1. A. Razmadze Mathematical Institute of I. Javakhishvili Tbilisi State University , 6 Tamarashvili Str. , Tbilisi 0186; and Georgian Technical University, 77 Kostava str., Tbilisi 0175 , Georgia
2. A. Razmadze Mathematical Institute of I. Javakhishvili Tbilisi State University , 6 Tamarashvili Str. , Tbilisi 0186; and Department of Mathematics, Faculty of Exact and Natural Sciences, I. Javakhishvili Tbilisi State University, 13 University Str., Tbilisi 0186 , Georgia