1. Department of Basic Science and Humanities, Indian Institute of Information Technology, Bhagalpur 813210, India
2. Department of Mathematics, Jaypee Institute of Information Technology, Noida, Uttar Pradesh 201301, India
3. Department of Mathematics, Institute of Science, Banaras Hindu University, Varanasi 221005, India
4. Department of Mathematics, University of Florida, Gainesville, FL 32611, USA