A comparison theorem for Clark-Aleksandrov measures on the torus

Abstract

Given two Clark-Aleksandrov measures $\sigma^1$ and $\sigma^2$ on $\bT^2$, we prove a theorem relating the property that $\sigma^1 \ll \sigma^2$ to containment of a concrete function in a certain de Branges-Rovnyak space. We show that our theorem is applicable for all rational inner functions on $\bD^2$, and we provide several examples of how the theorem can be applied to investigate Clark-Aleksandrov measures related to such functions.