Department of Mathematics, The University of Hong Kong, Hong Kong, 999077, China. whou@connect.hku.hk.|Hematology Oncology Division, Northwestern University, Evanston, IL 60208, USA.|Faculty of Science and Engineering, Soka University, Tokyo, 192-8577, Japan.|Department of Mathematics, The University of Hong Kong, Hong Kong, 999077, China.|Faculty of Science and Engineering, Soka University, Tokyo, 192-8577, Japan.