[1] Univ Cergy Pontoise, CNRS, UMR 8089, Lab Phys Theor & Modelisat, F-95302 Cergy Pontoise, France
[2] Heriot Watt Univ, Dept Math, Edinburgh EH14 4AS, Midlothian, Scotland
[3] Leibniz Univ Hannover, Inst Theoret Phys, Appelstr 2, D-30167 Hannover, Germany