1. Institute of Mathematics, Academy of Sciences of the Czech Republic, Zitná 25, CZ-115 67 Praha 1, Czech Republic
2. Institute of Mathematics, Johannes Gutenberg-University Mainz, Staudingerweg 9, 55 128 Mainz, Germany
3. Academy for Multidisciplinary Studies, Capital Normal University, West 3rd Ring North Road 105, 100048 Beijing, P. R. China