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Submitted on: 27 March 2012.
Revised on: 25 April 2012.
Accepted on: 25 May 2012.
___________________Communications of the Korean Mathematical Society
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Submitted on: 31 July 2006.
Revised on: 29 September 2006.
Accepted on: 13 November 2006.
___________________Communications of the Korean Mathematical Society
Submitted on: 09 March 2002.
Revised on: 06 June 2002.
Accepted on: 01 August 2002.
___________________Communications of the Korean Mathematical Society
Submitted on: 09 August 1999.
Revised on: 08 September 1999.
Accepted on: 30 October 1999.
___________________Communications of the Korean Mathematical Society
Submitted on: 23 April 2021.
Revised on: 22 June 2021.
Accepted on: 03 July 2021.
___________________Communications of the Korean Mathematical Society
Submitted on: 12 July 2013.
Revised on: 15 October 2013.
Accepted on: 15 November 2013.
___________________Communications of the Korean Mathematical Society
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