... andlower solutions with monotone iterative technique 11, and some fixed point theorems incones 12–14. On the other hand, in the last two years, some researchers have used variationalmethods ... Equations and Inclusions, vol. 2 ofContemporary Mathematics and Its Applications, Hindawi, New York, NY, USA, 2006.10 D. Qian and X. Li, “Periodic solutions for ordinary differential equations with ... “Variational approach to impulsive differential equations with periodic boundaryconditions,” Nonlinear Analysis: Real World Applications. In press.21 G. Bonanno and G. D’Agu`ı, “A Neumann boundary...