1. Department of Mathematics, Harbin Institute of Technology, Harbin, Heilongjiang 150006, China
2. School of Mathematics and Statistics, Northeast Petroleum University, Daqing, Heilongjiang 163318, China
3. Department of Mathematics, University of Iowa, Iowa City, IA 52242, USA
4. Imaging Diagnosis and Interventional Center, State Key Laboratory of Oncology in South China, Sun Yat-sen University Cancer Center, Guangzhou, Guangdong 510060, China