1. Department of Applied Mathematics, Bucharest University of Economic Studies, Piata Romana 6, 010374 Bucharest, Romania
2. Romanian Academy, Costin C. Kiriţescu National Institute of Economic Research—Centre of Mountain Economy (CE-MONT), 13 Calea 13 Septembrie, 030508 Bucharest, Romania
3. Department of Applied Mathematics, Wrocław University of Environmental and Life Sciences, Grunwaldzka 53, 50-537 Wrocław, Poland
4. Department of Mathematics, University of Hasselt, BE 3590 Diepenbeek, Belgium