Positive periodic solutions for a neutral differential system with feedback control *

Applying a fixed-point theorem of strict-set-contractions, some new criteria are established for the existence of positive periodic solutions of the following neutral differential system with feedback controldx(t)dt=x(t)[r(t)−a0(t)x(t)−a1(t)x(t−σ1(t))−a2(t)x′(t−σ2(t))−β(t)u(t−τ1(t))],du(t)dt=−a(t)u(t)+b(t)x(t−τ2(t)).