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3. V. A. Il’in, The Spectral Theory of Differential Operators (Nauka, Moscow, 1991) [in Russian].
4. M. Otelbaev, B. K. Kokebaev, and A. N. Shynybekov, Dokl. Akad. Nauk 271(4), 1307–1311 (1983).
5. I. S. Gradshtein and I. M. Ryzhik, Tables of Integrals of Sums, Series, and Products (Nauka, Moscow, 1971) [in Russian].