1. Department of Mathematics and Mechanics, China University of Mining and Technology, Xuzhou, Jiangsu 221008, China
2. Department of Chemical Engineering, University of Chemical Technology and Metallurgy, Sofia, 8 Kliment Ohridsky Boulevard, 1756 Sofia, Bulgaria
3. Department of Mathematics and Statistics, University of Victoria, Victoria, BC, Canada V8W 3R4
4. Department of Mathematics, Faculty of Science, King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia