Submitted on: 03 May 2012.
Revised on: 28 June 2012.
Accepted on: 21 July 2012.
___________________Differential Equations & Applications
Submitted on: 17 April 2000.
Revised on: 26 May 2000.
Accepted on: 08 June 2000.
___________________Differential Equations & Applications
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Submitted on: 11 August 2008.
Revised on: 21 November 2008.
Accepted on: 18 December 2008.
___________________Differential Equations & Applications