This paper is intended to make a systematic study of periodicfunctions and to popularize them in teaching and learning.
周期函数的周期性是中学数学中的教学内容，掌握了函数的周期性，对函数性质的研究会带来不少方便。
2
When both the nutrient and toxicant inputs are periodicfunctions with the same period, the periodic coexistence of populations has been investigated.
当营养输入与毒素输入均为周期变化且有共同周期时讨论了种群的周期共存性。
3
An epidemic SIS model whose coefficient is almost periodicfunctions is studied. A sufficient condition of existence and uniqueness of positive periodic solution is obtained.