Journal of Functional Analysis 278 (2020) 108318
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Journal of Functional Analysis
www.elsevier.com/locate/jfa
Canonical systems with discrete spectrum
Roman Romanov a,1, Harald Woracek b,∗,2 a Department of Mathematical Physics, Faculty of Physics, St Petersburg State University, 7/9 Universitetskaya nab., 199034 St. Petersburg, Russia b Institute for Analysis and Scientific Computing, Vienna University of Technology, Wiedner Hauptstraße 8-10/101, 1040 Wien, Austria a r t i c l e i n f o a b s t r a c t
Article history: We study spectral properties of two-dimensional canonical Received 15 February 2019 systems y(t) = zJH(t)y(t), t ∈ [a, b), where the Hamilto- Accepted 9 September 2019 nian H is locally integrable on [a, b), positive semidefinite, Available online 25 September 2019 and Weyl’s limit point case takes place at b. We answer the Communicated by K. Seip following questions explicitly in terms of H: MSC: 37J05 Is the spectrum of the associated selfadjoint operator dis- 34L20 crete? 45P05 If it is discrete, what is its asymptotic distribution? 46E22
Keywords: Here asymptotic distribution means summability and limit Canonical system superior conditions relative to comparison functions growing Discrete spectrum sufficiently fast. Making an analogy with complex analysis, Operator ideal this corresponds to convergence class and type w.r.t. proxi- de Branges space mate orders having order larger than 1. It is a surprising fact that these properties depend only on the diagonal entries of H. In 1968 L.de Branges posed the following question as a fun- damental problem:
Which Hamiltonians are the structure Hamiltonian of some de Branges space?
* Corresponding author. E-mail addresses: [email protected] (R. Romanov), [email protected] (H. Woracek). 1 R. Romanov gratefully acknowledges the support of Russian Science Foundation, Grant 17-11-01064 (Theorems B.1 and 1.3), of RFBR through grants 19-01-00657 and 19-01-00565 and of Saint Petersburg State University through Grant IAS-11.42.673.2017. 2 H. Woracek was supported by the project P30715-N35 of the Austrian Science Fund. https://doi.org/10.1016/j.jfa.2019.108318 0022-1236/© 2019 Elsevier Inc. All rights reserved. 2 R. Romanov, H. Woracek / Journal of Functional Analysis 278 (2020) 108318
We give a complete and explicit answer. © 2019 Elsevier Inc. All rights reserved.
1. Introduction
We study the spectrum of the selfadjoint model operator associated with a two- dimensional canonical system
y (t)=zJH(t)y(t),t∈ [a, b). (1.1)