1. Department of Mathematics, Huzhou University, Huzhou 313000, P. R. China
2. Key Laboratory of Computing and Stochastic, Mathematics (Ministry of Education), School of Mathematics and Statistics, Hunan Normal University, Changsha, Hunan 410081, P. R. China
3. School of Mathematics and Computer Science, Iran University of Science and Technology, Narmak, Tehran, Iran
4. Department of Mathematical Sciences, University of South Africa, Florida, South Africa