Simultaneous polynomial approximation in Beurling-Sobolev spaces via Blaschke products

TL;DR

This paper develops a new approximation lemma in Orlicz-Beurling-Sobolev spaces using Blaschke products, revealing how the Luxemburg norm's asymptotic behavior depends on the function  near zero, with applications to universal function properties.

Contribution

It introduces a novel simultaneous polynomial approximation result in Orlicz-Beurling-Sobolev spaces and analyzes the asymptotic behavior of Blaschke product norms, extending classical approximation theory.

Findings

Norms remain bounded when   t^2

Norms tend to zero when =o(t^2)

Norms diverge when t^2=o()

Abstract

Assuming that $\phi(t)=o(t^2)$ as $t\to0$, we establish a lemma on simultaneous polynomial approximation in Orlicz-Beurling-Sobolev spaces $\ell_a^{\phi}$. These spaces, endowed with the Luxemburg norm $\Vert \cdot \Vert_{\ell^{\phi}}$, generalize the classical Beurling-Sobolev spaces $\ell_a^p$ for $p>2$. More precisely, we prove that for every $\varepsilon>0$, every $v\in\mathbb{N}$ and every function $\varphi$ continuous on $\partial\mathbb{D}$, there exist a polynomial $P(z)=\sum_{k=v}^d a_k z^k$ and a compact set $K\subset\partial\mathbb{D}$ with $m(K)>1-\varepsilon$ such that \[\|P\|_{\ell^{\phi}}\le\varepsilon \quad \text{and}\quad \|P-\varphi\|_K\le\varepsilon.\] The proof relies on a result of independent interest describing the asymptotic behaviour of the Luxemburg norm $\|B^k\|_{\ell^{\phi}}$ of powers of a finite Blaschke product $B$ which is not a monomial. This behaviour is governed by the comparison between $\phi(t)$ and $t^2$ near $0$: the norms remain bounded when $\phi\asymp t^2$, tend to $0$ when $\phi=o(t^2)$, and diverge to $+\infty$ when $t^2=o(\phi(t))$. A key ingredient in the proof is the qualitative limit $\sup_{j\ge0}|\widehat{B^k}(j)|\to0$ as $k\to\infty$. As an application of the simultaneous approximation lemma, we derive the existence of functions in $\ell_a^{\phi}$ with universal properties, including Menshov universality of Taylor partial sums and universality with respect to radial boundary limits.