# Möbius Algebras¶

class sage.combinat.posets.moebius_algebra.BasisAbstract(R, basis_keys=None, element_class=None, category=None, prefix=None, names=None, **kwds)

Abstract base class for a basis.

class sage.combinat.posets.moebius_algebra.MoebiusAlgebra(R, L)

The Möbius algebra of a lattice.

Let $$L$$ be a lattice. The Möbius algebra $$M_L$$ was originally constructed by Solomon [Solomon67] and has a natural basis $$\{ E_x \mid x \in L \}$$ with multiplication given by $$E_x \cdot E_y = E_{x \vee y}$$. Moreover this has a basis given by orthogonal idempotents $$\{ I_x \mid x \in L \}$$ (so $$I_x I_y = \delta_{xy} I_x$$ where $$\delta$$ is the Kronecker delta) related to the natural basis by

$I_x = \sum_{x \leq y} \mu_L(x, y) E_y,$

where $$\mu_L$$ is the Möbius function of $$L$$.

Note

We use the join $$\vee$$ for our multiplication, whereas [Greene73] and [Etienne98] define the Möbius algebra using the meet $$\wedge$$. This is done for compatibility with QuantumMoebiusAlgebra.

REFERENCES:

 [Solomon67] Louis Solomon. The Burnside Algebra of a Finite Group. Journal of Combinatorial Theory, 2, 1967. doi:10.1016/S0021-9800(67)80064-4.
 [Greene73] Curtis Greene. On the Möbius algebra of a partially ordered set. Advances in Mathematics, 10, 1973. doi:10.1016/0001-8708(73)90106-0.
 [Etienne98] Gwihen Etienne. On the Möbius algebra of geometric lattices. European Journal of Combinatorics, 19, 1998. doi:10.1006/eujc.1998.0227.
class E(M, prefix='E')

The natural basis of a Möbius algebra.

Let $$E_x$$ and $$E_y$$ be basis elements of $$M_L$$ for some lattice $$L$$. Multiplication is given by $$E_x E_y = E_{x \vee y}$$.

one()

Return the element 1 of self.

EXAMPLES:

sage: L = posets.BooleanLattice(4)
sage: E = L.moebius_algebra(QQ).E()
sage: E.one()
E[0]

product_on_basis(x, y)

Return the product of basis elements indexed by x and y.

EXAMPLES:

sage: L = posets.BooleanLattice(4)
sage: E = L.moebius_algebra(QQ).E()
sage: E.product_on_basis(5, 14)
E[15]
sage: E.product_on_basis(2, 8)
E[10]

class I(M, prefix='I')

The (orthogonal) idempotent basis of a Möbius algebra.

Let $$I_x$$ and $$I_y$$ be basis elements of $$M_L$$ for some lattice $$L$$. Multiplication is given by $$I_x I_y = \delta_{xy} I_x$$ where $$\delta_{xy}$$ is the Kronecker delta.

one()

Return the element 1 of self.

EXAMPLES:

sage: L = posets.BooleanLattice(4)
sage: I = L.moebius_algebra(QQ).I()
sage: I.one()
I[0] + I[1] + I[2] + I[3] + I[4] + I[5] + I[6] + I[7] + I[8]
+ I[9] + I[10] + I[11] + I[12] + I[13] + I[14] + I[15]

product_on_basis(x, y)

Return the product of basis elements indexed by x and y.

EXAMPLES:

sage: L = posets.BooleanLattice(4)
sage: I = L.moebius_algebra(QQ).I()
sage: I.product_on_basis(5, 14)
0
sage: I.product_on_basis(2, 2)
I[2]

a_realization()

Return a particular realization of self (the $$B$$-basis).

EXAMPLES:

sage: L = posets.BooleanLattice(4)
sage: M = L.moebius_algebra(QQ)
sage: M.a_realization()
Moebius algebra of Finite lattice containing 16 elements
over Rational Field in the natural basis

lattice()

Return the defining lattice of self.

EXAMPLES:

sage: L = posets.BooleanLattice(4)
sage: M = L.moebius_algebra(QQ)
sage: M.lattice()
Finite lattice containing 16 elements
sage: M.lattice() == L
True

class sage.combinat.posets.moebius_algebra.MoebiusAlgebraBases(parent_with_realization)

The category of bases of a Möbius algebra.

INPUT:

• base – a Möbius algebra
class ElementMethods
class ParentMethods
one()

Return the element 1 of self.

EXAMPLES:

sage: L = posets.BooleanLattice(4)
sage: C = L.quantum_moebius_algebra().C()
sage: all(C.one() * b == b for b in C.basis())
True

product_on_basis(x, y)

Return the product of basis elements indexed by x and y.

EXAMPLES:

sage: L = posets.BooleanLattice(4)
sage: C = L.quantum_moebius_algebra().C()
sage: C.product_on_basis(5, 14)
q^3*C[15]
sage: C.product_on_basis(2, 8)
q^4*C[10]

super_categories()

The super categories of self.

EXAMPLES:

sage: from sage.combinat.posets.moebius_algebra import MoebiusAlgebraBases
sage: M = posets.BooleanLattice(4).moebius_algebra(QQ)
sage: bases = MoebiusAlgebraBases(M)
sage: bases.super_categories()
[Category of finite dimensional commutative algebras with basis over Rational Field,
Category of realizations of Moebius algebra of Finite lattice
containing 16 elements over Rational Field]

class sage.combinat.posets.moebius_algebra.QuantumMoebiusAlgebra(L, q=None)

The quantum Möbius algebra of a lattice.

Let $$L$$ be a lattice, and we define the quantum Möbius algebra $$M_L(q)$$ as the algebra with basis $$\{ E_x \mid x \in L \}$$ with multiplication given by

$E_x E_y = \sum_{z \geq a \geq x \vee y} \mu_L(a, z) q^{\operatorname{crk} a} E_z,$

where $$\mu_L$$ is the Möbius function of $$L$$ and $$\operatorname{crk}$$ is the corank function (i.e., $$\operatorname{crk} a = \operatorname{rank} L - \operatorname{rank}$$ a). At $$q = 1$$, this reduces to the multiplication formula originally given by Solomon.

class C(M, prefix='C')

The characteristic basis of a quantum Möbius algebra.

The characteristic basis $$\{ C_x \mid x \in L \}$$ of $$M_L$$ for some lattice $$L$$ is defined by

$C_x = \sum_{a \geq x} P(F^x; q) E_a,$

where $$F^x = \{ y \in L \mid y \geq x \}$$ is the principal order filter of $$x$$ and $$P(F^x; q)$$ is the characteristic polynomial of the (sub)poset $$F^x$$.

class E(M, prefix='E')

The natural basis of a quantum Möbius algebra.

Let $$E_x$$ and $$E_y$$ be basis elements of $$M_L$$ for some lattice $$L$$. Multiplication is given by

$E_x E_y = \sum_{z \geq a \geq x \vee y} \mu_L(a, z) q^{\operatorname{crk} a} E_z,$

where $$\mu_L$$ is the Möbius function of $$L$$ and $$\operatorname{crk}$$ is the corank function (i.e., $$\operatorname{crk} a = \operatorname{rank} L - \operatorname{rank}$$ a).

one()

Return the element 1 of self.

EXAMPLES:

sage: L = posets.BooleanLattice(4)
sage: E = L.quantum_moebius_algebra().E()
sage: all(E.one() * b == b for b in E.basis())
True

product_on_basis(x, y)

Return the product of basis elements indexed by x and y.

EXAMPLES:

sage: L = posets.BooleanLattice(4)
sage: E = L.quantum_moebius_algebra().E()
sage: E.product_on_basis(5, 14)
E[15]
sage: E.product_on_basis(2, 8)
q^2*E[10] + (q-q^2)*E[11] + (q-q^2)*E[14] + (1-2*q+q^2)*E[15]

class KL(M, prefix='KL')

The Kazhdan-Lusztig basis of a quantum Möbius algebra.

The Kazhdan-Lusztig basis $$\{ B_x \mid x \in L \}$$ of $$M_L$$ for some lattice $$L$$ is defined by

$B_x = \sum_{y \geq x} P_{x,y}(q) E_a,$

where $$P_{x,y}(q)$$ is the Kazhdan-Lusztig polynomial of $$L$$, following the definition given in [EPW14].

EXAMPLES:

We construct some examples of Proposition 4.5 of [EPW14]:

sage: M = posets.BooleanLattice(4).quantum_moebius_algebra()
sage: KL = M.KL()
sage: KL[4] * KL[5]
(q^2+q^3)*KL[5] + (q+2*q^2+q^3)*KL[7] + (q+2*q^2+q^3)*KL[13]
+ (1+3*q+3*q^2+q^3)*KL[15]
sage: KL[4] * KL[15]
(1+3*q+3*q^2+q^3)*KL[15]
sage: KL[4] * KL[10]
(q+3*q^2+3*q^3+q^4)*KL[14] + (1+4*q+6*q^2+4*q^3+q^4)*KL[15]

a_realization()

Return a particular realization of self (the $$B$$-basis).

EXAMPLES:

sage: L = posets.BooleanLattice(4)
sage: M = L.quantum_moebius_algebra()
sage: M.a_realization()
Quantum Moebius algebra of Finite lattice containing 16 elements
with q=q over Univariate Laurent Polynomial Ring in q
over Integer Ring in the natural basis

lattice()

Return the defining lattice of self.

EXAMPLES:

sage: L = posets.BooleanLattice(4)
sage: M = L.quantum_moebius_algebra()
sage: M.lattice()
Finite lattice containing 16 elements
sage: M.lattice() == L
True