Submitted on: 30 August 1999.
Revised on: 09 November 1999.
Accepted on: 23 November 1999.
___________________East European Journal of Physics
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Submitted on: 22 May 2012.
Revised on: 31 July 2012.
Accepted on: 13 September 2012.
___________________East European Journal of Physics
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Submitted on: 25 May 2012.
Revised on: 08 August 2012.
Accepted on: 02 September 2012.
___________________East European Journal of Physics