1. Universit de Lorraine, CRAN, UMR 7039, Vanduvre-les-Nancy Cedex, France
2. CNRS, CRAN, UMR 7039, Vanduvre-les-Nancy Cedex, France
3. Institute of Mathematics “Simion Stoilow” of the Romanian Academy, Bucharest, Romania
4. The Academy of the Romanian Scientists, Bucharest, Romania