1. School of Mathematics and Physics, China University of Geosciences, Wuhan, Hubei, China
2. Department of Mathematics, College of Science, Sana'a University, Sana'a, Sana'a, Yemen
3. Department of Mathematics, College of Science, Northwestern Polytechnical University, Xi'an, Shannxi, China
4. School of Information Engineering, Chang′an University, Xi′an, Shannxi, China