Thomson decompositions of measures in the disk

Abstract

We study the classical problem of identifying the structure of $P^2(\mu)$, the closure of analytic polynomials in the Lebesgue space $L^2(\mu)$ of a compactly supported Borel measure $\mu$ living in the complex plane. In his influential work, Thomson showed that the space decomposes into a full $L^2$-space and other pieces which are essentially spaces of analytic functions on domains in the plane. For a family of measures $\mu$ supported on the closed unit disk $\overline{\mathbb{D}}$ which have a part on the open disk $\mathbb{D}$ which is similar to the Lebesgue area measure, and a part on the unit circle $\mathbb{T}$ which is the restriction of the Lebesgue linear measure to a general measurable subset $E$ of $\mathbb{T}$, we extend the ideas of Khrushchev and calculate the exact form of the Thomson decomposition of the space $P^2(\mu)$. It turns out that the space splits according to a certain decomposition of measurable subsets of $\mathbb{T}$ which we introduce. We highlight applications to the theory of the Cauchy integral operator and de Branges-Rovnyak spaces.