1. Department of Mathematics, North Eastern Hill University, Shillong, Meghalaya 793022, India
2. Mathematics Division, Department of Basic Sciences and Social Sciences, North Eastern Hill University, Shillong, Meghalaya 793022, India
3. Department of Mathematics, Indian Institute of Engineering Science and Technology, Shibpur, Howrah-711 103, India
4. Department of Mathematics, St. Anthony’s College, Shillong, Meghalaya 793001, India