Affiliation: | a Laboratoire de Mathématiques, CNRS UMR 6620, Université Blaise Pascal, 63177 Aubière cedex, Franceb Department of Differential Equations, Faculty of Mechanics and Mathematics, Moscow Lomonosov State University, Moscow 119991, Russiac Narvik University College, Postboks 385, 8505 Narvik, Norwayd P.N. Lebedev Physical Institute RAS, Leninski pr., 53, Moscow 117924, Russia |