Bull. Korean Math. Soc. 2011; 48(6): 1291-1302
Printed November 1, 2011
https://doi.org/10.4134/BKMS.2011.48.6.1291
Copyright © The Korean Mathematical Society.
Shan Zhang, Zuhan Liu, and Zhongxue L\"u
Yangzhou University, Xuzhou Normal University, Xuzhou Normal University
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