1. College of Mathematics, Taiyuan University of Technology, Shanxi, China.
2. Department of Mathematics, Hong Kong Baptist University, Kowloon Tong, Hong Kong.
3. Department of Mathematics, Simon Fraser University, Burnaby, British Columbia, Canada V5A1S6.
4. Department of Mathematics, University of British Columbia, Vancouver, Canada, and Department of Mathematics, Hong Kong Baptist University, Kowloon Tong, Hong Kong.