1. Bellman, R.,Methods of Nonlinear Analysis, Vol. 2 Academic Press, New York, New York, 1973.
2. Bellman, R., andKalaba, R.,Quasilinearization and Nonlinear Boundary-Value Problems, American Elsevier, New York, New York, 1965.
3. Ladde, G. S., Lakshmikantham, V., andVatsala, A. S.,Monotone Iterative Techniques for Nonlinear Differnntial Equations, Pitman, Boston, Massachusetts, 1985.