Monomial symmetric functions#

class sage.combinat.sf.monomial.SymmetricFunctionAlgebra_monomial(Sym)#

Bases: sage.combinat.sf.classical.SymmetricFunctionAlgebra_classical

A class for methods related to monomial symmetric functions

INPUT:

  • self – a monomial symmetric function basis

  • Sym – an instance of the ring of the symmetric functions

class Element#

Bases: sage.combinat.sf.classical.SymmetricFunctionAlgebra_classical.Element

expand(n, alphabet='x')#

Expand the symmetric function self as a symmetric polynomial in n variables.

INPUT:

  • n – a nonnegative integer

  • alphabet – (default: 'x') a variable for the expansion

OUTPUT:

A monomial expansion of self in the \(n\) variables labelled by alphabet.

EXAMPLES:

sage: m = SymmetricFunctions(QQ).m()
sage: m([2,1]).expand(3)
x0^2*x1 + x0*x1^2 + x0^2*x2 + x1^2*x2 + x0*x2^2 + x1*x2^2
sage: m([1,1,1]).expand(2)
0
sage: m([2,1]).expand(3,alphabet='z')
z0^2*z1 + z0*z1^2 + z0^2*z2 + z1^2*z2 + z0*z2^2 + z1*z2^2
sage: m([2,1]).expand(3,alphabet='x,y,z')
x^2*y + x*y^2 + x^2*z + y^2*z + x*z^2 + y*z^2
sage: m([1]).expand(0)
0
sage: (3*m([])).expand(0)
3
exponential_specialization(t=None, q=1)#

Return the exponential specialization of a symmetric function (when \(q = 1\)), or the \(q\)-exponential specialization (when \(q \neq 1\)).

The exponential specialization \(ex\) at \(t\) is a \(K\)-algebra homomorphism from the \(K\)-algebra of symmetric functions to another \(K\)-algebra \(R\). It is defined whenever the base ring \(K\) is a \(\QQ\)-algebra and \(t\) is an element of \(R\). The easiest way to define it is by specifying its values on the powersum symmetric functions to be \(p_1 = t\) and \(p_n = 0\) for \(n > 1\). Equivalently, on the homogeneous functions it is given by \(ex(h_n) = t^n / n!\); see Proposition 7.8.4 of [EnumComb2].

By analogy, the \(q\)-exponential specialization is a \(K\)-algebra homomorphism from the \(K\)-algebra of symmetric functions to another \(K\)-algebra \(R\) that depends on two elements \(t\) and \(q\) of \(R\) for which the elements \(1 - q^i\) for all positive integers \(i\) are invertible. It can be defined by specifying its values on the complete homogeneous symmetric functions to be

\[ex_q(h_n) = t^n / [n]_q!,\]

where \([n]_q!\) is the \(q\)-factorial. Equivalently, for \(q \neq 1\) and a homogeneous symmetric function \(f\) of degree \(n\), we have

\[ex_q(f) = (1-q)^n t^n ps_q(f),\]

where \(ps_q(f)\) is the stable principal specialization of \(f\) (see principal_specialization()). (See (7.29) in [EnumComb2].)

The limit of \(ex_q\) as \(q \to 1\) is \(ex\).

INPUT:

  • t (default: None) – the value to use for \(t\); the default is to create a ring of polynomials in t.

  • q (default: \(1\)) – the value to use for \(q\). If q is None, then a ring (or fraction field) of polynomials in q is created.

EXAMPLES:

sage: m = SymmetricFunctions(QQ).m()
sage: (m[3]+m[2,1]+m[1,1,1]).exponential_specialization()
1/6*t^3

sage: x = 5*m[1,1,1] + 3*m[2,1] + 1
sage: x.exponential_specialization()
5/6*t^3 + 1

We also support the \(q\)-exponential_specialization:

sage: factor(m[3].exponential_specialization(q=var("q"), t=var("t")))
(q - 1)^2*t^3/(q^2 + q + 1)
principal_specialization(n=+ Infinity, q=None)#

Return the principal specialization of a symmetric function.

The principal specialization of order \(n\) at \(q\) is the ring homomorphism \(ps_{n,q}\) from the ring of symmetric functions to another commutative ring \(R\) given by \(x_i \mapsto q^{i-1}\) for \(i \in \{1,\dots,n\}\) and \(x_i \mapsto 0\) for \(i > n\). Here, \(q\) is a given element of \(R\), and we assume that the variables of our symmetric functions are \(x_1, x_2, x_3, \ldots\). (To be more precise, \(ps_{n,q}\) is a \(K\)-algebra homomorphism, where \(K\) is the base ring.) See Section 7.8 of [EnumComb2].

The stable principal specialization at \(q\) is the ring homomorphism \(ps_q\) from the ring of symmetric functions to another commutative ring \(R\) given by \(x_i \mapsto q^{i-1}\) for all \(i\). This is well-defined only if the resulting infinite sums converge; thus, in particular, setting \(q = 1\) in the stable principal specialization is an invalid operation.

INPUT:

  • n (default: infinity) – a nonnegative integer or infinity, specifying whether to compute the principal specialization of order n or the stable principal specialization.

  • q (default: None) – the value to use for \(q\); the default is to create a ring of polynomials in q (or a field of rational functions in q) over the given coefficient ring.

For q=1 and finite n we use the formula from Proposition 7.8.3 of [EnumComb2]:

\[ps_{n,1}(m_\lambda) = \binom{n}{\ell(\lambda)} \binom{\ell(\lambda)}{m_1(\lambda), m_2(\lambda),\dots},\]

where \(\ell(\lambda)\) denotes the length of \(\lambda\).

In all other cases, we convert to complete homogeneous symmetric functions.

EXAMPLES:

sage: m = SymmetricFunctions(QQ).m()
sage: x = m[3,1]
sage: x.principal_specialization(3)
q^7 + q^6 + q^5 + q^3 + q^2 + q

sage: x = 5*m[2] + 3*m[1] + 1
sage: x.principal_specialization(3, q=var("q"))
-10*(q^3 - 1)*q/(q - 1) + 5*(q^3 - 1)^2/(q - 1)^2 + 3*(q^3 - 1)/(q - 1) + 1
antipode_by_coercion(element)#

The antipode of element via coercion to and from the power-sum basis or the Schur basis (depending on whether the power sums really form a basis over the given ground ring).

INPUT:

  • element – element in a basis of the ring of symmetric functions

EXAMPLES:

sage: Sym = SymmetricFunctions(QQ)
sage: m = Sym.monomial()
sage: m[3,2].antipode()
m[3, 2] + 2*m[5]
sage: m.antipode_by_coercion(m[3,2])
m[3, 2] + 2*m[5]

sage: Sym = SymmetricFunctions(ZZ)
sage: m = Sym.monomial()
sage: m[3,2].antipode()
m[3, 2] + 2*m[5]
sage: m.antipode_by_coercion(m[3,2])
m[3, 2] + 2*m[5]

Todo

Is there a not too difficult way to get the power-sum computations to work over any ring, not just one with coercion from \(\QQ\)?

from_polynomial(f, check=True)#

Return the symmetric function in the monomial basis corresponding to the polynomial f.

INPUT:

  • self – a monomial symmetric function basis

  • f – a polynomial in finitely many variables over the same base ring as self. It is assumed that this polynomial is symmetric.

  • check – boolean (default: True), checks whether the polynomial is indeed symmetric

OUTPUT:

  • This function converts a symmetric polynomial \(f\) in a polynomial ring in finitely many variables to a symmetric function in the monomial basis of the ring of symmetric functions over the same base ring.

EXAMPLES:

sage: m = SymmetricFunctions(QQ).m()
sage: P = PolynomialRing(QQ, 'x', 3)
sage: x = P.gens()
sage: f = x[0] + x[1] + x[2]
sage: m.from_polynomial(f)
m[1]
sage: f = x[0]**2+x[1]**2+x[2]**2
sage: m.from_polynomial(f)
m[2]
sage: f = x[0]^2+x[1]
sage: m.from_polynomial(f)
Traceback (most recent call last):
...
ValueError: x0^2 + x1 is not a symmetric polynomial
sage: f = (m[2,1]+m[1,1]).expand(3)
sage: m.from_polynomial(f)
m[1, 1] + m[2, 1]
sage: f = (2*m[2,1]+m[1,1]+3*m[3]).expand(3)
sage: m.from_polynomial(f)
m[1, 1] + 2*m[2, 1] + 3*m[3]
from_polynomial_exp(p)#

Conversion from polynomial in exponential notation

INPUT:

  • self – a monomial symmetric function basis

  • p – a multivariate polynomial over the same base ring as self

OUTPUT:

  • This returns a symmetric function by mapping each monomial of \(p\) with exponents exp into \(m_\lambda\) where \(\lambda\) is the partition with exponential notation exp.

EXAMPLES:

sage: m = SymmetricFunctions(QQ).m()
sage: P = PolynomialRing(QQ,'x',5)
sage: x = P.gens()

The exponential notation of the partition \((5,5,5,3,1,1)\) is:

sage: Partition([5,5,5,3,1,1]).to_exp()
[2, 0, 1, 0, 3]

Therefore, the monomial:

sage: f = x[0]^2 * x[2] * x[4]^3

is mapped to:

sage: m.from_polynomial_exp(f)
m[5, 5, 5, 3, 1, 1]

Furthermore, this function is linear:

sage: f = 3 * x[3] + 2 * x[0]^2 * x[2] * x[4]^3
sage: m.from_polynomial_exp(f)
3*m[4] + 2*m[5, 5, 5, 3, 1, 1]
product(left, right)#

Return the product of left and right.

  • left, right – symmetric functions written in the monomial basis self.

OUTPUT:

  • the product of left and right, expanded in the monomial basis, as a dictionary whose keys are partitions and whose values are the coefficients of these partitions (more precisely, their respective monomial symmetric functions) in the product.

EXAMPLES:

sage: m = SymmetricFunctions(QQ).m()
sage: a = m([2,1])
sage: a^2
4*m[2, 2, 1, 1] + 6*m[2, 2, 2] + 2*m[3, 2, 1] + 2*m[3, 3] + 2*m[4, 1, 1] + m[4, 2]
sage: QQx.<x> = QQ['x']
sage: m = SymmetricFunctions(QQx).m()
sage: a = m([2,1])+x
sage: 2*a # indirect doctest
2*x*m[] + 2*m[2, 1]
sage: a^2
x^2*m[] + 2*x*m[2, 1] + 4*m[2, 2, 1, 1] + 6*m[2, 2, 2] + 2*m[3, 2, 1] + 2*m[3, 3] + 2*m[4, 1, 1] + m[4, 2]