Affiliation: | Department of Mathematical Sciences, University of Oulu, P.O. Box 3000, 90014 Oulu, Finland ; Department of Mathematical Sciences, University of Oulu, P.O. Box 3000, 90014 Oulu, Finland ; Department of Mechanics and Mathematics, Moscow Lomonosov State University, Vorobiovy Gory, GSP-2, 119992 Moscow, Russia |