1. Ilia Vekua Institute of Applied Mathematics of Ivane Javakhishvili Tbilisi State University, 2 University Street, 0186 Tbilisi, Georgia
2. Georgian Technical University, 77 Kostava Ave., 0175 Tbilisi, Georgia
3. Sokhumi State University, 12 Politkovskaya Street, 0186 Tbilisi, Georgia
4. Ivane Javakhishvili Tbilisi State University, 2 University Street, 0186 Tbilisi, Georgia