1. Institute of Mathematics “Simion Stoilow” of the Romanian Academy, P.O. Box 1-764, RO-014700 Bucharest, Romania
2. Academy of Romanian Scientists, 3 Ilfov, 050044 Bucharest, Romania
3. Institute of Mathematics of the Romanian Academy, CRAN, UMR 7039, Campus Sciences, BP 70239, VandÏuvre-les-Nancy CEDEX, 54506 Nancy, France
4. CNRS, CRAN, UMR 7039, 54500 Nancy, France