Institution: | (1) Department of Applied Mathematics, Pukyong National University, Pusan, 608-737, Korea;(2) Department of Mathematics, Hanoi Pedagogical Institute No. 2, Xuan Hoa, Me Linh, Vinh Phuc, Vietnam;(3) Institute of Mathematics, Vietnamese Academy of Science and Technology, 18 Hoang Quoc Viet Road, 10307 Hanoi, Vietnam |