Affiliation: | 1. Department of Mathematics, University of Science and Technology of China, Hefei, Anhui, 230026, China;2. Department of Computer Science, The University of Hong Kong, Hong Kong, China;3. Department of Mathematics, Southeast Missouri State University, Cape Girardeau, MO 63701, United States;4. Computer Science Department, Rice University, 6100 Main Street, MS-132, Houston, TX 77251, United States |