1. School of Mathematical Sciences, Zhejiang Normal University, Jinhua, Zhejiang 321004, P. R. China
2. Department of Mathematics, Bahauddin Zakariya University, Vehari Campus, Vehari 61100, Pakistan
3. Department of Mathematics, King Abdulaziz University, Jeddah 21589, Saudi Arabia
4. Department of Mathematics and Statistics, University of South Florida, Tampa, FL 33620-5700, USA
5. School of Mathematical and Statistical Sciences, North-West University, Mafikeng Campus, Private Bag X2046, Mmabatho 2735, South Africa