Affiliation: | A1 Mathematics Department, Krasnoyarsk University pr. Svobodnyi 79, 660 062, Krasnoyarsk, Russia. E-mail: rybakov@math.kgu.krasnoyarsk.su A2 Mathematics Dept., Science Faculty, Ege University, Bornova-Izmir, 35100, Turkey. E-mail: terziler@fenfak.ege.edu.tr |