1. Department of Mathematical Physics, A. Razmadze Mathematical Institute, I. Javakhishvili Tbilisi State University, Tbilisi, Georgia
2. Department of Mathematics, Georgian Technical University, Tbilisi, Georgia
3. I. Vekua Institute of Applied Mathematics, I. Javakhishvili Tbilisi State University, Tbilisi, Georgia
4. Department of Mathematics, Business and Technology University, Tbilisi, Georgia