1. Department of Mathematics, Faculty of Sciences and Arts, Ağri University, Ağri, Turkey
2. Department of Mathematics, Faculty of Sciences and Arts, Ordu University, Ordu, Turkey
3. Dipartimento di Matematica e Informatica, Universitá di Catania, Catania, Italy
4. Rudn University, 6 Miklukho, Maklay St., Moscow 117198, Russia