METHOD OF UPPER AND LOWER SOLUTIONS FOR NONLINEAR DELAY INTEGRAL-EQUATIONS MODELING EPIDEMICS AND POPULATION-GROWTH

In this paper we prove some results about the existence of positive periodic solutions of nonlinear delay integral equations which formulate a model to explain the evolution of some infectious diseases and which have also been used as a growth equation for single species population when the birth rate varies seasonally. We use monotone methods (upper and lower solutions) to obtain our results and we compare them with other results obtained by different authors which have used topological methods to study the mentioned equation.