1. Faculty of Mathematics and Computer Science, University of Bucharest, Str. Academiei 14, 010014 Bucharest, Romania
2. “Gheorghe Mihoc-Caius Iacob” Institute of Mathematical Statistics and Applied Mathematics, Calea 13 Septembrie 13, 050711 Bucharest, Romania
3. “Costin C. Kiriţescu” National Institute of Economic Research, Calea 13 Septembrie 13, 050711 Bucharest, Romania