1. I. Javakhishvili Tbilisi State University A. Razmadze Mathematical Institute, Tbilisi, Georgia
2. Georgian Technical University Department of Mathematics, Tbilisi, Georgia
3. I. Javakhishvili Tbilisi State University Faculty of Exact and Natural Sciences, Tbilisi, Georgia
4. Gori State Teaching University Faculty of Education, Exact and Natural Sciences, Gori, Georgia