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Submitted on: 07 November 2007.
Revised on: 15 January 2008.
Accepted on: 04 March 2008.
___________________APPLIED MATHEMATICAL MODELLING
Submitted on: 28 August 2014.
Revised on: 11 October 2014.
Accepted on: 26 November 2014.
___________________APPLIED MATHEMATICAL MODELLING
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