1. Department of Mathematics, Hooghly Engineering & Technology College, Hooghly, West Bengal 712 103, India
2. Department of Mathematics, Gobardanga Hindu College, 24 Parganas (North), West Bengal 743 273, India
3. Makhla Debiswari Vidyaniketan, Hooghly, West Bengal 712 245, India
4. Department of Applied Mathematics, University of Calcutta, 92 A. P. C. Road, Kolkata 700 009, India