# Generic code for bases¶

This is a collection of code that is shared by bases of noncommutative symmetric functions and quasisymmetric functions.

AUTHORS:

• Jason Bandlow
• Franco Saliola
• Chris Berg
class sage.combinat.ncsf_qsym.generic_basis_code.AlgebraMorphism(domain, on_generators, position=0, codomain=None, category=None, anti=False)

A class for algebra morphism defined on a free algebra from the image of the generators

class sage.combinat.ncsf_qsym.generic_basis_code.BasesOfQSymOrNCSF(parent_with_realization)

TESTS:

sage: from sage.categories.realizations import Category_realization_of_parent
sage: A = Sets().WithRealizations().example(); A
The subset algebra of {1, 2, 3} over Rational Field
sage: C = A.Realizations(); C
Category of realizations of The subset algebra of {1, 2, 3} over Rational Field
sage: isinstance(C, Category_realization_of_parent)
True
sage: C.parent_with_realization
The subset algebra of {1, 2, 3} over Rational Field
sage: TestSuite(C).run()

class ElementMethods
degree()

The maximum of the degrees of the homogeneous summands.

EXAMPLES:

sage: S = NonCommutativeSymmetricFunctions(QQ).S()
sage: (x, y) = (S[2], S[3])
sage: x.degree()
2
sage: (x^3 + 4*y^2).degree()
6
sage: ((1 + x)^3).degree()
6

sage: F = QuasiSymmetricFunctions(QQ).F()
sage: (x, y) = (F[2], F[3])
sage: x.degree()
2
sage: (x^3 + 4*y^2).degree()
6
sage: ((1 + x)^3).degree()
6


TESTS:

sage: S = NonCommutativeSymmetricFunctions(QQ).S()
sage: S.zero().degree()
Traceback (most recent call last):
...
ValueError: The zero element does not have a well-defined degree.
sage: F = QuasiSymmetricFunctions(QQ).F()
sage: F.zero().degree()
Traceback (most recent call last):
...
ValueError: The zero element does not have a well-defined degree.

duality_pairing(y)

The duality pairing between elements of NSym and elements of QSym The complete basis is dual to the monomial basis.

INPUT:

• y – an element of the dual Hopf algebra of self

OUTPUT:

• The result of pairing self with y.

EXAMPLES:

sage: R = NonCommutativeSymmetricFunctions(QQ).Ribbon()
sage: F = QuasiSymmetricFunctions(QQ).Fundamental()
sage: R[1,1,2].duality_pairing(F[1,1,2])
1
sage: R[1,2,1].duality_pairing(F[1,1,2])
0

sage: L = NonCommutativeSymmetricFunctions(QQ).Elementary()
sage: F = QuasiSymmetricFunctions(QQ).Fundamental()
sage: L[1,2].duality_pairing(F[1,2])
0
sage: L[1,1,1].duality_pairing(F[1,2])
1

skew_by(y, side='left')

The operation which is dual to multiplication by y, where y is an element of the dual space of self.

This is calculated through the coproduct of self and the expansion of y in the dual basis.

INPUT:

• y – an element of the dual Hopf algebra of self
• side – (Default=’left’) Either ‘left’ or ‘right’

OUTPUT:

• The result of skewing self by y, on the side side

EXAMPLES:

Skewing an element of NCSF by an element of QSym:

sage: R = NonCommutativeSymmetricFunctions(QQ).ribbon()
sage: F = QuasiSymmetricFunctions(QQ).Fundamental()
sage: R([2,2,2]).skew_by(F[1,1])
R[1, 1, 2] + R[1, 2, 1] + R[1, 3] + R[2, 1, 1] + 2*R[2, 2] + R[3, 1] + R[4]
sage: R([2,2,2]).skew_by(F[2])
R[1, 1, 2] + R[1, 2, 1] + R[1, 3] + R[2, 1, 1] + 3*R[2, 2] + R[3, 1] + R[4]


Skewing an element of QSym by an element of NCSF:

sage: S = NonCommutativeSymmetricFunctions(QQ).S()
sage: R = NonCommutativeSymmetricFunctions(QQ).R()
sage: F = QuasiSymmetricFunctions(QQ).F()
sage: F[3,2].skew_by(R[1,1])
0
sage: F[3,2].skew_by(R[1,1], side='right')
0
sage: F[3,2].skew_by(S[1,1,1], side='right')
F[2]
sage: F[3,2].skew_by(S[1,2], side='right')
F[2]
sage: F[3,2].skew_by(S[2,1], side='right')
0
sage: F[3,2].skew_by(S[1,1,1])
F[2]
sage: F[3,2].skew_by(S[1,1])
F[1, 2]
sage: F[3,2].skew_by(S[1])
F[2, 2]

sage: S = NonCommutativeSymmetricFunctions(QQ).S()
sage: R = NonCommutativeSymmetricFunctions(QQ).R()
sage: M = QuasiSymmetricFunctions(QQ).M()
sage: M[3,2].skew_by(S[2])
0
sage: M[3,2].skew_by(S[2], side='right')
M[3]
sage: M[3,2].skew_by(S[3])
M[2]
sage: M[3,2].skew_by(S[3], side='right')
0

class BasesOfQSymOrNCSF.ParentMethods
alternating_sum_of_fatter_compositions(composition)

Returns the alternating sum of fatter compositions in a basis of the non- commutative symmetric functions.

INPUT:

• composition – a composition

OUTPUT:

• Returns the alternating sum of the fatter compositions of composition, in the basis self. The alternation is upon the length of the compositions, and is normalized so that composition has coefficient one.

EXAMPLES:

sage: NCSF=NonCommutativeSymmetricFunctions(QQ)
sage: elementary = NCSF.elementary()
sage: elementary.alternating_sum_of_fatter_compositions(Composition([2,2,1]))
L[2, 2, 1] - L[2, 3] - L[4, 1] + L[5]
sage: elementary.alternating_sum_of_fatter_compositions(Composition([1,2]))
L[1, 2] - L[3]


TESTS:

sage: complete = NonCommutativeSymmetricFunctions(ZZ).complete()
sage: I = Composition([1,1])
sage: x = complete.alternating_sum_of_fatter_compositions(I)
sage: [c.parent() for c in x.coefficients()]
[Integer Ring, Integer Ring]

alternating_sum_of_finer_compositions(composition, conjugate=False)

Returns the alternating sum of finer compositions in a basis of the non- commutative symmetric functions.

INPUT:

• composition – a composition
• conjugate – (default: False) a boolean

OUTPUT:

• Returns the alternating sum of the finer compositions of composition, in the basis self. The alternation is upon the length of the compositions, and is normalized so that composition has coefficient one.

EXAMPLES:

sage: NCSF = NonCommutativeSymmetricFunctions(QQ)
sage: elementary = NCSF.elementary()
sage: elementary.alternating_sum_of_finer_compositions(Composition([2,2,1]))
L[1, 1, 1, 1, 1] - L[1, 1, 2, 1] - L[2, 1, 1, 1] + L[2, 2, 1]
sage: elementary.alternating_sum_of_finer_compositions(Composition([1,2]))
-L[1, 1, 1] + L[1, 2]


TESTS:

sage: complete = NonCommutativeSymmetricFunctions(ZZ).complete()
sage: I = Composition([2])
sage: x = complete.alternating_sum_of_finer_compositions(I)
sage: [c.parent() for c in x.coefficients()]
[Integer Ring, Integer Ring]

counit_on_basis(I)

The counit is defined by sending all elements of positive degree to zero.

EXAMPLES:

sage: S = NonCommutativeSymmetricFunctions(QQ).S()
sage: S.counit_on_basis([1,3])
0
sage: M = QuasiSymmetricFunctions(QQ).M()
sage: M.counit_on_basis([1,3])
0


TESTS:

sage: S.counit_on_basis([])
1
sage: S.counit_on_basis(Composition([]))
1
sage: M.counit_on_basis([])
1
sage: M.counit_on_basis(Composition([]))
1

degree_on_basis(I)

Return the degree of the basis element indexed by $$I$$.

INPUT:

• I – a composition

OUTPUT:

• Returns the degree of the non-commutative symmetric function basis element of self indexed by I. By definition, this is the size of the composition I.

EXAMPLES:

sage: R = NonCommutativeSymmetricFunctions(QQ).ribbon()
sage: R.degree_on_basis(Composition([2,3]))
5
sage: M = QuasiSymmetricFunctions(QQ).Monomial()
sage: M.degree_on_basis(Composition([3,2]))
5
sage: M.degree_on_basis(Composition([]))
0

duality_pairing(x, y)

The duality pairing between elements of NSym and elements of QSym.

This is a default implementation and assumes that self.realizations_of().a_realization() is dual to self.realizations_of().dual().a_realization().

INPUT:

• x – an element of self
• y – an element in the dual basis of self

OUTPUT:

• The result of pairing the function x from self with the function y from the dual basis of self

EXAMPLES:

sage: R = NonCommutativeSymmetricFunctions(QQ).Ribbon()
sage: F = QuasiSymmetricFunctions(QQ).Fundamental()
sage: R.duality_pairing(R[1,1,2], F[1,1,2])
1
sage: R.duality_pairing(R[1,2,1], F[1,1,2])
0
sage: F.duality_pairing(F[1,2,1], R[1,1,2])
0

sage: S = NonCommutativeSymmetricFunctions(QQ).Complete()
sage: M = QuasiSymmetricFunctions(QQ).Monomial()
sage: S.duality_pairing(S[1,1,2], M[1,1,2])
1
sage: S.duality_pairing(S[1,2,1], M[1,1,2])
0
sage: M.duality_pairing(M[1,1,2], S[1,1,2])
1
sage: M.duality_pairing(M[1,2,1], S[1,1,2])
0

sage: S = NonCommutativeSymmetricFunctions(QQ).Complete()
sage: F = QuasiSymmetricFunctions(QQ).Fundamental()
sage: S.duality_pairing(S[1,2], F[1,1,1])
0
sage: S.duality_pairing(S[1,1,1,1], F[4])
1

duality_pairing_by_coercion(x, y)

The duality pairing between elements of NSym and elements of QSym.

This is a default implementation that uses self.realizations_of().a_realization() and its dual basis.

INPUT:

• x – an element of self
• y – an element in the dual basis of self

OUTPUT:

• The result of pairing the function x from self with the function y from the dual basis of self

EXAMPLES:

sage: L = NonCommutativeSymmetricFunctions(QQ).Elementary()
sage: F = QuasiSymmetricFunctions(QQ).Fundamental()
sage: L.duality_pairing_by_coercion(L[1,2], F[1,2])
0
sage: F.duality_pairing_by_coercion(F[1,2], L[1,2])
0
sage: L.duality_pairing_by_coercion(L[1,1,1], F[1,2])
1
sage: F.duality_pairing_by_coercion(F[1,2], L[1,1,1])
1

duality_pairing_matrix(basis, degree)

The matrix of scalar products between elements of NSym and elements of QSym.

INPUT:

• basis – A basis of the dual Hopf algebra
• degree – a non-negative integer

OUTPUT:

• The matrix of scalar products between the basis self and the basis basis in the dual Hopf algebra of degree degree.

EXAMPLES:

The ribbon basis of NCSF is dual to the fundamental basis of QSym:

sage: R = NonCommutativeSymmetricFunctions(QQ).ribbon()
sage: F = QuasiSymmetricFunctions(QQ).Fundamental()
sage: R.duality_pairing_matrix(F, 3)
[1 0 0 0]
[0 1 0 0]
[0 0 1 0]
[0 0 0 1]
sage: F.duality_pairing_matrix(R, 3)
[1 0 0 0]
[0 1 0 0]
[0 0 1 0]
[0 0 0 1]


The complete basis of NCSF is dual to the monomial basis of QSym:

sage: S = NonCommutativeSymmetricFunctions(QQ).complete()
sage: M = QuasiSymmetricFunctions(QQ).Monomial()
sage: S.duality_pairing_matrix(M, 3)
[1 0 0 0]
[0 1 0 0]
[0 0 1 0]
[0 0 0 1]
sage: M.duality_pairing_matrix(S, 3)
[1 0 0 0]
[0 1 0 0]
[0 0 1 0]
[0 0 0 1]


The matrix between the ribbon basis of NCSF and the monomial basis of QSym:

sage: R = NonCommutativeSymmetricFunctions(QQ).ribbon()
sage: M = QuasiSymmetricFunctions(QQ).Monomial()
sage: R.duality_pairing_matrix(M, 3)
[ 1 -1 -1  1]
[ 0  1  0 -1]
[ 0  0  1 -1]
[ 0  0  0  1]
sage: M.duality_pairing_matrix(R, 3)
[ 1  0  0  0]
[-1  1  0  0]
[-1  0  1  0]
[ 1 -1 -1  1]


The matrix between the complete basis of NCSF and the fundamental basis of QSym:

sage: S = NonCommutativeSymmetricFunctions(QQ).complete()
sage: F = QuasiSymmetricFunctions(QQ).Fundamental()
sage: S.duality_pairing_matrix(F, 3)
[1 1 1 1]
[0 1 0 1]
[0 0 1 1]
[0 0 0 1]


A base case test:

sage: R.duality_pairing_matrix(M,0)
[1]

one_basis()

This returns the empty composition.

OUTPUT

• The empty composition.

EXAMPLES:

sage: L=NonCommutativeSymmetricFunctions(QQ).L()
sage: parent(L)
<class 'sage.combinat.ncsf_qsym.ncsf.NonCommutativeSymmetricFunctions.Elementary_with_category'>
sage: parent(L).one_basis()
[]

skew(x, y, side='left')

Skews a function by another function in the Hopf dual.

INPUT:

• x – A non-commutative or quasi-symmetric function. It is an element of self
• y – A quasi-symmetric or non-commutative symmetric function. It is an element of the dual algebra of self
• side – (Default=’left’) Either ‘left’ or ‘right’

OUTPUT:

• Returns the result of skewing the element x written in the basis self by the Hopf algebra element y written in the dual basis of self.

EXAMPLES:

sage: S = NonCommutativeSymmetricFunctions(QQ).complete()
sage: F = QuasiSymmetricFunctions(QQ).Fundamental()
sage: S.skew(S[2,2,2], F[1,1])
S[1, 1, 2] + S[1, 2, 1] + S[2, 1, 1]
sage: S.skew(S[2,2,2], F[2])
S[1, 1, 2] + S[1, 2, 1] + S[2, 1, 1] + 3*S[2, 2]

sage: R = NonCommutativeSymmetricFunctions(QQ).ribbon()
sage: F = QuasiSymmetricFunctions(QQ).Fundamental()
sage: R.skew(R[2,2,2], F[1,1])
R[1, 1, 2] + R[1, 2, 1] + R[1, 3] + R[2, 1, 1] + 2*R[2, 2] + R[3, 1] + R[4]
sage: R.skew(R[2,2,2], F[2])
R[1, 1, 2] + R[1, 2, 1] + R[1, 3] + R[2, 1, 1] + 3*R[2, 2] + R[3, 1] + R[4]

sage: S = NonCommutativeSymmetricFunctions(QQ).S()
sage: R = NonCommutativeSymmetricFunctions(QQ).R()
sage: M = QuasiSymmetricFunctions(QQ).M()
sage: M.skew(M[3,2], S[2])
0
sage: M.skew(M[3,2], S[2], side='right')
M[3]
sage: M.skew(M[3,2], S[3])
M[2]
sage: M.skew(M[3,2], S[3], side='right')
0


TESTS:

sage: R = NonCommutativeSymmetricFunctions(QQ).R()
sage: R.skew([2,1], [1])
Traceback (most recent call last):
...
AssertionError: x must be an element of Non-Commutative Symmetric Functions over the Rational Field
sage: R([2,1]).skew_by([1])
Traceback (most recent call last):
...
AssertionError: y must be an element of Quasisymmetric functions over the Rational Field
sage: F = QuasiSymmetricFunctions(QQ).F()
sage: F([2,1]).skew_by([1])
Traceback (most recent call last):
...
AssertionError: y must be an element of Non-Commutative Symmetric Functions over the Rational Field

sum_of_fatter_compositions(composition)

Returns the sum of all fatter compositions.

INPUT:

• composition – a composition

OUTPUT:

• Returns the sum of all basis elements which are indexed by compositions fatter (coarser?) than composition.

EXAMPLES:

sage: L=NonCommutativeSymmetricFunctions(QQ).L()
sage: L.sum_of_fatter_compositions(Composition([2,1]))
L[2, 1] + L[3]
sage: R=NonCommutativeSymmetricFunctions(QQ).R()
sage: R.sum_of_fatter_compositions(Composition([1,3]))
R[1, 3] + R[4]

sum_of_finer_compositions(composition)

Returns the sum of all finer compositions.

INPUT:

• composition – a composition

OUTPUT:

• Returns the sum of all basis self elements which are indexed by compositions finer than composition.

EXAMPLES:

sage: L=NonCommutativeSymmetricFunctions(QQ).L()
sage: L.sum_of_finer_compositions(Composition([2,1]))
L[1, 1, 1] + L[2, 1]
sage: R=NonCommutativeSymmetricFunctions(QQ).R()
sage: R.sum_of_finer_compositions(Composition([1,3]))
R[1, 1, 1, 1] + R[1, 1, 2] + R[1, 2, 1] + R[1, 3]

sum_of_partition_rearrangements(par)

Returns the sum of all basis elements indexed by compositions which can be shuffled to obtain a partition.

INPUT:

• par – a partition

OUTPUT:

• Returns the sum of all self basis elements indexed by compositions which are shuffles of par.

EXAMPLES:

sage: NCSF=NonCommutativeSymmetricFunctions(QQ)
sage: elementary = NCSF.elementary()
sage: elementary.sum_of_partition_rearrangements(Partition([2,2,1]))
L[1, 2, 2] + L[2, 1, 2] + L[2, 2, 1]
sage: elementary.sum_of_partition_rearrangements(Partition([3,2,1]))
L[1, 2, 3] + L[1, 3, 2] + L[2, 1, 3] + L[2, 3, 1] + L[3, 1, 2] + L[3, 2, 1]
sage: elementary.sum_of_partition_rearrangements(Partition([]))
L[]

BasesOfQSymOrNCSF.super_categories()

TESTS:

sage: from sage.combinat.ncsf_qsym.generic_basis_code import BasesOfQSymOrNCSF
sage: QSym = QuasiSymmetricFunctions(QQ)
sage: BasesOfQSymOrNCSF(QSym).super_categories()
[Category of realizations of Quasisymmetric functions over the Rational Field,
Category of graded hopf algebras with basis over Rational Field,
Join of Category of graded hopf algebras over Rational Field and Category of realizations of hopf algebras over Rational Field]

class sage.combinat.ncsf_qsym.generic_basis_code.GradedModulesWithInternalProduct(base, name=None)

Constructs the class of modules with internal product. This is used to give an internal product structure to the non-commutative symmetric functions.

EXAMPLES:

sage: from sage.combinat.ncsf_qsym.generic_basis_code import GradedModulesWithInternalProduct
sage: N = NonCommutativeSymmetricFunctions(QQ)
sage: R = N.ribbon()
sage: R in GradedModulesWithInternalProduct(QQ)
True

class ElementMethods
internal_product(other)

Returns the internal product of two non-commutative symmetric functions.

INPUT:

• other – another non-commutative symmetric function

OUTPUT:

• Returns the result of taking the internal product of self with other.

EXAMPLES:

sage: N = NonCommutativeSymmetricFunctions(QQ)
sage: S = N.complete()
sage: x = S.an_element(); x
2*S[] + 2*S[1] + 3*S[1, 1]
sage: x.internal_product(S[2])
3*S[1, 1]
sage: x.internal_product(S[1])
2*S[1]
sage: S[1,2].internal_product(S[1,2])
S[1, 1, 1] + S[1, 2]

class GradedModulesWithInternalProduct.ParentMethods
internal_product()

Internal product as an endomorphism of self.

This is constructed by extending the method internal_product_on_basis() bilinearly, if available, or using the method internal_product_by_coercion().

OUTPUT:

• The internal product endomorphism of the non-commutative symmetric functions.

EXAMPLES:

sage: N = NonCommutativeSymmetricFunctions(QQ)
sage: S = N.complete()
sage: S.internal_product
Generic endomorphism of Non-Commutative Symmetric Functions over the Rational Field in the Complete basis
sage: S.internal_product(S[2,2], S[1,2,1])
2*S[1, 1, 1, 1] + S[1, 1, 2] + S[2, 1, 1]
sage: S.internal_product(S[2,2], S[1,2])
0

sage: N = NonCommutativeSymmetricFunctions(QQ)
sage: R = N.ribbon()
sage: R.internal_product
<bound method ....internal_product_by_coercion ...>
sage: R.internal_product_by_coercion(R[1, 1], R[1,1])
R[2]
sage: R.internal_product(R[2,2], R[1,2])
0

internal_product_on_basis(I, J)

The internal product of the two basis elements indexed by I and J (optional)

INPUT:

• I, J – compositions indexing two elements of the basis of self

Returns the internal product of the corresponding basis elements. If this method is implemented, the internal product is defined from it by linearity.

EXAMPLES:

sage: N = NonCommutativeSymmetricFunctions(QQ)
sage: S = N.complete()
sage: S.internal_product_on_basis([2,2], [1,2,1])
2*S[1, 1, 1, 1] + S[1, 1, 2] + S[2, 1, 1]
sage: S.internal_product_on_basis([2,2], [2,1])
0

class GradedModulesWithInternalProduct.Realizations(category, *args)

TESTS:

sage: from sage.categories.covariant_functorial_construction import CovariantConstructionCategory
sage: class FooBars(CovariantConstructionCategory):
...       _functor_category = "FooBars"
sage: Category.FooBars = lambda self: FooBars.category_of(self)
sage: C = FooBars(ModulesWithBasis(ZZ))
sage: C
Category of foo bars of modules with basis over Integer Ring
sage: C.base_category()
Category of modules with basis over Integer Ring
sage: latex(C)
\mathbf{FooBars}(\mathbf{ModulesWithBasis}_{\Bold{Z}})
sage: import __main__; __main__.FooBars = FooBars # Fake FooBars being defined in a python module
sage: TestSuite(C).run()

ParentMethods

alias of Realizations.ParentMethods

GradedModulesWithInternalProduct.super_categories()

EXAMPLES:

sage: from sage.combinat.ncsf_qsym.generic_basis_code import GradedModulesWithInternalProduct
sage: GradedModulesWithInternalProduct(ZZ).super_categories()
[Category of graded modules over Integer Ring]


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