1. College of Mathematics, Changchun Normal University, Changchun, People's Republic of China
2. College of Mathematics and Informatics, Fujian Normal University, Qishan Campus, Fuzhou, People's Republic of China
3. Dipartimento di Matematica e Informatica, Università degli Studi di Perugia, Perugia, Italy
4. College of Mathematics and System Science, Shandong University of Science and Technology, Qingdao, People's Republic of China