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Submitted on: 10 December 2007.
Revised on: 30 December 2007.
Accepted on: 17 January 2008.
___________________Communications of the Korean Mathematical Society
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Submitted on: 05 August 1999.
Revised on: 22 October 1999.
Accepted on: 21 December 1999.
___________________Communications of the Korean Mathematical Society
Submitted on: 02 April 2010.
Revised on: 02 June 2010.
Accepted on: 24 June 2010.
___________________Communications of the Korean Mathematical Society
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