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Favard theorem for reproducing kernels

Authors: Bultheel, Adhemar; González-Vera, Pablo; Hendriksen, Erik; Njåstad, Olav;

Favard theorem for reproducing kernels

Abstract

Consider for \(n = 0, 1, \dots\) the nested spaces \({\mathcal L}_n\) of rational functions of degree \(n\) at most with given poles \(1/ \overline \alpha_i\), \(|\alpha_i |< 1\), \(i = 1, \dots, n\). Let \({\mathcal L} = \cup^\infty_0 {\mathcal L}_n\). Given a finite positive measure \(\mu\) on the unit circle, we associate with it an inner product on \({\mathcal L}\) by \(\langle f,g \rangle = \int f \overline gd \mu\). Suppose \(k_n (z,w)\) is the reproducing kernel for \({\mathcal L}_n\) i.e., \(\langle f(z), k_n (z,w) \rangle = f(w)\), for all \(f \in {\mathcal L}_n\), \(|w |< 1\), then it is known that they satisfy a coupled recurrence relation. In this paper we shall prove a Favard type theorem which says that if you have a sequence of kernel functions \(k_n (z,w)\) which are generated by such a recurrence, then there will be a measure \(\mu\) supported on the unit circle so that \(k_n\) is the reproducing kernel for \({\mathcal L}_n\). The measure is unique under certain extra conditions on the points \(\alpha_i\).

Keywords

Reproducing kernel, Mathematics, Applied, Numerical & Computational Mathematics, 0102 Applied Mathematics, Zeros of polynomials, rational functions, and other analytic functions of one complex variable (e.g., zeros of functions with bounded Dirichlet integral), Orthogonal rational functions, Favard theorem, REPRODUCING KERNEL, 4901 Applied mathematics, 4613 Theory of computation, Science & Technology, 0103 Numerical and Computational Mathematics, Applied Mathematics, reproducing kernel, orthogonal rational functions, ORTHOGONAL RATIONAL FUNCTIONS, FAVARD THEOREM, 0906 Electrical and Electronic Engineering, Computational Mathematics, Physical Sciences, 4903 Numerical and computational mathematics, Mathematics

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citations
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!
2
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