1. Department of Mathematics The Chinese University of Hong Kong Shatin Hong Kong P.R. CHINA
2. School of Mathematics and Statistics Central China Normal University Wuhan 430079 P.R. CHINA
3. Department of Mathematics Jinan Unviersity Guangzhou 510632 P.R. CHINA
4. Mathematical Institute Tohoku University Sendai 980‐0856 JAPAN
5. Department of Mathematics University of Pennsylvania, David Rittenhouse Lab 209 South 33rd Street Philadelphia PA 19104‐6395 USA