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The Klein-Gordon equation, the Hilbert transform and Gauss-type maps: H ∞ approximation
KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Mathematics (Div.).ORCID iD: 0000-0002-4971-7147
2021 (English)In: Journal d'Analyse Mathematique, ISSN 0021-7670, E-ISSN 1565-8538, Vol. 144, no 1, p. 119-190Article in journal (Refereed) Published
Abstract [en]

The impetus to this work is the need to characterize when the system {ϕ1m, ϕ2n} where m, n = 0, 1, 2,…., is complete in the weak-star topology of H∞ on the unit disk (or the half-plane). Here, ϕ1 and ϕ2 are two atomic inner functions, of the form ϕ1(z)=exp(λ1z+1z−1)andϕ2(z)=exp(λ2z−1z+1), where λ1, λ2 are positive reals. Our main result asserts that the system of non-negative integral powers {ϕ1m, ϕ2n} is weak-star dense in H∞ of the unit disk if and only if λ1λ2 ≤ π2. In earlier work in the L∞ setting on the unit circle all the integer powers were considered, and the corresponding result was obtained (Hedenmalm and Montes-Rodríguez, 2011). The approach was to first transfer the completeness problem to the real line via the Cayley transform, and to then connect with the dynamics of Gauss-type transformations on the interval [−1, 1]. Indeed, the nonexistence of nontrivial finite absolutely continuous invariant measures for the Gauss-type map was the key ingredient of the analysis. Moreover, it was shown that the answer to the completeness problem has striking consequences for the Klein-Gordon equation. Here, the analysis is much more subtle as a result of the required finer topology. To appreciate the difference, we observe that the standard quotient space L1/H1 used as the predual of H∞ is not appropriate for our purposes. Instead we model the predual in the real way, as L1 plus the Hilbert transform of L1, in analogy with the decomposition of BMO. The next step is to analyze carefully the iterates of the transfer operator applied to the Hilbert kernel. The approach involves a splitting of the Hilbert kernel which is induced by the transfer operator. The careful analysis of this splitting involves detours to the Hurwitz zeta function as well as to the theory of totally positive matrices. 

Place, publisher, year, edition, pages
Springer Nature , 2021. Vol. 144, no 1, p. 119-190
National Category
Computational Mathematics
Identifiers
URN: urn:nbn:se:kth:diva-313846DOI: 10.1007/s11854-021-0173-4ISI: 000730059800003Scopus ID: 2-s2.0-85121285699OAI: oai:DiVA.org:kth-313846DiVA, id: diva2:1668301
Note

QC 20220613

Available from: 2022-06-13 Created: 2022-06-13 Last updated: 2022-06-25Bibliographically approved

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Hedenmalm, Håkan

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