1. LAMFA, UMR 7352, CNRS; University of Picardie; 33 Rue Saint Leu Amiens 80039 France
2. Mathematics Centre for Mathematical Sciences, Lund Institute of Technology; Lund University; Box 118 Lund SE-221 00 Sweden
3. Aix Marseille University, CPT, IML; Frumam, Luminy, Case 907 Marseille Cedes 9 13288 France
4. Université de Toulon, CPT; La Garde 83957 France