An inductive Julia-Caratheodory theorem for Pick functions in two variables

Abstract

We study the asymptotic behavior of Pick functions, analytic functions which take the upper half plane to itself. We show that if a two variable Pick function $f$ has real residues to order $2N-1$ at infinity and the imaginary part of the remainder between $f$ and this expansion is of order $2N+1,$ then $f$ has real residues to order $2N$ and directional residues to order $2N+1.$ Furthermore, $f$ has real residues to order $2N+1$ if and only if the $2N+1$-th derivative is given by a polynomial, thus obtaining a two variable analogue of a higher order Julia-Carathéodory type theorem.