1. Formerly University of California, Department of Mathematics, Los Angeles, California, USA; presently Harbin Institute of Technology, Department of Mathematics, Harbin, China..
2. University of California, Department of Mathematics, Los Angeles, California, USA..
3. Harbin Institute of Technology, Department of Mathematics, Harbin, China..
4. Formerly University of California, Department of Mathematics, Los Angeles, California, USA; presently Tsinghua University, Department of Mathematics, Beijing, China..