1. 1. Ion Mihai, Department of Mathematics, Faculty of Mathematics and Computer Science, University of Bucharest, 010014 Bucharest, Romania. E-mail : imihai@fmi.unibuc.ro
2. 2. Cihan Ozgur, Department of Mathematics, Faculty of Arts and Sciences, Balıkesir University, Balıkesir, Turkey. E-mail : cozgur@balikesir.edu.tr