Abstract: In this paper, we study the neoclassical Solow-Swan model where the natural capital is introduced as a factor of production and modeled as a renewable resource. In contrast with the standard literature, the labor growth rate is assumed to be non constant over time. In this framework, we investigate the conditions under which the economy may be sustainable or unsustainable in the long run, we derive the set of sustainable marginal propensity to consume for any given tax rate, we determine the nature of the non-trivial steady states of the economy. In this framework some new models are introduced to analyse some economic aspects belong to the economic growth theory and the relationship with the environment.

Brief Biography of the Speaker:
MASSIMILIANO FERRARA, was born in Pisa (Italy) on June 8, 1972. He graduated in 1995 in Economics at the University of Messina with academic honors. In 2001 follows the title of Ph.D. with academic honors in "Tools for Mathematical Economics and Finance". Professor in "mathematical methods for the economical, financial and actuarial sciences." since 2002. Chief of the Chairs of Mathematical Economics and Economic Statistics at the Faculty of Law - Economics Degree - Mediterranean University of Reggio Calabria since 2007. Professor in the degree course on European Economics at the Faculty of Political Science, University of Milan, where he also is Professor of Decision Theory on the Master by title “Marketing Intelligence and Data Analysis”. Head of the Economics Degree of the Mediterranean University of Reggio Calabria. Invited Speaker by American Mathematical Society (Western Michigan University, USA) and Calcutta Mathematical Society, INDIA and Visiting Professor at the Lomonosov Moscow State University (Department of Mathematics), the New Jersey Institute of Technology in NewArk (NJ) (USA), (Department of Mathematical Sciences), the Eotvos Lorand University of Budapest (Department of Atomic Physics, Faculty of Sciences), Politehnica of Bucharest (Department of Mathematics). Author of 74 publications on international journals many of them "high impact Scientific International (ISI)” and 4 monographs. His biography is inserted, finding scientific merit in the international collections Who's Who in the world 2006, 2007 and 2008 published by Marquis (since 1899) in the United States, in the collection 2000 Outstanding Intellectuals of the 21st century (years 2006 and 2007), in turn, published by the Biographical Centre, University of Cambridge, England and the prestigious collection Accomplished International Profiles of Leaders, published by American Biographical Institute, Inc. (Year 2008). He has lectured on invitation at the Lomonosov Moscow State University, University of Budapest, University of Hagen, Politehnica of Bucharest, University "Ovidius" Costanza, Bocconi University of Milan, University of Milan. Member of Accademia Peloritana dei Pericolanti (2003-current), Member of the Balkan Society of Geometers (2003- current), Member of the Scientific SET - Advances Center for Studies on Economic Theory - (Center for Advanced Studies Theoretical Economics) at the University of Milan Bicocca (2005-current), Member of the Mathematical Association of America (2007-current), Member of the SIEP (Societa italiana di Economia Pubblica) (2008-current),. Scientific Coordinator of international projects financed by the Ministry of Foreign Affairs: The Executive Programme of scientific and techonological cooperation between Italy and Romania during 2006- 2008 and of the Executive Programme of scientific and techonological cooperation between Italy and Estonia during 2005-2007. Editor and referree of several International Journals. Official Reviewer of Mathematical Reviews (MathSciNet), Division of the American Mathematical Society and Zentralblatt MATH, reviews scientific journal published by the European Mathematical Society, the Heidelberg Academy of Sciences and Fachinformationszentrum Karlshruhe.