1. A. Razmadze Mathematical Institute; Tbilisi State University; Tbilisi Tamarashvili str. 60177 Georgia
2. University of Georgia; IV, 77a Merab Kostava St Tbilisi Georgia
3. Department of Engineering Sciences and Mathematics; Luleå University of Technology; Luleå SE-971 87 Sweden
4. Department of Mathematics, Faculty of Exact and Natural Sciences; Tbilisi State University; Chavchavadze str. 1 Tbilisi 0128 Georgia