Positive periodic solutions for second-order neutral differential equations with functional delay

We use Krasnoselskii's fixed point theorem to prove the existence of positive periodic solutions of the second-order nonlinear neutral differential equation $$ frac{d^2}{dt^2}x(t)+p(t)frac{d}{dt}x(t)+q(t)x(t) =cfrac{d}{dt}x(t- au(t))+f(t,h(x(t)),g(x(t- au(t)))). $$