1. W Wasow, On the Asymptotic Solution of Boundary Value Problems for Ordinary Differential Equations Containing a Small Parameter., J. Math. Phys. 32 (1944), 173–183
2. W Wasow, Asymptotic Solution of Boundary Value Problems for the Differential Equation ΔU + λδU/δx =λf(x, y), Duke Math. J. 11 (1944), 405–415.
3. KO Friedrichs and W Wasow, Singular Perturbations of Non-Linear Oscillations, Duke Math J. 13 (1946), 367–381.
4. DA Flanders and JJ Stoker, The Limit Case of Relaxation Oscillations, Studies in Nonlinear Vibration Theory, New York University (1946).
5. A Tihonov, On the Dependence of the Solutions of Differential Equations on a Small Parameter, Mat. Sb. 22 (1948), 193–204. (Russian)