1. Institute of Technical Physics and Materials Science, Research Centre for Natural Sciences, Hungarian Academy of Sciences, PO Box 49, 1525 Budapest, Hungary
2. Department of Applied Mathematics, School of Mathematics, University of Leeds, Leeds LS2 9JT, UK
3. College of Physics and Electronic Information Engineering, Wenzhou University, 325035 Wenzhou, People's Republic of China
4. Faculty of Natural Sciences and Mathematics, University of Maribor, Koroška cesta 160, 2000 Maribor, Slovenia