[1] Fudan Univ, Sch Math Sci, 220 Handan Rd, Shanghai 200433, Peoples R China
[2] Moscow MV Lomonosov State Univ, Fac Mech & Math, Moscow 119991, Russia
[3] Natl Res Ctr, Kurchatov Inst, Inst Theoret & Expt Phys, Bolshaya Cheremushkinskaya Ul 25, Moscow 117218, Russia
[4] Russian Acad Sci, Kharkevich Inst, Inst Informat Transmiss Problems, Bolshoi Karetnyi Per 19,Str 1, Moscow 127051, Russia