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<article article-type="research-article" dtd-version="1.3" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xml:lang="en"><front><journal-meta><journal-id journal-id-type="publisher-id">najo</journal-id><journal-title-group><journal-title xml:lang="en">Nanosystems: Physics, Chemistry, Mathematics</journal-title><trans-title-group xml:lang="ru"><trans-title>Наносистемы: физика, химия, математика</trans-title></trans-title-group></journal-title-group><issn pub-type="ppub">2220-8054</issn><issn pub-type="epub">2305-7971</issn><publisher><publisher-name>Университет ИТМО</publisher-name></publisher></journal-meta><article-meta><article-id pub-id-type="doi">10.17586/2220-8054-2019-10-5-511-519</article-id><article-id custom-type="elpub" pub-id-type="custom">najo-583</article-id><article-categories><subj-group subj-group-type="heading"><subject>Research Article</subject></subj-group><subj-group subj-group-type="section-heading" xml:lang="en"><subject>MATHEMATICS</subject></subj-group><subj-group subj-group-type="section-heading" xml:lang="ru"><subject>МАТЕМАТИКА</subject></subj-group></article-categories><title-group><article-title>Analytic description of the essential spectrum of a family of 3x3 operator matrices</article-title><trans-title-group xml:lang="ru"><trans-title></trans-title></trans-title-group></title-group><contrib-group><contrib contrib-type="author" corresp="yes"><name-alternatives><name name-style="western" xml:lang="en"><surname>Rasulov</surname><given-names>T. H.</given-names></name></name-alternatives><bio xml:lang="en"><p>M. Ikbol str. 11, 200100 Bukhara</p><p> </p></bio><email xlink:type="simple">rth@mail.ru</email><xref ref-type="aff" rid="aff-1"/></contrib><contrib contrib-type="author" corresp="yes"><name-alternatives><name name-style="western" xml:lang="en"><surname>Tosheva</surname><given-names>N. A.</given-names></name></name-alternatives><bio xml:lang="en"><p>M. Ikbol str. 11, 200100 Bukhara</p></bio><email xlink:type="simple">nargiza_n@mail.ru</email><xref ref-type="aff" rid="aff-1"/></contrib></contrib-group><aff xml:lang="en" id="aff-1"><institution>Bukhara State University, Faculty of Physics and Mathematics</institution><country>Uzbekistan</country></aff><pub-date pub-type="collection"><year>2019</year></pub-date><pub-date pub-type="epub"><day>23</day><month>08</month><year>2025</year></pub-date><volume>10</volume><issue>5</issue><fpage>511</fpage><lpage>519</lpage><permissions><copyright-statement>Copyright &amp;#x00A9; Rasulov T.H., Tosheva N.A., 2025</copyright-statement><copyright-year>2025</copyright-year><copyright-holder xml:lang="ru">Rasulov T.H., Tosheva N.A.</copyright-holder><copyright-holder xml:lang="en">Rasulov T.H., Tosheva N.A.</copyright-holder><license xml:lang="ru" license-type="creative-commons-attribution" xlink:href="https://creativecommons.org/licenses/by/4.0/" xlink:type="simple"><license-p>Данная работа распространяется под лицензией Creative Commons Attribution 4.0.</license-p></license><license xml:lang="en" license-type="creative-commons-attribution" xlink:href="https://creativecommons.org/licenses/by/4.0/" xlink:type="simple"><license-p>This work is licensed under a Creative Commons Attribution 4.0 License.</license-p></license></permissions><self-uri xlink:href="https://nanojournal.ifmo.ru/jour/article/view/583">https://nanojournal.ifmo.ru/jour/article/view/583</self-uri><abstract><p>We consider the family of 3 х 3 operator matrices H(K), K ϵ Td := (-π; π]d arising in the spectral analysis of the energy operator of the spin-boson model of radioactive decay with two bosons on the torus Td. We obtain an analog of the Faddeev equation for the eigenfunctions of H(K). An analytic description of the essential spectrum of H(K) is established. Further, it is shown that the essential spectrum of H(K) consists the union of at most three bounded closed intervals.</p></abstract><kwd-group xml:lang="en"><kwd>family of operator matrices</kwd><kwd>generalized Friedrichs model</kwd><kwd>bosonic Fock space</kwd><kwd>annihilation and creation operators</kwd><kwd>channel operator</kwd><kwd>decomposable operator</kwd><kwd>fiber operators</kwd><kwd>the Faddeev equation</kwd><kwd>essential spectrum</kwd><kwd>Weyl criterion</kwd></kwd-group></article-meta></front><back><ref-list><title>References</title><ref id="cit1"><label>1</label><citation-alternatives><mixed-citation xml:lang="ru">Minlos R.A., Spohn H. The three-body problem in radioactive decay: the case of one atom and at most two photons. Topics in Statistical and Theoretical Physics. Amer. Math. Soc. 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