Existence of positive periodic solutions for second-order functional differential equations

In this paper, we show the existence of positive T-periodic solutions of second-order functional differential equations u ''(t) - rho(2)u(t)+lambda g(t)f(u(t-tau(t))) = 0, t is an element of R, where rho > 0 is a constant, g is an element of C (R, [0, infinity)), tau is an element of C(R, R) are T-periodic functions, f is an element of C([0, infinity), [0, infinity)) and lambda is a positive parameter. Our approach based on global bifurcation theorem.