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Submitted on: 17 October 2005.
Revised on: 20 January 2006.
Accepted on: 11 February 2006.
___________________Differential and Integral Equations
Submitted on: 03 November 2007.
Revised on: 21 December 2007.
Accepted on: 26 January 2008.
___________________Differential and Integral Equations
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