Affiliation: | aDepartment of Mathematics, Umes Chandra college, 13, Surya Sen Street, Calcutta 700012, India;bDepartment of Mathematics, Vivekananda College, 269, D. H. Road, Calcutta 700063, India;cDepartment of Mathematics, Heritage Institute of Technology, Chowbaga Road, Anandapur, P.O. East Kolkata Township, Kolkata 700107, India |