[1] Univ Picardie Jules Verne, LAMFA CNRS Umr 6140, F-80039 Amiens 1, France
[2] Federat Rech Unites Math Marseille, Inst Math Luminy, Ctr Phys Theor, F-13288 Marseille 9, France
[3] Univ Mediterranee, F-13288 Marseille 9, France
[4] Univ Toulon & Var, Ctr Phys Theor, F-13288 Marseille 9, France
[5] Federat Rech Unites Math Marseille, CNRS Luminy, F-13288 Marseille 9, France