1Department of Mathematics, Umes Chandra College, 13, Surya Sen Street, Calcutta-700012, India.
2Department of Mathematics, Vivekananda College, 269, D. H. Road, Calcutta-700063, India.
3Department of Applied Mathematics, University of Calcutta, 92 A. P. C. Road, Kolkata-700009, India.
4Department of Mathematics, Heritage Institute of Technology, Chowbaga Road, Anandapur, P.O. East Kolkata Township, Kolkata-700107, India.
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