# Monomial symmetric functions¶

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

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

TESTS:

sage: m = SymmetricFunctions(QQ).m()
True
sage: TestSuite(m).run(skip=['_test_associativity', '_test_distributivity', '_test_prod'])
sage: TestSuite(m).run(elements = [m[1,1]+m[2], m[1]+2*m[1,1]])

class Element(M, x)

Create a combinatorial module element. This should never be called directly, but only through the parent combinatorial free module’s __call__() method.

TESTS:

sage: F = CombinatorialFreeModule(QQ, ['a','b','c'])
sage: B = F.basis()
sage: f = B['a'] + 3*B['c']; f
B['a'] + 3*B['c']
True

expand(n, alphabet='x')

Expands the symmetric function as a symmetric polynomial in $$n$$ variables.

INPUT:

• self – an element of the monomial symmetric function basis
• n – a positive integer
• alphabet – a variable for the expansion (default: $$x$$)

OUTPUT: a monomial expansion of an instance of self in $$n$$ variables

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

SymmetricFunctionAlgebra_monomial.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 $$\QQ$$?

SymmetricFunctionAlgebra_monomial.from_polynomial(f, check=True)

Returns 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]

SymmetricFunctionAlgebra_monomial.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]


..SEEALSO:: Partition(), Partition.to_exp()

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