\Bibitem{Ote79} \by M.~O.~Otelbaev \paper Estimates of $s$-numbers and completeness conditions for systems of root vectors of non-self-adjoint Sturm--Liouville operators \jour Mat. Zametki \yr 1979 \vol 25 \issue 3 \pages 409--418 \mathnet{http://mi.mathnet.ru/mzm8317} \mathscinet{http://mathscinet.ams.org/mathscinet-getitem?mr=534096} \zmath{https://zbmath.org/?q=an:0425.47030|0405.47033} \transl \jour Math. Notes \yr 1979 \vol 25 \issue 3 \pages 216--221 \crossref{https://doi.org/10.1007/BF01159514}