Existence of positive periodic solution of a periodic cooperative model with delays and impulses

where ai, bi, and ci (i= 1,2) are nonnegative ω-periodic continuous functions. It is well known that more realistic and interesting species models should take into account both the seasonality of the changing environment and time delays [4, 8, 9], and that the birth of many species is not continuous but occurs at fixed time intervals (some wild animals have seasonal births), in the long run; the birth of these species can be considered as an impulse to the system [1, 2, 5, 7]. To describe this phenomenon exactly, we proposed