1. A. Razmadze Mathematical Institute of I. Javakhishvili Tbilisi State University , 6, Tamarashvili Str., 0177; and Sokhumi State University, 9 Politkovskaia Str. , Tbilisi 0186 , Georgia
2. Department of Mathematics , Brunel University London , Uxbridge UB8 3PH , United Kingdom
3. Department of Mathematics , Georgian Technical University , 77, Kostava Str., 0175; and I. Vekua Institute of Applied Mathematics, I. Javakhishvili Tbilisi State University, 2 University Str. , Tbilisi 0186 , Georgia