1. Faculty of Mathematics and Computer Science, University of Bucharest , Academiei 14, RO-010014 Bucharest , Romania
2. Department of Applied Mathematics, The Bucharest University of Economic Studies , Piata Romana 6, RO-010374 Bucharest , Romania
3. “Gheorghe Mihoc-Caius Iacob” Institute of Mathematical Statistics and Applied Mathematics of Romanian Academy, 2 . Calea 13 Septembrie, nr.13, sect. 5, RO-050711 , Bucharest , Romania
4. “Costin C. Kiritescu” National Institute of Economic Research of Romanian Academy , 3. Calea 13 Septembrie, nr.13, sect. 5, RO-050711 , Bucharest , Romania