1. Department of Mathematical Analysis A. Razmadze Mathematical Institute , I. Javakhishvili Tbilisi State University , 6. Tamarashvili St., 0177 , Tbilisi , Georgia
2. Faculty of Mathematics and Computer Science , Adam Mickiewicz University, Poznań , Uniwersytetu Poznańskiego 4 61-614 , Poznań , Poland
3. Department of Mathematics Faculty of Informatics and Control Systems , Georgian Technical University , 77, Kostava St ., Tbilisi , Georgia