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Submitted on: 26 January 2000.
Revised on: 10 March 2000.
Accepted on: 15 April 2000.
___________________BRAZILIAN JOURNAL OF PHYSICS---SPRINGER
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Submitted on: 01 February 2002.
Revised on: 01 June 2002.
Accepted on: 28 June 2002.
___________________BRAZILIAN JOURNAL OF PHYSICS---SPRINGER
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