Affiliation: | 1. Department of Mathematics, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, PR China;2. College of Science, Xi’an Jiaotong University, Xi’an 710049, PR China;3. Department of Mathematics, University of Iowa, Iowa City, IA 52242, USA;4. Department of Mathematics, Shanghai Jiao Tong University, Shanghai 200030, PR China |