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An Introduction to Nonlinear Boundary Value Problems. Academic Press, Inc., New York, 1974.
MR 0445048
[3] G. S. Ladde, V. Lakshmikantham and A. S. Vatsala:
Monotone Iterative Techniques for Nonlinear Differential Equations. Pitman, Boston, 1985.
MR 0855240