作者单位:Pham Ky Anh (1) Le Tuan Hoa (2) Hoang Xuan Phu (2)
1. Faculty of Mathematics, Mechanics, and Informatics, Vietnam National University, Hanoi, 334 Nguyen Trai, Thanh Xuan, Hanoi, Vietnam 2. Institute of Mathematics, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Hanoi, Vietnam