On de Branges--Rovnyak Kernels Admitting a Complete Pick Factor

Abstract: For a contractive multiplier $\varphi$ in the multiplier algebra $M(k)$ of a kernel $k$, the associated de Branges--Rovnyak kernel is given by $$k^\varphi(x,y) = (1-\varphi(x)\overline{\varphi(y)})k(x,y)$$. Motivated by recent developments clarifying the structural and geometric features of reproducing kernel Hilbert spaces associated with kernels admitting a complete Pick factor, we investigate the precise conditions for a general de Branges-Rovnyak kernel to admit a complete Pick factor, thereby extending the framework introduced by Ahmed, Das and Panja (\textit{J. Geom. Anal.}, 2025). In this paper, we characterize the existence of a complete Pick factor for $k<sup>\varphi$ across a broad class of base kernels encompassing both complete Pick and non-complete Pick architectures (such as the Szegő kernel on the polydisk). Our first characterization is formulated in terms of operator-valued holomorphic functions satisfying an interpolation condition. We also show that $k<sup>\varphi$ admits a complete Pick factor if and only if $(\widetilde k)<sup>\varphi$ is itself a complete Pick kernel, where $\widetilde k$ is an auxiliary kernel constructed from the given data. Notably, our main result is completely new even when specialized to the classical Szegö kernel of the unit disk. As an application of our framework, we obtain a structural insight into a classical theorem of Chu (\textit{J. Funct. Anal.}, 2020) and provide an alternative proof of a recent result by Luo and Zhu (\textit{Canad. J. Math.}, 2024). The results are illustrated by concrete examples.