Submitted on: 25 April 2004.
Revised on: 13 August 2004.
Accepted on: 29 September 2004.
___________________European Physical Journal Plus
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Submitted on: 11 February 2012.
Revised on: 25 March 2012.
Accepted on: 12 May 2012.
___________________European Physical Journal Plus
Submitted on: 03 August 1998.
Revised on: 23 November 1998.
Accepted on: 11 December 1998.
___________________European Physical Journal Plus
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Submitted on: 16 August 2006.
Revised on: 17 October 2006.
Accepted on: 17 November 2006.
___________________European Physical Journal Plus
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Submitted on: 08 August 2013.
Revised on: 06 September 2013.
Accepted on: 19 September 2013.
___________________European Physical Journal Plus