Pseudo-Taylor expansions and the Carathéodory-Fejér problem

Abstract

We give a new solvability criterion for the boundary Carathéodory-Fejér 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.

18 pages