Comparing Two Generalized Noncommutative Nevanlinna–Pick Theorems

We explore the relationship between two noncommutative generalizations of the classical Nevanlinna–Pick theorem: one proved by Constantinescu and Johnson in 2003 and the other proved by Muhly and Solel in 2004. To make the comparison, we generalize Constantinescu and Johnson’s theorem to the context of W^∗-correspondences and Hardy algebras. After formulating the so-called displacement equation in this context, we are able to follow Constantinescu and Johnson’s line of reasoning in our proof. Though our result is similar in appearance to Muhly and Solel’s, closer inspection reveals differences. Nevertheless, when the given data lie in the center of the dual correspondence, the theorems are essentially the same.