Pseudo-Taylor expansions and the Carath\'{e}odory-Fej\'{e}r problem

TL;DR

This paper introduces a new criterion for solving the boundary Carathéodory-Fejér problem, using positivity of Hankel matrices and a reduction method, to construct functions with prescribed boundary derivatives in the Pick class.

Contribution

It provides a novel solvability criterion for the boundary Carathéodory-Fejér problem based on Hankel matrix positivity and reduction techniques.

Findings

The criterion is expressed in terms of positivity of an associated Hankel matrix.

The proof utilizes a reduction method due to Julia and Nevanlinna.

The approach offers a new perspective on boundary interpolation problems.

Abstract

We give a new solvability criterion for the boundary Carath\'{e}odory-Fej\'{e}r problem: given a point $x \in \mathbb{R}$ and, a finite set of target values $a^0,a^1,...,a^n \in \mathbb{R}$, to construct a function $f$ in the Pick class such that the limit of $f^{(k)}(z)/k!$ as $z \to x$ nontangentially in the upper half plane is $a^k$ for $k= 0,1,...,n$. The criterion is in terms of positivity of an associated Hankel matrix. The proof is based on a reduction method due to Julia and Nevanlinna.