Affiliation: | 1. Department of Mathematics and Computational Science, Hengyang Normal University, Hengyang 421008, Hunan, PR China;2. Department of Mathematics, Honghe University, Mengzi 661100, Yunnan, PR China;3. Department of Mathematics, University of Oradea, Str. Universitaˇ?ii, No. 1, 410087, Romania |