作者机构:[1]School of Mathematics, Yangzhou University, Yangzhou, Jiangsu, 225002
相关基金:The NSF (11071208 and 11126046) of China and the Postgraduate Innovation Project (CXZZ13_0888) of Jiangsu Province. Acknowledgements The authors are very grateful to Professor Wang Hong-yu for his insightful discussions and the reviewers for having pointed out several improvements in writting this paper.