1. School of Mechanics and Civil Engineering, China University of Mining and Technology, Xuzhou 221116, People’s Republic of China. dyangxiaojun@163.com
2. Institute of Engineering, Polytechnic of Porto, Department of Electrical Engineering, Rua Dr. António Bernardino de Almeida, 4249-015 Porto, Portugal
3. Departamento de Analisis Matematico, Facultad de Matematicas, Universidad de Santiago de Compostela, 15782 Santiago, Spain
4. Faculty of Science, King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia