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2. Nonlinear two point boundary value problems;Bailey,1968
3. An Introduction to Nonlinear Boundary Value Problems;Bernfeld,1974
4. Sur lese´quations du calcul des variations;Bernstein;Ann. Sci.E´cole Norm. Sup.,1912
5. Bifurcation of limit cycles of a family of plane vector fields;Bogdanov;Selecta Math. Soviet,1981