1. School of Mathematical Sciences, University of Jinan, Jinan, Shandong 250022, China
2. Mathematics Department, School of Science and Technology, University of Trás-os-Montes e Alto Douro, Vila Real, Portugal
3. Department of Mathematics and Physical Sciences, Prince Sultan University, P.O. Box 66833, Riyadh 11586, Saudi Arabia
4. Department of Mathematics, University of Texas at San Antonio, San Antonio, TX 78249, USA