作者机构:[1]Chern Institute of Mathematics & LPMC, Nankai University, Tianjin 300071, China, [2]Department of Mathematics, Yunnan Normal University, Kunming650092, China
相关基金:Acknowledgements This work was supported in part by the National Natural Science Foundation of China (Grant Nos. 10621101, 10921061), the National Key Basic Research Development Project (2006CB805905), and the Specialized Research Fund for the Doctoral Program (200800550015).