Submitted on: 28 March 2016.
Revised on: 17 May 2016.
Accepted on: 15 June 2016.
___________________EUROPEAN JOURNAL OF APPLIED MATHEMATICS
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Submitted on: 28 February 2001.
Revised on: 13 May 2001.
Accepted on: 18 June 2001.
___________________EUROPEAN JOURNAL OF APPLIED MATHEMATICS
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Submitted on: 13 August 2006.
Revised on: 22 October 2006.
Accepted on: 11 November 2006.
___________________EUROPEAN JOURNAL OF APPLIED MATHEMATICS