First Author:   Nabil Aflouk University of Paris North aflouknabil@yahoo.fr   Avenue Jean Baptiste Clément, 93430, France   + 33 1 49 40 33 13(Phone) + 33 1 49 40 33 13(Fax) http://www.univ-paris13.fr/CEPN/ Second Author: Jacques Mazier University of Paris North mazier@univ-paris13.fr   Villetaneuse, 93430, France   + 33 1 49 40 33 13(Phone) + 33 1 49 40 33 34(Fax) http://www.univ-paris13.fr/CEPN/