1. Department of Mathematics, School of Mathematics and Statistics, Nanjing University of Science and Technology, Nanjing 210094, China
2. Laboratory of Mathematics, Djillali Liabes University of Sidi Bel-Abbès, P. O. Box 89, Sidi Bel-Abbès 22000, Algeria
3. Department of Mathematics, University of Texas at San Antonio, San Antonio, TX 78249, USA
4. Department of Economic Sciences, University of Tiaret, Tiaret 14035, Algeria