作者机构:[1]School of Mathematical Sciences, Xiamen University, Xiamen 361005, China
相关基金:The third author was partially supported by NSF of China (10931006) and a grant from the PhD Programs Foundation of Ministry of Education of China (20100121110014). The fourth author was partially supported by NSF of China (11371024), Natural Science Foundation of Fujian Province (2013J01018) and Fundamental Research Funds for the Central University (2013121001).