1. Faculty of Mathematics, Chemnitz University of Technology, D-09107 Chemnitz, Germany
2. Department of Physics and Mathematics, Bashkir State Pedagogical University, October rev. st., 3a, 450000 Ufa, Russia
3. Department of Mathematics, Faculdade de Motricidade Humana (TU Lisbon) and Group of Mathematical Physics of the University of Lisbon, Complexo Interdisciplinar, Av. Prof. Gama Pinto 2, P-1649-003 Lisboa, Portugal