Nguyen Manh Hung Toulouse School of Economics (LERNA)   CERMSEM, Université de Paris1 106-112 bd de l'Hopital Paris, 75013 France   manh-hung.nguyen@tse-fr.eu             JEL Areas: C6 : Mathematical Methods and Programming: General E6 : Macroeconomic Policy, Macroeconomic Aspects of Public Finance, and General