\Bibitem{Pol15} \by D.~M.~Polyakov \paper Spectral analysis of a~fourth order differential operator with periodic and antiperiodic boundary conditions \jour Algebra i Analiz \yr 2015 \vol 27 \issue 5 \pages 117--152 \mathnet{http://mi.mathnet.ru/aa1457} \mathscinet{http://mathscinet.ams.org/mathscinet-getitem?mr=3582944} \elib{https://elibrary.ru/item.asp?id=24849917} \transl \jour St. Petersburg Math. J. \yr 2016 \vol 27 \issue 5 \pages 789--811 \crossref{https://doi.org/10.1090/spmj/1417} \isi{https://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcApp=Publons&SrcAuth=Publons_CEL&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=000383058900005} \scopus{https://www.scopus.com/record/display.url?origin=inward&eid=2-s2.0-84981332780}