| The periodic phenomenon appears frequently in the biological phenomenon and the production practice,thus the cyclical question is an important aspect of the dynamic system research.This thesis is composed of four Chapters.In the first Chapter,we introduce the existed related works and the origin of the problems we discussed.The main works of this paper are also simply introduced in this Chapter.In the second Chapter,we consider the existence of three periodic solutions of the high-dimensional differential system.By utilizing the Leggett-william fixed point theorem, we obtain new sufficient conditions for the existence of positive periodic solutions.In the third Chapter,we study a second-order high-dimensional differential system with a parameter and obtain sufficient conditions for the existence of positive periodic solutions when the parameter changes in a certain range.In the fourth Chapter,we consider n-dimensional impulsive Lotka-Volterra neutral system with distributed delays.We obtain new sufficient conditions for the existence of positive periodic solutions by means of concidence degree theory and inequatily. |
