1. E. A. Coddington and N. Levinson, Theory of Ordinary Differential Equations (McGraw-Hill, 1955; IL, Moscow, 1958).
2. W. Wasow, Asymptotic Expansions for Ordinary Differential Equations (R. E. Krieger Publ. Company, Huntington, N.Y., 1976; Mir, Moscow 1968).
3. M. V. Fedoryuk, Asymptotic Methods for Linear Ordinary Differential Equations (Nauka, Moscow, 1983) [in Russian].
4. Kh. K. Ishkin, “Necessary Conditions for the Localization of the Spectrum of the Sturm-Liouville Problem on a Curve,” Matem. Zametki 78 (1), 72 (2005). [Math. Notes 78 (1), 64 (2005)].