1. Perturbation Methods in Applied Mathematics;Cole,1968
2. Topics in singular perturbations;O'Malley;Advances in Math.,1968
3. Asymptotic Expansions of Solutions to Singularly Perturbed Equations;Vasil'eva,1973
4. Asymptotic behavior and existence of a solution of the Cauchy problem for a first order differential equation with a small parameter;Trenogin;Dokl. Akad. Nauk SSSR,1963
5. Linear Differential Equations in a Banach Space;Krein,1967