Giorgio Fabbri Université d'Evry-Val-d'Essonne, France   4 bd. François Mitterrand Evry, 91025 France   giorgio.fabbri@univ-evry.ff   https://sites.google.com/site/giorgiofabbri1979/           JEL Areas: C6 : Mathematical Methods and Programming: General Q2 : Renewable Resources and Conservation: General