Submitted on: 14 January 1998.
Revised on: 12 February 1998.
Accepted on: 26 March 1998.
___________________APPLIED MATHEMATICAL MODELLING
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Submitted on: 25 June 2021.
Revised on: 04 October 2021.
Accepted on: 28 November 2021.
___________________APPLIED MATHEMATICAL MODELLING
Submitted on: 17 February 2015.
Revised on: 10 April 2015.
Accepted on: 25 May 2015.
___________________APPLIED MATHEMATICAL MODELLING
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