Periodic solutions of a nonlinear second-order differential equation with deviating argument

With the help of the coincidence degree continuation theorem, the existence of periodic solutions of a nonlinear second-order differential equation with deviating argument x″(t)+f1(x(t))x′(t)+f2(x(t))(x′(t))2+g(x(t−τ(t)))=0, under some assumptions are obtained.