Positive periodic solutions for a periodic neutral differential equation with feedback control

We study the existence of periodic solutions for a neutral differential equation with feedback control of the form dN(t)dt=r(t)N(t)1-N(t-τ(t))+c(t)N′(t-τ(t))K(t)-β(t)u(t-δ(t)),du(t)dt=-a(t)u(t)+b(t)N(t-σ(t)).