1. 1Department of Mathematics, Hanoi University of Science, Vietnam National University 334 Nguyen Trai, Thanh Xuan, Hanoi, Vietnam
2. 2Thang Long Institute of Mathematics and Applied Sciences, Thang Long University
3. 3Department of Mathematics, Thuyloi University 175 Tay Son, Dong Da, Hanoi, Vietnam
4. 4Department of Mathematics, The University of Danang - University of Science and Education 459 Ton Duc Thang, Danang, Vietnam