Affiliation: | 1. Department of Applied Mathematics, Nanjing Forestry University, Nanjing 210037, PR China;2. Department of Mathematics, Nanjing University, Nanjing 210093, PR China;3. Department of Mathematics, Inha University, Incheon 402-751, Republic of Korea;4. School of Mathematical Sciences, Xiamen University, Xiamen 361005, PR China |