1. Department of Mathematics, Burdwan University, Golapbag, Burdwan 713 104, West Bengal, India
2. Centre for Foundation Studies in Science, University of Malaya, 50603 Kuala Lumpur, Malaysia
3. Department of Mathematics, Kazi Nazrul University, B.C.W. Campus, Asansol 713 304, West Bengal, India
4. Institute of Mathematical Sciences, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia