[1] Fudan Univ, Sch Math Sci, Shanghai 200433, Peoples R China
[2] Moscow State Univ Leninskie Gory, Dept Math & Mech, Moscow 119991, Russia
[3] Inst Theoret & Expt Phys, Main Cheremushkinskaya St, Moscow 117218, Russia
[4] Russian Acad Sci, Inst Informat Transmiss Problems, Bolshoy Karetny Lane, Moscow 127051, Russia