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Submitted on: 10 December 2011.
Revised on: 12 February 2012.
Accepted on: 18 March 2012.
___________________COMPTES RENDUS PHYSIQUE---ACAD SCIENCES
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Submitted on: 23 January 2008.
Revised on: 06 March 2008.
Accepted on: 21 April 2008.
___________________COMPTES RENDUS PHYSIQUE---ACAD SCIENCES
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Submitted on: 02 April 2010.
Revised on: 10 June 2010.
Accepted on: 26 July 2010.
___________________COMPTES RENDUS PHYSIQUE---ACAD SCIENCES
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