arXiv subjects

Jacob S. Christiansen

Publications and source records attributed to Jacob S. Christiansen.

24 records · Page 2Linked to original sources

arXiv2011

A question by Chihara about shell polynomials and indeterminate moment problems

The generalized Stieltjes--Wigert polynomials depending on parameters 0\le p<1 and 0<q<1 are discussed. By removing the mass at zero of the N-extremal solution concentrated in the zeros of the D-function from the Nevanlinna parametrization, we obtain a discrete measure μ^M which is uniquely determined by its moments. We calculate the coefficients of the corresponding orthonormal polynomials (P^M_n). As noticed by Chihara, these polynomials are the shell polynomials corresponding to the maximal parameter sequence for a certain chain sequence. We also find the minimal parameter sequence, as well as the parameter sequence corresponding to the generalized Stieltjes--Wigert polynomials, and compute the value of related continued fractions. The mass points of μ^M have been studied in recent papers of Hayman, Ismail--Zhang and Huber. In the special case of p=q, the maximal parameter sequence is constant and the determination of μ^M and (P^M_n) gives an answer to a question posed by Chihara in 2001

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arXiv2009

Finite Gap Jacobi Matrices, II. The Szegő Class

Let $\fre\subset\bbR$ be a finite union of disjoint closed intervals. We study measures whose essential support is $\fre$ and whose discrete eigenvalues obey a 1/2-power condition. We show that a Szegő condition is equivalent to \[ \limsup \f{a_1... a_n}{\ca(\fre)^n}>0 \] (this includes prior results of Widom and Peherstorfer--Yuditskii). Using Remling's extension of the Denisov--Rakhmanov theorem and an analysis of Jost functions, we provide a new proof of Szegő asymptotics, including $L^2$ asymptotics on the spectrum. We use heavily the covering map formalism of Sodin--Yuditskii as presented in our first paper in this series.

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arXiv2008

Finite Gap Jacobi Matrices, I. The Isospectral Torus

Let $\frak{e}\subset\mathbb{R}$ be a finite union of disjoint closed intervals. In the study of OPRL with measures whose essential support is $\frak{e}$, a fundamental role is played by the isospectral torus. In this paper, we use a covering map formalism to define and study this isospectral torus. Our goal is to make a coherent presentation of properties and bounds for this special class as a tool for ourselves and others to study perturbations. One important result is the expression of Jost functions for the torus in terms of theta functions.

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arXiv2007

Finite Gap Jacobi Matrices: An Announcement

We consider Jacobi matrices whose essential spectrum is a finite union of closed intervals. We focus on Szego's theorem, Jost solutions, and Szego asymptotics for this situation. This announcement describes talks the authors gave at OPSFA 2007.

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arXiv2007

Self-adjoint difference operators and symmetric Al-Salam--Chihara polynomials

The symmetric Al-Salam--Chihara polynomials for $q>1$ are associated with an indeterminate moment problem. There is a self-adjoint second order difference operator on $\ell^2(\Z)$ to which these polynomials are eigenfunctions. We determine the spectral decomposition of this self-adjoint operator. This leads to a class of discrete orthogonality measures, which have been obtained previously by Christiansen and Ismail using a different method, and we give an explicit orthogonal basis for the corresponding weighted $\ell^2$-space. In particular, the orthocomplement of the polynomials is described explicitly. Taking a limit we obtain all the $N$-extremal solutions to the $q^{-1}$-Hermite moment problem, a result originally obtained by Ismail and Masson in a different way. Some applications of the results are discussed.

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arXiv2005

Self-adjoint difference operators and classical solutions to the Stieltjes--Wigert moment problem

The Stieltjes-Wigert polynomials, which correspond to an indeterminate moment problem on the positive half-line, are eigenfunctions of a second order q-difference operator. We consider the orthogonality measures for which the difference operator is symmetric in the corresponding weighted $L^2$-spaces. These measures are exactly the solutions to the q-Pearson equation.In the case of discrete and absolutely continuous measures the difference operator is essentially self-adjoint, and the corresponding spectral decomposition is given explicitly. In particular, we find an orthogonal set of q-Bessel functions complementing the Stieltjes-Wigert polynomials to an orthogonal basis for $L^2(μ)$ when $μ$ is a discrete orthogonality measure solving the q-Pearson equation. To obtain the spectral decomposition of the difference operator in case of an absolutely continuous orthogonality measure we use the results from the discrete case combined with direct integral techniques.

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