Carathéodory interpolation on the non-commutative polydisk

Abstract

The Carathéodory problem in the $N$-variable non-commutative Herglotz--Agler class and the Carathéodory--Fejér problem in the $N$-variable non-commutative Schur--Agler class are posed. It is shown that the Carathéodory (resp., Carathéodory--Fejér) problem has a solution if and only if the non-commutative polynomial with given operator coefficients (the data of the problem indexed by an admissible set $\Lambda$) takes operator values with positive semidefinite real part (resp., contractive operator values) on $N$-tuples of $\Lambda$-jointly nilpotent contractive $n\times n$ matrices, for all $n\in \mathbb{N}$.

to appear in J. Funct. Anal