1. S. Yu. Slavyanov, Asymptotic Solutions of the One-Dimensional Schrödinger Equation [in Russian], Leningrad State Univ. Press, Leningrad (1990); English transl., Amer. Math. Soc., Providence, R. I. (1996).
2. 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).
3. V. M. Babich, “Mathematical theory of diffraction (a survey of research carried out at the Laboratory of Mathematical Problems of Geophysics of the Leningrad Branch of the Institute of Mathematics),” Proc. Steklov Inst. Math., 175, (47–63) (1988).
4. V. M. Babich and V. S. Buldyrev, Asymptotic Methods in Problems of the Diffraction of Short Waves [in Russian], Nauka, Moscow (1972); English transl
5. V. M. Babich and V. S. Buldyrev, Short-Wavelength Diffraction Theory: Asymptotic Methods (Springer Ser. Wave Phenom., Vol. 4), Springer, Berlin (1991).