Institution: | (1) Sobolev Institute of Mathematics, Russian Academy of Sciences Siberian Branch, Novosibirsk, 630090, Russia;(2) Faculty of Science, University of Kragujevac, PO Box 60, YU-34000 Kragujevac, Yugoslavia;(3) Department of Mathematics, PEF, University of Maribor, Koroka 160, SI-2000 Maribor, Slovenia |