1. School of Mathematics and Statistics, Tianshui Normal University, Tianshui 741001, China
2. School of Sciences, Xi’an University of Architecture and Technology, Xi’an 710055, China
3. State Key Laboratory for Strength and Vibration of Mechanical Structures, Xi’an Jiaotong University, Xi’an 710049, China
4. School of Arts and Sciences, Shaanxi University of Science and Technology, Xi’an 710021, China