Submitted on: 04 August 2009.
Revised on: 27 November 2009.
Accepted on: 02 January 2010.
___________________ALEA-Latin American Journal of Probability and Mathematical Statistics
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Submitted on: 01 December 2007.
Revised on: 31 December 2007.
Accepted on: 16 February 2008.
___________________ALEA-Latin American Journal of Probability and Mathematical Statistics