1. Watson, G., A Treatise on the Theory of Bessel Functions, Cambridge, 1945. Translated under the title Teoriya besselevykh funktsii, Moscow: Inostrannaya Literatura, 1949.
2. Erdélyi, A., Magnus, W., Oberhettinger, F., and Tricomi, F.G., Higher Transcendental Functions (Bateman Manuscript Project), New York: McGraw-Hill, 1953. Translated under the title Vysshie transtsendentnye funktsii, Moscow, 1974, vol. 2.
3. Olver, F.W., Asymptotics and Special Functions, New York–London: Academic Press, 1974. Translated under the title Vvedenie v asimptoticheskie metody i spetsial’nye funktsii, Moscow: Nauka, 1978.
4. Langer, R., On the asymptotic solutions of differential equations, with an application to the Bessel functions of large complex order, Trans. Amer. Math. Soc., 1932, vol. 34, no. 3, pp. 447–480.
5. Makin, A.S., On the absence of the basis property for the root function system of the Sturm–Liouville operator with degenerate boundary conditions, Dokl. Math., 2016, vol. 93, no. 2, pp. 220–222.