1. Institute for Problems in Mechanics of Russian Academy of Sciences and Moscow Institute of Physics and Technology, Prosp. Vernadskogo 101-1, 119526 Moscow, Russia. E-mail: dobr@ipmnet.ru
2. Centre de Physique Théorique and Université du Sud Toulon-Var, UMR 6207 Campus de Luminy, Case 907, 13288 Marseille Cedex 9, France. E-mail: rouleux@cpt.univ-mrs.fr