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Submitted on: 26 December 2002.
Revised on: 18 March 2003.
Accepted on: 11 May 2003.
___________________Journal of Mathematics and Computer Science-JMCS
Submitted on: 10 March 2011.
Revised on: 03 July 2011.
Accepted on: 31 July 2011.
___________________Journal of Mathematics and Computer Science-JMCS
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Submitted on: 22 November 2010.
Revised on: 29 January 2011.
Accepted on: 19 February 2011.
___________________Journal of Mathematics and Computer Science-JMCS
Submitted on: 12 August 2001.
Revised on: 10 September 2001.
Accepted on: 26 September 2001.
___________________Journal of Mathematics and Computer Science-JMCS
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