1. Department of Mathematical Sciences, Indian Institute of Technology (BHU), Varanasi 221005, India
2. Department of Mathematical Sciences, Indian Institute of Technology (BHU), Varanasi 221005, India e-mail:
3. Institute of Mathematical Sciences, University of Malaya, Kuala Lumpur 50603, Malaysia;
4. Department of Actuarial Science and Applied Statistics, UCSI University, Kuala Lumpur 56000, Malaysia
5. Department of Mathematical Sciences, University of South Africa (UNISA), Pretoria 0003, South Africa