A new perspective on Riemann surfaces and von Neumann algebras via thin Blaschke products

Abstract

In 2014, K.Guo and the second author constructed the first example of an infinite Blaschke product $B$ (a thin Blaschke product), for which the associated multiplication operator $M_B$ is irreducible on the Bergman space $L_a^2(\mathbb{D})$ over the unit disk $\mathbb D$. We take a substantial step forward toward characterizing the irreducibility of the operator $M_B$ on $L_a^2(\mathbb{D})$, or equivalently, the trivialness of the von Neumann algebra $\{M_B,M_B^*\}'$. This characterization indicates a close connection with the primeness of $B$ and with the two-component property of the Riemann surface $\mathcal{S}_B$ associated with $B$. The approach is built upon a nested family of connected components of the sublevel set $\{z\in \mathbb{D}:|B(z)|<t\}$ for $0<t<1$, which captures the function-theoretic and geometric feature of $B$.