1. School of Mathematics, Southeast University, Nanjing 210096, P. R. China
2. Department of Cardiology, Nanjing Medical University, Nanjing 210029, P. R. China
3. China Information Tech. Designing & Consulting Institute Co., Ltd., Beijing 100048, P. R. China
4. Mathematical Sciences Department, Worcester Polytechnic Institute, Worcester, MA 01609, USA