1. Department of Differential Equations , Faculty of Mechanics and Mathematics , Moscow Lomonosov State University , Vorobyovy Gory , Moscow , 119992 Russia
2. Center of Nonlinear Problems of Mathematical Physics , RUDN University , Miklukho-Maklaya str. 6 , Moscow , 117198 Russia ; and Institute of Applied Mathematics and Mechanics of NAS of Ukraine, Dobrovol’skogo str. 1, Slavyansk, 84116 Ukraine