1. P. B. Bailey, L. F. Shampine, and P. E. Waltman, Nonlinear Two Point Boundary Value Problems, Academic Press, New York (1968).
2. S. N. Bernshtein, ?On the equations of the calculus of variations,? Uspekhi Matem. Nauk,8, 32?74 (1940).
3. M. I. El'shin and L. I. Smolich, ?On a condition for the solvability of a boundary value problem,? Matem. Zametki,13, No. 2, 247?250 (1973).
4. H. Epheser, ?Über die Existenz der Lösungen von Randwertaufgaben mit gewöhnlichen, nichtlinearen Differentialgleichungen zweiter Ordnung,? Math. Z.,61, No. 4, 435?454 (1955).
5. I. T. Kiguradze, ?On some singular boundary-value problems for second-order nonlinear ordinary differential equations,? Differents. Uravnen.,4, No. 10, 1753?1773 (1968).