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Submitted on: 13 August 2005.
Revised on: 21 November 2005.
Accepted on: 20 December 2005.
___________________Journal de l Ecole Polytechnique-Mathematiques
Submitted on: 12 March 2005.
Revised on: 10 May 2005.
Accepted on: 17 June 2005.
___________________Journal de l Ecole Polytechnique-Mathematiques
Submitted on: 22 December 2003.
Revised on: 09 February 2004.
Accepted on: 29 February 2004.
___________________Journal de l Ecole Polytechnique-Mathematiques
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Submitted on: 29 July 2009.
Revised on: 26 November 2009.
Accepted on: 09 December 2009.
___________________Journal de l Ecole Polytechnique-Mathematiques
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