1. J. D. Cole, Perturbation Methods in Applied Mathematics, Blaisdell, Waltham, MA, 1968.
2. W. Eckhaus, Asymptotic Analysis of Singular Perturbations, North Holland, Amsterdam, 1979.
3. Reduction of the order of a linear ordinary differential equation containing a small parameter
4. S. Kaplun, Fluid Mechanics and Singular Perturbations, edited by P. A. Lagerstrom. L. N. Howard and C.S. Liu, Academic Press, New York, 1967.
5. On the Approximation of Double Limits by Single Limits and the Kaplun Extension Theorem