1. Department of Mathematics, Faculty of Science, University of Hradec Králové, Rokitanského 62, 500 03 Hradec Králové, Czech Republic
2. Lepage Research Institute, 783 42 Slatinice, Czech Republic
3. Department of Mathematics, LaTrobe University, Melbourne, Bundoora, Victoria 3086, Australia
4. School of Mathematics, Beijing Institute of Technology, 5 South Zhongguancun Street, Haidian Zone, Beijing 100081, P. R. China