1. Department of Mathematics, Bidhan Chandra College, Asansol — 4, West Bengal 713304, India
2. Department of Mathematics, University of Tabuk, K.S.A.
3. Department of Mathematics, Jadavpur University, Kolkata 700032, India
4. Department of Applied Mathematics, Nanjing University of Science and Technology, Nanjing 210094, P. R. China