1. Agmon, S., Lectures on Elliptic Boundary Value Problems, Van Nostrand, Princeton, N. J., 1965.
2. Besjes, J. G., Singular perturbation problems for linear elliptic differential operators of arbitrary order. I. Degeneration to elliptic operators, J. Math. Anal. Appl., 49, 24–46, (1975).
3. Lecture Notes in Mathematics;P. P. N. Groen de,1979
4. North-Holland Mathematics Studies;W. Eckhaus,1973
5. Eckhaus, W., Asymptotic Analysis of Singular Perturbations, North-Holland, Amsterdam, 1979.