Department of Mathematics, Hong Kong Baptist University, Kowloon Tong, Hong Kong.|Department of Biostatistics, Yale University, New Haven, 06510, CT, USA.|Department of Mathematics, Hong Kong Baptist University, Kowloon Tong, Hong Kong.|Department of Computer Science and Institute of Computational and Theoretical Studies, Hong Kong Baptist University, Kowloon Tong, Hong Kong. xwan@comp.hkbu.edu.hk.