Power sum symmetric functions#

class sage.combinat.sf.powersum.SymmetricFunctionAlgebra_power(Sym)#

Bases: sage.combinat.sf.multiplicative.SymmetricFunctionAlgebra_multiplicative

A class for methods associated to the power sum basis of the symmetric functions

INPUT:

self – the power sum basis of the symmetric functions
Sym – an instance of the ring of symmetric functions

class Element#

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

adams_operation(n)#

Return the image of the symmetric function self under the \(n\)-th Frobenius operator.

The \(n\)-th Frobenius operator \(\mathbf{f}_n\) is defined to be the map from the ring of symmetric functions to itself that sends every symmetric function \(P(x_1, x_2, x_3, \ldots)\) to \(P(x_1^n, x_2^n, x_3^n, \ldots)\). This operator \(\mathbf{f}_n\) is a Hopf algebra endomorphism, and satisfies

\[\mathbf{f}_n m_{(\lambda_1, \lambda_2, \lambda_3, \ldots)} = m_{(n\lambda_1, n\lambda_2, n\lambda_3, \ldots)}\]

for every partition \((\lambda_1, \lambda_2, \lambda_3, \ldots)\) (where \(m\) means the monomial basis). Moreover, \(\mathbf{f}_n (p_r) = p_{nr}\) for every positive integer \(r\) (where \(p_k\) denotes the \(k\)-th powersum symmetric function).

The \(n\)-th Frobenius operator is also called the \(n\)-th Frobenius endomorphism. It is not related to the Frobenius map which connects the ring of symmetric functions with the representation theory of the symmetric group.

The \(n\)-th Frobenius operator is also the \(n\)-th Adams operator of the \(\Lambda\)-ring of symmetric functions over the integers.

The \(n\)-th Frobenius operator can also be described via plethysm: Every symmetric function \(P\) satisfies \(\mathbf{f}_n(P) = p_n \circ P = P \circ p_n\), where \(p_n\) is the \(n\)-th powersum symmetric function, and \(\circ\) denotes (outer) plethysm.

INPUT:

n – a positive integer

OUTPUT:

The result of applying the \(n\)-th Frobenius operator (on the ring of symmetric functions) to self.

EXAMPLES:

sage: Sym = SymmetricFunctions(ZZ)
sage: p = Sym.p()
sage: p[3].frobenius(2)
p[6]
sage: p[4,2,1].frobenius(3)
p[12, 6, 3]
sage: p([]).frobenius(4)
p[]
sage: p[3].frobenius(1)
p[3]
sage: (p([3]) - p([2]) + p([])).frobenius(3)
p[] - p[6] + p[9]

See also

plethysm()

eval_at_permutation_roots(rho)#

Evaluate at eigenvalues of a permutation matrix.

Evaluate an element of the power sum basis at the eigenvalues of a permutation matrix with cycle structure \(\rho\).

This function evaluates an element at the roots of unity

\[\Xi_{\rho_1},\Xi_{\rho_2},\ldots,\Xi_{\rho_\ell}\]

where

\[\Xi_{m} = 1,\zeta_m,\zeta_m^2,\ldots,\zeta_m^{m-1}\]

and \(\zeta_m\) is an \(m\) root of unity. These roots of unity represent the eigenvalues of permutation matrix with cycle structure \(\rho\).

INPUT:

rho – a partition or a list of non-negative integers

OUTPUT:

an element of the base ring

EXAMPLES:

sage: p = SymmetricFunctions(QQ).p()
sage: p([3,3]).eval_at_permutation_roots([6])
0
sage: p([3,3]).eval_at_permutation_roots([3])
9
sage: p([3,3]).eval_at_permutation_roots([1])
1
sage: p([3,3]).eval_at_permutation_roots([3,3])
36
sage: p([3,3]).eval_at_permutation_roots([1,1,1,1,1])
25
sage: (p[1]+p[2]+p[3]).eval_at_permutation_roots([3,2])
5

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: p = SymmetricFunctions(QQ).p()
sage: a = p([2])
sage: a.expand(2)
x0^2 + x1^2
sage: a.expand(3, alphabet=['a','b','c'])
a^2 + b^2 + c^2
sage: p([2,1,1]).expand(2)
x0^4 + 2*x0^3*x1 + 2*x0^2*x1^2 + 2*x0*x1^3 + x1^4
sage: p([7]).expand(4)
x0^7 + x1^7 + x2^7 + x3^7
sage: p([7]).expand(4,alphabet='t')
t0^7 + t1^7 + t2^7 + t3^7
sage: p([7]).expand(4,alphabet='x,y,z,t')
x^7 + y^7 + z^7 + t^7
sage: p(1).expand(4)
1
sage: p(0).expand(4)
0
sage: (p([]) + 2*p([1])).expand(3)
2*x0 + 2*x1 + 2*x2 + 1
sage: p([1]).expand(0)
0
sage: (3*p([])).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: p = SymmetricFunctions(QQ).p()
sage: x = p[8,7,3,1]
sage: x.exponential_specialization()
0
sage: x = p[3] + 5*p[1,1] + 2*p[1] + 1
sage: x.exponential_specialization(t=var("t"))
5*t^2 + 2*t + 1

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

sage: factor(p[3].exponential_specialization(q=var("q"), t=var("t")))
(q - 1)^2*t^3/(q^2 + q + 1)

frobenius(n)#

Return the image of the symmetric function self under the \(n\)-th Frobenius operator.

\[\mathbf{f}_n m_{(\lambda_1, \lambda_2, \lambda_3, \ldots)} = m_{(n\lambda_1, n\lambda_2, n\lambda_3, \ldots)}\]

The \(n\)-th Frobenius operator is also the \(n\)-th Adams operator of the \(\Lambda\)-ring of symmetric functions over the integers.

INPUT:

n – a positive integer

OUTPUT:

The result of applying the \(n\)-th Frobenius operator (on the ring of symmetric functions) to self.

EXAMPLES:

sage: Sym = SymmetricFunctions(ZZ)
sage: p = Sym.p()
sage: p[3].frobenius(2)
p[6]
sage: p[4,2,1].frobenius(3)
p[12, 6, 3]
sage: p([]).frobenius(4)
p[]
sage: p[3].frobenius(1)
p[3]
sage: (p([3]) - p([2]) + p([])).frobenius(3)
p[] - p[6] + p[9]

See also

plethysm()

omega()#

Return the image of self under the omega automorphism.

The omega automorphism is defined to be the unique algebra endomorphism \(\omega\) of the ring of symmetric functions that satisfies \(\omega(e_k) = h_k\) for all positive integers \(k\) (where \(e_k\) stands for the \(k\)-th elementary symmetric function, and \(h_k\) stands for the \(k\)-th complete homogeneous symmetric function). It furthermore is a Hopf algebra endomorphism and an involution, and it is also known as the omega involution. It sends the power-sum symmetric function \(p_k\) to \((-1)^{k-1} p_k\) for every positive integer \(k\).

The images of some bases under the omega automorphism are given by

\[\omega(e_{\lambda}) = h_{\lambda}, \qquad \omega(h_{\lambda}) = e_{\lambda}, \qquad \omega(p_{\lambda}) = (-1)^{|\lambda| - \ell(\lambda)} p_{\lambda}, \qquad \omega(s_{\lambda}) = s_{\lambda^{\prime}},\]

where \(\lambda\) is any partition, where \(\ell(\lambda)\) denotes the length (length()) of the partition \(\lambda\), where \(\lambda^{\prime}\) denotes the conjugate partition (conjugate()) of \(\lambda\), and where the usual notations for bases are used (\(e\) = elementary, \(h\) = complete homogeneous, \(p\) = powersum, \(s\) = Schur).

omega_involution() is a synonym for the omega() method.

OUTPUT:

the image of self under the omega automorphism

EXAMPLES:

sage: p = SymmetricFunctions(QQ).p()
sage: a = p([2,1]); a
p[2, 1]
sage: a.omega()
-p[2, 1]
sage: p([]).omega()
p[]
sage: p(0).omega()
0
sage: p = SymmetricFunctions(ZZ).p()
sage: (p([3,1,1]) - 2 * p([2,1])).omega()
2*p[2, 1] + p[3, 1, 1]

omega_involution()#

Return the image of self under the omega automorphism.

The images of some bases under the omega automorphism are given by

omega_involution() is a synonym for the omega() method.

OUTPUT:

the image of self under the omega automorphism

EXAMPLES:

sage: p = SymmetricFunctions(QQ).p()
sage: a = p([2,1]); a
p[2, 1]
sage: a.omega()
-p[2, 1]
sage: p([]).omega()
p[]
sage: p(0).omega()
0
sage: p = SymmetricFunctions(ZZ).p()
sage: (p([3,1,1]) - 2 * p([2,1])).omega()
2*p[2, 1] + p[3, 1, 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.

We use the formulas from Proposition 7.8.3 of [EnumComb2]:

\[ \begin{align}\begin{aligned}ps_{n,q}(p_\lambda) = \prod_i (1-q^{n\lambda_i}) / (1-q^{\lambda_i}),\\ps_{n,1}(p_\lambda) = n^{\ell(\lambda)},\\ps_q(p_\lambda) = 1 / \prod_i (1-q^{\lambda_i}),\end{aligned}\end{align} \]

where \(\ell(\lambda)\) denotes the length of \(\lambda\), and where the products range from \(i=1\) to \(i=\ell(\lambda)\).

EXAMPLES:

sage: p = SymmetricFunctions(QQ).p()
sage: x = p[8,7,3,1]
sage: x.principal_specialization(3, q=var("q"))
(q^24 - 1)*(q^21 - 1)*(q^9 - 1)/((q^8 - 1)*(q^7 - 1)*(q - 1))

sage: x = 5*p[1,1,1] + 3*p[2,1] + 1
sage: x.principal_specialization(3, q=var("q"))
5*(q^3 - 1)^3/(q - 1)^3 + 3*(q^6 - 1)*(q^3 - 1)/((q^2 - 1)*(q - 1)) + 1

By default, we return a rational function in \(q\):

sage: x.principal_specialization(3)
8*q^6 + 18*q^5 + 36*q^4 + 38*q^3 + 36*q^2 + 18*q + 9

If n is not given we return the stable principal specialization:

sage: x.principal_specialization(q=var("q"))
3/((q^2 - 1)*(q - 1)) - 5/(q - 1)^3 + 1

scalar(x, zee=None)#

Return the standard scalar product of self and x.

INPUT:

x – a power sum symmetric function
zee – (default: uses standard zee function) optional input specifying the scalar product on the power sum basis with normalization \(\langle p_{\mu}, p_{\mu} \rangle = \mathrm{zee}(\mu)\). zee should be a function on partitions.

Note that the power-sum symmetric functions are orthogonal under this scalar product. With the default value of zee, the value of \(\langle p_{\lambda}, p_{\lambda} \rangle\) is given by the size of the centralizer in \(S_n\) of a permutation of cycle type \(\lambda\).

OUTPUT:

the standard scalar product between self and x, or, if the optional parameter zee is specified, then the scalar product with respect to the normalization \(\langle p_{\mu}, p_{\mu} \rangle = \mathrm{zee}(\mu)\) with the power sum basis elements being orthogonal

EXAMPLES:

sage: p = SymmetricFunctions(QQ).p()
sage: p4 = Partitions(4)
sage: matrix([ [p(a).scalar(p(b)) for a in p4] for b in p4])
[ 4  0  0  0  0]
[ 0  3  0  0  0]
[ 0  0  8  0  0]
[ 0  0  0  4  0]
[ 0  0  0  0 24]
sage: p(0).scalar(p(1))
0
sage: p(1).scalar(p(2))
2

sage: zee = lambda x : 1
sage: matrix( [[p[la].scalar(p[mu], zee) for la in Partitions(3)] for mu in Partitions(3)])
[1 0 0]
[0 1 0]
[0 0 1]

verschiebung(n)#

Return the image of the symmetric function self under the \(n\)-th Verschiebung operator.

The \(n\)-th Verschiebung operator \(\mathbf{V}_n\) is defined to be the unique algebra endomorphism \(V\) of the ring of symmetric functions that satisfies \(V(h_r) = h_{r/n}\) for every positive integer \(r\) divisible by \(n\), and satisfies \(V(h_r) = 0\) for every positive integer \(r\) not divisible by \(n\). This operator \(\mathbf{V}_n\) is a Hopf algebra endomorphism. For every nonnegative integer \(r\) with \(n \mid r\), it satisfies

\[\mathbf{V}_n(h_r) = h_{r/n}, \quad \mathbf{V}_n(p_r) = n p_{r/n}, \quad \mathbf{V}_n(e_r) = (-1)^{r - r/n} e_{r/n}\]

(where \(h\) is the complete homogeneous basis, \(p\) is the powersum basis, and \(e\) is the elementary basis). For every nonnegative integer \(r\) with \(n \nmid r\), it satisfes

\[\mathbf{V}_n(h_r) = \mathbf{V}_n(p_r) = \mathbf{V}_n(e_r) = 0.\]

The \(n\)-th Verschiebung operator is also called the \(n\)-th Verschiebung endomorphism. Its name derives from the Verschiebung (German for “shift”) endomorphism of the Witt vectors.

The \(n\)-th Verschiebung operator is adjoint to the \(n\)-th Frobenius operator (see frobenius() for its definition) with respect to the Hall scalar product (scalar()).

The action of the \(n\)-th Verschiebung operator on the Schur basis can also be computed explicitly. The following (probably clumsier than necessary) description can be obtained by solving exercise 7.61 in Stanley’s [STA].

Let \(\lambda\) be a partition. Let \(n\) be a positive integer. If the \(n\)-core of \(\lambda\) is nonempty, then \(\mathbf{V}_n(s_\lambda) = 0\). Otherwise, the following method computes \(\mathbf{V}_n(s_\lambda)\): Write the partition \(\lambda\) in the form \((\lambda_1, \lambda_2, \ldots, \lambda_{ns})\) for some nonnegative integer \(s\). (If \(n\) does not divide the length of \(\lambda\), then this is achieved by adding trailing zeroes to \(\lambda\).) Set \(\beta_i = \lambda_i + ns - i\) for every \(s \in \{ 1, 2, \ldots, ns \}\). Then, \((\beta_1, \beta_2, \ldots, \beta_{ns})\) is a strictly decreasing sequence of nonnegative integers. Stably sort the list \((1, 2, \ldots, ns)\) in order of (weakly) increasing remainder of \(-1 - \beta_i\) modulo \(n\). Let \(\xi\) be the sign of the permutation that is used for this sorting. Let \(\psi\) be the sign of the permutation that is used to stably sort the list \((1, 2, \ldots, ns)\) in order of (weakly) increasing remainder of \(i - 1\) modulo \(n\). (Notice that \(\psi = (-1)^{n(n-1)s(s-1)/4}\).) Then, \(\mathbf{V}_n(s_\lambda) = \xi \psi \prod_{i = 0}^{n - 1} s_{\lambda^{(i)}}\), where \((\lambda^{(0)}, \lambda^{(1)}, \ldots, \lambda^{(n - 1)})\) is the \(n\)-quotient of \(\lambda\).

INPUT:

n – a positive integer

OUTPUT:

The result of applying the \(n\)-th Verschiebung operator (on the ring of symmetric functions) to self.

EXAMPLES:

sage: Sym = SymmetricFunctions(ZZ)
sage: p = Sym.p()
sage: p[3].verschiebung(2)
0
sage: p[4].verschiebung(4)
4*p[1]

The Verschiebung endomorphisms are multiplicative:

sage: all( all( p(lam).verschiebung(2) * p(mu).verschiebung(2)
....:           == (p(lam) * p(mu)).verschiebung(2)
....:           for mu in Partitions(4) )
....:      for lam in Partitions(4) )
True

Testing the adjointness between the Frobenius operators \(\mathbf{f}_n\) and the Verschiebung operators \(\mathbf{V}_n\):

sage: Sym = SymmetricFunctions(QQ)
sage: p = Sym.p()
sage: all( all( p(lam).verschiebung(2).scalar(p(mu))
....:           == p(lam).scalar(p(mu).frobenius(2))
....:           for mu in Partitions(2) )
....:      for lam in Partitions(4) )
True

antipode_on_basis(partition)#

Return the antipode of self[partition].

The antipode on the generator \(p_i\) (for \(i > 0\)) is \(-p_i\), and the antipode on \(p_\mu\) is \((-1)^{length(\mu)} p_\mu\).

INPUT:

self – the power sum basis of the symmetric functions
partition – a partition

OUTPUT:

the result of the antipode on self(partition)

EXAMPLES:

sage: Sym = SymmetricFunctions(QQ)
sage: p = Sym.p()
sage: p.antipode_on_basis([2])
-p[2]
sage: p.antipode_on_basis([3])
-p[3]
sage: p.antipode_on_basis([2,2])
p[2, 2]
sage: p.antipode_on_basis([])
p[]

bottom_schur_function(partition, degree=None)#

Return the least-degree component of s[partition], where s denotes the Schur basis of the symmetric functions, and the grading is not the usual grading on the symmetric functions but rather the grading which gives every \(p_i\) degree \(1\).

This least-degree component has its degree equal to the Frobenius rank of partition, while the degree with respect to the usual grading is still the size of partition.

This method requires the base ring to be a (commutative) \(\QQ\)-algebra. This restriction is unavoidable, since the least-degree component (in general) has noninteger coefficients in all classical bases of the symmetric functions.

The optional keyword degree allows taking any homogeneous component rather than merely the least-degree one. Specifically, if degree is set, then the degree-th component will be returned.

REFERENCES:

ClSt03: Peter Clifford, Richard P. Stanley, Bottom Schur functions. arXiv math/0311382v2.

EXAMPLES:

sage: Sym = SymmetricFunctions(QQ)
sage: p = Sym.p()
sage: p.bottom_schur_function([2,2,1])
-1/6*p[3, 2] + 1/4*p[4, 1]
sage: p.bottom_schur_function([2,1])
-1/3*p[3]
sage: p.bottom_schur_function([3])
1/3*p[3]
sage: p.bottom_schur_function([1,1,1])
1/3*p[3]
sage: p.bottom_schur_function(Partition([1,1,1]))
1/3*p[3]
sage: p.bottom_schur_function([2,1], degree=1)
-1/3*p[3]
sage: p.bottom_schur_function([2,1], degree=2)
0
sage: p.bottom_schur_function([2,1], degree=3)
1/3*p[1, 1, 1]
sage: p.bottom_schur_function([2,2,1], degree=3)
1/8*p[2, 2, 1] - 1/6*p[3, 1, 1]

coproduct_on_generators(i)#

Return coproduct on generators for power sums \(p_i\) (for \(i > 0\)).

The elements \(p_i\) are primitive elements.

INPUT:

self – the power sum basis of the symmetric functions
i – a positive integer

OUTPUT:

the result of the coproduct on the generator \(p(i)\)

EXAMPLES:

sage: Sym = SymmetricFunctions(QQ)
sage: p = Sym.powersum()
sage: p.coproduct_on_generators(2)
p[] # p[2] + p[2] # p[]

eval_at_permutation_roots_on_generators(k, rho)#

Evaluate \(p_k\) at eigenvalues of permutation matrix.

This function evaluates a symmetric function p([k]) at the eigenvalues of a permutation matrix with cycle structure \rho.

This function evaluates a \(p_k\) at the roots of unity

\[\Xi_{\rho_1},\Xi_{\rho_2},\ldots,\Xi_{\rho_\ell}\]

where

\[\Xi_{m} = 1,\zeta_m,\zeta_m^2,\ldots,\zeta_m^{m-1}\]

and \(\zeta_m\) is an \(m\) root of unity. This is characterized by \(p_k[ A , B ] = p_k[A] + p_k[B]\) and \(p_k[ \Xi_m ] = 0\) unless \(m\) divides \(k\) and \(p_{rm}[\Xi_m]=m\).

INPUT:

k – a non-negative integer
rho – a partition or a list of non-negative integers

OUTPUT:

an element of the base ring

EXAMPLES:

sage: p = SymmetricFunctions(QQ).p()
sage: p.eval_at_permutation_roots_on_generators(3, [6])
0
sage: p.eval_at_permutation_roots_on_generators(3, [3])
3
sage: p.eval_at_permutation_roots_on_generators(3, [1])
1
sage: p.eval_at_permutation_roots_on_generators(3, [3,3])
6
sage: p.eval_at_permutation_roots_on_generators(3, [1,1,1,1,1])
5