基金Part of this work was completed when the second author was visiting the Univer- sity of Western Ontario, and he would like to thank the staff in the Department of Applied Mathematics for their help and thank the University for its excellent facilities and support during his stay.
The second author was supported by China Scholarship Council, partially sup- ported by NNSF of China (No. 11031002), by the Heilongjiang Provincial Natural Science Foundation (No. A200806), and by the Program of Excellent Team and the Science Research Foundation in Harbin Institute of Technology.