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Submitted on: 04 June 2009.
Revised on: 24 September 2009.
Accepted on: 17 November 2009.
___________________Italian Journal of Pure and Applied Mathematics
Submitted on: 25 January 2013.
Revised on: 24 May 2013.
Accepted on: 12 July 2013.
___________________Italian Journal of Pure and Applied Mathematics
Submitted on: 19 March 2006.
Revised on: 23 May 2006.
Accepted on: 22 June 2006.
___________________Italian Journal of Pure and Applied Mathematics
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Submitted on: 01 January 2001.
Revised on: 24 April 2001.
Accepted on: 14 May 2001.
___________________Italian Journal of Pure and Applied Mathematics
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