1. Department of Occupational Safety and Health, College of Public Health, China Medical University, Taichung 404, Taiwan
2. Department of Mathematics, Aligarh Muslim University, Aligarh 202 002, India
3. Department of Mathematics and Statistics, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia
4. Department of Mathematics, National Tsing Hua University, Hsinchu 300, Taiwan