1. University of Nantes/CNRS , UMR 6629 , Jean Leray Laboratory of Mathematics , 2 rue de Houssiniere, BP 92208, 44322 Nantes , France
2. Business and Technology University , 82 I. Chavchavadze Avenue , Tbilisi 0162 ; and I. Vekua Institute of Applied Mathematics of Iv. Javakhishvili Tbilisi State University, 11 University St., Tbilisi 0186 , Georgia
3. Department of Mathematics and I. Vekua Institute of Applied Mathematics , Iv. Javakhishvili Tbilisi State University , 11 University St. , Tbilisi 0186 , Georgia