Coefficients of Orthogonal Polynomials on the Unit Circle and Higher Order Szego Theorems

Abstract

Let $\mu$ be a non-trivial probability measure on the unit circle $\partial\bbD$, $w$ the density of its absolutely continuous part, $\alpha_n$ its Verblunsky coefficients, and $\Phi_n$ its monic orthogonal polynomials. In this paper we compute the coefficients of $\Phi_n$ in terms of the $\alpha_n$. If the function $\log w$ is in $L^1(d\theta)$, we do the same for its Fourier coefficients. As an application we prove that if $\alpha_n \in \ell^4$ and $Q(z) = \sum_{m=0}^N q_m z^m$ is a polynomial, then with $\bar Q(z) = \sum_{m=0}^N \bar q_m z^m$ and $S$ the left shift operator on sequences we have $|Q(e^{i\theta})|^2 \log w(\theta) \in L^1(d\theta)$ if and only if $\{\bar Q(S)\alpha\}_n \in \ell^2$. We also study relative ratio asymptotics of the reversed polynomials $\Phi_{n+1}^*(\mu)/\Phi_n^*(\mu)-\Phi_{n+1}^*(\nu)/\Phi_n^*(\nu)$ and provide a necessary and sufficient condition in terms of the Verblunsky coefficients of the measures $\mu$ and $\nu$ for this difference to converge to zero uniformly on compact subsets of $\bbD$.