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Submitted on: 17 February 2018.
Revised on: 27 March 2018.
Accepted on: 01 April 2018.
Vietnam Journal of Mathematics
Submitted on: 12 November 2005.
Revised on: 19 December 2005.
Accepted on: 14 January 2006.
Vietnam Journal of Mathematics
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Submitted on: 16 January 2000.
Revised on: 20 February 2000.
Accepted on: 03 March 2000.
Vietnam Journal of Mathematics
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