1. Department of Mathematics, COMSATS University Islamabad, Attock Campus, Attock, Pakistan;
2. Department of Systems Engineering and Engineering Management, City University of Hong Kong, Kowloon, Hong Kong;
3. Department of Mathematics and Statistics, King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia;
4. Department of Statistics, Faculty of Science, University of Sargodha, Sargodha, Pakistan