1. Department of Mathematics, West Bengal State University, Barasat, Kolkata 700126, India
2. Department of Instrumentation Science, Jadavpur University, Kolkata 700 032, India
3. Department of Mathematics, Visva-Bharati, Santiniketan - 731 204, India and Advanced Centre for Nonlinear and Complex Phenomena, 1175 Survey Park, Kolkata 700 075, India