1. Department of Mathematics and RIRCM, Kyungpook National University, Daegu-41566, South Korea
2. Department of Mathematics, Kabi Sukanta Mahavidyalaya, The University of Burdwan, Bhadreswar, P.O.-Angus, Hooghly, Pin 712221, West Bengal, India
3. Department of Pure Mathematics, University of Calcutta, West Bengal, India