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Submitted on: 03 June 2004.
Revised on: 02 August 2004.
Accepted on: 20 September 2004.
___________________EUROPEAN JOURNAL OF PHYSICS---IOP PUBLISHING LTD
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Submitted on: 05 July 2006.
Revised on: 07 October 2006.
Accepted on: 27 November 2006.
___________________EUROPEAN JOURNAL OF PHYSICS---IOP PUBLISHING LTD
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Submitted on: 07 August 2009.
Revised on: 09 November 2009.
Accepted on: 24 December 2009.
___________________EUROPEAN JOURNAL OF PHYSICS---IOP PUBLISHING LTD
Submitted on: 27 December 2016.
Revised on: 24 March 2017.
Accepted on: 22 April 2017.
___________________EUROPEAN JOURNAL OF PHYSICS---IOP PUBLISHING LTD
Submitted on: 28 January 2000.
Revised on: 15 April 2000.
Accepted on: 16 May 2000.
___________________EUROPEAN JOURNAL OF PHYSICS---IOP PUBLISHING LTD