1. Department of Mathematics, Harbin Institute of Technology, Harbin 150006, P. R. China
2. School of Mathematical Sciences, Harbin Normal University, Harbin 150025, P. R. China
3. Applied Mathematics Department, The University of Western Ontario, London, Canada N6A 5B7, Canada
4. Department of Mathematics and Computer Science, North Carolina Central University, Durham, NC 27713, USA