Department of Mathematics, Shanghai Normal University, Shanghai 200234, China.|Department of Biostatistics and Bioinformatics, Rollins School of Public Health, Emory University, Atlanta, GA 30322, USA.|Department of Biostatistics and Bioinformatics, Rollins School of Public Health, Emory University, Atlanta, GA 30322, USA.|Department of Mathematics, Shanghai Normal University, Shanghai 200234, China.