Bull. Korean Math. Soc. 2011; 48(1): 95-104
Printed January 1, 2011
https://doi.org/10.4134/BKMS.2011.48.1.95
Copyright © The Korean Mathematical Society.
Vu Nhat Huy and Qu\^{o}\llap{\raise 0.4ex\hbox{\'{}}}c-Anh Ng\^{o}
Vi\^{e}t Nam National University, Vi\^{e}t Nam National University
2015; 52(3): 987-998
2001; 38(1): 121-128
2010; 47(1): 103-111
2012; 49(1): 33-47
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