1. Ahlberg, J. H., Nilson, E. N., Walsh, J. L.: The theory of splines and their applications. New York: Academic Press 1967
2. Bailey, P. B., Shampine, L. F., Waltman, P. E.: Nonlinear two-point boundary value problems. New York: Academic Press 1968.
3. Bellman, R. E., Kalaba, R. E.: Quasilinearization and nonlinear boundary-value problems. New York: Elsevier 1965.
4. Bramble, J. H., Hubbard, B. E.: On a finite difference analogue of an elliptic boundary problem which is neither diagonally dominant nor of nonnegative type. J. Math. and Phys.43, 117?132 (1964).
5. Carrier, G. F.: Singular perturbation theory and geophysics. SIAM Review12, 175?193 (1970).