1. Department of Mathematics, Vivekananda College, Kolkata 700063, India
2. School of Mathematics and Statistics, Xi'an Jiaotong University, Xi'an 710049, China
3. PG and Research Department of Physics, Bishop Heber College, Tiruchirappalli 620017, Tamil Nadu, India
4. Physics and Applied Mathematics Unit, Indian Statistical Institute, Kolkata 700 108, India