1. College of Science, Yanshan University, Qinhuangdao 066004, China
2. Department of Mathematics and Statistics, University of Victoria, Victoria, British Columbia, Canada V8W 3R4
3. Department of Mathematics, Faculty of Mathematical Sciences, University of Mazandaran, Babolsar 47415-416, Iran
4. Department of Mathematics and Mechanics, China University of Mining and Technology, Jiangsu, Xuzhou 221008, China