1. Department of Mathematics and Computer Science, University of Science, Vietnam National University, 227 Nguyen Van Cu Q.5, Ho Chi Minh City, Vietnam
2. Tan Tao University, Tan Tao Avenue, Tan Duc e-City, Long An Province, Vietnam
3. Department of Mathematics, National University of Singapore, Blk S17, 10 Lower Kent Ridge Road, Singapore 119076, Singapore
4. Department of Mathematical Sciences, SCEN 327, 1 University of Arkansas, Fayetteville, AR 72701, USA