1. Physics and Applied Mathematics Unit, Indian Statistical Institute, Kolkata 700108, India
2. Department of Physics, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India
3. Department of Control Theory, Nizhni Novgorod State University, Gagarin Avenue 23, 606950 Nizhni Novgorod, Russia
4. Department of Mathematics, Jadavpur University, Kolkata 700032, India