The Model of Population Projection by Delay Differential Equation with Two Lags

In order to give the more reasonable models of population projections, delay differential equations with two lags were considered. By analysing the relative growth rate of population, the growth function of population which determined by the maximum age and minimum age of new mothers might be more suitable. Because of the model does not have analytic solution, numerical method gives the result of the delay differential equation, and the population of America and population in the world tests the rationality of the model. The result of the model agrees with the actual when give the appropriate parameters and some reasonable assumptions.