1. Department of Mathematics, Cooch Behar PanchananBarma University, Cooch Behar, West Bengal 736101, India
2. Department of Mathematics and Statistical Sciences, Botswana International University of Science and Technology, Private Bag 16, Palapye, Botswana
3. Department of Mathematics, Mathabhanga College, Mathabhanga, Cooch Behar, West Bengal 736146, India