Operator noncommutative functions

We establish a theory of noncommutative (NC) functions on a class of von Neumann algebras with a particular direct sum property, e.g., B(H). In contrast to the theory's origins, we do not rely on appealing to results from the matricial case. We prove that the kth directional derivative of any NC function at a scalar point is a k-linear homogeneous polynomial in its directions. Consequences include the fact that NC functions defined on domains containing scalar points can be uniformly approximated by free polynomials as well as realization formulas for NC functions bounded on particular sets, e.g., the NC polydisk and NC row ball.

Automated summary

This article establishes a theory of noncommutative functions on a certain class of von Neumann algebras and proves that the directional derivative of any NC function is a linear homogeneous polynomial in its directions. These results have important implications for the approximation and realization of NC functions.