Simultaneous Approximation by Finite Blaschke Products and Bounded Universal Functions

TL;DR

This paper advances approximation theory in complex analysis by demonstrating how finite Blaschke products can simultaneously approximate bounded holomorphic functions and unimodular continuous functions on the unit circle, extending classical results.

Contribution

It introduces new simultaneous approximation results using finite Blaschke products, including analogues for singular inner functions and applications to universal boundary behavior.

Findings

Existence of finite Blaschke products approximating prescribed functions

Analogues for singular inner functions established

Application to bounded holomorphic functions with universal boundary behavior

Abstract

This paper complements the work done on simultaneous approximation results in classical Banach spaces, by focusing on approximation by finite Blaschke products. We prove the existence of a finite Blaschke product that approximates a prescribed holomorphic function bounded by 1 locally uniformly on the unit disc, and simultaneously approximates a prescribed unimodular continuous function uniformly on a compact subset of the unit circle of arclength measure 0. We also prove an analogue where the continuous function is bounded by 1 and the the approximation is achieved by an appropriate dilate of the finite Blaschke product. These results are essentially combinations of classical results of Caratheodory and Fisher on approximation by finite Blaschke products. We also give analogues for singular inner functions. Finally, we apply our results to prove the existence of bounded holomorphic functions on the unit disc that exhibit a certain universal boundary behaviour.