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Submitted on: 15 October 2008.
Revised on: 03 January 2009.
Accepted on: 15 February 2009.
___________________Bulletin of Mathematical Sciences
Submitted on: 02 February 2009.
Revised on: 22 March 2009.
Accepted on: 14 April 2009.
___________________Bulletin of Mathematical Sciences
Submitted on: 10 February 2004.
Revised on: 27 March 2004.
Accepted on: 20 May 2004.
___________________Bulletin of Mathematical Sciences
Submitted on: 11 November 2018.
Revised on: 26 December 2018.
Accepted on: 30 January 2019.
___________________Bulletin of Mathematical Sciences
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Submitted on: 30 April 2010.
Revised on: 13 July 2010.
Accepted on: 29 August 2010.
___________________Bulletin of Mathematical Sciences