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Submitted on: 13 September 2013.
Revised on: 01 November 2013.
Accepted on: 16 November 2013.
___________________EUROPEAN JOURNAL OF PHYSICS---IOP PUBLISHING LTD
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Submitted on: 14 October 2011.
Revised on: 10 January 2012.
Accepted on: 19 February 2012.
___________________EUROPEAN JOURNAL OF PHYSICS---IOP PUBLISHING LTD
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Submitted on: 07 June 2003.
Revised on: 24 July 2003.
Accepted on: 14 August 2003.
___________________EUROPEAN JOURNAL OF PHYSICS---IOP PUBLISHING LTD
Submitted on: 30 September 2015.
Revised on: 04 January 2016.
Accepted on: 29 January 2016.
___________________EUROPEAN JOURNAL OF PHYSICS---IOP PUBLISHING LTD