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Submitted on: 12 July 2006.
Revised on: 04 October 2006.
Accepted on: 22 November 2006.
___________________Italian Journal of Pure and Applied Mathematics
Submitted on: 09 November 2000.
Revised on: 10 January 2001.
Accepted on: 16 February 2001.
___________________Italian Journal of Pure and Applied Mathematics
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