1. Andrea Razmadze Mathematical Institute, Ivane Javakhishvili Tbilisi State University, Tbilisi 0193, Georgia
2. Kutaisi International University, Kutaisi 4600, Georgia
3. Department of mathematics and computer science, University of Catania, Catania 6-95125, Italy
4. Faculty of Fundamental Science, Industrial University, Ho Chi Minh City, Vietnam