1. Centre for Mathematical and Statistical Sciences, Peechi Campus, Peechi-680653, Kerala, India and Department of Mathematics, Aligarh Muslim University, Aligarh-202002, India
2. Department of Mathematics, D. A-V. Postgraduate College, Kanpur-208001, U.P., India
3. Department of Mathematics, L.N.C.T., Bhopal, M. P., India