\Bibitem{Mil62}
\by V.~D.~Milman
\paper A~transformation operator for Sturm--Liouville differential equations in the non-selfadjoint case
\jour Sb. Math.
\yr 1962
\vol 101
\mathnet{http://mi.mathnet.ru/eng/sm4695}
\mathscinet{http://mathscinet.ams.org/mathscinet-getitem?mr=167671}
\zmath{https://zbmath.org/?q=an:0213.36303}