# On the Nevanlinna problem -- Characterization of all Schur-Agler class solutions affiliated with a given kernel

## TL;DR

This paper generalizes Nevanlinna's classical interpolation characterization to multivariable domains for functions in the Schur-Agler class, focusing on the bidisc, symmetrized bidisc, and annulus, using kernel methods.

## Contribution

It extends Nevanlinna's interpolation results to several complex variables within the Schur-Agler class, linked to specific kernels and domains.

## Key findings

- Characterization of Schur-Agler class solutions on the bidisc
- Extension of Nevanlinna's theorem to the symmetrized bidisc
- Interpolation results for the annulus domain

## Abstract

Given a domain $\Omega$ in $\mathbb{C}^m$, and a finite set of points $z_1,z_2,\ldots, z_n\in \Omega$ and $w_1,w_2,\ldots, w_n\in \mathbb{D}$ (the open unit disc in the complex plane), the \textit{Pick interpolation problem} asks when there is a holomorphic function $f:\Omega \rightarrow \overline{\mathbb{D}}$ such that $f(z_i)=w_i,1\leq i\leq n$. Pick gave a condition on the data $\{z_i, w_i:1\leq i\leq n\}$ for such an $interpolant$ to exist if $\Omega=\mathbb{D}$. Nevanlinna characterized all possible functions $f$ that \textit{interpolate} the data. We generalize Nevanlinna's result to a domain $\Omega$ in $\mathbb{C}^m$ admitting holomorphic test functions when the function $f$ comes from the Schur-Agler class and is affiliated with a certain completely positive kernel. The Schur class is a naturally associated Banach algebra of functions with a domain. The success of the theory lies in characterizing the Schur class interpolating functions for three domains - the bidisc, the symmetrized bidisc and the annulus - which are affiliated to given kernels.