1. R. E. Langer, The asymptotic solutions of ordinary linear differential equations of the second order with special reference to a turning point, Trans. Amer. Math. Soc., 67, 461 – 490 (1949), https://doi.org/10.2307/1990486
2. T. M. Cherry, Uniform asymptotic formulae for functions with transition points, Trans. Amer. Math. Soc., 68, 224 – 257 (1950), https://doi.org/10.2307/1990443
3. A. A. Dorodniczy`n, Asimptoticheskie zakony` raspredeleniya sobstvenny`kh znachenij dlya nekotory`kh osoby`kh vidov differenczial`ny`kh uravnenij vtorogo poryadka, Uspekhi mat. nauk, 7, вып. 6(52), 3 – 96 (1952).
4. W. Wasow, Asymptotic expansions for ordinary differential equations, Interscience Publishes, New York (1965).
5. A. M. Samojlenko, Ob asimptoticheskom integrirovanii odnoj sistemy` linejny`kh differenczial`ny`kh uravnenij s maly`m parametrom pri chasti proizvodny`kh, Ukr. mat. zhurn., 54, № 11, 1505 – 1517 (2002).