1. School of Mathematics and Statistics, Shaanxi Normal University, Xi’an 710062, P. R. China
2. Department of Mathematics, Faculty of Science, Al-Azhar University, Assiut 71524, Egypt
3. School of Mathematics and Statistics, Northwestern Polytechnical University, Xi’an 710072, P. R. China
4. Department of Mathematics, Faculty of Science, Sohag University, Sohag 82524, Egypt