1. Department of Mathematics, Hong Kong Baptist University, Kowloon Tong, Hong Kong.
2. Hong Kong Center for Cerebro-Cardiovascular Health Engineering COCHE, Hong Kong Science Park, Hong Kong, and NORCE Norwegian Research Centre, Nygårdstangen, NO-5838 Bergen, Norway.
3. Computer Science Department, Technion, Haifa, 32000, Israel.
4. The author is deceased. Former addresses: Department of Mathematics, University of Houston, Houston, TX 77204-3008 USA, and Department of Mathematics, Hong Kong Baptist University, Kowloon Tong, Hong Kong.