Affiliations: Indrajit Lahiri (corresponding author), Department of Mathematics, University of Kalyani, Kalyani, West Bengal 741235, India, e-mail: ilahiri@hotmail.com; Sujoy Majumder, Department of Mathematics, Raiganj University, Raiganj, West Bengal 733134, India, e-mail: sm05math@gmail.com, sujoy.katwa@gmail.com