\Bibitem{GolNazOle88} \by Yu.~D.~Golovatyj, S.~A.~Nazarov, O.~A.~Oleinik \paper The asymptotic behaviour of~eigenvalues and eigenfunctions in~problems on vibrations of a~medium with singular perturbation of the density \jour Russian Math. Surveys \yr 1988 \vol 43 \issue 5 \pages 229--230 \mathnet{http://mi.mathnet.ru/eng/rm2008} \crossref{https://doi.org/10.1070/RM1988v043n05ABEH001922} \mathscinet{http://mathscinet.ams.org/mathscinet-getitem?mr=971478} \zmath{https://zbmath.org/?q=an:0677.35075} \adsnasa{https://adsabs.harvard.edu/cgi-bin/bib_query?1988RuMaS..43..229G} \isi{https://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcApp=Publons&SrcAuth=Publons_CEL&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=A1988AN96100018}