Affiliation: | (1) Department of Mathematical Sciences, Norwegian University of Science and Technology, N-7491 Trondheim, Norway;(2) Institute for Low Temperature Physics & Engineering, 47, Lenin PR, 310164 Kharkov, Ukraine;(3) Department of Mathematics and Mechanics, St. Petersburg State University, Bibliotechnaja Pl., 2, Stary Peterhof, 198904 St. Petersburg, Russia |