1. S. A. Lomov, Introduction to the General Theory of Singular Perturbations (Nauka, Moscow, 1981) [in Russian].
2. V. F. Safonov and A. A. Bobodzhanov, Course in Higher Mathematics. Singularly Perturbed Equations and the Regularization Method (Izd. dom MÉI, Moscow, 2012) [in Russian].
3. S. A. Lomov and I. S. Lomov, Foundations of Mathematical Boundary-Layer Theory (Izd.Moskov. Univ., Moscow, 2011) [in Russian].
4. O. E. Omel’chenko and N. N. Nefedov, “Boundary-layer solutions to quasilinear integro-differential equations of the second order,” Zh. Vychisl. Mat. i Mat. Fiz. 42 (4), 491–503 (2002) [Comput. Math. Math. Phys. 42 (4), 470–482 (2002)].