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Submitted on: 25 August 1999.
Revised on: 15 November 1999.
Accepted on: 12 January 2000.
___________________APPLIED MATHEMATICAL MODELLING
Submitted on: 14 October 2010.
Revised on: 20 December 2010.
Accepted on: 24 January 2011.
___________________APPLIED MATHEMATICAL MODELLING
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Submitted on: 22 July 2001.
Revised on: 10 November 2001.
Accepted on: 23 November 2001.
___________________APPLIED MATHEMATICAL MODELLING
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Submitted on: 16 April 2007.
Revised on: 01 July 2007.
Accepted on: 02 August 2007.
___________________APPLIED MATHEMATICAL MODELLING
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