Finite Gap Jacobi Matrices, III. Beyond the Szeg\H{o} Class

TL;DR

This paper extends the analysis of finite gap Jacobi matrices beyond the Szeg"H{o} class, establishing conditions under which ratios of orthogonal polynomials converge outside the real line, even for non-Szeg"H{o} class cases.

Contribution

It introduces new spectral conditions involving harmonic measures and frequency modules that guarantee the convergence of polynomial ratios beyond the Szeg"H{o} class.

Findings

Ratios of orthogonal polynomials converge under specified spectral conditions.

Non-Szeg"H{o} class Jacobi matrices can exhibit similar convergence properties.

Conditions involve finite limits of weighted sums and subexponential growth constraints.

Abstract

Let $\fre\subset\bbR$ be a finite union of $\ell+1$ disjoint closed intervals and denote by $\omega_j$ the harmonic measure of the $j$ leftmost bands. The frequency module for $\fre$ is the set of all integral combinations of $\omega_1,..., \omega_\ell$. Let $\{\tilde{a}_n, \tilde{b}_n\}_{n=1}^\infty$ be a point in the isospectral torus for $\fre$ and $\tilde{p}_n$ its orthogonal polynomials. Let $\{a_n,b_n\}_{n=1}^\infty$ be a half-line Jacobi matrix with $a_n = \tilde{a}_n + \delta a_n$, $b_n = \tilde{b}_n + \delta b_n$. Suppose \[ \sum_{n=1}^\infty %(\abs{a_n-\tilde{a}_n}^2 + \abs{b_n-\tilde{b}_n}^2) <\infty \abs{\delta a_n}^2 + \abs{\delta b_n}^2 <\infty \] and $\sum_{n=1}^N e^{2\pi i\omega n} \delta a_n$, $\sum_{n=1}^N e^{2\pi i\omega n} \delta b_n$ have finite limits as $N\to\infty$ for all $\omega$ in the frequency module. If, in addition, these partial sums grow at most subexponentially with respect to $\omega$, then for $z\in\bbC\setminus\bbR$, $p_n(z)/\tilde{p}_n(z)$ has a limit as $n\to\infty$. Moreover, we show that there are non-Szeg\H{o} class $J$'s for which this holds.