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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 custom-type="elpub" pub-id-type="custom">najo-1202</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>YOUNG RESEARCHES SIMPOSIUM</subject></subj-group><subj-group subj-group-type="section-heading" xml:lang="ru"><subject>YOUNG RESEARCHES SIMPOSIUM</subject></subj-group></article-categories><title-group><article-title>Negative eigenvalues of the Y-type chain of weakly coupled ball   resonators</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>Anikevich</surname><given-names>A. S.</given-names></name></name-alternatives><bio xml:lang="en"><p>St. Petersburg</p></bio><email xlink:type="simple">shadowpoos@gmail.com</email><xref ref-type="aff" rid="aff-1"/></contrib></contrib-group><aff xml:lang="en" id="aff-1"><institution>St. Petersburg National Research University of Information Technologies, Mechanics and Optics</institution><country>Russian Federation</country></aff><pub-date pub-type="collection"><year>2013</year></pub-date><pub-date pub-type="epub"><day>20</day><month>08</month><year>2025</year></pub-date><volume>4</volume><issue>4</issue><issue-title>Special Issue.  INTERNATIONAL CONFERENCE   "MATHEMATICAL CHALLENGE OF QUANTUM  TRANSPORT IN NANOSYSTEMS - 2013.   PIERRE DUCLOS WORKSHOP"</issue-title><fpage>545</fpage><lpage>549</lpage><permissions><copyright-statement>Copyright &amp;#x00A9; Anikevich A.S., 2025</copyright-statement><copyright-year>2025</copyright-year><copyright-holder xml:lang="ru">Anikevich A.S.</copyright-holder><copyright-holder xml:lang="en">Anikevich A.S.</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/1202">https://nanojournal.ifmo.ru/jour/article/view/1202</self-uri><abstract><p>Spectral properties of a system are strongly associated with its geometry. The spectral problem for the Y-bent chain of weakly-coupled ball resonators is investigated. The Y-bent system can be described as a central ball linking three chains consisting of balls of the same radius. There is a δ-coupling condition with parameter α at every contact point. Specifically, it is assumed that the axis passing through the center of each ball lies in the same plane and the centers of balls that are the closest to the central ball form an equilateral triangle. The transfer-matrix approach and the theory of extensions are employed to solve the spectral problem for this system. It is shown that such system with a certain value of parameter α has at most one negative eigenvalue in the case of δ-coupling in contact points.</p></abstract><kwd-group xml:lang="en"><kwd>negative eigenvalue</kwd><kwd>delta-coupling</kwd><kwd>operator extensions theory</kwd></kwd-group><funding-group><funding-statement xml:lang="en">This work was supported by FTP “Scientific and Educational Human Resources  for Innovation-Driven Russia”(contract 16.740.11.0030), grant 11-08-00267 of RFBR, FTP  “Researches and Development in the Priority Directions Developments of a Scientific and  Technological Complex of Russia 2007-2013”(state contract 07.514.11.4146) and grants of  the President of Russia (state contract 14.124.13.2045-MK and grant MK-1493.2013.1)</funding-statement></funding-group></article-meta></front><back><ref-list><title>References</title><ref id="cit1"><label>1</label><citation-alternatives><mixed-citation xml:lang="ru">P. Duclos, P. Exner, O. Turek. 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