1. Institute of Mathematics ‘Simion Stoilow’ of the Romanian Academy; RO-014700 Bucharest Romania
2. Université de Lorraine, CRAN, UMR 7039, Campus Sciences; Vandœvre-lés-Nancy Cedex 54506 France
3. CNRS, CRAN, UMR 7039; Vandœvre-lés-Nancy Cedex France
4. Faculty of Economics and Business Administration; Sofia University; Sofia 1113 Bulgaria