[1] Lomonosov Moscow State Univ, Fac Mech & Math, Moscow, Russia
[2] Demidov Yaroslavl State Univ, Ctr Integrable Syst, Yaroslavl, Russia
[3] Natl Res Univ, Moscow Inst Phys & Technol, Ctr Fundamental Math, Moscow, Moscow, Russia
[4] Kazan Volga Reg Fed Univ, Lobachevsky Inst Math & Mech, Kazan, Russia