\Bibitem{GubKov81} \by G.~M.~Gubreev, A.~I.~Kovalenko \paper Tests for completeness of root subspaces of a differentiation operator with abstract boundary conditions \jour Mat. Zametki \yr 1981 \vol 30 \issue 4 \pages 543--552 \mathnet{http://mi.mathnet.ru/mzm6179} \mathscinet{http://mathscinet.ams.org/mathscinet-getitem?mr=638427} \zmath{https://zbmath.org/?q=an:0481.47029} \transl \jour Math. Notes \yr 1981 \vol 30 \issue 4 \pages 765--770 \crossref{https://doi.org/10.1007/BF01137805} \isi{https://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcApp=Publons&SrcAuth=Publons_CEL&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=A1981NS56600020}