2] + 1/6*[2, 1, 3] + 1/6*[2, 3, 1] + 1/6*[3, 1, 2] + 1/6*[3, 2, 1] sage: QS3.dft()*vector(a) (1, 0, 0, 0, 0, 0) sage: a = QS3.epsilon_ik([[1,2],[3]], [[1,2],[3]]); a 1/3*[1, 2, 3] - 1/6*[1, 3, 2] + 1/3*[2, 1, 3] - 1/6*[2, 3, 1] - 1/6*[3, 1, 2] - 1/6*[3, 2, 1] sage: QS3.dft()*vector(a) (0, 0, 0, 0, 1, 0)
Let us take some properties of the seminormal basis listed in the docstring of :meth:`seminormal_basis`, and verify them on the situation of `S_3`.
First, check the formula
.. MATH::
\epsilon(T) = \frac{1}{\kappa_{\mathrm{sh}(T)}} \epsilon(\overline{T}) e(T) \epsilon(\overline{T}).
In fact::
sage: from sage.combinat.symmetric_group_algebra import e sage: def test_sn1(n): ....: QSn = SymmetricGroupAlgebra(QQ, n) ....: QSn1 = SymmetricGroupAlgebra(QQ, n - 1) ....: for T in StandardTableaux(n): ....: TT = T.restrict(n-1) ....: eTT = QSn1.epsilon_ik(TT, TT) ....: eT = QSn.epsilon_ik(T, T) ....: kT = prod(T.shape().hooks()) ....: if kT * eT != eTT * e(T) * eTT: ....: return False ....: return True sage: test_sn1(3) True sage: test_sn1(4) # long time True
Next, we check the identity
.. MATH::
\epsilon(T, S) = \frac{1}{\kappa_{\mathrm{sh}(T)}} \epsilon(\overline S) \pi_{T, S} e(T) \epsilon(\overline T)
which we used to define `\epsilon(T, S)`. In fact::
sage: from sage.combinat.symmetric_group_algebra import e sage: def test_sn2(n): ....: QSn = SymmetricGroupAlgebra(QQ, n) ....: mul = QSn.left_action_product ....: QSn1 = SymmetricGroupAlgebra(QQ, n - 1) ....: for lam in Partitions(n): ....: k = prod(lam.hooks()) ....: for T in StandardTableaux(lam): ....: for S in StandardTableaux(lam): ....: TT = T.restrict(n-1) ....: SS = S.restrict(n-1) ....: eTT = QSn1.epsilon_ik(TT, TT) ....: eSS = QSn1.epsilon_ik(SS, SS) ....: eTS = QSn.epsilon_ik(T, S) ....: piTS = [0] * n ....: for (i, j) in T.cells(): ....: piTS[T[i][j] - 1] = S[i][j] ....: piTS = QSn(Permutation(piTS)) ....: if k * eTS != mul(mul(eSS, piTS), mul(e(T), eTT)): ....: return False ....: return True sage: test_sn2(3) True sage: test_sn2(4) # long time True
Let us finally check the identity
.. MATH::
\epsilon(T, S) \epsilon(U, V) = \delta_{T, V} \epsilon(U, S)
In fact::
sage: def test_sn3(lam): ....: n = lam.size() ....: QSn = SymmetricGroupAlgebra(QQ, n) ....: mul = QSn.left_action_product ....: for T in StandardTableaux(lam): ....: for S in StandardTableaux(lam): ....: for U in StandardTableaux(lam): ....: for V in StandardTableaux(lam): ....: lhs = mul(QSn.epsilon_ik(T, S), QSn.epsilon_ik(U, V)) ....: if T == V: ....: rhs = QSn.epsilon_ik(U, S) ....: else: ....: rhs = QSn.zero() ....: if rhs != lhs: ....: return False ....: return True sage: all( test_sn3(lam) for lam in Partitions(3) ) True sage: all( test_sn3(lam) for lam in Partitions(4) ) # long time True """
raise TypeError("it must be a standard tableau of size %s"%self.n)
raise TypeError("kt must be a standard tableau of size %s"%self.n)
raise ValueError("it and kt must be of the same shape")
else: return z.map_support(lambda x: x.inverse())
epsilon_ik_cache = {} def epsilon_ik(itab, ktab, star=0): """ Return the seminormal basis element of the symmetric group algebra `\QQ S_n` corresponding to the pair of tableaux ``itab`` and ``ktab`` (or restrictions of these tableaux, if the optional variable ``star`` is set).
INPUT:
- ``itab``, ``ktab`` -- two standard tableaux of same size.
- ``star`` -- integer (default: `0`).
OUTPUT:
The element `\epsilon(I, K) \in \QQ S_n`, where `I` and `K` are the tableaux obtained by removing all entries higher than `n - \mathrm{star}` from ``itab`` and ``ktab``, respectively (where `n` is the size of ``itab`` and ``ktab``). Here, we are using the notations from :meth:`~sage.combinat.symmetric_group_algebra.SymmetricGroupAlgebra_n.seminormal_basis`.
EXAMPLES::
sage: from sage.combinat.symmetric_group_algebra import epsilon_ik sage: epsilon_ik([[1,2],[3]], [[1,3],[2]]) 1/4*[1, 3, 2] - 1/4*[2, 3, 1] + 1/4*[3, 1, 2] - 1/4*[3, 2, 1] sage: epsilon_ik([[1,2],[3]], [[1,3],[2]], star=1) Traceback (most recent call last): ... ValueError: the two tableaux must be of the same shape """
else: epsilon(kt, star+1)) * (1/kappa(it.shape()))
epsilon_cache = {} def epsilon(tab, star=0): r""" The `(T, T)`-th element of the seminormal basis of the group algebra `\QQ[S_n]`, where `T` is the tableau ``tab`` (with its ``star`` highest entries removed if the optional variable ``star`` is set).
See the docstring of :meth:`~sage.combinat.symmetric_group_algebra.SymmetricGroupAlgebra_n.seminormal_basis` for the notation used herein.
EXAMPLES::
sage: from sage.combinat.symmetric_group_algebra import epsilon sage: epsilon([[1,2]]) 1/2*[1, 2] + 1/2*[2, 1] sage: epsilon([[1],[2]]) 1/2*[1, 2] - 1/2*[2, 1] """
else: epsilon_cache[t] = e(t) res = epsilon_cache[t] else:
def pi_ik(itab, ktab): r""" Return the permutation `p` which sends every entry of the tableau ``itab`` to the respective entry of the tableau ``ktab``, as an element of the corresponding symmetric group algebra.
This assumes that ``itab`` and ``ktab`` are tableaux (possibly given just as lists of lists) of the same shape.
EXAMPLES::
sage: from sage.combinat.symmetric_group_algebra import pi_ik sage: pi_ik([[1,3],[2]], [[1,2],[3]]) [1, 3, 2] """
def kappa(alpha): r""" Return `\kappa_\alpha`, which is `n!` divided by the number of standard tableaux of shape `\alpha` (where `\alpha` is a partition of `n`).
INPUT:
- ``alpha`` -- integer partition (can be encoded as a list).
OUTPUT:
The factorial of the size of ``alpha``, divided by the number of standard tableaux of shape ``alpha``. Equivalently, the product of all hook lengths of ``alpha``.
EXAMPLES::
sage: from sage.combinat.symmetric_group_algebra import kappa sage: kappa(Partition([2,1])) 3 sage: kappa([2,1]) 3 """
def a(tableau, star=0, base_ring=QQ): r""" The row projection operator corresponding to the Young tableau ``tableau`` (which is supposed to contain every integer from `1` to its size precisely once, but may and may not be standard).
This is the sum (in the group algebra of the relevant symmetric group over `\QQ`) of all the permutations which preserve the rows of ``tableau``. It is called `a_{\text{tableau}}` in [EtRT]_, Section 4.2.
REFERENCES:
.. [EtRT] Pavel Etingof, Oleg Golberg, Sebastian Hensel, Tiankai Liu, Alex Schwendner, Dmitry Vaintrob, Elena Yudovina, "Introduction to representation theory", :arXiv:`0901.0827v5`.
INPUT:
- ``tableau`` -- Young tableau which contains every integer from `1` to its size precisely once.
- ``star`` -- nonnegative integer (default: `0`). When this optional variable is set, the method computes not the row projection operator of ``tableau``, but the row projection operator of the restriction of ``tableau`` to the entries ``1, 2, ..., tableau.size() - star`` instead.
- ``base_ring`` -- commutative ring (default: ``QQ``). When this optional variable is set, the row projection operator is computed over a user-determined base ring instead of `\QQ`. (Note that symmetric group algebras currently don't preserve coercion, so e. g. a symmetric group algebra over `\ZZ` does not coerce into the corresponding one over `\QQ`; so convert manually or choose your base rings wisely!)
EXAMPLES::
sage: from sage.combinat.symmetric_group_algebra import a sage: a([[1,2]]) [1, 2] + [2, 1] sage: a([[1],[2]]) [1, 2] sage: a([]) [] sage: a([[1, 5], [2, 3], [4]]) [1, 2, 3, 4, 5] + [1, 3, 2, 4, 5] + [5, 2, 3, 4, 1] + [5, 3, 2, 4, 1] sage: a([[1,4], [2,3]], base_ring=ZZ) [1, 2, 3, 4] + [1, 3, 2, 4] + [4, 2, 3, 1] + [4, 3, 2, 1] """ t = t.restrict(t.size()-star)
# Ugly hack for the case of an empty tableau, due to the # annoyance of Permutation(Tableau([]).row_stabilizer()[0]) # being [1] rather than [] (which seems to have its origins in # permutation group code). # TODO: Fix this.
def b(tableau, star=0, base_ring=QQ): r""" The column projection operator corresponding to the Young tableau ``tableau`` (which is supposed to contain every integer from `1` to its size precisely once, but may and may not be standard).
This is the signed sum (in the group algebra of the relevant symmetric group over `\QQ`) of all the permutations which preserve the column of ``tableau`` (where the signs are the usual signs of the permutations). It is called `b_{\text{tableau}}` in [EtRT]_, Section 4.2.
INPUT:
- ``tableau`` -- Young tableau which contains every integer from `1` to its size precisely once.
- ``star`` -- nonnegative integer (default: `0`). When this optional variable is set, the method computes not the column projection operator of ``tableau``, but the column projection operator of the restriction of ``tableau`` to the entries ``1, 2, ..., tableau.size() - star`` instead.
- ``base_ring`` -- commutative ring (default: ``QQ``). When this optional variable is set, the column projection operator is computed over a user-determined base ring instead of `\QQ`. (Note that symmetric group algebras currently don't preserve coercion, so e. g. a symmetric group algebra over `\ZZ` does not coerce into the corresponding one over `\QQ`; so convert manually or choose your base rings wisely!)
EXAMPLES::
sage: from sage.combinat.symmetric_group_algebra import b sage: b([[1,2]]) [1, 2] sage: b([[1],[2]]) [1, 2] - [2, 1] sage: b([]) [] sage: b([[1, 2, 4], [5, 3]]) [1, 2, 3, 4, 5] - [1, 3, 2, 4, 5] - [5, 2, 3, 4, 1] + [5, 3, 2, 4, 1] sage: b([[1, 4], [2, 3]], base_ring=ZZ) [1, 2, 3, 4] - [1, 2, 4, 3] - [2, 1, 3, 4] + [2, 1, 4, 3] sage: b([[1, 4], [2, 3]], base_ring=Integers(5)) [1, 2, 3, 4] + 4*[1, 2, 4, 3] + 4*[2, 1, 3, 4] + [2, 1, 4, 3]
With the ``l2r`` setting for multiplication, the unnormalized Young symmetrizer ``e(tableau)`` should be the product ``b(tableau) * a(tableau)`` for every ``tableau``. Let us check this on the standard tableaux of size 5::
sage: from sage.combinat.symmetric_group_algebra import a, b, e sage: all( e(t) == b(t) * a(t) for t in StandardTableaux(5) ) True """ t = t.restrict(t.size()-star)
# Ugly hack for the case of an empty tableau, due to the # annoyance of Permutation(Tableau([]).row_stabilizer()[0]) # being [1] rather than [] (which seems to have its origins in # permutation group code). # TODO: Fix this.
e_cache = {} def e(tableau, star=0): r""" The unnormalized Young projection operator corresponding to the Young tableau ``tableau`` (which is supposed to contain every integer from `1` to its size precisely once, but may and may not be standard).
If `n` is a nonnegative integer, and `T` is a Young tableau containing every integer from `1` to `n` exactly once, then the unnormalized Young projection operator `e(T)` is defined by
.. MATH::
e(T) = a(T) b(T) \in \QQ S_n,
where `a(T) \in \QQ S_n` is the sum of all permutations in `S_n` which fix the rows of `T` (as sets), and `b(T) \in \QQ S_n` is the signed sum of all permutations in `S_n` which fix the columns of `T` (as sets). Here, "signed" means that each permutation is multiplied with its sign; and the product on the group `S_n` is defined in such a way that `(pq)(i) = p(q(i))` for any permutations `p` and `q` and any `1 \leq i \leq n`.
Note that the definition of `e(T)` is not uniform across literature. Others define it as `b(T) a(T)` instead, or include certain scalar factors (we do not, whence "unnormalized").
EXAMPLES::
sage: from sage.combinat.symmetric_group_algebra import e sage: e([[1,2]]) [1, 2] + [2, 1] sage: e([[1],[2]]) [1, 2] - [2, 1] sage: e([]) []
There are differing conventions for the order of the symmetrizers and antisymmetrizers. This example illustrates our conventions::
sage: e([[1,2],[3]]) [1, 2, 3] + [2, 1, 3] - [3, 1, 2] - [3, 2, 1]
To obtain the product `b(T) a(T)`, one has to take the antipode of this::
sage: QS3 = parent(e([[1,2],[3]])) sage: QS3.antipode(e([[1,2],[3]])) [1, 2, 3] + [2, 1, 3] - [2, 3, 1] - [3, 2, 1]
.. SEEALSO::
:func:`e_hat` """ # TODO: # The current method only computes the e's over QQ. There should be # a way to compute them over other base rings as well. Be careful # with the cache.
t = t.restrict(t.size()-star)
else:
# Ugly hack for the case of an empty tableau, due to the # annoyance of Permutation(Tableau([]).row_stabilizer()[0]) # being [1] rather than [] (which seems to have its origins in # permutation group code). # TODO: Fix this.
ehat_cache = {} def e_hat(tab, star=0): r""" The Young projection operator corresponding to the Young tableau ``tab`` (which is supposed to contain every integer from `1` to its size precisely once, but may and may not be standard). This is an idempotent in the rational group algebra.
If `n` is a nonnegative integer, and `T` is a Young tableau containing every integer from `1` to `n` exactly once, then the Young projection operator `\widehat{e}(T)` is defined by
.. MATH::
\widehat{e}(T) = \frac{1}{\kappa_\lambda} a(T) b(T) \in \QQ S_n,
where `\lambda` is the shape of `T`, where `\kappa_\lambda` is `n!` divided by the number of standard tableaux of shape `\lambda`, where `a(T) \in \QQ S_n` is the sum of all permutations in `S_n` which fix the rows of `T` (as sets), and where `b(T) \in \QQ S_n` is the signed sum of all permutations in `S_n` which fix the columns of `T` (as sets). Here, "signed" means that each permutation is multiplied with its sign; and the product on the group `S_n` is defined in such a way that `(pq)(i) = p(q(i))` for any permutations `p` and `q` and any `1 \leq i \leq n`.
Note that the definition of `\widehat{e}(T)` is not uniform across literature. Others define it as `\frac{1}{\kappa_\lambda} b(T) a(T)` instead.
EXAMPLES::
sage: from sage.combinat.symmetric_group_algebra import e_hat sage: e_hat([[1,2,3]]) 1/6*[1, 2, 3] + 1/6*[1, 3, 2] + 1/6*[2, 1, 3] + 1/6*[2, 3, 1] + 1/6*[3, 1, 2] + 1/6*[3, 2, 1] sage: e_hat([[1],[2]]) 1/2*[1, 2] - 1/2*[2, 1]
There are differing conventions for the order of the symmetrizers and antisymmetrizers. This example illustrates our conventions::
sage: e_hat([[1,2],[3]]) 1/3*[1, 2, 3] + 1/3*[2, 1, 3] - 1/3*[3, 1, 2] - 1/3*[3, 2, 1]
.. SEEALSO::
:func:`e` """ t = t.restrict(t.size()-star) res = ehat_cache[t] else:
e_ik_cache = {} def e_ik(itab, ktab, star=0): """ EXAMPLES::
sage: from sage.combinat.symmetric_group_algebra import e_ik sage: e_ik([[1,2,3]], [[1,2,3]]) [1, 2, 3] + [1, 3, 2] + [2, 1, 3] + [2, 3, 1] + [3, 1, 2] + [3, 2, 1] sage: e_ik([[1,2,3]], [[1,2,3]], star=1) [1, 2] + [2, 1] """
raise ValueError("the two tableaux must be of the same shape")
def seminormal_test(n): """ Run a variety of tests to verify that the construction of the seminormal basis works as desired. The numbers appearing are results in James and Kerber's 'Representation Theory of the Symmetric Group' [JamesKerber]_.
EXAMPLES::
sage: from sage.combinat.symmetric_group_algebra import seminormal_test sage: seminormal_test(3) True """ #Theorem 3.1.10 raise ValueError("3.1.10 - %s"%tab)
#Lemma 3.2.12 (ii) print(value) raise ValueError("3.2.12.2 - %s" % tab)
#3.2.8 (i) raise ValueError("3.2.8.1 - %s, %s"%(tab, tab2))
#3.2.8 (ii) raise ValueError("3.2.8.2 - %s, %s"%(tab, tab2))
#Lemma 3.1.20 raise ValueError("3.1.20 - %s, %s"%(tab, tab2)) raise ValueError("3.1.20 - %s, %s"%(tab, tab2))
#######################
def HeckeAlgebraSymmetricGroupT(R, n, q=None): r""" Return the Hecke algebra of the symmetric group `S_n` on the T-basis with quantum parameter ``q`` over the ring `R`.
If `R` is a commutative ring and `q` is an invertible element of `R`, and if `n` is a nonnegative integer, then the Hecke algebra of the symmetric group `S_n` over `R` with quantum parameter `q` is defined as the algebra generated by the generators `T_1, T_2, \ldots, T_{n-1}` with relations
.. MATH::
T_i T_{i+1} T_i = T_{i+1} T_i T_{i+1}
for all `i < n-1` ("braid relations"),
.. MATH::
T_i T_j = T_j T_i
for all `i` and `j` such that `| i-j | > 1` ("locality relations"), and
.. MATH::
T_i^2 = q + (q-1) T_i
for all `i` (the "quadratic relations", also known in the form `(T_i + 1) (T_i - q) = 0`). (This is only one of several existing definitions in literature, not all of which are fully equivalent. We are following the conventions of [GS93]_.) For any permutation `w \in S_n`, we can define an element `T_w` of this Hecke algebra by setting `T_w = T_{i_1} T_{i_2} \cdots T_{i_k}`, where `w = s_{i_1} s_{i_2} \cdots s_{i_k}` is a reduced word for `w` (with `s_i` meaning the transposition `(i, i+1)`, and the product of permutations being evaluated by first applying `s_{i_k}`, then `s_{i_{k-1}}`, etc.). This element is independent of the choice of the reduced decomposition, and can be computed in Sage by calling ``H[w]`` where ``H`` is the Hecke algebra and ``w`` is the permutation.
The Hecke algebra of the symmetric group `S_n` with quantum parameter `q` over `R` can be seen as a deformation of the group algebra `R S_n`; indeed, it becomes `R S_n` when `q = 1`.
.. WARNING::
The multiplication on the Hecke algebra of the symmetric group does *not* follow the global option ``mult`` of the :class:`Permutations` class (see :meth:`~sage.combinat.permutation.Permutations.options`). It is always as defined above. It does not match the default option (``mult=l2r``) of the symmetric group algebra!
REFERENCES:
.. [GS93] David M. Goldschmidt. *Group characters, symmetric functions, and the Hecke algebras*. AMS 1993.
EXAMPLES::
sage: HeckeAlgebraSymmetricGroupT(QQ, 3) Hecke algebra of the symmetric group of order 3 on the T basis over Univariate Polynomial Ring in q over Rational Field
::
sage: HeckeAlgebraSymmetricGroupT(QQ, 3, 2) Hecke algebra of the symmetric group of order 3 with q=2 on the T basis over Rational Field
The multiplication on the Hecke algebra follows a different convention than the one on the symmetric group algebra does by default::
sage: H3 = HeckeAlgebraSymmetricGroupT(QQ, 3) sage: H3([1,3,2]) * H3([2,1,3]) T[3, 1, 2] sage: S3 = SymmetricGroupAlgebra(QQ, 3) sage: S3([1,3,2]) * S3([2,1,3]) [2, 3, 1]
sage: TestSuite(H3).run() """
class HeckeAlgebraSymmetricGroup_generic(CombinatorialAlgebra): def __init__(self, R, n, q=None): """ TESTS::
sage: HeckeAlgebraSymmetricGroupT(QQ, 3) Hecke algebra of the symmetric group of order 3 on the T basis over Univariate Polynomial Ring in q over Rational Field
::
sage: HeckeAlgebraSymmetricGroupT(QQ, 3, q=1) Hecke algebra of the symmetric group of order 3 with q=1 on the T basis over Rational Field """
else: raise ValueError("q must be in R (= {})".format(R))
# _repr_ customization: output the basis element indexed by [1,2,3] as [1,2,3]
_repr_option_bracket = False
def q(self): """ EXAMPLES::
sage: HeckeAlgebraSymmetricGroupT(QQ, 3).q() q sage: HeckeAlgebraSymmetricGroupT(QQ, 3, 2).q() 2 """
def _coerce_start(self, x): """ EXAMPLES::
sage: H3 = HeckeAlgebraSymmetricGroupT(QQ, 3) sage: H3._coerce_start([2,1]) T[2, 1, 3] """ ################################################### # Coerce permutations of size smaller that self.n # ################################################### return self.one() list(range(len(x)+1, self.n+1)))) raise TypeError
class HeckeAlgebraSymmetricGroup_t(HeckeAlgebraSymmetricGroup_generic):
def __init__(self, R, n, q=None): """ TESTS::
sage: H3 = HeckeAlgebraSymmetricGroupT(QQ, 3) sage: H3 == loads(dumps(H3)) True """
def t_action_on_basis(self, perm, i): r""" Return the product `T_i \cdot T_{perm}`, where ``perm`` is a permutation in the symmetric group `S_n`.
EXAMPLES::
sage: H3 = HeckeAlgebraSymmetricGroupT(QQ, 3) sage: H3.t_action_on_basis(Permutation([2,1,3]), 1) q*T[1, 2, 3] + (q-1)*T[2, 1, 3] sage: H3.t_action_on_basis(Permutation([1,2,3]), 1) T[2, 1, 3] sage: H3 = HeckeAlgebraSymmetricGroupT(QQ, 3, 1) sage: H3.t_action_on_basis(Permutation([2,1,3]), 1) T[1, 2, 3] sage: H3.t_action_on_basis(Permutation([1,3,2]), 2) T[1, 2, 3] """ raise ValueError("i (= %(i)d) must be between 1 and n (= %(n)d)" % {'i': i, 'n': self.n})
# This used to be perm_i = t_i * perm. I have changed it to # perm_i = t_i.right_action_product(perm) because it would # otherwise cause TestSuite(H3) to fail when # Permutations.options(mult) would be set to "r2l". # -- Darij, 19 Nov 2013
else: #Ti^2 = (q - q^(-1))*Ti - q1*q2
def t_action(self, a, i): r""" Return the product `T_i \cdot a`.
EXAMPLES::
sage: H3 = HeckeAlgebraSymmetricGroupT(QQ, 3) sage: a = H3([2,1,3])+2*H3([1,2,3]) sage: H3.t_action(a, 1) q*T[1, 2, 3] + (q+1)*T[2, 1, 3] sage: H3.t(1)*a q*T[1, 2, 3] + (q+1)*T[2, 1, 3] """
def _multiply_basis(self, perm1, perm2): """ EXAMPLES::
sage: H3 = HeckeAlgebraSymmetricGroupT(QQ, 3, 1) sage: a = H3([2,1,3])+2*H3([1,2,3])-H3([3,2,1]) sage: a^2 #indirect doctest 6*T[1, 2, 3] + 4*T[2, 1, 3] - T[2, 3, 1] - T[3, 1, 2] - 4*T[3, 2, 1]
::
sage: QS3 = SymmetricGroupAlgebra(QQ, 3) sage: a = QS3([2,1,3])+2*QS3([1,2,3])-QS3([3,2,1]) sage: a^2 6*[1, 2, 3] + 4*[2, 1, 3] - [2, 3, 1] - [3, 1, 2] - 4*[3, 2, 1] """
def t(self, i): """ Return the element `T_i` of the Hecke algebra ``self``.
EXAMPLES::
sage: H3 = HeckeAlgebraSymmetricGroupT(QQ,3) sage: H3.t(1) T[2, 1, 3] sage: H3.t(2) T[1, 3, 2] sage: H3.t(0) Traceback (most recent call last): ... ValueError: i (= 0) must be between 1 and n-1 (= 2) """
# The permutation here is simply the transposition (i, i+1).
def algebra_generators(self): """ Return the generators of the algebra.
EXAMPLES::
sage: HeckeAlgebraSymmetricGroupT(QQ,3).algebra_generators() [T[2, 1, 3], T[1, 3, 2]] """
def jucys_murphy(self, k): """ Return the Jucys-Murphy element `J_k` of the Hecke algebra.
These Jucys-Murphy elements are defined by
.. MATH::
J_k = (T_{k-1} T_{k-2} \cdots T_1) (T_1 T_2 \cdots T_{k-1}).
More explicitly,
.. MATH::
J_k = q^{k-1} + \sum_{l=1}^{k-1} (q^l - q^{l-1}) T_{(l, k)}.
For generic `q`, the `J_k` generate a maximal commutative sub-algebra of the Hecke algebra.
.. WARNING::
The specialization `q = 1` does *not* map these elements `J_k` to the Young-Jucys-Murphy elements of the group algebra `R S_n`. (Instead, it maps the "reduced" Jucys-Murphy elements `(J_k - q^{k-1}) / (q - 1)` to the Young-Jucys-Murphy elements of `R S_n`.)
EXAMPLES::
sage: H3 = HeckeAlgebraSymmetricGroupT(QQ,3) sage: j2 = H3.jucys_murphy(2); j2 q*T[1, 2, 3] + (q-1)*T[2, 1, 3] sage: j3 = H3.jucys_murphy(3); j3 q^2*T[1, 2, 3] + (q^2-q)*T[1, 3, 2] + (q-1)*T[3, 2, 1] sage: j2*j3 == j3*j2 True sage: j0 = H3.jucys_murphy(1); j0 == H3.one() True sage: H3.jucys_murphy(0) Traceback (most recent call last): ... ValueError: k (= 0) must be between 1 and n (= 3) """
q ** l - q ** (l-1) ) for l in range(1, k) ), distinct=True )
#old algorithm: # left = 1 # right = 1 # for j in range(1, k): # left *= self.t(k-j) # right *= self.t(j) # return left*right
# For unpickling backward compatibility (Sage <= 4.1) from sage.structure.sage_object import register_unpickle_override register_unpickle_override('sage.combinat.symmetric_group_algebra', 'HeckeAlgebraSymmetricGroupElement_t', CombinatorialFreeModule.Element) register_unpickle_override('sage.combinat.symmetric_group_algebra', 'SymmetricGroupAlgebraElement_n', CombinatorialFreeModule.Element) |