1. Laboratoire de Mathématiques Appliquées, CNRS-UMR5142, Université de Pau, Av. de l'Université, 64000 Pau, France
2. Department of Mathematics, B. Verkin Institut for Low Temperature Physics and Engineering, 47, av. Lenin, 61103 Kharkov, Ukraine
3. Narvik University College, Postbox 385, Narvik, 8505, Norway and Lebedev Physical Institute RAS, Leninski prospect 53, Moscow 119991, Russia