1. Department of Mathematics, Gebze Technical University, 41400 Gebze/Kocaeli, Turkey
2. Department of Mathematics, The University of Danang - University of Education, 459 Ton Duc Thang, Danang city, Vietnam
3. Department of Algebra and Mathematical Logic, Kazan (Volga Region) Federal University, 18 Kremlyovskaya str., Kazan, 420008 Russia