Affiliation: | Faculty of Electronic Engineering, University of Niš 18 000 Niš, Yugoslavia;Institute of Mathematics, University of Novi Sad 21 000 Novi Sad, Yugoslavia;Department of Mathematics and Statistics University of Pittsburgh, Pittsburgh,PA 15260 USA;Institute of Mathematics, Bulgarian Academy of Sciences, ‘Acad. G. Bonchev’ str., bl. 8, BG-1113 Sofia, Bulgaria |