Submitted on: 21 December 1999.
Revised on: 17 January 2000.
Accepted on: 09 February 2000.
Thai Journal of Mathematics
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Submitted on: 28 January 2016.
Revised on: 26 February 2016.
Accepted on: 06 March 2016.
Thai Journal of Mathematics
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Submitted on: 09 May 2007.
Revised on: 13 June 2007.
Accepted on: 04 July 2007.
Thai Journal of Mathematics
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