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, 10307, Hanoi, Vietnam
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