... Sh. Guseinov, “On the existence of positive solutions for nonlinear differ-ential equations with periodic boundary conditions,” Journal of Computational and Applied Mathematics,vol. 132, no. 2, ... Torres, and M. Zhang, Periodic solutions of second order non-autonomous singular dy-namical systems, ” Journal of Differential Equations, vol. 239, no. 1, pp. 196–212, 2007.4 M. A. del Pino and ... ∈ 0, 2π × 0, ∞1.2 and gu > 0 is nonincreasing and hu/gu is nondecreasing in u;G3 there exist continuous, nonnegative functions g1u and h1u on 0, ∞ such thatFt,...