“…Perturbation theory is proved to be a powerful tool in the qualitative and quantitative study of periodic solutions to the Hill, Mathieu, van der Pol equations, and many other important mathematical models in nonlinear mechanics and physics (see, e.g., [3]). Without pretending to be complete, we also mention the existence results concerning periodic boundary value problems for nonlinear differential equations with singularities [4,5], second-order non-autonomous nonlinear equations on the positive cone [6], superlinear second-order equations with positive solutions [7], first-order problem with resonance and nonlinear impulses [8], Hamiltonian systems with nonsmooth potentials [9], and second-order equations with a convection term [10].…”