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Submitted on: 02 March 2001.
Revised on: 25 June 2001.
Accepted on: 03 August 2001.
___________________BRAZILIAN JOURNAL OF PHYSICS---SPRINGER
Submitted on: 07 June 2018.
Revised on: 25 August 2018.
Accepted on: 17 September 2018.
___________________BRAZILIAN JOURNAL OF PHYSICS---SPRINGER
Submitted on: 15 April 2014.
Revised on: 18 July 2014.
Accepted on: 29 August 2014.
___________________BRAZILIAN JOURNAL OF PHYSICS---SPRINGER
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Submitted on: 25 August 2013.
Revised on: 17 November 2013.
Accepted on: 05 December 2013.
___________________BRAZILIAN JOURNAL OF PHYSICS---SPRINGER