1. College of Mathematics and Systems Science; Shandong University of Science and Technology; Qingdao 266590 China
2. Department of Mathematics and Mathematical Statistics; Umeå University; SE-90187 Umeå Sweden
3. Laboratory of Mine Disaster Prevention and Control, Key Laboratory of Ministry of Education Co-sponsored by Province and Ministry; Shandong University of Science and Technology; Qingdao 266590 China