1. V. A. Vinokurov and V. A. Sadovnichii, “Asymptotics of Any Order for Eigenvalues and Eigenfunctions of the Sturm–Liouville Boundary Value Problem on a Segment with Summable Potential,” Differ. Uravn. 34 (10), 1424 (1998).
2. V. A. Vinokurov and V. A. Sadovnichii, “Asymptotics of any Order for Eigenvalues and Eigenfunctions of the Sturm–Liouville Boundary Value Problem on a Segment with Summable Potential,” Izvestiya RAN. Ser. Matem. 64 (4), 47 (2000).
3. G. D. Birkhoff, “On the Asymptotic Chr of the Solutions of the Certain Linear Differential Equations Containing Parameter,” Trans. Amer. Math. Soc. 9, 219 (1908).
4. Ya. D. Tamarkin, Some General Problems of the Theory of Ordinary Linear Differential Equations (M. P. Frolova’s Printing House, Petrograd, 1917) [in Russian].
5. A. B. Vasil’eva and V. F. Butuzov, Asymptotic Expansions of Solutions to Singular Perturbed Equations (Nauka, Moscow, 1973) [in Russian].