arXiv subjects

Friedrich Knop

Publications and source records attributed to Friedrich Knop.

43 records · Page 3Linked to original sources

arXiv2001

Semisymmetric polynomials and the invariant theory of matrix vector pairs

In this paper we introduce and investigate a one-parameter family of polynomials. They are semisymmetric, i.e. symmetric in the variables with odd and even index separately. In fact, the family forms a basis of the space of semisymmetric polynomials. For two values of the parameter r, namely r=1/2 and r=1, the polynomials have a representation theoretic meaning. In general, they form the semisymmetric analogue of (shifted) Jack polynomials.

math.RT↗

arXiv2001

Construction of commuting difference operators for multiplicity free spaces

We study root systems equipped with a basis of dominant weights such that certain axioms hold. This formalism allows to define a linear basis P of the space of Weyl group invariant polynomials. This basis is actually a family depending on at least one parameter. Our main result is the construction of difference operators which are simultaneously diagonalized by P. From this, Pieri type rules are derived. This generalizes results for shifted Jack polynomials. Even though the approach is purely combinatorial, the main motivation comes from multiplicity free actions of reductive groups on vector spaces. Then the algebra of invariant differential operators has a distinguished basis, the Capelli operators, which gives rise to a basis P as above. The paper ends with a comprehensive table detailing the combinatorial structure of multiplicity free actions.

math.RT↗

arXiv1997

Weyl groups of Hamiltonian manifolds, I

We consider a connected compact Lie group K acting on a symplectic manifold M such that a moment map m exists. A pull-back function via m Poisson commutes with all K-invariants. Guillemin-Sternberg raised the problem to find a converse. In this paper, we solve this problem by determining the Poisson commutant of the algebra of K-invariants. It is completely controlled by the image of m and a certain subquotient W_M of the Weyl group of K. The group W_M is also a reflection group and forms a symplectic analogue of the little Weyl group of a symmetric space. The proof rests ultimately on techniques from algebraic geometry. In fact, a major part of the paper is of independent interest: it establishes connectivity and reducedness properties of the fibers of the (complex algebraic) moment map of a complex cotangent bundle.

dg-ga↗

arXiv1996

A recursion and a combinatorial formula for Jack polynomials

Heckman and Opdam introduced a non-symmetric analogue of Jack polynomials using Cherednik operators. In this paper, we derive a simple recursion formula for these polynomials and formulas relating the symmetric Jack polynomials with the non-symmetric ones. These formulas are then implemented by a closed expression of symmetric and non-symmetric Jack polynomials in terms of certain tableaux. The main application is a proof of a conjecture of Macdonald stating certain integrality and positivity properties of Jack polynomials.

q-alg↗

arXiv1996

Difference equations and symmetric polynomials defined by their zeros

In this paper, we introduce a new family of symmetric polynomials which depends on a parameter r. They are defined by specifying certain of their zeros. For the parameter values 1/2, 1, and 2 they have an interpretation in terms of Capelli identities. First, we give explicit formulas in some special cases. Then we show that the polynomials can also be defined in terms of difference equations. As a corollary we obtain that their top homogeneous part is a Jack polynomial. This is used to give a new proof of the Pieri formula for Jack polynomials.

q-alg↗

arXiv1996

Integrality of two variable Kostka functions

The two variable Kostka functions are the scalar products of the Macdonald polynomials with the Schur polynomials with respect to the scalar product which makes the Hall-Littlewood polynomials pairwise orthogonal. A conjecture of Macdonald states that they are polynomials in q and t with non-negative integral coefficients. We prove that the Kostka functions are at least polynomials with integral coefficients. The main idea is to prove an analogous statement for the non-symmetric Macdonald polynomials by establishing recursion relations via the affine Hecke algebra.

q-alg↗

arXiv1996

Symmetric and non-symmetric quantum Capelli polynomials

We define a family of symmetric and a family of non-symmetric polynomials in terms of vanishing conditions. These families depend on two paramters, q and t. Their main feature is that they consist of non-homogeneous polynomials. The symmetric polynomials form the quantized version of polynomials occuring in the context of generalized Capelli identities. We show that these quantum Capelli polynomials are also characterized by q-difference equations. More precisely, they are eigenfunctions of Cherednik type operators and transform under an affine Hecke algebra. Thus, we are able to identify their top homogeneous component as a Macdonald polynomial (symmetric or non-symmetric, respectively).

q-alg↗