1. M. I. Vishik and L. A. Lyusternik, “Regular degeneration and boundary layer for linear differential equations with small parameter” [in Russian], Usp. Mat. Nauk 12, No. 5, 3–122 (1957).
2. M. I. Vishik and L. A. Lyusternik, “the asymptotic behavior of solutions of linear differential equations with large or quickly changing coefficients and boundary conditions” [in Russian], Usp. Mat. Nauk 15, No. 4, 27–95 (1960); English transl.: Russ. Math. Surv. 15, No. 4, 23–91 (1960).
3. A. S. Demidov, “Asymptotic behavior of the solution of a boundary value problem for elliptic pseudodifferential equations with a small parameter multiplying the leading operator” [in Russian], Tr. Mosk. Mat. Obshch. 32, 119–146 (1975); English transl.: Trans. Mosc. Math. Soc. 32, 115–142 (1977).
4. M. I. Vishik, “On strongly elliptic systems of differential equations” [in Russian], Mat. Sb. 29, No. 3, 615–676 (1951).
5. V. D. Shafranov, “On magnetohydrodynamical equilibrium configurations” [in Russian], Zhurn. Ehksper. Teor. Fiz. 33, No. 3, 710–722 (1957); English transl.: Sov. Phys., JETP 6, 545–554 (1958).