1. International College Beijing, China Agricultural University, No. 17 Qinghua East Road, Haidian District, Beijing 100083, P. R. China
2. Department of Mathematics, School of Applied Mathematical and Physical Sciences, National Technical University of Athens, Zografou Campus, GR-15780 Athens, Greece
3. Department of Mathematics, George Washington University, Phillips Hall, Room 739, DC 20052 Washington, USA
4. University of Gdańsk, Jana Bażyńskiego 8, 80-309 Gdańsk, Poland