1. Department of Mathematics and Engineering, Agrarian State University of Moldova, Mircesti Street 44, 2049 Chisinau, Moldova
2. Laboratoire de Modélisation et Simulation Multi-Echelle FRE 3160 CNRS, Université Paris Est, 5 boulevard Descartes, 77454 Marne la Vallée Cedex, France
3. Department of Mathematics, Agrarian State University of Moldova, Mircesti Street 44, 2049 Chisinau, Moldova