1. Makin, A.S., On basis properties of the system of root functions of the Sturm–Liouville operator with degenerate boundary conditions. I, Differ. Equations, 2018, vol.54, no. 10, pp. 1338–1353.
2. Nikiforov, A.F. and Uvarov, V.B., Spetsial’nye funktsii matematicheskoi fiziki (Special Functions of Mathematical Physics), Moscow: Nauka, 1978.
3. Fikhtengol’ts, G.M., Kurs differentsial’nogo i integral’nogo ischisleniya (Course of Differential and Integral Calculus), Moscow: Gos. Izd. Fiz. Mat. Lit., 1969, Vol.3.
4. Marchenko, V.A., Operatory Shturma–Liuvillya i ikh prilozheniya (Sturm–Liouville Operators and Their Applications), Kiev: Naukova Dumka, 1977.
5. Smirnov, V.I., Kurs vysshei matematiki (Course of Higher Mathematics), Moscow: Nauka, 1974, Vol.3, Pt. 2.