Theorems of Szegö–Verblunsky type in the multivariate and almost periodic settings

The classical Szegö–Verblunsky theorem relates integrability of the logarithm of the absolutely continuous part of a probability measure on the circle to square summability of the sequence of recurrence coefficients for the orthogonal polynomials determined by the measure. The present paper constructs orthogonal polynomials on the torus of arbitrary finite dimension in order to prove theorems of Szegő-Verblunsky type in the multivariate and almost periodic settings. The results are applied to the one-dimensional Schrödinger equation in impedance form to yield a new trace formula valid for piecewise constant impedance, a case where the classical trace formula breaks down. As a byproduct, the analysis gives an explicit formula for the Taylor coefficients of a bounded holomorphic function on the open disk in terms of its continued fraction expansion.