Affiliation: | a School of Mathematical Sciences and Institute of Mathematics and Interdisciplinary Science, Capital Normal University, Beijing 100037, People's Republic of China b Department of Mathematics and Statistics, Utah State University, Logan, UT 84322, USA c School of Mathematics and Computer Sciences, Fujian Normal University, Fuzhou 350007, People's Republic of China d Institut de Mathématique, Université Catholique de Louvain, 1348 Louvain-la-Neuve, Belgium |