摘要

In this paper, the problem of the existence of periodic solutions is studied for the second-order differential equations with a singularity of repulsive type, x ''(t) + f(x'(t)) + phi(t)x(t) - 1/x'(t) = h(t), where phi and h are T-periodic functions. By using topological degree theory, a new result on the existence of positive periodic solutions is obtained. The interesting thing is that the sign of the function phi(t) is allowed to be changed for t is an element of [0,T].