1. Département de Mathématiques, Faculty of Science, Université de Strasbourg, Strasbourg, France;
2. Department of Mathematics, Faculty of Science, University of Zagreb, Zagreb, Croatia;
3. Univ. Paris-Est Marne-la-Vallée, LIGM (UMR 8094), CNRS, ENPC, ESIEE Paris, France;
4. Univ Lyon, EnsL, UCBL, CNRS, Inria, LIP, LYON Cedex 07, France