1. Systems of Differential Equations Containing Small Parameters Multiplying Some of the Derivatives;Tihonov;MATH. SB,1952
2. Singular Perturbations of Non-Linear Systems of Differential Equations and an Associated Boundary Layer Equations;Levin;J. RAT. MECH. ANAL,1954
3. Asymptotic Behavior of Solutions to Certain Problems Involving Nonlinear Differential Equations Containing a Small Parameter Multiplying the Highest Derivatives;Vasileva;RUSS. MATH. SURVEYS,1963
4. Asymptotic Expansions for Ordinary Differential Equations;Wasow,1965
5. Butuzov, V.F., Vasileva, A.B., and Fedoryuk, M.V., Asymptotic Methods in Theory of Ordinary Differential Equations, ITOGI NAUKI, MATHEMATICAL ANALYSIS 1967, Moscow, 1969 (Russian). English translation to appear in PROGRESS IN MATHEMATICS, New York, Plenum Press.