1. V. P. Maslov, The Complex WKB Method for Nonlinear Equations (Nauka, Moscow, 1977; Birkhäuser, Basel, 1994).
2. A. Nayfeh, Introduction to Perturbation Techniques (New York, Wiley, 1981; Mir, Moscow, 1984).
3. A. B. Vasil’eva and V. F. Butuzov, Asymptotic Expansions of Solutions of Singularly Perturbed Equations (Nauka, Moscow, 1973) [in Russian].
4. S. A. Lomov, Introduction to the General Theory of Singular Perturbations (Nauka, Moscow, 1981) [in Russian].
5. V. F. Safonov and A. A. Bobodzhanov, Singularly Perturbed Problems and the Regularization Method (Izd. MÉI, Moscow, 2010) [in Russian].