A necessary and sufficient condition for the existence of positive periodic solutions of a model of hematopoiesis

In this paper, we derive a sufficient condition as well as a necessary condition for existence of positive periodic solutions of the hematopoiesis model with periodic coefficients x^'(t)[email protected](t)x(t)+p(t)@?x([email protected](t))1+[x([email protected](t))]^n,t>=0, where @d,[email protected]?C(R^+,(0,~)) and @[email protected]?C(R^+,R^+) are T-periodic functions. When p(t)[email protected]@d(t) with @c>0, a sufficient and necessary condition for the existence of a positive T-periodic solution is established.