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zbMATH Open
Article
Data sources: zbMATH Open
MATHEMATICA SCANDINAVICA
Article . 1996 . Peer-reviewed
Data sources: Crossref
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Approximation by Interpolating Blaschke Products.

Approximation by interpolating Blaschke products
Authors: Oyma, Knut;

Approximation by Interpolating Blaschke Products.

Abstract

Suppose that \(H^\infty\) is the set of bounded analytic functions in the unit disk \(D\) and that \(T=\partial D\). A Blaschke product \(B(z)\) is a bounded analytic function of the form \[ B(z)= z^m \prod_{n=1}^\infty \frac{-\overline{z}_n} {|z_n|} \frac{z- z_n} {1-\overline{z}_n} \quad \text{where } \sum(1-|z_n|)< \infty,\;|B(e^{i\theta})|=1 \text{ a.e.} \] A sequence \(\{z_n\}\subset D\) is called interpolating if every interpolation problem \(f(z_n)= w_n\), \(\{w_n\}\in l^\infty\) has a solution in \(H^\infty\). A Blaschke product whose zero set is an interpolating sequence is called an interpolating Blaschke product. The author gives a new proof of the theorem of Jones which says that every unimodular function in \(L^\infty(T)\) can be uniformly approximated by the ratio of two interpolating Blaschke products. The proof uses ideas of Garnett and Nicolau which were used to prove that every function in \(H^\infty\) can be uniformly approximated by a linear combination of interpolating Blaschke products. In particular, let \(K\) be the closed convex hull of the interpolating Blaschke products. Garnett and Nicolau proved that for any \(h\in H^\infty\), there exists a constant \(c_f\) such that \(f\in c_f K\). It is unknown whether every \(H^\infty\) function of unit norm is contained in \(K\). The author proves that every \(H^\infty\) function of sufficiently small norm is contained in \(K\). He remarks that \(|f|< 10^{-1000}\) is sufficiently small.

Country
Germany
Related Organizations
Keywords

510.mathematics, Blaschke products, etc., Blaschke product, Article, interpolation

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selected citations
These citations are derived from selected sources.
This is an alternative to the "Influence" indicator, which also reflects the overall/total impact of an article in the research community at large, based on the underlying citation network (diachronically).
BIP!Citations provided by BIP!
popularity
This indicator reflects the "current" impact/attention (the "hype") of an article in the research community at large, based on the underlying citation network.
BIP!Popularity provided by BIP!
influence
This indicator reflects the overall/total impact of an article in the research community at large, based on the underlying citation network (diachronically).
BIP!Influence provided by BIP!
impulse
This indicator reflects the initial momentum of an article directly after its publication, based on the underlying citation network.
BIP!Impulse provided by BIP!
3
Average
Top 10%
Average
Green
bronze