1. Department of Applied Mathematics I, University of Vigo, E. E. Forestal, 36005 Pontevedra, Spain
2. Department of Algebra, University of Santiago de Compostela, 15782 Santiago de Compostela, Spain
3. A. Razmadze Mathematical Institute, Tbilisi State University, Tamarashvili St. 6, 0177 Tbilisi, Georgia