Nguyen thang Dao Center for Operations Research and Econometrics   Terrasse des Ardennais 13 Louvain la Neuve, 1348 Belgium   nguyen.dao@uclouvain.be   https://sites.google.com/site/nguyenthangdaocore/home           JEL Areas: O : Economic Development, Technological Change, and Growth J1 : Demographic Economics: General