Submitted on: 03 October 1999.
Revised on: 07 November 1999.
Accepted on: 24 December 1999.
___________________East European Journal of Physics
Submitted on: 10 February 2016.
Revised on: 03 April 2016.
Accepted on: 21 April 2016.
___________________East European Journal of Physics
Submitted on: 10 October 2002.
Revised on: 02 December 2002.
Accepted on: 29 December 2002.
___________________East European Journal of Physics
Submitted on: 12 October 2020.
Revised on: 29 December 2020.
Accepted on: 26 February 2021.
___________________East European Journal of Physics
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