Positive Periodic Solutions of Second-Order Differential Equations with Delays

The existence results of positive omega-periodic solutions are obtained for the second-order differential equation with delays -u(n) + a(t) = f (t, u(t - tau(1)), ... , u(t - tau(n))), where a epsilon C (R, (0, infinity)) is a omega-periodic function, f : R x [0,infinity)(n) -> [0, infinity) is a continuous function, which is omega-periodic in t, and tau(1), tau(2), ... , tau(n) are positive constants. Our discussion is based on the fixed point index theory in cones.