1. Department of Biostatistics, Bioinformatics and Biomathematics, Georgetown University, Washington, DC 20057, USA
2. Department of Statistics and Actuarial Science, University of Western ontario, London, ON, Canada N6A 5B7
3. Department of Mathematics and Statistics, Georgia State University, Atlanta, GA 30303, USA
4. Academy of Mathematics and Systems Science, Chinese Academy of Sciences, Beijing 100190, China