[1] Nanjing Univ Aeronaut & Astronaut, Dept Math, Coll Sci, Nanjing 210016, Peoples R China
[2] Natl Univ Singapore, Dept Math, Level 4,Block S17,10 Lower Kent Ridge Rd, Singapore 119076, Singapore
[3] Peking Univ, Dept Engn, Beijing 100871, Peoples R China