1. Center for Nonlinear Studies and Department of Mathematics, Northwest University, Xi'an 710069, P.R. China. E-mail: zhenhua.guo.math@gmail.com
2. LCP, Institute of Applied Physics and Computational Mathematics, P.O. Box 8009, Beijing 100088, P.R. China. E-mails: jiang@iapcm.ac.cn, tzxief@yahoo.com