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In mathematics, Macdonald polynomials Pλ(x; t,q) are a family of orthogonal polynomials in several variables, introduced by Macdonald (1987). He later introduced a non-symmetric generalization in 1995. Macdonald originally associated his polynomials with weights λ of finite root systems and used just one variable t, but later realized that it is more natural to associate them with affine root systems rather than finite root systems, in which case the variable t can be replaced by several different variables t=(t1,...,tk), one for each of the k orbits of roots in the affine root system. The Macdonald polynomials are polynomials in n variables x=(x1,...,xn), where n is the rank of the affine root system. They generalize many other families of orthogonal polynomials, such as Jack polynomials and Hall–Littlewood polynomials and Askey–Wilson polynomials, which in turn include most of the named 1-variable orthogonal polynomials as special cases. Koornwinder polynomials are Macdonald polynomials of certain non-reduced root systems. They have deep relationships with affine Hecke algebras and Hilbert schemes, which were used to prove several conjectures made by Macdonald about them.
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Definition
First fix some notation:
The Macdonald polynomials Pλ for λ ∈ P+ are uniquely defined by the following two conditions:
In other words, the Macdonald polynomials are obtained by orthogonalizing the obvious basis for AW. The existence of polynomials with these properties is easy to show (for any inner product). A key property of the Macdonald polynomials is that they are orthogonal: 〈Pλ, Pμ〉 = 0 whenever λ ≠ μ. This is not a trivial consequence of the definition because P+ is not totally ordered, and so has plenty of elements that are incomparable. Thus one must check that the corresponding polynomials are still orthogonal. The orthogonality can be proved by showing that the Macdonald polynomials are eigenvectors for an algebra of commuting self adjoint operators with 1-dimensional eigenspaces, and using the fact that eigenspaces for different eigenvalues must be orthogonal.
In the case of non-simply-laced root systems (B, C, F, G), the parameter t can be chosen to vary with the length of the root, giving a three-parameter family of Macdonald polynomials. One can also extend the definition to the nonreduced root system BC, in which case one obtains a six-parameter family (one t for each orbit of roots, plus q) known as Koornwinder polynomials. It is sometimes better to regard Macdonald polynomials as depending on a possibly non-reduced affine root system. In this case there is one parameter t associated to each orbit of roots in the affine root system, plus one parameter q. The number of orbits of roots can vary from 1 to 5.
Examples
1, C1), the Macdonald polynomials are the Askey–Wilson polynomials, which in turn include as special cases most of the named families of orthogonal polynomials in 1 variable.
n, Cn), the Macdonald polynomials are the
Koornwinder polynomials.
The Macdonald constant term conjecture
If t = qk for some positive integer k, then the norm of the Macdonald polynomials is given by
This was conjectured by Macdonald (1982) as a generalization of the Dyson conjecture, and proved for all (reduced) root systems by Cherednik (1995) using properties of double affine Hecke algebras. The conjecture had previously been proved case-by-case for all roots systems except those of type En by several authors.
There are two other conjectures which together with the norm conjecture are collectively referred to as the Macdonald conjectures in this context: in addition to the formula for the norm, Macdonald conjectured a formula for the value of Pλ at the point tρ, and a symmetry
Again, these were proved for general reduced root systems by Cherednik (1995), using double affine Hecke algebras, with the extension to the BC case following shortly thereafter via work of van Diejen, Noumi, and Sahi.
The Macdonald positivity conjecture
In the case of roots systems of type An−1 the Macdonald polynomials are simply symmetric polynomials in n variables with coefficients that are rational functions of q and t. A certain transformed version
It is still a central open problem in algebraic combinatorics to find a combinatorial formula for the qt-Kostka coefficients.
n! conjecture
The n! conjecture of Adriano Garsia and Mark Haiman states that for each partition μ of n the space
spanned by all higher partial derivatives of
has dimension n!, where (pj, qj) run through the n elements of the diagram of the partition μ, regarded as a subset of the pairs of non-negative integers. For example, if μ is the partition 3 = 2 + 1 of n = 3 then the pairs (pj, qj) are (0, 0), (0, 1), (1, 0), and the space Dμ is spanned by
which has dimension 6 = 3!.
Haiman's proof of the Macdonald positivity conjecture and the n! conjecture involved showing that the isospectral Hilbert scheme of n points in a plane was Cohen–Macaulay (and even Gorenstein). Earlier results of Haiman and Garsia had already shown that this implied the n! conjecture, and that the n! conjecture implied that the Kostka–Macdonald coefficients were graded character multiplicities for the modules Dμ. This immediately implies the Macdonald positivity conjecture because character multiplicities have to be non-negative integers.
Ian Grojnowski and Mark Haiman found another proof of the Macdonald positivity conjecture by proving a positivity conjecture for LLT polynomials.
Combinatorial formula for the Macdonald polynomials
In 2005, J. Haglund, M. Haiman and N. Loehr gave the first proof of a combinatorial interpretation of the Macdonald polynomials. While very useful for computation and interesting in its own right, this combinatorial formula does not immediately imply positivity of the Kostka-Macdonald coefficients Kλμ(q,t), as it gives the decomposition of the Macdonald polynomials into monomial symmetric functions rather than into Schur functions.
The formula, which involves the transformed Macdonald polynomials
where σ is a filling of the Young diagram of shape μ, inv and maj are certain combinatorial statistics (functions) defined on the filling σ. This formula expresses the Macdonald polynomials in infinitely many variables. To obtain the polynomials in n variables, simply restrict the formula to fillings that only use the integers 1,2,...,n. The term xσ should be interpreted as
The transformed Macdonald polynomials
where
The transformed Macdonald polynomials
where
This formula can be used to prove Knop and Sahi's formula for the Jack polynomials. T
Non-symmetric Macdonald polynomials
In 1995, Macdonald introduced a non-symmetric analogue of the symmetric Macdonald polynomials, and the symmetric Macdonald polynomials can easily be recovered from the non-symmetric counterpart. In his original definition, he shows that the non-symmetric Macdonald polynomials are a unique family of polynomials orthogonal to a certain inner product, as well as satisfying a triangularity property when expanded in the monomial basis.
In 2007, Haglund, Haiman and Loehr gave a combinatorial formula for the non-symmetric Macdonald polynomials.
The non-symmetric Macdonald polynomials specialize to Demazure characters by taking q=t=0, and to key polynomials when q=t=∞.