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2. J. Weidmann, Spectral Theory of Ordinary Differential Operators (Springer, Berlin, 1987).
3. A. D. Polyanin and V. F. Zaitsev, Exact Solutions for Ordinary Differential Equations, 2nd ed. (Chapman and Hall, London, 2000).
4. E. Kamke, Handbook of Ordinary Differential Equations (Nauka, Moscow, 1977) [in Russian].
5. A catalogue of Sturm–Liouville differential equations;W. N. Everitt,2004